Rate enhancing effect of hydrogen chloride and methanesulfonic acid on the intramolecular asymmetric reduction of o-aminoaceto- and -benzophenones with diisopinocampheylborane

While o-hydroxyaceto- and -benzophenones undergo reduction with B-chlorodiisopinocampheylborane (1) and diisopinocampheylborane (2) at the same rate, the reduction of o-carboxyaceto- and -benzophenones is faster with 2 as compared with 1. The rate retardation for the reaction of keto acids with 1 was accounted by a competition between an intra- and intermolecular reduction. In contrast, o-aminoaceto- and -benzophenone undergo faster reduction with 1 and 2. The rate enhancing influence of HCl and MeSO₃H on the intramolecular asymmetric reduction of these amino ketones with 2 has been tested and confirmed.