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Reactions of Aliphatic Ketones R 2 CO (R=Me, Et, i Pr, and t Bu) with the MCl 4 /Li(Hg) System (M=U or Ti): Mechanistic Analogies between the McMurry, Wittig, and Clemmensen Reactions
Author(s) -
Villiers Claude,
Ephritikhine Michel
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20010716)7:14<3043::aid-chem3043>3.0.co;2-d
Subject(s) - chemistry , ketyl , carbenoid , wittig reaction , pinacol , reactivity (psychology) , medicinal chemistry , photochemistry , steric effects , radical , benzophenone , stereochemistry , organic chemistry , rhodium , catalysis , medicine , alternative medicine , pathology
Analysis of the products of the reactions of ketones R 2 CO (R=Me, Et, i Pr, t Bu) with the MCl 4 /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= i Pr, formation of i Pr 2 C=C i Pr 2 as the only coupling product, the nonproduction of this alkene by reduction of the uranium pinacolate [U]−OCR 2 CR 2 O−[U] (R= i Pr) at 20 °C, and the instability of the corresponding titanium pinacolate towards rupture of the pinacolic C−C bond indicated that reductive coupling of i Pr 2 CO 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= t Bu, no coupling product was obtained and the alkane t Bu 2 CH 2 was the major product. The involvement of the carbenoid species [M]=C t Bu 2 was confirmed by its trapping with H 2 O, leading to t Bu 2 CH 2 , and with the aldehydes RCHO, giving the cross‐coupling products t Bu 2 C=C(R)H (R=Me, t Bu). 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.