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

On the basis of the 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 [(eta(5)-tetrahydrofluorenyl)RhX2](2) (2 a: X=Br; 2 b: X=I) were synthesized by treatment of the bis(ethylene) derivative (eta(5)-tetrahydrofluorenyl)Rh(C2H4)(2) (1 a) with halogens. An analogous reaction of the cyclooctadiene complex (eta(5)-tetrahydrofluorenyl)Rh(cod) (1 b) with I-2 is complicated by the side formation of [(cod)RhI](2). The reaction of 2 b with 2,2 '-bipyridyl leads to cation [(eta(5)-tetrahydrofluorenyl)Rh(2,2 '-bipyridyl)I](+) (3). The halide abstraction from 2 a,b with thallium or silver salts allowed us to prepare sandwich compounds with incoming cyclopentadienyl, dicarbollide and mesityleneligands [(eta(5)-tetrahydrofluorenyl)RhCp](+) (4), (eta(5)-tetrahydrofluorenyl)Rh(eta-7,8-C2B9H11) (5), and [(eta(5)-tetrahydrofluorenyl)Rh(eta-mesitylene)](2+) (6). The structures of 1 b, 2 b . 2I(2), 3PF(6), 4TlI(4), 5, and [(cod)RhI](2) were determined by X-ray diffraction. Compounds 2 a,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 2 b 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 %).

Automated summary

A simple approach was developed to new effective rhodium catalysts for the construction of C-C and C-N bonds based on the 1,2,3,4-tetrahydrofluorenyl ligand. These catalysts showed efficient catalytic activity in various reactions, such as oxidative coupling of benzoic acids with alkynes and reductive amination of aldehydes and ketones, yielding high yields of the desired products.