Hydride Reduction of NAD(P)⁺ Model Compounds with a Ru(II)–Hydrido Complex

In order to better understand the regioselective hydride transfer of metal hydrido complexes to NAD­(P)⁺ model compounds, reactions of [Ru­(tpy)­(bpy)­H]⁺ (<b>Ru-H</b>: tpy = 2,2′:6″,2″-terpyridine, bpy = 2,2′-bipyridine) with various substituent NAD­(P)⁺ model compounds were investigated in detail. All of the NAD­(P)⁺ model compounds accepted hydride from <b>Ru-H</b>, yielding 1:1 adducts, where the dihydro form(s) of the model compounds coordinated with the carbamoyl group to the Ru­(II) center of [Ru­(tpy)­(bpy)]²⁺, with very different reaction rates. Some reactions produced the adduct with only the 1,4-dihydro structure, whereas others produced a mixture of two adducts, with a 1,4- or 1,2-dihydro structure. In particular, temperature-dependent adduct formation kinetics studies provided important information on the transition state(s) of the hydride transfer reactions and factors for determining the regioselectivity. Most adducts were cleaved to the corresponding free dihydro product(s) with the same distribution of the regioisomers to the adduct(s)

Formation of η²‑Coordinated Dihydropyridine–Ruthenium(II) Complexes by Hydride Transfer from Ruthenium(II) to Pyridinium Cations

Reactions between various pyridinium cations with and without a −CF₃ substituent at the 3-position and [Ru­(tpy)­(bpy)­H]⁺ (tpy = 2,2′:6′,2″-terpyridine and bpy = 2,2′-bipyridine) were investigated in detail. The corresponding 1,4-dihydropyridines coordinating to a Ru­(II) complex in η² mode through a CC bond were quantitatively formed at the initial stage. The only exception observed was in the case of the 1-benzylpyridinium cation, where a mixture of two adducts with 1,4-dihydropyridine and 1,2-dihydropyridine was formed in the ratio 96:4. Cleavage of the Ru–(CC) bond proceeded at a slower rate in all reactions, giving the corresponding dihydropyridine and [Ru­(tpy)­(bpy)­(NCCH₃)]²⁺ when acetonitrile was used as a solvent. Kinetic activation parameters for the adduct formation indicated that the 1,4-regioselectivities were induced by formation of sterically constrained structures