ORGANOMETALLIC CHEMISTRY SANS ORGANOMETALLIC REAGENTS - MODULATED ELECTRON-TRANSFER REACTIONS OF SUBVALENT EARLY TRANSITION-METAL SALTS

The potential of low-valent, early transition-metal reagents as select ive reductants in organic chemistry has been foreshadowed by intensive research on the ill-defined and heterogeneous subvalent titanium inte rmediates generated in the McMurry reaction and its numerous variants. As part of a long-term research effort to develop soluble, well-defin ed transition-metal reductants of modulated and selective activity tow ard organic substrates, the THF-soluble reductant, titanium dichloride , has been thoroughly examined, as well as the analogous ZrCl2 and HfC l2 reagents, all of which are readily obtainable by the alkylative red uction of the Group 4 tetrachloride by butyllithium in THF. Noteworthy is that such interactions of MCl4 with butyllithium in hydrocarbon me dia lead, in contrast, to M(III) or M(IV) halide hydrides. Analogous a lkylative reductions in THF applied to VCl4, CrCl3, and MoCl5 have yie lded reducing agents similar to those obtained from MCl4 but gradated in their reactivity. Such reductants have proved capable of coupling c arbonyl derivatives, benzylic halides, acetylenes and certain olefins in a manner consistent with an oxidative addition involving a two-elec tron transfer (TET). Such a reaction pathway is consistent with the ob served stereochemistry for pinacol formation from ketones and for the reductive dimerization of alkynes. In contrast to the reaction of CrCl 3 with two equivalents of butyllithium, which leads to a CrCl intermed iate, the interaction of CrCl3 in THF with four equivalents of butylli thium at -78 degrees C yields a reagent of the empirical formulation, LiCrH4 . 2 LiCl . 2 THF, as supported by elemental and gasometric anal ysis of its protolysis, This hydridic reductant cleaves a wide gamut o f sigma carbon-heteroatom bonds (C-X, C-O, C-S and C-N), towards which the CrC1 reductant is unreactive. The type of cleavage and/or coupled products resulting from the action of ''LiCrH4'' on these substrates is best understood as arising from single-electron transfer (SET). In light of the aforementioned findings, the gradated reducing action not ed among TiCl2, ZrCl2, HfCl2 and CrCl, as well as the contrasting redu cing behavior between CrC1 and LiCrH4, there is no doubt that future r esearch with early transition metals will continue to yield novel redu ctants of modulated and site-selective reactivity.