Catalytic Transfer Hydrogenation of Arenes and Heteroarenes

Abstract

Transfer hydrogenation reactions are of great interest to reduce diverse molecules under mild reaction conditions. To date, this type of reaction has only been successfully applied to alkenes, alkynes and polarized unsaturated compounds such as ketones, imines, pyridines, etc. The reduction of benzene derivatives by transfer hydrogenation has never been described, which is likely due to the high energy barrier required to dearomatize these compounds. In this context, we have developed a catalytic transfer hydrogenation reaction for the reduction of benzene derivatives and heteroarenes to form complex 3-dimensional scaffolds bearing various functional groups at room temperature without needing compressed hydrogen gas.

Keywords: arene reduction; catalysis; organic chemistry; rhodium; transfer hydrogenation.

Scheme 1

Transfer hydrogenation reactions.

Scheme 2

Scope of the transfer hydrogenation of arenes. All data are reported as isolated yields. d.r. values are determined by GC or ¹H NMR. a) Reaction time of 48 h. b) Reaction performed at 50 °C. c) Undetermined diastereomeric ratio because of complex mixture. d) 2 equivalent of ammonia borane were used. e) 1 equivalent of ammonia borane was used. f) 2.5 equivalent of ammonia borane were used. g) 3 mol % of catalyst were used.

Scheme 3

Scope of the transfer hydrogenation of arenes bearing free amines. All data are reported as isolated yields. d.r. values are determined by GC or ¹H NMR. a) Reaction time of 48 h. b) Reaction performed at 50 °C. c) Undetermined diastereomeric ratio because of complex mixture. d) 2 equivalent of ammonia borane were used. e) 1.0 equivalents of ammonia borane was used. f) 2.5 equivalents of ammonia borane were used. g) 3 mol % of catalyst were used.

Scheme 4

Mechanistic studies.