Tellurium Promoted Formation of Anionic Species and Reaction with Electrophiles

About a quarter of a century ago, Tellurium was considered almost as an exotic element and was treated with certain diffidence by the most serious chemists. Then the explosive development of Selenium chemistry called attention to the potentiality of Tellurium analogues, resulting in studies on preparation methods for inorganic and organic tellurium compounds and particularly on their applications in Organic Synthesis. In the mid 50’s, H. Rheinboldt published the first review (about 50 pages) on selenium and tellurium compounds in organic chemistry, then in 1990 K. Irgolic dedicated as much as 1000 pages in a review on tellurium compounds. The impressive number of publications on this subject during the last few years shows that Tellurium already plays a new role as a powerful chemical tool. Herein we present an overview of the most important applications of Tellurium compounds in Organic Synthesis.


Introduction
About a quarter of a century ago, Tellurium was considered almost as an exotic element and was treated with certain diffidence by the most serious chemists.Then the explosive development of Selenium chemistry called attention to the potentiality of Tellurium analogues, resulting in studies on preparation methods for inorganic and organic tellurium compounds and particularly on their applications in Organic Synthesis.In the mid 50's, H. Rheinboldt published the first review (about 50 pages) on selenium and tellurium compounds in organic chemistry 1a , then in 1990 K. Irgolic dedicated as much as 1000 pages in a review on tellurium compounds 1b .The impressive number of publications on this subject during the last few years shows that Tellurium already plays a new role as a powerful chemical tool.Herein we present an overview of the most important applications of Tellurium compounds in Organic Synthesis.

Reduction
A large variety of organic substrates can be reduced by tellurium reagents, for instance carbonyl compounds and their α,β-conjugated analogues, aryl alkenes and alkynes, nitro compounds and other nitrogenated derivatives.Compounds bearing heteroatoms can be submitted to reductive cleavage of the carbon-heteroatom bond.The most commonly used reagents are H2Te, NaHTe, Na2Te, PhTeH and PhTeNa.They offer some advantages over conventional reagents such as milder reaction conditions, better regiose-lectivity, in situ preparation of the reducing reagent, sometimes in catalytic conditions, and the recovery of tellurium in the elemental form or as diphenyl ditelluride.

Reduction of carbonyl compounds
Aldehydes and ketones can be reduced to the corresponding primary and secondary alcohols by H2Te and PhTeH.The former is prepared by in situ hydrolysis of Al2Te3 and the latter by treatment of PhTeLi with CF3CO2H or methanolysis of PhTeSiMe3 [2][3][4] .An additional method employs the Bu2Te/TiCl4 system to generate Ti (III) species, which is the actual reducing agent 5 .
Selective reduction of the C=C bond of α,β-conjugated carbonyl compounds (aldehydes, ketones, esters, acids and amides) is achieved by treatment with H2Te, 2 PhTeH 3,4 and NaHTe. 3,6The latter is prepared by treatment of elemental tellurium with NaBH4 in ethanol under controlled acidification with acetic acid.are reduced to the corresponding amines by H2Te, NaHTe and PhTeH.An interesting and useful application is the one-step reductive alkylation of amines with carbonyl compounds 10a,11 (Eq.1).

Reduction of oxiranes
Oxiranes can either be directly deoxygenated to alkenes by (EtO)2P(O)TeM (M = Na, Li) 12 in a stereoselective manner or via a two-step methodology, by reduction with NaHTe to a telluro alcohol (1), then a tosylation-elimination sequence furnishes an unstable epitelluride intermediate (2) which easily detellurates to an alkene 13 .Telluro alcohol (1) can also undergo reductive detelluration with nickel boride affording the corresponding alcohol, which in turn can be further oxidized to ketones 13 (Scheme 1).
α,β-Epoxyketones are reduced to the corresponding β-hydroxy ketones by NaHTe 14 .Epoxides bearing leaving groups in a suitable position are converted to allylic alcohols via formation of an epitelluride intermediate 15 .This methodology, combined with the Sharpless kinetic resolution of secondary allylic alcohols, provides a useful procedure for the conversion of racemic alcohols to pure single enantiomers 16 (Scheme 2).

Debromination of vicinal dibromides
This reaction proceeds via a typical anti-E2 elimination by usage of reagents such as diaryl tellurides 17a and ditellurides 17b , NaHTe 17c , Na2Te 17d , (EtO)2P(O)TeNa 17e , 2-ThTeNa 17f and (Ph3Sn)2Te 17g .The advantages over the conventional methods are milder experimental conditions, higher yields, lack of side reactions and inertness of many functional groups towards the tellurium reagents.Several procedures can also be carried out using catalytic quantites of tellurium compound in the presence of a suitable reducing reagent, for instance potassium metabisulfite, for the regeneration of tellurium reagent 17h (Scheme 3).

Reductive cleavage of carbon-heteroatom bonds
Several tellurium reagents promote the title reductive cleavage on α-halo ketones, whereas thienyl tellurolate can also remove other heteroatom substituted leaving groups 18 (Eq.2).

Tellurium Promoted Formation of Anionic Species and Reaction with Electrophiles
Nucleophilic attack of Na2Te at the halogen on α-halo esters or nitriles leads to the corresponding carbanions which react with aldehydes giving α,β-unsaturated compounds according to Reformatsky-type reactions 19 .The same tellurium reagent undergoes reaction with Ph2S2 or ArSO2Cl affording thiolate and sulfinate anions through an electron transfer from the telluride ion.Further in situ reaction with α-halo carbonyl compounds or alkyl halides leads to α-thio carbonyl compounds and sulfones 20 (Scheme 4).
Bu2Te promotes in situ generation of triphenyl methylene phosphorane from the parent phosphonium iodide for further Wittig methylenation reactions.The peculiarity of this methodology is the formation of phosphorane in neutral conditions 21 (Scheme 5).

Deprotection of Organic Functionalities
Anionic tellurium species offer great possibilities for the regeneration of protected functional groups, the reactions proceeding under mild conditions in non-hydrolytic media.Carboxylic acids protected as alkyl or benzyl esters 22a , as well as phenacyl 22c , allyl 22d and β-halo ethyl carboxylates 22b,e,f can be regenerated by NaHTe, Na2Te and Na2Te2.Phenols can be regenerated from aryl carboxylates 23a , halo acetates 22e or carbonates 23b by NaHTe and from phenyl allyl ethers 22d by Na2Te.Amines protected as trichloro-t-butyl carbamates can be regenerated by 2-ThTeNa 24 (Scheme 6).

Oxidation
An2TeO is an easily prepared, stable crystalline compound which has been successfully employed as a mild and selective oxidizing reagent, with the advantage of possible regeneration from the corresponding telluride after the oxidative process.(ArTeO)2O has recently been introduced as an oxidizing reagent, similarly to the former oxidant.Typical oxidations 25 are: thiols to disulfides, hidroquinones to quinones, diols and benzyl alcohols to aldehydes, acyl hydrazines to acyl hydrazides, N-phenyl hydroxylamine to nitrosobenzene, benzophenone hydrazone to diphenyl diazomethane and phenyl isothiocyanate to diphenylurea.One important transformation is the oxidation of thio and seleno carbonyl compounds to the oxo-analogues.These oxidations can also be accomplished by a catalytic procedure (Scheme 7).
By treatment with (PhTeO)2O in boiling AcOH under sulfuric acid catalysis, alkenes give rise to syn-diacetoxylated products 26 .Other methodology employs TeO2/LiBr or TeCl4/LiOAc in AcOH.Aryl alkenes can also be dimethoxylated by treatment with Ph2Te2 and t-butyl peroxide in MeOH and sulfuric acid 27 .

Telluro Cyclofunctionalization
Addition of an electrophilic tellurium reagent, for instance ArTeCl3, to alkenes C=C bond bearing a nucleophilic moiety at a suitable position, followed by intramolecular trapping of the telluronium intermediate furnishes cyclic structures where the tellurium moiety lies outside the newly ring.Such reactions are known as tellurocyclofunctionalization and they can be either lactonization 28 or etherification 29  R Other useful reagents for these cyclizations are: PhTeOSO2C6H4NO2-4 30a for lactonizations and etherifications, and ArTe(O)OAc 30b , TeO2/AcOH/LiCl 30c and TeO2/HCl/MeOH 30d for etherifications.Similar reactions are also well known with the selenium analogs.Reductive removal of the tellurium moiety from the cyclic products can be achieved by further reaction with Bu3SnH 31 .

Replacement of Tellurium Moiety with Other Atoms
Replacement by halogens: halogenodetelluration ArTeCl3 and Ar2TeCl2 bearing a para-electron-donating group on the aromatic ring undergo Te/I exchange in the presence of fluoride anions through an electrophilic mechanism 32a .A similar reaction has also been carried out on vinylic tellurium trichlorides (3) with I2 or NBS.Since compounds 3 are easily prepared from phenylacetylenes, this two-step sequence offers a simple methodology for the syn-iodo-or bromo-chlorination of alkynes 32b (Scheme 9).
Ipso-substitution (or α-elimination) on tellurium (IV) halides, i.e. selective transfer of halogen from tellurium to the carbon moiety 33 , can be performed on suitable substrates such as RTeCl3 (R= alkyl, vinyl, aryl), Ar2TeCl2 and PhTe(Br)2R (R= alkyl) by oxidative, photolytic or pyrolytic procedures depending on the tellurium substrates.The latter procedure is useful for homologation of alkyl halides (Scheme 10).Other halogenodetellurations can also be accomplished by reaction of α-dichloro telluro ketones 34a and acetylenic tellurides 34b with halogens.

Replacement by alkoxy group
Alkyl phenyl telluroxides are easily obtained by oxidation of parent tellurides with mCPBA or by alkaline hydrolysis of the corresponding dibromides.Treatment of telluroxides with excess mCPBA or CF3CO3H in alcohols leads to detelluration and formation of alkyl ethers.Alkyl phenyl tellurones, obtained by oxidation of telluroxides with NaIO4 give similar results under the same conditions 35a (Scheme 11).
Phenyl migration 35a has been reported on structures bearing a phenyl group vicinal to the tellurium moiety.This  methodology has been used to prepare α-aryl propanoic acids, which are important anti-inflamatory pharmaceuticals, from aryl ethyl ketones 35b .Ring contractions of 1-tellurium-2-alkoxy-cyclohexanes has also been accomplished under similar detellurative methoxylation conditions 35a .

Replacement by hydrogen
Phenyl alkyl tellurides undergo mild reductions with Ph3SnH furnishing the corresponding alkanes 36 .Since the starting tellurides are easily prepared from nucleophilic substitution of alkyl halides with tellurolate anions, the overall sequence offers a mild reduction methodology for alkyl halides.

Detellurative coupling reaction
The extrusion of tellurium from organotellurium compounds leading to coupled products can be achieved by Raney-Ni 37a,b , Pd (0) 37c or Li2PdCl4 37d (Scheme 12).

Detellurative cross-coupling reaction
Alkenes bearing electron-withdrawing groups undergo arylation or vinylation by treatment with Ar2TeCl2, ArTeCl3, Ar2Te or vinylic tellurides 38 in the presence of PdCl2 or Pd(OAc)2 (Scheme 13).These cross-coupling reactions can be carried out using catalytic amounts of Pd (II) salts in the presence of a suitable oxidant such as t-BuOOH, CuCl2 or AgOAc.Under Ni (II) or Co (II) phosphine complex catalysis, some organotellurides react with Grignard reagents furnishing both cross-coupled and homo-coupled products 39 (Scheme 13).

Detellurative carbonylation
Pd (II) also promotes detellurative carbonylations 40 of alkyl, vinyl, alkynyl and phenyl tellurides with CO/MeOH/Et3N giving methyl carboxylates.While performed in the presence of CuCl2 as oxidant, only catalytical amount of Pd (II) is needed.The same methodology is also suitable to prepare substituted butenolides starting from hydroxy vinyl tellurides (Scheme 14).

Vinylic Tellurides
Vinylic tellurides are important compounds due to their peculiar behaviour as synthons and intermediates.The general preparative procedure is the addition of organyl tellurolates, obtained from reduction of the corresponding ditellurides with NaBH4, to acetylenes.This addition reaction is highly stereoselective, occurring exclusively in anti mode giving Z adducts 41 (Scheme 15), which is in sharp contrast to the well known hydrostannylation 42a , hydrozirconation 42b and hydroalumination 42c of acetylenes, characterized by syn addition modes furnishing E products. 42-Divinyl tellurides, in turn, are prepared by addition of Na2Te to activated terminal alkynes 41d (Scheme 15).

Detellurative cross-coupling reaction with organocuprates
Substitution of tellurium moiety on vinylic tellurides with retention of the C=C bond geometry has been per- formed with different types of organocuprates.Lithium diorganocuprates 43a furnish cross-coupled products with vinylic tellurides (Eq.3, Scheme 16), and upon reaction with 1,2-ditelluro substituted alkenes, only the terminal tellurium moiety is detellurated.Lower order lithium or magnesium cyanocuprates 43b (Eq.4, Scheme 16) as well as higher order mixed lithium-magnesium, dimagnesium 43c and zinc cyanocuprates 43d (Eqs.5 and 6, Scheme 16) also promote similar reactions.Interestingly, the latter reagents led to complete inversion of the C=C bond geometry in the coupled products.

Olefin Synthesis
Although not so well known as the familiar selenoxide syn-elimination, the corresponding telluroxide elimination 44 has recently gained more attention among the olefin synthesis methodologies.The starting telluroxides are prepared by hydrolysis of parent dibromides or by oxidation of tellurides with mCPBA.The elimination reaction, which is preferential towards the less substituted carbon, can occur either at room temperature or upon heating over 200 °C depending on the starting telluroxide.Combination of this methodology with the easy obtention of alkyl phenyl tellurides from the corresponding halides provides a straightforward route to the dehydrobromination of alkyl bromides.Moreover, this methodology can provide vinyl ethers, allyl ethers and alcohols when combined with the alkoxy telluration of alkenes, or with the oxirane ring opening by tellurolates (Scheme 17).Some noteworthy details have been reported in respect to the elimination reaction, they are: addition of Et3N increasing the yields of elimination products, pyridinyl tellurium 45a moiety as a better leaving group than phenyl tellurium moiety, and alkyl ferrocenyl tellurides 45b being successfully submitted to elimination sequences in the presence of Et3N or tetracyano ethylene.
Allylic telluroxides undergo [2,3]-sigmatropic rearrangements 46 furnishing allylic alcohols after hydrolysis.The same sequence when performed on chiral allylic ferrocenyl tellurides shows evidence of chirality transfer to the allylic alcohol as the rearranged product (Scheme 18).Olefin synthesis via tellurium compounds can also be accomplished by usage of telluronium ylides.They are prepared by treatment of telluronium salts bearing electronwithdrawing groups with bases and can be submitted to Wittig-type olefination reactions with a variety of carbonyl compounds 47 (Scheme 19).A high E stereoselectivity is observed in in reactions with aldehydes.These results are especially noteworthy when compared to those from stabilized sulfonium ylides which are non reactive towards carbonyl compounds.

Transmetalation Reaction
Te/Li exchange -organolithium reagents By treatment of diorganyl tellurides with alkyllithiums, a Te/Li exchange reaction takes place generating the most stable organolithium derivative 41e,49 .This transmetalation is very useful for the preparation of lithium reagents which cannot be achieved by conventional methods such as allyl, benzyl, propargyl, acyl, aroyl and heteroatom substituted methyllithiums.The Te/Li exchange on Z-vinylic tellurides occurs with retention of the original geometry of the C=C bond providing a useful methodology for preparation of Z-vinyllithiums.The resulting new organolithiums can be trapped with a variety of different electrophiles, as shown in the following representative examples (Scheme 20).

Te/Na, K, Mg, Ca, Zn exchange
Transmetalations with Na, K, Mg and Ca were performed on alkyl, alkenyl, aryl, allyl and benzyl tellurides, 50 while Te/Zn exchange reaction 51 followed by coupling with aryl iodide was achieved under palladium catalysis (Scheme 21).

Te/Cu exchange -organocopper reagents
The most promising synthetic application of Z-vinylic tellurides is their transmetalation with higher order cyanocuprates.The counter-ion in the cuprate plays as important role in this reaction.Instead of a detellurative cross-coupling reaction as observed with lithium-magnesium and di-magnesium derivatives, the dilithium cyanocuprate promotes Te/Cu exchange giving higher order vinylic cyanocuprates with retention of the original C=C bond geometry.The resulting Z-vinylic cuprates readily undergo conjugate addition to α,β-unsaturated ketones 52a,b , epoxide ope-ning 52b and coupling reaction with bromo alkynes in the presence of ZnCl2 52c (Scheme 22).
There are some special higher order cyanocuprates such as dimethyl, (2-thienyl, butyl) and (imidazoyl, butyl) dilithium cyanocuprates which carry the so-called ''dummy ligands'' that are non-transferable from the cuprate, therefore avoiding formation of undesired addition by-products.

Radical Reaction
By treatment with Li2Te, allylic halides furnishes 1,5dienes 53 via a coupling reaction of the corresponding allylic radicals.Upon irradiation of a mixture containing the acetyl derivative of N-hydroxy-2-thiopyridone (4), alkyl anisyl telluride and an electrophilic olefin, a radical chain mechanism 54 takes place, which is useful in intramolecular cyclizations affording six-membered rings (Scheme 23).The same mixture is also useful in tellurium mediated addition of carbohydrates to olefins.Recently, telluro methyl and methylene substituted tetrahydrofurans 55 were obtained from epoxides by combination of tellurolate promoted oxirane ring opening and radical cyclizations (Scheme 24).