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

Abstract

Reactions between various pyridinium cations with and without a −CF 3 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 η 2 mode through a CC bond were quantitatively formed at the initial stage. The only exception observed was in the case of the 1benzylpyridinium 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)(NCCH 3 )] 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.

Thermodynamic and Kinetic Hydricity of Ruthenium(II) Hydride Complexes

“…75 In addition, with the three acceptors studied here, we detected intermediate species (Supporting Information, Figures S8 and S9), attributed to η 2 -imidazole and η 2 -dihydropyridine complexes. 75 For example, with BzIm + , the bpy-6H resonance of the hydride (δ 9.75 ppm) decreased while the corresponding resonance of the acetonitrile complex (δ 9.60) increased. A broad peak at δ 7.35 grew in with time, finally narrowing to a triplet after about 2 days.…”

A Neutral RuII Hydride Complex for the Regio‐ and Chemoselective Reduction of N‐Silylpyridinium Ions

“…Even this step is reversible and is as such a rare example of a retro‐hydrosilylation . We cannot fully exclude temporary η 2 ‐coordination of 6 to the Ru II center . We further think that the replacement of any ligand at the Ru II center is dissociative through the short‐lived, coordinatively unsaturated Ru II complex 4 + .…”

“…We performed this particular experiment to fulfill two main purposes: i) We intended to underline the conclusion drawn from experiments B and E that the 1,4‐dihydropyridine does not exist next to the cationic Ru II catalyst. ii) We searched for a Ru II complex with the 1,4‐dihydropyridine coordinated to the Ru II center, which could be another intermediate in the catalytic cycle …”

“…On the other hand, a variety of models for NAD + with different substituents in the C3 position were investigated, revealing that coordination of the amide moiety to the transition‐metal center is essential for reactivity and regioselectivity . Ishitani and co‐workers also established η 2 ‐coordination of the formed 1,4‐dihydropyridines to Ru II centers when the Lewis‐basic site at C3 is missing . Also, the mechanism of the hydride transfer onto the respective pyridinium ion was investigated .…”

Dearomatization of Transition Metal-Coordinated N-Heterocyclic Ligands and Related Chemistry

“…In the field of transition metal organometallic chemistry, dihydropyridines have been either formed on the coordination sphere of a metal to which they are bound through the C=C double bonds, or used as substrates. Ishitani studied the formation of intermediate complexes containing 1,4-dihydropyridines η 2 -coordinated to ruthenium fragments in the reaction of pyridinium cations with ruthenium hydride complexes, 24 Myers employed complexes of a strongly π-basic metal fragment with η 2 -coordinated pyridinium cations to access metalcomplexed dihydropyridines, 25 Rudler found dihydropyridines to be useful reagents in the chemistry of Fischer carbenes, 26 and Liebeskind employed dihydropyridine π-complexes as intermediated in the synthesis of more reduced heterocycles, such as piperidines. 27 The reaction of pyridines with organolithium reagents afforded mainly 2-substituted pyridines through alkyl addition to generate a dearomatized lithium complex of dihydropyridine.…”