ORGANOMETALLIC COMPOUNDS OF GROUP-III .49. REDUCTIVE DEOXYGENATION OF KETONES AND SECONDARY ALCOHOLS BY ORGANOALUMINUM LEWIS-ACIDS

The reductive deoxygenation of ketones and secondary alcohols to the corresponding methylene hydrocarbons has been achieved in good to excellent yield by the combined action of an aluminum hydride source and a strongly Lewis-acidic aluminum reagent. Such reductions were successful with diaryl ketones, alkyl aryl ketones, and dialkyl ketones, as exemplified by the reduction of benzophenone, acetophenone and 5-nonanone, respectively. The corresponding secondary alcohols of these ketones, namely benzhydrol, 1-phenyl-1-ethanol, and 5-nonanol, could also be converted into their respective methylene hydrocarbons by Lewis-acidic sources of aluminum hydride. All such reductions of ketones could be conducted in a single reaction flask in a one-, two-, or three-step process. In the one-step process, which is most suitable for diaryl ketones, i-BuAlCl2 may be employed as both the hydride source and the Lewis acid. For alkyl aryl ketones a two-step process, consisting first of reduction with i-Bu2AlH and then treatment with AlBr3 (with or without catalysis by Cp2TiCl2), leads to better yields. Finally, for dialkyl ketones a three-step process proved to be preferred, wherein a sequential treatment with i-Bu2AlH, AlBr3 and then additional i-Bu2AlH (with a Ni(acac)2 catalyst) gives the highest conversion to alkane. If required, residual alkene may be removed by a brief catalytic hydrogenation or treatment with BH3.THF. The ease of deoxygenating the foregoing ketones and secondary alcohols appears to be governed by the ease of forming, and the relative stability of, the corresponding carbenium ion intermediates, namely Ar2HC+ > ArRHC+ > R2HC+. The driving force for such deoxygenations by these aluminum reagents undoubtedly is the exothermic formation of the dialuminoxane system, R2Al-O-AlR2.