Secondary reaction rhodium catalysis

That Rh-allyl complexes can also act as nucleophiles in addition to aldehydes has been demonstrated by Oshima et al. in 2006 [198]. Retro-aUylation of the homoallyl alcohol 191 under rhodium catalysis generates a nucleophilic aUylrhodium species that reacts with aldehydes 190 to give the corresponding secondary alcohols 192 in situ (Scheme 12.94). Subsequent isomerization of these alcohols proceeds under the reaction conditions to furnish the corresponding saturated ketones 193 in modest to good yields. 

In contrast to the case of the water soluble [RhClP3] complexes (P = PTA, TPPMS or TPPTS) which did not promote the reduction of C=0 function in aldehydes or ketones in biphasic systems, [RhCl(PPh3)3] was found an active catalyst for reduction of ketones with aqueous HCOONa (Scheme 3.32). The reaction was aided by phase transfer catalysis using Aliquat-336 and required a large excess of PPh3 to prevent reduction of rhodium into inactive metal. Substrates like acetophenone, butyrophenone, cyclohexanone and dibenzyl-ketone were reduced to the corresponding secondary carbinols with turnover frequencies of 10-40 h 1 [251]. 

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