Reactions of Aliphatic Ketones R2CO (R=Me, Et,iPr, andtBu) with the MCl4/Li(Hg) System (M=U or Ti): Mechanistic Analogies between the McMurry, Wittig, and Clemmensen Reactions

Abstract

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 degrees 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 degrees 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 degrees 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.

Thorium(IV) alkyl synthesis from a thorium(III) cyclopentadienyl complex and an N-heterocyclic olefin

“…It is notable, however, that all uranium-and thoriumcarbenes prepared on macroscopic scales reported to date exhibit at least one or, more usually, two phosphanyl-substituents. For both uranium and thorium the only examples of 'true' or 'pure' alkylidene species, that is where the carbene α-substituents are hydrogen or alkyl groups, remain restricted to cryogenic matrix isolation studies, where compounds are prepared on microscopic scales, or are proposed as reaction intermediates [37][38][39][40][41][42][43][44][45][46][47][48]. In order to progress the area away from phosphanyl-substituted carbenes towards 'true' alkylidenes, we became interested in probing the coordination and organometallic chemistry of N-heterocyclic olefins (NHOs) of the general form H 2 C=C(NRCH) 2 [49].…”

Uranium–Carbon Multiple Bonding: Facile Access to the Pentavalent Uranium Carbene [U{C(PPh2NSiMe3)2}(Cl)2(I)] and Comparison of UVC and UIVC Bonds

“…, C(1)-P(1) 1.676 (11), C(1)-P(2) 1.701 (10), P(1)-N(1) 1.636(7), P(2)-N(2) 1.648(8); P(1)-C(1)-P(2) 154.5 (6). lengths and angles.…”

Low-Valent Titanium-Mediated Reductive Coupling of Carbonyl Compounds

“…While a mechanism involving titanapinacolate intermediates is most common for alkene formation by McMurry reaction, examples are known where a metallapinacolate can be excluded as an intermediate [136][137][138]. Sterically hindered ketones, such as iPr 2 CO and Bu t 2 CO, have been found to undergo McMurry coupling reaction via carbenoid intermediates.…”