Diastereospecific Coupling of Imines by Low‐Valent Titanium: An Experimental and Computational Study

Abstract

The reaction of phenylsilane (PhSiH3) and titanium(IV) isopropoxide [Ti(OiPr)4] generates low‐valent titanium alkoxides that reduce and reductively couple imines. The C–C coupling reaction is diastereospecific, with exclusive formation of the (±)‐isomer. The yield is dependent on the concentration of titanium(IV) isopropoxide used. Using imines with varying electronic demand revealed that the coupling is most efficient for unsubstituted imines. The involvement of a trimeric titanium biradical species is invoked to explain the observed concentration dependence and diastereospecificity in the reaction. Multilayer ONIOM (“our own n‐layered integrated molecular orbital and molecular mechanics”) calculations were carried out on the plausible intermediates involved by using the Guassian 09 suite of programs. Formation of the trimeric biradical species leading to the formation of the (±)‐isomer is more favored than formation of the trimeric biradical species, which leads to the meso‐isomer. Similar calculations with dimeric intermediates do not predict the (±)‐selectivity. Mass spectral analysis of the reaction mixture indicates the presence of a trimeric titanium alkoxide species.

Practical C–P bond formation via heterogeneous photoredox and nickel synergetic catalysis

“…of the radical scavenger 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was added to the reaction under standard conditions, the formation of 3aa was completely inhibited, and the phosphine-TEMPO adduct was detected by HRMS, which indicated the involvement of radicals in this reaction. Based on this result and previous reports [30,[32][33][34][35][36][37][38][39][40][41][42][43][44]51], a plausible reaction mechanism was proposed for the formation of the desired C-P bond (Fig. 3).…”

“…This leads to the problem of the inability to separate, recover, and reuse compared with heterogeneous photocatalysts (Fig. 1(a)) [36][37][38][39][40][41][42][43][44]. In this work, because of continuing interest in visible-light photocatalysis [45][46][47][48][49][50], this C-P formation reaction is updated by the joint use of Ni catalysts and heterogeneous cadmium sulfide (CdS, 2.4-eV bandgap) photocatalysts ( Fig.…”

Visible light mediated homo- and heterocoupling of benzyl alcohols and benzyl amines on polycrystalline cadmium sulfide

“…Typical visible light absorbing semiconductors are dye-sensitized or element-doped titanium dioxide, other transition metal oxides, carbon nitride and cadmium sulfide. They have been applied to visible light water splitting, 5 reduction of nitrate to ammonia, 6 carbon dioxide activation, 7 hydrogen production, 8 oxidation of alcohols to aldehydes, 9 and amines to imines. 10 Photocatalytic C-C bond formations using heterogeneous photocatalysts are rare.…”

Phenylsilane

“…5 The reductive coupling of aldimines has been achieved by the use of a combination of PhSiH 3 and titanium isopropoxide [Ti(OiPr) 4 ]. 6 Reductions of organic halides to dehalogenated alkanes with the In(OAc) 3 -PhSiH 3 system are readily accomplished. 7 It can also act as an in situ carboxylic acid activating agent to prepare carboxamides and peptides from carboxylic acids and amines.…”