Easy Access to Versatile Catalytic Systems for C-H Activation and Reductive Amination Based on Tetrahydrofluorenyl Rhodium(III) Complexes.

Abstract

On the basis of 1,2,3,4-tetrahydrofluorenyl ligand, a simple approach was developed to new effective rhodium catalysts for the construction of C-C and C-N bonds. The halide compounds [(η5-tetrahydrofluorenyl)RhX2]2 (2a: X = Br; 2b: X = I) were synthesized by treatment of the bis(ethylene) derivative (η5-tetrahydrofluorenyl)Rh(C2H4)2 (1a) with halogens. An analogous reaction of the cyclooctadiene complex (η5-tetrahydrofluorenyl)Rh(cod) (1b) with I2 is complicated by the side formation of [(cod)RhI]2. The reaction of 2b with 2,2 -bipyridyl leads to cation [(η5-tetrahydrofluorenyl)Rh(2,2 -bipyridyl)I]+ (3). The halide abstraction from 2a,b with thallium or silver salts allowed us to prepare sandwich compounds with incoming cyclopentadienyl, dicarbollide and mesitylene ligands [(η5-tetrahydrofluorenyl)RhCp]+ (4), (η5-tetrahydrofluorenyl)Rh(η-7,8-C2B9H11) (5), and [(η5-tetrahydrofluorenyl)Rh(η-mesitylene)]2+ (6). The structures of 1b, 2b∙2I2, 3PF6, 4TlI4, 5, and [(cod)RhI]2 were determined by X-ray diffraction. Compounds 2a,b efficiently catalyze the oxidative coupling of benzoic acids with alkynes to selectively give isocoumarins or naphthalenes, depending on the reaction temperature. Moreover, they showed moderate catalytic activity in other annulations of alkynes with aromatic compounds (such as benzamide, acetanilide, etc.) which proceed through CH activation. Compound 2b also effectively catalyzes the reductive amination of aldehydes and ketones in the presence of carbon monoxide and water via water-gas shift reaction, giving amines in high yields (67-99%).