Preparations and asymmetric, reducing abilities of chiral polymeric amine‐boranes

Abstract

Abstract Polymeric amine‐boranes 2b, 4b , and 8c and their corresponding low‐molecular‐weight model compounds 3b, 5b , and 6b were obtained by reaction of chiral amines with diborane; these amines were poly(pyrrolidine‐1,2‐diylmethylene) ( 2a ), poly(imino‐1‐isobutylethylene) ( 4a ), poly{[1,2‐bis(4‐vinylphenyl)ethyl]amine} ( 8b ), 1‐methylpyrrolidin‐2‐yl‐1‐pyrrolidinyl‐methane ( 3a ), N 1 ‐isopentyl‐ N 2 ‐methyl‐4‐methyl‐1,2‐pentanediamine ( 5a ), and α‐phenylphenethylamine ( 6a ). The polymeric amine‐boranes are white powders, stable to be kept for several weeks at room temperature, whereas the low‐molecular‐weight model compounds are less stable syrup‐like materials. The reduction of acetophenone to optically active 1‐phenylethanol was carried out with the chiral amine‐boranes in the presence of boron trifluoride etherate at various reaction temperatures. The product resulting from the reduction with 8c can easily be separated from the reaction mixture by filtration. The optical yield of the product obtained with 8c was 8,9% at −40°C. The highest optical yield (25,3%) was obtained with α‐phenylphenethylamineborane 6b at 0°C.