Using a two-step hydride transfer to achieve 1,4-reduction in the catalytic hydrogenation of an acyl pyridinium cation.
The stoichiometric reduction of N-carbophenoxypyridinium tetraphenylborate (6) by CpRu(P-P)H (Cp = eta(5)-cyclopentadienyl; P-P = dppe, 1,2-bis(diphenylphosphino)ethane, or dppf, 1,1'-bis(diphenylphosphino)ferrocene), and Cp*Ru(P-P)H (Cp* = eta(5)-pentamethylcyclopentadienyl; P-P = dppe) gives mixtures of 1,2- and 1,4-dihydropyridines. The stoichiometric reduction of 6 by Cp*Ru(dppf)H (5) gives only the 1,4-dihydropyridine, and 5 catalyzes the exclusive formation of the 1,4-dihydropyridine from 6, H(2), and 2,2,6,6-tetramethylpiperidine. In the stoichiometric reductions, the ratio of 1,4 to 1,2 product increases as the Ru hydrides become better one-electron reductants, suggesting that the 1,4 product arises from a two-step (e(-)/H(*)) hydride transfer. Calculations at the UB3LYP/6-311++G(3df,3pd)//UB3LYP/6-31G* level support this hypothesis, indicating that the spin density in the N-carbophenoxypyridinium radical (13) resides primarily at C4, while the positive charge in 6 resides primarily at C2 and C6. The isomeric dihydropyridines thus result from the operation of different mechanisms: the 1,2 product from a single-step H(-) transfer and the 1,4 product from a two-step (e(-)/H(*)) transfer.
Shaw
Anthony P
AP
Department of Chemistry, Columbia University, New York, New York 10027, USA.
Ryland
Bradford L
BL
Franklin
Mary J
MJ
Norton
Jack R
JR
Chen
Judy Y-C
JY
Hall
Michelle Lynn
ML
D002384:Catalysis; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D006865:Hydrogenation; D007202:Indicators and Reagents; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D010084:Oxidation-Reduction; D011522:Protons; D011726:Pyridinium Compounds / Q000737:chemistry*; D012428:Ruthenium / Q000737:chemistry
D007202:Indicators and Reagents; D011522:Protons; D011726:Pyridinium Compounds; D012428:Ruthenium