Influence of External Ligands and Auxiliaries on the Reactivity of Low-Valent Titanium in McMurry Reaction: Selectivity and Mechanistic Profile

Abstract

Influence of various external ligands/auxiliaries on the reductive carbonyl coupling reactions mediated by low-valent titanium (LVT) reagents (McMurry coupling) has been explored. The LVT species generated from Tyrlik's TiCl3−Mg−THF system has been selected for the reactivity-tuning trials. Our studies show that incorporation of about 10 equiv of pyridine (a π-acceptor ligand) with respect to TiCl3 to the THF solvated LVT reagent could arrest the reductive dimerization of acetophenone at the intermediate pinacol stage. Modulation of the LVT-species by covalently-binding monohydroxy, 1,2- and 1,3-dihydroxy auxiliaries also resisted deoxygenation completely to give the pinacols in higher yields and better diastereoselectivity, as compared to pyridine-modified LVT system. Amongst these modified reagents, the LVT−catechol (1:1) system was found to be the most efficient combination for total pinacolization of aromatic carbonyl compounds, even under refluxing conditions. The actual titanium reagent responsible for this transformation is proposed to be a Ti(II) complex formed in situ. Further enhancement of stereoselectivity (threo-selectivity) of the pinacols has been achieved by carrying out the reactions with LVT-complexes modified by a variety of covalently-linking auxiliaries at low temperatures. The results demonstrate that by the judicious incorporation of various ligands/auxiliaries into LVT reagents it is possible to widen the scope and applicability of the classical McMurry reaction.