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

The rate constants k H ( k D ) have been determined at 27 °C for H· (D·) transfer from CpCr(CO) 3 H(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.