Allylation of aldehydes catalyzed by chiral N,N'-bis(N-methyl-2-methylene-4,5-bisphenyl-imidazole)-1,2-cyclohexane diamine rhodium(III) complexes

Abstract

Chiral bis-imidazole rhodium(III) complexes catalyze the allylation of aldehydes by allyltributyltin. The pre-catalyst was readily prepared from chiral N,N'-bis(N-methyl-2-methylene-4,5-bisphenylimidazole)-1,2-cyclohexanediamine, potassium carbonate, and rhodium(III) chloride trihydrate. The rhodium(III) complex produced showed no activity in an allyl transfer process in the presence of the allyltin reagent. However, when silver tetrafluoroborate was added to the pre-catalyst and stirred for 1 h, the resulting system became an efficient catalyst for the allyl transfer process. The reductions produced homo-allyl alcohols with good to excellent yield, although generally with poor facial selectivity (8–10% ee, aryl aldehydes, 4 examples; 99% ee, aliphatic aldehyde, 1 example).Key words: allylation, aldehydes, enantioselectivity, rhodium(III) tetramine complex.