Using a Two-Step Hydride Transfer To Achieve 1,4-Reduction in the Catalytic Hydrogenation of an Acyl Pyridinium Cation

Abstract

The stoichiometric reduction of N-carbophenoxypyridinium tetraphenylborate (6) by CpRu(P-P)H (Cp = η 5 -cyclopentadienyl; P-P = dppe, 1,2-bis(diphenylphosphino)ethane or dppf, 1,1′-bis (diphenylphosphino)ferrocene) and Cp*Ru(P-P)H (Cp* = η 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.

Facile Catalytic Hydrosilylation of Pyridines

“…Although we believe that the hydride is predominantly transferred to the pyridinium cation in a single step (a two‐electron process), we cannot exclude the possibility of a sequence of one‐electron processes. For the closely related system RO 2 CNC 5 H 5 + /[Cp(dppe)RuH] (dppe=1,2‐bis(diphenylphosphanyl)ethane), Norton and co‐workers found that both pathways are feasible 17. In fact, the mechanism of single‐electron transfer (SET) can operate preferentially for substrates with substituents in the 4‐position.…”

Facile Catalytic Hydrosilylation of Pyridines

“…For the closely related system RO 2 C À NC 5 H 5 + /[Cp(dppe)RuH] (dppe = 1,2-bis(diphenylphosphanyl)ethane), Norton and co-workers found that both pathways are feasible. [17] In fact, the mechanism of single-electron transfer (SET) can operate preferentially for substrates with substituents in the 4-position. For example, the SET mechanism can account for the formation of the pinacol product in the hydrosilylation of 4-acetylpyridine (Table 1, entry 11).…”

In‐Situ Nanostructuring and Stabilization of Polycrystalline Copper by an Organic Salt Additive Promotes Electrocatalytic CO2 Reduction to Ethylene

“…‐Br ) at the para or ortho position, respectively. The amount of para‐para coupled product is higher than the ortho‐ortho product, as expected from the higher spin density at the para position of the precursor radical (Figure S10) . Combined, these species account for >95 % of the organic material that could be extracted from the copper electrode .…”