Kinetics of H· Transfer from CpCr(CO)3H to Various Enamides: Application to Construction of Pyrrolidines

• Published in: JACS Au 2023-12, Vol.3 (12), p.3366-3373
• Main Authors: Li, Guangchen, Shi, Shicheng, Qian, Jin, Norton, Jack R., Xu, Guo-Xiong, Liu, Ji-Ren, Hong, Xin
• Format: Article
• Language: English
• ISSN: 2691-3704; 2691-3704
• DOI: 10.1021/jacsau.3c00529

The rate constants k H (k D) have been determined at 27 °C for H· (D·) transfer from CpCr­(CO)3H­(D) to the CC bonds of various enamides. This process leads to the formation of α-amino radicals. Vinyl enamides with N-alkyl and N-phenyl substituents have proven to be good H· acceptors, with rate constants close to those of styrene and methyl methacrylate. A methyl substituent on the incipient radical site decreases k H by a factor of 4; a methyl substituent on the carbon that will receive the H· decreases k H by a factor of 380. The measured k H values indicate that these α-amino radicals can be used for the cyclization of enamides to pyrrolidines. A vanadium hydride, HV­(CO)4(dppe), has proven more effective at the cyclization of enamides than Cr or Co hydridespresumably because the weakness of the V–H bond leads to faster H· transfer. The use of the vanadium hydride is operationally simple, employs mild reaction conditions, and has a broad substrate scope. Calculations have confirmed that H· transfer is the slowest step in these cyclization reactions.