Reactions of aliphatic ketones R2CO (R = Me, Et, iPr, and tBu) with the MCl4/Li(Hg) system (M = U or Ti): Mechanistic analogies between the McMurry, Wittig, and Clemmensen reactions

Article abstract of DOI:10.1002/1521-3765(20010716)7:14<3043::AID-CHEM3043>3.0.CO;2-D

Analysis of the products of the reactions of ketones R₂CO (R = Me, Et, iPr, tBu) with the MCl₄/Li(Hg) system (M = U, Ti) at 20°C revealed significant differences. For R = Me, the reaction proceeded exclusively (M = U) or preferentially (M = Ti) via a metallopinacol intermediate resulting from dimerization of ketyl radicals. Pinacol was liberated by hydrolysis, and tetramethylethylene was obtained after further reduction at 65°C. For R = iPr, formation of iPr₂C=CiPr₂ as the only coupling product, the nonproduction of this alkene by reduction of the uranium pinacolate [U]-OCR₂CR₂O-[U] (R = iPr) at 20°C, and the instability of the corresponding titanium pinacolate towards rupture of the pinacolic C-C bond indicated that reductive coupling of iPr₂CO did not proceed by dimerization of ketyl radicals. Formation of 2,4-dimethyl-2-pentene was in favor of a carbenoid intermediate resulting from deoxygenative reduction of the ketyl. These results revealed that for sterically hindered ketones, McMurry reactions can be viewed as Wittig-like olefination reactions. For R = tBu, no coupling product was obtained and the alkane tBu₂CH₂ was the major product. The involvement of the carbenoid species [M]=CtBu₂ was confirmed by its trapping with H₂O, leading to tBu₂CH₂, and with the aldehydes RCHO, giving the cross-coupling products tBu₂C=C(R)H (R = Me, tBu). Therefore, in the case of severely congested ketones, McMurry reactions present strong similarities to the Clemmensen reduction of ketones, owing to the involvement in both reactions of carbenoid species which exhibit similar reactivity. Wiley-VCH Verlag GmbH, 2001.