Mechanistic insight into conjugated N–N bond cleavage by Rh(iii)-catalyzed redox-neutral C–H activation of pyrazolones

Abstract

Density functional theory (DFT) calculations have been performed to investigate the detailed mechanism of Rh(III)-catalyzed redox-neutral C–H activation of pyrazolones with PhCCPh. It is found that (1) the methylene C–H activation is prior to the phenyl C–H activation, (2) the N–N bond cleavage is realized via Rh(III) → Rh(I) → Rh(III) rather than via Rh(III) → Rh(V) → Rh(III). The zwitterionic Rh(I) complex is identified to be a key intermediate in promoting the N–N bond cleavage. (3) Different from the Rh(III)-catalyzed hydrazine-directed C–H activation for indole synthesis, the rate-determining step of the reaction studied in this work is the Rh(III) → Rh(I) → Rh(III) process resulting in the N–N bond cleavage rather than the alkyne insertion step. The present theoretical study provides new insight into the mechanism of the conjugated N–N bond cleavage.