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Organometallic Chemistry sans Organometallic Reagents: Modulated Electron‐Transfer Reactions of Subvalent Early Transition Metal Salts
Author(s) -
Eisch John J.,
Shi Xian,
Alila Joseph R.,
Thiele Sven
Publication year - 1997
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19971300903
Subject(s) - chemistry , reagent , reactivity (psychology) , pinacol , butyllithium , transition metal , halide , oxidative addition , organic synthesis , organic chemistry , medicinal chemistry , photochemistry , inorganic chemistry , catalysis , medicine , alternative medicine , pathology
The potential of low‐valent, early transition‐metal reagents as selective reductants in organic chemistry has been foreshadowed by intensive research on the ill‐defined and heterogeneous subvalent titanium intermediates generated in the McMurry reaction and its numerous variants. As part of the long‐term research effort to develop soluble, well‐defined transition‐metal reductants of modulated and selective activity toward organic substrates, the THF‐soluble reductant, titanium dichloride, has been throughly examined, as well as the analogous ZrCl 2 and HfCl 2 reagents, all of which are readily obtainable by the alkylative reduction of the Group tetrachloride by butyllithium in THF. Noteworthy is that such interactions of MCl 4 with butyllithium in hydrocarbon media lead, in contrast, to M(III) or M(IV) halide hydrides. Analogous alkylative reductions in THF applied to VCl 4 , CrCl 3 , and MoCl 5 have yielded reducing agents similar to those obtained from Mcl 4 but gradated in their reactivity. Such reductants have proved capable of coupling carbonyl derivatives, benzylic halides, acetylenes and certain olefins in a manner consistent with an oxidative addition involving a two‐electron transfer (TET). Such a reaction pathway is consistent with the observed stereochemistry of pinacol formation from ketones and for the reductive dimerization of alkynes. In contrast of the reaction of CrCl 3 with two equivalents of butyllithium, which leads to a CrCl intermediate, the interaction of CrCl 3 in THF with four equivalents of butyllithium at –78°C yields a reagent of the empirical formulation, LiCrH 4 .2LiCl.2 THF, as supported by elemental and gasometric analysis of its protolysis. This hydridic reductant cleaves a wide gamut of s̀ carbon–heteroatom bonds (C–X, C–O, C–S and C–N), towards which the CrCl reductant is unreactive. The type of cleavage and/or coupled products resulting from the action of “LiCrH 4 ” on these substrates in best understood as arising from single‐electron transfer (SET). In light of the aforementioned findings, the gradated reducing action noted among TiCl 2 , HfCl 2 and CrCl, as well as the contrasting reducing behavior between CrCl and LiCrH 4 , there is no doubt that future research with early transition metals will continue to yield novel reductants of modulated and site‐selective reactivity.