Interaction of acyl chlorides and triethylsilane catalysed by rhodium complexes

Abstract

The aroyl chlorides XC₆H₄COCl react with triethylsilane in the presence of mer-[RhCl₃(PBu₂Ph)₃], (A), or better, trans-[RhCl(CO)(PEtPh₂)₂], (B), at 120 °C to give ketones, (XC₆H₄)₂CO, and aldehydes, XC₆H₄CHO, in a ratio which varies with the nature of X, e.g., with catalyst (B), for X =p-OMe the yields of aldehyde and ketone are 2 and 63% respectively while for X =p-NO₂ the yields are 66 and 0%. For X =p-MeO, the aldehyde : ketone ratio varies as x in the catalyst trans-[RhCl(CO)(PEt_xPh_3–x)₂] is varied progressively from 0 to 3. With catalyst (B), the chlorides RCOCl (R = PhCHCH₂, Prⁱ, and n-C₅H₁₁) also give fair yields of aldehydes, while for R = Et comparable amounts of aldehyde and ketone are produced. The expected dialdehydes are obtained in good yield from m- and p-phthaloyl chloride, but products of a different type are formed from o-phthaloyl and succinyl chloride. For p-nitrobenzoyl chloride with catalyst (B) in toluene, studies have been made of the effect of varying the reaction temperature and the nature of the silicon hydride in the series SiHEt₃, SiHPh₃, SiH₂Ph₂, and SiH(OEt)₃; in all cases some nitrobenzene is formed together with p-nitrobenzaldehyde, the proportion varying with the nature of the silane and being especially high for triphenylsilane. A catalytic cycle is suggested involving known types of oxidative additions and reductive eliminations, reduction of Rh^III to Rh^I species by the silicon hydride, and rearrangement of Rh(COR) to Rh(CO)R systems.