Organotellurium chemistry. 7. Reductive removal of electronegative .alpha.-substituents of ketones and acids by a tellurolate reagent

Abstract

Lithium and sodium 2-thiophenetellurolates have been used for the reductive dehalogenation of a variety of -halo ketones and acids. Acetoxy, mesyloxy, and phenylthio groups were also successfully removed from the a position of an acetophenone. The reductions could in many cases be carried out by using sodium borohydride as the reducing agent in the presence of only a catalytic amount of the organotellurium reagent. Evidence is presented in support of a two-step mechanism involving the formation of -aryltelluro carbonyl derivatives and enolate anions in succession.Methods for the selective removal of certain functional groups adjacent to a carbonyl have recently received considerable attention. In particular, a number of procedures have been developed to bring about the reductive dehalogenation of -halo ketones. Zinc in acetic acid,1 11phos-phines2-5 and various inorganic phosphorus compounds,6 sodium borohydride,7 iodide ion,8-11 thiols12 and selenols,13 titanium trichloride,14 vanadium(II) chloride,15 N¿f-dimethylaniline,16 organotin hydrides,17 sodium dithionite,18 and various metal carbonyls19-21 are some of the reagents that have been used.The recent report of the use of sodium hydrogen telluride for the denomination of uic-dibromides22 and the early observation by Bergson23 that sodium telluride and sodium ditelluride reduced chloroacetic acid to acetic acid suggested that anionic organotellurium reagents might be capable of effecting the reductive dehalogenation of «-halo carbonyl compounds. We now describe the reductions of some «-substituted ketones and carboxylic acids using lithium and sodium 2-thiophenetellurolate (1). Results and DiscussionLithium arenetellurolates are readily available by the insertion of elemental tellurium into the carbon-metal bond of various organolithium compounds. This reaction is especially effective when the aromatic moiety is heter-

α-Telluration of 2-acetylthiophene: Electronic influence of the heteroaromatic moiety on solid state structures

“…Iodo analogues are prepared either by oxidative addition of a-iodo-2-acetylthiophene to tellurium powder to afford 1c or by metathesis of the bromo/chloro compounds (1b, 2a, 3a) with alkali metal iodides. Reduction of these dihalides with Na 2 S 2 O 5 affords the corresponding tellurides, (1,2,3) which are readily oxidized with dihalogens to afford the corresponding diorganotellurium(IV) dihalides (Scheme 1).…”

“…This synthetic protocol is especially effective in the ortho telluration of heteroaromatic substrates [1]. Lithium or sodium arenetellurolates are very good nucleophilic species and can be easily alkylated by alkyl halides to obtain alkylaryltellurides [2]. However, the reaction of lithium 2-thiophenetellurolates with acylmethyl halides results in reductive dehalogenation to give parent ketones [3] instead of the expected acylmethyl(2-thiophenyl)tellurides.…”

Phosphorus(III) Iodide

“…This PI 3 reduction can be coupled with the regioselective opening of terminal epoxides by TMSI and alcohol oxidation for a three-step transformation of a terminal epoxide to a methyl ketone. 15,16 P 2 I 4 , 15 triphenylphosphine, 17 1,3-Dimethyl-2-phenylbenzimidazoline, 18 PhSiH 3 /Mo 0 , 19 Samarium(II) Iodide, 20 Zinc-Acetic Acid, 21 Iron-Graphite, 22 Tributylstannane, 23 Sodium O,O-Diethyl Phosphorotelluroate, 24 lithium 2-thiophenetellurolates, 25 and a wide variety of other reagents 26,27 can also be used for the reduction of α-halo carbonyl compounds. Elimination Reactions.…”

Organotellurium Compounds in Organic Synthesis

“…72 Bearing in mind that, in contrast to other reagents (for example HgCl 2 6S ), which give rise to E-isomers as a result of trans-attack, the addition of TeCl^ to substituted acetylenes always leads to Z-isomers (XLVI) and that the elimination of tellurium from the latter proceeds with retention of configuration, the above reaction sequence can be regarded as a convenient method of synthesis of Z-chloro(or bromo)alkanes from alkynes.…”