Ring opening reactions of heterocycles with selenium and tellurium nucleophiles

Damiano Tanini and Antonella Capperucci *
Dipartimento di Chimica “Ugo Schiff”, Università di Firenze, Via della Lastruccia 3-13, 50019 Sesto Fiorentino, Italy. E-mail: antonella.capperucci@unifi.it

First published on 25th June 2019

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1 Introduction

Organoselenium and organotellurium compounds are of significant utility in organic synthesis, materials science, food chemistry, medicinal chemistry, and biology.¹

Since the pioneering studies reported by Sharpless in 1973² on the ring opening reaction of epoxides with selenium nucleophiles, the use of chalcogens in nucleophilic ring opening reactions (NRORs) has been attracting growing interest among chemists.³ Built upon this basis, a number of procedures for the conversion of strained heterocycles into functionalised valuable chalcogen-containing molecules, including β-hydroxy-, and β-amino-selenides and tellurides, have been developed. In addition, several chalcogen-mediated functional-group transformations have been reported and applied in organic synthesis and in organocatalysis. NRORs of epoxides and aziridines represent the method of choice for the synthesis of β-hydroxy and β-amino chalcogenides. These valuable compounds are versatile reagents which, owing to the possibility to selectively functionalise the carbon–chalcogen bond, find broad application in organic synthesis. For example, total syntheses of lycorine,⁴ (−)-galanthamine,⁵ (+)-7-deoxypancratistatin,⁶ and plumisclerin A⁷ (Fig. 1) all share a selenium-mediated ring opening reaction key step.

Furthermore, owing to their biological properties, functionalised organoselenides and organotellurides are interesting drug candidates with potential applications in medicinal chemistry.^8,9

In this Perspective, the range of ring-opening reactions currently available for the synthesis of functionalised organoselenium and organotellurium compounds is reviewed. The methodologies covered include NRORs of three-, four-, five-, and six-membered heterocycles with selenium and tellurium nucleophiles; particular attention is devoted to novel methodologies and to the use of “unconventional” reagents, developed over the last two decades.

2 Ring opening of three-membered heterocycles

2.1 Synthesis of unsymmetric selenides and tellurides

Several synthetic methodologies based on the ring opening reactions of three-membered heterocycles with selenium and tellurium nucleophiles have been reported. The reductive cleavage of the chalcogen–chalcogen bond of diselenides or ditellurides and the insertion of a chalcogen atom into a metal–carbon bond are arguably the most widely employed methods to generate nucleophilic metal organic selenolates and tellurolates.^3a,10 A large variety of reagents have been used to reduce diselenides and ditellurides. Metal-hydrides, such as NaBH₄, NaH, LiBEt₃H, and LiAlH₄, are, along with sodium, the most commonly employed reducing agents (Scheme 1). The harsh reaction conditions and the poor functional group tolerance are the main drawbacks of metal-hydride-based procedures.

An interesting example of generation of nucleophilic selenium and tellurium reagents under simple and mild conditions was reported by Lenardão and co-workers.^11a Chalcogenated anions were in situ obtained from the corresponding diselenides 1, or ditellurides 2, and NaBH₄/Al₂O₃ under solvent-free conditions, and then reacted with epoxides and lactones to provide β-chalcogenated alcohols 3 and 4 and acids 5 and 6 (Scheme 2). The enantiopurity and absolute configuration of secondary alcohols were determined through NMR spectroscopy using BINOL/DMAP as a chiral solvating agent (CSA) and naproxen derivatives as chiral derivatizing agents.^11b

Ring opening of unactivated 2-methylaziridine was more efficient with NaBH₄/PEG-400, leading to the β-seleno amine 7 in high yield, while lower yields were observed for the corresponding β-telluro derivative 8 (Scheme 2).

Recently, besides these “traditional” reagents a number of new methods have emerged for the preparation of selenium and tellurium nucleophile species, which often avoid the use of strong basic conditions, which could not be tolerated by some functional groups.

Chandrasekaran and co-workers reported a rongalite-promoted synthesis of β-hydroxy and β-amino selenides from diaryl diselenides and epoxides or aziridines (Scheme 3).¹²

The methodology is stereospecific and regioselective; the NROR generally occurs on the less hindered carbon of the three-membered heterocycle. A mixture of regioisomers is formed when using 2-aryl-substituted (products 7e and 7e′, Scheme 3) or some di- or tri-substituted substrates. The same procedure could also be applied to the reduction of diaryl disulfides, enabling to access β-functionalised sulfides, upon treatment of the in situ generated thiolate with epoxides or aziridines. However, although variously substituted epoxides and protected aziridines could be employed, the reaction is limited to diaryl diselenides and diaryl disulfides. A related rongalite promoted procedure, developed by Xu, Yu, and co-workers, allowed the regioselective synthesis of β-hydroxy tellurides by using diaryl ditellurides and monosubstituted epoxides.¹³

Over the last two decades, a number of zinc-mediated ring opening procedures for the synthesis of functionalised selenides have been reported. Such procedures proceed through the reaction of epoxides and aziridines with zinc selenolates, generated by reductive cleavage of the corresponding diselenides. Movassagh and Shamsipoor reported that phenyl and benzyl selenolates can be easily generated from the corresponding diselenides upon treatment either with Zn/AlCl₃ or with zinc powder in NaOH aqueous solution. In situ reaction of the so generated zinc selenolates with epoxides afforded β-hydroxy selenides in high yields with generally good regioselectivity.¹⁴

Santi, Tiecco and co-workers reported an efficient zinc-mediated reduction of diselenides into the corresponding selenols, occurring in a biphasic system under acidic conditions (Scheme 4).¹⁵ Selenols generated under these conditions react in situ with epoxides, enabling the synthesis of β-hydroxy selenides. The NROR was found to be highly regioselective, generally occurring on the less substituted carbon of the oxirane. Notably, the nucleophilic attack occurred almost exclusively on the more hindered carbon (r.r. 19:1) when using styrene oxide.¹⁵

This zinc-mediated methodology was also applied by Braga and co-workers to the synthesis of β-seleno amines through the ring opening of unprotected aziridines with diaryl and dialkyl diselenides (Scheme 5).¹⁶ The reaction was amenable to variously substituted diselenides, although lower yields were observed when using dialkyl diselenides and diaryl diselenides bearing electron-donating groups or ortho-substituted hindered rings. Notably, β-phenyltelluro amine 8a was obtained from diphenyl ditelluride under the same reaction conditions, although in rather low yield.

Furthermore, Santi, Tiecco and co-workers found that (phenylselenenyl)zinc chloride, a bench-stable selenolate obtained from zinc and phenylselenyl chloride by oxidative insertion of Zn into the Se–Cl bond, could be efficiently employed in epoxide ring-opening reactions, occurring “on water” at room temperature with high yields and high regioselectivity (Scheme 6, reaction a).¹⁷

In a related study, Braga and co-workers reported an efficient NROR of protected and unprotected aziridines with (phenylselenenyl)zinc bromide in ionic liquids (Scheme 6, reaction b).¹⁸

Ionic liquids were also employed as reaction media and promoters in CuO nano particle-mediated ring opening reaction of aziridines with diorganyl diselenides, enabling the synthesis of chiral β-seleno amino derivatives.¹⁹

Procter and co-workers reported the ring opening reaction of epoxides with ytterbium(III) chalcogenolate complexes 9, conveniently prepared by insertion of ytterbium metal into the chalcogen–chalcogen bond of disulfides, diselenides, and ditellurides.²⁰ Such lanthanide(III) chalcogenolate complexes are believed to play a dual role in the reaction with epoxides, both delivering the nucleophile species and activating the electrophile due to their Lewis acid behaviour. Furthermore, owing to the oxophilicity of lanthanides, the formation of ytterbium–oxygen bonds probably represents the driving force of the reaction. Variously alkyl-substituted epoxides were converted into the corresponding β-arylchalcogeno alcohols through this regio- and chemo-selective route (Scheme 7). On the other hand, a mixture of regioisomers was formed when styrene oxide was used. Furthermore, the scope of the reaction is limited to the use of diaryl dichalcogenides, as only poor yields were achieved employing alkyl analogues.

Chiral β-seleno amine derivatives were also synthesized under different conditions upon treatment of aziridines with indium(III)-chalcogenolates, obtained from indium(I) chloride and diselenides.²¹ Indium(I)-compounds are able to generate the related In(III) derivatives through their oxidative insertion into suitable substrates (Scheme 8).

Indium selenolate proved to be faster than the selenolate anion generated with NaBH₄/EtOH in promoting the ring opening of aziridines. This could be ascribed to the coordination of the In(III)-complex through the oxygen of the N-Boc, promoting the nucleophilic attack of the selenium (Fig. 2). In fact, when trityl was used as a protecting group, no reaction was observed.²¹

It is worthwhile to mention that using aziridine-2-carboxylates (R¹ = CO₂Me) as a starting material, a series of selenocysteine and selenothreonine derivatives with interesting structural diversity were straightforwardly prepared, without loss of optical purity.

Vargas and Comasseto reported the synthesis of chiral β-telluroamines via the NROR of N-Boc and N-Ts protected aziridines with lithium organotellurolates, in situ generated from elemental tellurium and organolithium compounds (Scheme 9).²² The authors found this method to be more efficient with respect to generating tellurium nucleophiles by reduction of ditellurides or by insertion of a tellurium into carbon–magnesium bonds. The same procedure could also be applied to the synthesis of related aminoselenides and aminosulfides.²²

Ring-opening of epoxides was also achieved by treatment of diselenides in the presence of benzyl(triethyl)ammonium tetrathiomolibdate ([BnNEt₃]₂MoS₄), which enabled cleavage of the Se–Se bond.²³ The reaction was carried out also with precursors of diselenides, such as benzyl bromide (or benzyl alcohol) and KSeCN, which were treated with tetrathiomolibdate to in situ form the selenium anion, able to induce the ring opening of epoxides. β-Hydroxy selenides were so obtained under mild conditions through a one-pot, tandem, multistep procedure (Scheme 10).²³

Selenosilanes and tellurosilanes have also been employed as efficient reagents in ring opening reactions of strained heterocycles. Our interest in the chemistry of silyl chalcogenides²⁴ led us to develop silicon-mediated procedures to access functionalised selenium- and tellurium-containing molecules under very mild conditions (PhOⁿNBu₄ catalysis). In this context, (phenylseleno)trimethylsilane 11 proved to be an effective selenium-transfer reagent,²⁵ providing alternative access to β-phenylseleno alcohols (Scheme 11).²⁶ Interestingly, the reaction could also be performed in ionic liquids.

Tellurosilanes also behave as efficient reagents for the delivery of tellurium functionalities. Ogura and co-workers reported that (phenyltelluro)trimethylsilane 12 behaved as an efficient reagent for the transfer of the phenyltelluro moiety onto epoxides.²⁷

Recently, in order to disclose novel ring-opening-based protocols for the synthesis of functionalised organotellurides, we extended the scope of the reactivity of silyl tellurides to aziridines and thiiranes (Schemes 12 and 13).^28,29 The NRORs occurred with stereospecificity and excellent regioselectivity in the presence of TBAF.

Interestingly, besides N-Boc and N-Ts protected aziridines, the reactivity of arylchalcogeno silanes was successfully extended to N-H unactivated analogues to enable the synthesis of selenides, tellurides, and sulfides bearing free amino functionalities, although with lower yields with respect to their N-activated analogues (Scheme 13). Taking into account that chalcogenoamines are valuable small molecules both in medicinal chemistry^8a,30 and in organic synthesis,³¹ the development of a general and simple method for their synthesis appears highly desirable.

Furthermore, besides silyl selenides, also organoselenides containing Se–B or Se–Sn bonds have been employed in ring opening reactions of epoxides. Nishiyama and Sonoda reported the synthesis of β-hydroxy selenides through the Lewis acid-promoted reaction of tributylstannyl selenide 16 with oxiranes (Scheme 14, reaction a).³² The formation of β-hydroxy selenides could also be accomplished by using selenoboranes 17. However, as the formation of variable amounts of the corresponding olefins was also observed depending on the substrate and the reaction conditions, this transformation proved to be less synthetically useful (Scheme 14, reaction b).³³

The reactivity of silyl selenides with strained heterocycles has also been exploited for the synthesis of enantioenriched β-hydroxy- and β-amino-selenides through desymmetrization of meso-epoxides³⁴ and meso-aziridines.³⁵ Enantioenriched β-phenylseleno alcohols were achieved upon treatment of meso-epoxides with tert-butyl(dimethyl)(phenylseleno)silane in the presence of chiral salen(chromium) complexes (Scheme 15).

The enantioselectivity of the process proved to be strongly influenced by the structure of the starting material. Rather good results were obtained in the synthesis of variously substituted stilbene derivatives (Scheme 15, a, 3f–h). On the other hand, the ring opening reaction of cyclic or alkyl epoxides generally gave lower enantioselectivities (Schemes 15, 3c).³⁴

In related work, enantioenriched β-phenylseleno amines 7s–u were prepared by desymmetrization of meso-N-acylaziridines employing (phenylseleno)silanes as nucleophiles and VAPOL-derived phosphoric acid (R)-18 as the catalyst (Scheme 15, b). The best results in terms of yield, enantioselectivity, and convenient reaction time were obtained upon using a mixture of (phenylseleno)trimethylsilane 11 and benzeneselenol 19a as nucleophiles (Scheme 15, b).³⁵

Selenols have also been used as nucleophiles in NRORs of three-membered heterocycles. Rao and co-workers developed a procedure for the synthesis of β-hydroxy selenides through the ring opening reaction of epoxides with benzeneselenol, occurring under mild conditions in the presence of β-cyclodextrin in water.³⁶ Such a methodology enables the synthesis of variously substituted β-phenylseleno alcohols under supramolecular catalysis. The ring opening reaction proceeds with high regioselectivity via the formation of epoxide–β-CD inclusion complexes, activating the oxirane and leading to the nucleophilic attack on the less hindered carbon of the strained heterocycle.³⁶

Ionic liquid-promoted ring opening reactions of epoxides with aryl selenols have also been described.³⁷ Benzeneselenol and 1-naphthaleneselenol efficiently reacted with substituted epoxides in [bmim]BF₄ to afford the corresponding β-arylseleno alcohols in good yield with moderate to good regioselectivity. ¹H NMR studies suggested that [bmim]BF₄ promotes the ring opening reaction by activating the epoxide through hydrogen bond interactions involving the C2 proton of the imidazolium cation and the oxygen of the epoxide.³⁷

A heterometallic Ti–Ga–Salen catalysed enantioselective ring opening reaction of meso-epoxides with aryl selenols (benzeneselenol and 1-naphthaleneselenol) was also reported by Zhu and co-workers (Scheme 16).³⁸

In the proposed reaction mechanism the two metal Lewis acids of 20 act synergistically, activating the epoxide and directing the nucleophilic attack of the selenol. This methodology enables the synthesis of optically active arylseleno alcohols in high yields and with generally good enantioselectivity. The best results were obtained when benzeneselenol was employed as a nucleophile, lower enantioselectivity levels being observed when 1-naphthaleneselenol was used (Scheme 16).³⁸

2.2 Three-component ring-opening reactions

Recently, alternative procedures based on three-component ring-opening reactions of oxiranes with selenium powder and alkyl iodides, arylboronic acids, or terminal alkynes have also been reported.

Wu, Liu, and co-workers developed a silver-catalysed one-pot procedure for the synthesis of β-hydroxy selenides from arylboronic acids 21, selenium powder and epoxides.³⁹ This methodology allows the synthesis of a wide range of functionalised 2-arylseleno alcohols from readily available starting materials. β-Hydroxy selenides bearing substituted arylseleno or heteroarylseleno moieties could be achieved in moderate to good yields. However, although the ring opening arylselenation of epoxides could be performed with a wide range of arylboronic acids, the related alkenylselenation reaction proved to be less effective (compound 3q, Scheme 17).

This one-pot protocol was also applied to the synthesis of natural-product-derived selenides 3t and 3u, from the corresponding sesamyl- and tocopheryl-glycidyl ethers and phenylboronic acid (Scheme 17).

The proposed mechanism for the three-component cascade reaction involves the base-promoted disproportionation of elemental selenium, followed by a silver-catalysed radical selenation of the organoboronic acid, and a final selenium-mediated ring opening of the epoxide (Scheme 18).³⁹

Wu, Liu, and co-workers reported the synthesis of β-arylseleno alcohols via a copper catalysed three-component ring opening reaction of epoxides with selenium powder and aryl iodides.⁴⁰ The methodology provides straightforward access to a wide variety of hydroxy-substituted aryl–alkyl selenides with good functional group tolerance (Scheme 19). The reaction is thought to proceed through a S_N2-type NROR of the epoxide by the arylselenium–copper intermediate, in situ formed from elemental selenium and the aryl iodide.⁴⁰

The same group also reported a metal-free synthesis of alkynyl alkyl selenides 3ab–3aj through the reaction of terminal alkynes, selenium, and epoxides.⁴¹ Such a methodology is amenable to a variety of terminal alkynes and epoxides, including bioactive molecules such as pargyline (Scheme 20).

The reaction proceeds via double C–Se formation and the proposed mechanism involves an initial disproportionation of elemental selenium to generate Se²⁻, which reacts with the epoxide to form an alkylselenolate anion. The latter is then oxidised to the corresponding diselenide, which, upon reaction with the terminal alkyne in the presence of the base, affords the alkynyl alkyl selenide (Scheme 21).⁴¹ Notably, the first step is analogous to that proposed for the reaction of Se(0) with arylboronic acids (Scheme 18).

2.3 Synthetic applications of NRORs with chalcogens

As stated in the introduction, NRORs of suitably substituted strained heterocycles with selenium and tellurium nucleophiles have been applied to the synthesis of heterocycles with different ring-size and biologically relevant organochalcogenides. For example, β-functionalised organochalcogenides have been employed by Engman and co-workers as valuable intermediates for the synthesis of tetrahydrofuran- and pyrrolidin-derivatives.⁴²

β-Hydroxyalkyl phenyl selenides, obtained through the reaction of phenylselenide anions with epoxides, behaved as efficient Michael donors with ethyl propiolate to afford substituted alkyl vinyl ethers as pure (E)-isomers. These compounds were found to behave as radical precursors to give 2,5-disubstituted tetrahydrofuran-3-ones via carbonylation/reductive cyclization (Scheme 22).⁴²

It is noteworthy that also phenyltellurolate was reacted with epoxides, and the obtained β-hydroxyalkyl phenyl tellurides afforded tetrahydrofuran-3-ones through the same route. Interestingly, this route was also extended to aziridines, enabling the synthesis of pyrrolidin-3-ones (Scheme 22).⁴³

An interesting example of the application of selenolates in NRORs is represented by the stereoselective formation of chiral β-phenylseleno amine, which was used as a starting material to prepare enantiopure γ-aminocyclopentene derivatives (Scheme 23). These compounds represent important intermediates for the synthesis of carbocyclic nucleosides and γ-amino acid analogues.⁴³

Ring opening of the chiral aziridine with phenyl senolate afforded the trans-aminoselenide (1S,2S,1′S)-22 as major stereoisomer, which was transformed into the corresponding N-benzyl-allyl amine (1S,1′S)-23 by oxidation with hydrogen peroxide. This compound was then treated with OsO₄/NMO and NaIO₄ to afford selectively the desired γ-aminocyclopentene aldehyde (S)-24 and the related γ-amino acid (S)-25via an intramolecular selective aldol-condensation in the absence of an external base, the amino group behaving as an internal base able to catalyse the ring closure.⁴⁴

Also ring opening of a chiral epoxide (26) by diphenyldiselenide in the presence of N-acetylcysteine represents one of the key steps for the formal synthesis of the hepatite B virus inhibitor entecavir (Scheme 24). Other key features of the procedure include a Sharpless asymmetric epoxidation, a Morita–Baylis–Hillman reaction and a Riley selenium dioxide oxidation, leading in ten steps to entecavir.⁴⁴

Another example of a chalcogen-promoted reductive-epoxide ring opening reaction was reported for the total synthesis of the enantiomer of the marine sponge diterpenoid (−)-nakamurol A. In this case, a suitable chiral epoxide was treated with tellurium in the presence of rongalite (HOCH₂SO₂Na).⁴⁵ The so generated Te²⁻ induces a S_N upon the Ts-group followed by opening of the epoxide, with the formation of an epi-telluride, which loses Te⁰ through a stereocontrolled process, leading to ent-nakamurol A (Scheme 25).

Ring opening of chiral epoxides bearing an oxygenated aliphatic chain by phenylselenolate was used to obtain differently substituted hydroxy selenides as precursors of cis-disubstituted tetrahydrofurans 27via 5-endo cyclization of seleniranium ions (Scheme 26).⁴⁶

A particular example of intramolecular cyclization of selenated δ-lactones was induced by silica gel during their purification, leading to the γ-lactone 28 with high d.r. The acidic conditions caused the protonation of δ-lactones 29 followed by intramolecular Se-attack at C(6) and subsequent ring opening, with the formation of the intermediate seleniranium ion 30, which then cyclized to yield 28 (Scheme 27).⁴⁷

Selenosteroids, containing a combination of oxysterols and selenium, could have great potential for biological applications as pro-oxidant biomolecules. Therefore, ring opening of the epoxide of cholesterol by treatment with diselenides under NaBH₄ or Zn(0)/HCl conditions represented an interesting way to introduce stereoselectively selenium in the steroidal nucleus, through an anti-epoxide ring-opening and formation of trans-hydroxy selenides 31 (Scheme 28).⁴⁸

Similarly, treatment of the epoxide with dilithium diselenide (Li₂Se₂) led to the β-hydroxy cholesterol diselenide.

Furthermore, a methodology that combines sugar and steroid scaffolds linked by a selenium atom was developed using a sterereoselective nucleophilic substitution of modified human steroids, such as cholesterol and pregnenolone, and plant steroids (stigmasterol and sitosterol), with carbohydrate diselenide derivatives (Scheme 29). The substitution of oxygen by sulfur or selenium in conjugate linkage of biomolecules is frequently used to modify their properties. Cholesterol-derived selenoglycoconjugates 32 were prepared by the ring-opening of the steroid-epoxide with selenium–sugar nucleophiles (Se-pyranosides and furanosides), generated in situ by reductive cleavage of the diselenide bond with NaBH₄ (Scheme 29).⁴⁹

Steroidal selenoglycoconjugates 32 were obtained as single isomers through regioselective epoxide opening by sugar-selenolates.

Another biologically relevant class of selenated compounds is represented by non-natural amino acid derivatives. To access these molecules, β-phenylseleno amines prepared from N-Boc aziridines were reacted with anhydrides of N-protected amino acids to furnish non-natural selenium-containing amino acid derivatives and peptides (Scheme 30).⁵⁰

In this context, thiiranes have been scarcely studied with respect to their oxygen- and nitrogen-containing analogues. Punniyamurthy and co-workers reported the construction of chalcogen-containing heterocycles exploiting the reactivity of thiiranes with isoselenocyanates.⁵¹ The formation of 2-imino-thiaselenolanes 33 and 33′ proceeded through a metal free [3+2] cycloaddition under BF₂OTf·OEt₂ catalysis (Scheme 31). Analogously, isothiocyantes led to the corresponding 2-imino-dithiolanes.

The regioselectivity was dependent on the nature of the thiirane. Phenyl-substituted substrates underwent attack by the nucleophile at the benzylic carbon under electronic control, leading to compound 33. Conversely, when alkyl thiiranes were employed, the attack occurred at the less hindered side due to steric effects yielding regioisomer 33′.

The proposed catalytic mechanism foresees a coordination of the hard Lewis acid with the hard base (N atom), affording the intermediate 34, which reacts with the thiirane via regioselective nucleophilic ring opening at positions 2 or 3, followed by chemoselective [3+2] cyclization to afford the target compounds (Scheme 32).⁵¹

2.4 Synthesis of functionalised dialkyl selenides, diselenides, and selenols

Although numerous ring-opening routes are available to produce aryl-alkyl selenides, the synthesis of functionalised dialkyl selenides and diselenides is significantly less developed. Commonly used procedures towards these compounds employ metal dichalcogenides or chalcogenides, such as lithium and sodium derivatives, normally generated in the presence of metal hydrides.

Braga and co-workers reported the synthesis of enantioenriched β-amino diselenides through the NROR of N-Boc protected aziridines with Li₂Se₂, in situ generated upon treatment of elemental selenium with LiBEt₃H (Scheme 33). The so obtained bi-dentate systems have been applied in asymmetric catalysis as chiral ligands for the enantioselective addition of diethylzinc to aldehydes (Scheme 33).⁵²

Alcohols were obtained in high yields with aromatic aldehydes, while lower yields were achieved with aliphatic aldehydes. In all cases the alcohol with (R)-configuration was the predominant isomer observed. From the mechanistic point of view, zinc-selenolate 35 was proposed as the active catalyst of the reaction.⁵²

The harsh reaction conditions represent the main drawback of the metal hydride-based methodologies, therefore limiting their application to substrates which do not contain labile or base-sensitive moieties.

In this scenario, we recently addressed this issue by disclosing convenient alternative mild silicon-mediated routes towards functionalised dialkyl chalcogenides and dichalcogenides. Particularly, bis(trimethylsilyl)selenide 36 – or hexamethyldisilaselenane (HMDSS), (Me₃Si)₂Se – behaved as a very versatile selenium transfer reagent, enabling the synthesis of different classes of organoselenium compounds upon reaction with strained heterocycles in the presence of a catalytic amount of fluoride or phenolate anions. Epoxides, aziridines, and thiiranes could be selectively converted into the corresponding β-hydroxy-, β-amino-, and β-mercapto-selenides (37–39) or diselenides (40–42) by just tuning the amount of (Me₃Si)₂Se.⁵³ Indeed, selenides were the exclusive reaction products observed when using an excess of the electrophile (ca. 2.0 eq.) with respect to the selenosilane. On the other hand, diselenides were solely formed upon using an excess of HMDSS (Scheme 34). Notably, these silicon-mediated NRORs occurred stereospecifically with excellent regioselectivity, only the product arising from the nucleophilic attack onto the less hindered carbon atom of the strained heterocycle being observed.

Furthermore, the reactivity of (Me₃Si)₂Se 36 with thiiranes also enabled the synthesis of 3,7-disubstituted dithiaselenepanes 43, reasonably arising from the oxidative ring closure of the bis-silyl-sulfide intermediate. Notably, this procedure also allowed the synthesis of trithiepanes 44 when (Me₃Si)₂S, the sulfurated analogue of HMDSS, was employed (Scheme 35).⁵⁴

A plausible mechanism for these silicon-mediated ring opening reactions (Scheme 36) involves the formation of silicon hypervalent species. In the first step, a pentacoordinate species 45 is formed by coordination of the fluoride ion with the silicon atom of (Me₃Si)₂Se 36. The hexacoordinate silicon intermediate 46, formed upon interaction of Si with the heteroatom of the strained heterocycle, undergoes regioselective nucleophilic attack of Se leading to the key intermediate 47. TBAF is also released in this step, thus accounting for the catalytic role played in such reactions. The fate of the intermediate 47 depends on both the stoichiometry of the reaction and the reaction conditions. Indeed, selenides 37–39 and diselenides 40–42 can be selectively achieved as stated above.

Notably, we recently found that, owing to the very mild reaction conditions of this novel silicon-mediated route, β-hydroxy-, β-amino-, and β-mercapto-selenols 48–50 could be efficiently achieved from the corresponding three-membered heterocycles and HMDSS under strictly controlled reaction conditions, using citric acid as the proton source (Scheme 37).⁵⁵

The synthesised β-functionalised alkyl selenols exhibited unexpected stability and could be easily handled and employed in further unexplored transformations. Indeed, owing to the unique features of the selenol moiety, selenols can be selectively functionalised with a wide array of electrophiles under very mild conditions, enabling the synthesis of a plethora of organoselenium compounds^55–5851–58, which would have been difficult to access through other methodologies (Scheme 38).

Recently, we also evaluated the reactivity of (Me₃Si)₂Te with strained heterocycles. However, its instability prompted us to employ a different Te-nucleophile such as Li₂Te, in situ generated by reduction of elemental tellurium with LiEt₃BH. The reaction of epoxides and aziridines with Li₂Te enabled the first synthesis of β-hydroxy- and β-amino-dialkyl tellurides 59 and 60; on the other hand, under these conditions, thiiranes were directly converted into the corresponding 3,7-disubstituted 1,2,5-dithiatellurepanes 61 (Scheme 39).²⁹

The good functional group tolerance of these procedures allows their application to the synthesis of complex and densely functionalised systems. These structural features are often one of the key criteria to meet in order to develop new effective catalysts and drug candidates. For example, very recently we have applied ring-opening procedures to the synthesis of novel chalcogen-containing carbonic anhydrase modulators^8a,30,59 and catalytic or chain-breaking antioxidants.^56,60

As an alternative method to the use of (Me₃Si)₂Se or Se(0)/LiBEt₃H conditions, β-amino diselenides could also be achieved relying on the reactivity of tetraethylammonium tetraselenotungstate 62 with aziridines. Chandrasekaran and co-workers developed a regioselective and stereospecific procedure for the efficient conversion of monosubstituted and disubstituted N-tosyl activated aziridines into the corresponding diselenides (Scheme 40).⁶¹ However, this route could not be applied to the ring opening of NH unactivated aziridines, which proved to be unreactive upon treatment with selenotungstate 62. The proposed mechanism involves the formation of a key intermediate 63, which affords the diselenide through an internal redox process (Scheme 40).

Tetraethylammonium tetraselenotungstate 62 behaved as a very efficient selenium transfer reagent, enabling also the synthesis of conformationally locked bridged bicyclic diselenides 64 from the corresponding substituted cis-aziridino epoxides (Scheme 41).^61b,62 Interestingly, whilst the aziridine ring-opening always occurs at the more hindered carbon atom, the regiochemistry of the selenium nucleophilic attack on the epoxide strongly depends on the structure of the substrate and may take place at either the less substituted or at the more substituted carbon atom of the oxirane (structures 64f and 64f′, Scheme 41).^61b,62

Furthermore, the functionalised cyclic diselenide 65 was also obtained exploiting the reactivity of tetraethylammonium tetraselenotungstate with the bis-aziridine 66 (Scheme 42).^61b,62

Chandrasekaran and co-workers also reported an unusual selena-aza-Payne-type rearrangement, occurring upon treatment of (N-tosylaziridinyl)methyl tosylates 67 with tetraethylammonium tetraselenotungstate 62.^61b,63 The reaction proceeds via the formation of a selenirane intermediate, which undergoes selenium elimination, leading to the formation of allyl amine derivatives 68 with good regio- and stereo-selectivity. Intriguingly, for most of the substrates employed in the study, the first step of the proposed mechanism is represented by the regioselective nucleophilic attack of the selenotungstate onto the aziridine. The displacement of the OTs group, which generally occurs first in the well-studied Payne, aza-Payne, and thia-Payne, follows the NROR and leads to the formation of products 68 and 69 reported in Scheme 43. The overall result of this rearrangement is nitrogen migration from the C(2), C(3) to the C(1) carbon (Scheme 43). Interestingly, while aziridinemethanoltosylates 67a were exclusively converted into the corresponding allyl amines 68a with high yields and excellent regioselectivity, trans-aziridinemethanoltosylates trans-67b led to the formation of a mixture of the corresponding allyl amine trans-68b (major product) and the cyclic diselenide trans-69 (minor product). Furthermore, reaction of cis-aziridines cis-67b with selenotungstate 62 furnished a mixture of regioisomers of allyl amines cis-68b and 68b′, together with the cyclic diselenide cis-69.^61b,63

A simple and high yielding telluride-triggered procedure for the conversion of aziridinemethanoltosylates into racemic and non-racemic allyl amines has been reported by Dittmer and co-workers (Scheme 44).⁶⁴ In a related procedure, N-trityl- or N-benzhydryl-protected allyl amines were obtained upon treatment of the corresponding aziridinemethanol sulfonate esters with sodium telluride, in situ generated from elemental tellurium and NaBH₄. Notably, the formed Te(0) can be recovered and reused, thus removing the need for its disposal.⁶⁵

Dittmer and co-workers also developed tellurium-mediated methodologies for the synthesis of allylic alcohols exploiting the reactivity of telluride ions, generated upon treatment of elemental tellurium with various reducing agents, with sulfonate esters of oxiranemethanols. The regiochemistry of these reactions is strongly influenced by electronic and steric factors, as well as by the nature of the nucleophile and the reaction medium, including the presence of Lewis acids and bases.⁶⁶ The reactivity of 2-(chloromethyl)oxiranes with selenide ions has also been investigated.⁶⁷ Furthermore, a catalytic tellurium-based procedure for the transposition of allylic hydroxyl groups and carbon–carbon double bonds has also been developed by Dittmer and co-workers.⁶⁸ Tellurium-mediated methodologies represent useful tools for the synthesis of complex or biologically active molecules. For example, a tellurium transposition reaction has been used as a key step in Dittmer and co-workers’ synthesis of optically active boivinose.⁶⁹

3 Ring-opening of four-, five-, and six-membered heterocycles with chalcogen nucleophiles

Not only three membered heterocycles, but also larger rings (four-, five-, and six-membered derivatives) were found to undergo ring opening with chalcogen-containing nucleophiles.

It was reported that cyclic ethers (THF) upon treatment with dialkyl- or alkylphenylselenium dibromides in the presence of sodium borohydride furnished ω-hydroxyalkyl alkyl or phenyl selenides (Scheme 45).⁷⁰

Reactions of cyclic ethers with tellurium halides have been rarely described. However, an unexpected ring opening of tetrahydrofuran was observed when TeBr₄ was reacted with triphenylphosphine in THF, providing Ph₃PO(CH₂)₄TeBr₄70 as reported in Scheme 46.⁷¹

The structure of 70 was elucidated by X-ray analysis. It is formally a tellurium containing zwitterionic structure, which represents a very rare compound, with the negative charge on the Te atom and the positive charge on the P atom.⁷¹

Ring-opening of tetrahydrofuran was also observed when the in situ formed PhSeZnSePh 71a was reacted in THF with acyl chlorides to prepare selenol esters. In fact, when the seleniun–zinc complex was generated by reduction of diphenyl diselenide in the presence of zinc(0) and trifluoromethansulfonic acid (TfOH) in THF, followed by addition of the acyl chloride, besides the desired selenol ester 72, products arising from the ring opening of THF were unexpectedly observed (Scheme 47).⁷²

Depending on the nature of the acyl chloride (EDG or EWG on the aromatic rings) both side products 73 and 74 were observed in comparable amounts with respect to the selenol ester 72, showing that the ring-opening of THF competes with the reaction of the selenolate with acyl chlorides. Despite several attempts being performed, the selectivity of the reaction was not improved.⁷²

The ring opening reaction of selenium nucleophiles was also carried out with 2-oxazolines 75 to prepare chiral β-seleno amides.⁷³ The reaction proceeded through the regio- and chemoselective attack of aryl or alkyl selenide anions at the C(5) of the ring, affording chiral β-seleno amides 7, bearing different functional groups. It was demonstrated that the nature of the substituent R¹ on the oxazolidine does not play a significant role, and that the aromatic diselenides with the EWG group (Cl) or EDG (MeO) do not evidence a significant electronic influence (Scheme 48).⁷³

It is noteworthy that through this procedure a selenocysteine derivative was synthesized when 2-oxazoline-4-carboxylate was reacted with phenylselenolate.

Taking into account that mixed Se,N compounds represent an efficient class of ligands in asymmetric synthesis, the Pd-catalyzed asymmetric allylic alkylation of propenyl acetate with dimethyl malonate was studied in the presence of suitably substituted chiral β-seleno amides 7. Alkylated products 76 were obtained in good yield with high stereoselectivity (98% ee). The reaction was performed with a variety of dialkyl malonates and different β-seleno amides as chiral ligands, allowing one to obtain the alkylated products in high yields and with different levels of enantioselectivity (Scheme 49).⁷³

Ligands containing other functional groups such as thioether, alcohol, and ester behaved as less efficient catalytic systems in terms of yields and enantioselectivity (essentially racemic products were obtained).⁷³

Ring-opening of oxazolines by selenium nucleophiles was also performed with diselenides under indium-mediated conditions. Both indium iodide and indium metal/RHal were used to form indium selenolates via oxidative insertion (Scheme 50).⁷⁴

As proposed for the ring-opening of aziridines, an In(III) complex is generated, which promotes a regio- and chemoselective ring-opening of oxazolines to obtain non-racemic β-seleno amides and selenocysteine derivatives under mild conditions. Diphenylditelluride was also reacted, to afford β-telluro amides.⁷⁴

The reaction of 2-oxazoline-4-carboxylate with Woolins’ reagent (WR) and NaBH₄ was also investigated by Iwaoka and co-workers, and represents an alternative procedure to access optically pure L-selenocysteine and α-methyl-L-selenocysteine derivatives (Scheme 51).⁷⁵

Chiral β-seleno amines 7 were also obtained via a regio- and chemoselective ring-opening of various N-protected-2-oxazolidinones 77 by phenyl selenolate, followed by decarboxylation. In a similar manner phenyl tellurolate gave β-telluro amines 8 (Scheme 52).⁷⁶

The reaction was also performed with Li₂Se₂ as a nucleophile, affording the β-amino diselenide 41c (Scheme 53), which was demonstrated to act as a catalyst in the reaction of Et₂Zn with aldehydes.⁷⁶

Selenium anions (Na₂Se₂ and Na₂Se) prepared from selenium and sodium borohydride, were employed as nucleophiles in the ring-opening of lactones 78.⁷⁷ Krief and co-workers reported the reaction with γ-butyrolactone 78a to obtain diselenide 79a or selenide 80a as major compounds depending on the ratio Se(0)/NaBH₄ and on the ring opening reaction conditions (Scheme 54).⁷⁴

γ- and δ-lactones were efficiently subjected to nucleophilic cleavage with zinc selenolates, prepared from aryl diselenides and Zn/AlCl₃, affording the corresponding carboxylic acids 81 and 82 (Scheme 55).⁷⁸

When acidic protons were contained in the substrate (78b), the loss of carbon dioxide was observed, leading to the isolation of the corresponding seleno ketones 83 (Scheme 56).⁷⁸

An interesting example involving ring-opening of lactones is the treatment of the serine β-lactone 84 with dilithium chalcogenides, obtained by reduction of elemental selenium or tellurium with super hydride (LiEt₃BH). Optically active L-selenocystine 85 and L-tellurocystine 86 (with Li₂Y₂) or L-selenolanthionine 87 and L-tellurolanthionine 88 with (Li₂Y) were readily prepared in good yields (Scheme 57).⁷⁹

The reaction of serine β-lactone with phenyl telluride anions allowed one to synthesize tellurocysteine conjugates 8t.⁷⁹

Other five membered heterocycles such as 5-membered cyclic sulfamidates 89 were found to behave as versatile electrophiles towards phenyl selenolate, generated by reaction of diphenyl diselenide with rongalite/K₂CO₃, to give enantiopure β-amino phenyl selenides 7 in high yields (Scheme 58).⁸⁰ A regioselective attack of the nucleophile at C(4) was observed, irrespective of the type of substituents. In addition, differently from what was observed with activated and non-activated aziridines, the reactivity of sulfamidates was not dependent on the nature of the group on the nitrogen.

Sterically hindered substrates reacted slowly to afford the desired products in comparable yields (79–81%). Ring cleavage of 6-membered cyclic sulfamidates was achieved as well through this approach, affording γ-amino selenides in good yield.

Also cyclic sulfamidates underwent nucleophilic cleavage upon treatment with potassium selenocyanate followed by reductive dimerization induced by ammonium thiomolybdate [(BnNEt₃)₂MoS₄] through a one-pot multistep procedure. Differently substituted 5-membered cyclosulfamidates 89 were reacted to provide a wide variety of N-alkyl-β-amino diselenides 41 in high yields, including selenocystine derivatives (R² = CO₂Me) (Scheme 59).⁸¹

Exclusive attack of KSeCN at the C–O bond of sulfamidate was observed, followed by attack of MoS₄⁻ on the Se-atom. Hydrolysis of the intermediate complex gave amino diselenides 41.

4 Reactions of activated cyclopropanes with selenium nucleophiles

Chalcogenated nucleophiles were also employed in the reaction with carbocycles such as activated cyclopropane derivatives.

Methylenecyclopropanes 90 (MCPs) found applications as useful building blocks in synthetic organic chemistry. Heating MCPs in the presence of diphenyl diselenide 1 without solvent allowed one to obtain the corresponding ring-opened unsaturated products 91 (Scheme 60).^82a The reaction performed with diphenylphosphine oxide in the presence of a Lewis acid and Se (or S) gave homoallylic selenides (or sulfides) bearing the SeP(O)Ph₂ (or SP(O)Ph₂) moiety.^82b

Based on the finding that PhSeSePh gave the phenylselenyl radical by thermal cleavage, a radical mechanism was proposed for this reaction. A primary addition of the radical to the double bond caused a fast ring-opening via a homoallylic rearrangement. The reaction of the formed alkyl radical with a second equivalent of diselenide through a homolitic substitution (S_H) afforded alkyl-vinyl-1,3-bis(phenyl selenides) 91, with the regeneration of the selenyl radical (Scheme 60).

The reaction with unsymmetric methylenecyclopropanes afforded an almost equimolar mixture of E- and Z-isomers.⁸²

Under similar conditions, reaction of arylvinylidenecyclopropanes 92 with diphenyl diselenide at 150 °C led to 1,2-diarylselenocyclopentene derivatives 93 (Scheme 61).⁸³

As already reported for lactones, zinc selenolates 71, obtained from reductive cleavage of diaryl diselenides with Zn/AlCl₃, were efficient also in the nucleophilic ring-opening of monoactivated cyclopropanes bearing ketone, aldehyde, acid or nitrile moieties, to give the corresponding γ-arylselenyl ketone, acid, nitrile, and aldehyde in good to high yields under mild conditions (Scheme 62).⁸⁴ In the case of the ester derivative, no product was observed due to the competitive cleavage of the acyl-oxygen bond with the formation of phenol in a rather quantitative yield.⁸⁴

A variety of organoselenium compounds were prepared by nucleophilic ring opening of doubly activated cyclopropanes by diselenides/NaBH₄ (Scheme 63).⁸⁵

Besides the most commonly used diphenyl diselenide, differently substituted aryl diselenides were reduced by sodium hydride. The arylselenolates were reacted with varius EWG-activated cyclopropanes, to yield a wide range of γ-bi-functionalized selenides 94.⁸⁵

5 Conclusions and future perspectives

Organoselenides and organotellurides are valuable compounds both in biology and in organic synthesis. The capability of organochalcogenides to undergo chemo-, regio-, and stereo-selective transformations has attracted considerable interest during the last decades. A growing number of organoselenides and organotellurides find wide application in medicinal chemistry, biology, and materials science. Consequently, new methods to synthesise functionalised selenium- and tellurium-containing small molecules have been developed. The availability of a variety of chalcogen-containing nucleophiles, also employable under mild conditions, increases the synthetic utility of ring-opening-based procedures, enabling a facile and convenient synthesis of polyfunctionalised systems. In this perspective more recent developments in the nucleophilic ring opening reactions (NRORs) of heterocycles by chalcogenated nucleophiles, developed in the last twenty years, have been thoroughly discussed. A systematic description is reported, in order to gather methods for the synthesis of chalcogen-containing products through the use of different nucleophiles, arising from various sources, and reacted under different conditions. The aim was to give an overview of the reported methodologies, which can be applied depending on the synthetic needs and the tolerance of the groups present on the specific substrates. Particular attention was focused on the selectivity of the reported methodologies, both from the regiochemical and stereochemical point of view, leading to a wide variety of enantioenriched polyfunctionalized selenium compounds. Some examples are also reported on tellurium-containing derivatives. Additionally, significant synthetic applications have been reviewed to synthesize a variety of heterocyclic and linear systems, such as for example natural and non-natural amino acid analogues, carbocyclic nucleosides, enzyme inhibitors, and terpenoid derivatives. The synthesis of selenosteroids and steroidal selenoglycoconjugates has also been achieved through NRORs.

The results clearly show that ring-opening-based procedures represent a convenient and versatile tool to access a plethora of variously functionalised systems, and significant developments have been accomplished in recent years. Nevertheless, in order to enhance this kind of reactivity and to enlarge its application, attention could be focused on other possible directions: (i) investigation of the behaviour of aliphatic diselenides is scarce, and a deeper study on their use could be convenient; (ii) in this regard, also the application of alternative selenium (or tellurium) nucleophiles (such as for example selenols) could be interesting, to avoid the basic conditions necessary to reduce diselenides or elemental Se (or Te-analogues); (iii) besides the large number of oxygenated and nitrogen-containing strained heterocycles (epoxides and aziridines) in the reaction with selenium (or tellurium) nucleophiles, a very limited number of examples are reported on the reactivity of thiiranes, despite the interest that mixed Se/S systems can exhibit; (iv) further insights into the behaviour of tellurium-containing nucleophiles could be desirable, and can lead to wider application of these derivatives, as well as to deeper investigation of their properties and understanding of their reactivity; and (v) as outlined, most current studies focus on the behaviour of three-membered heterocycles, and the reactivity with larger rings is less explored. Thus, more attention should be paid in this sense, aiming to prepare more functionalized and more complex molecules, with significant structural diversity.

In summary, the behaviour of chalcogenated nucleophiles towards heterocycles can offer new challenges and opportunities towards novel interesting selenated (or tellurated) molecules, or to more complex derivatives accessible starting from selenium (or tellurium)-intermediates.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank MIUR-Italy (“Progetto Dipartimenti di Eccellenza 2018–2022” allocated to Department of Chemistry “Ugo Schiff”).

Notes and references

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