A comparative study of the McMurry reaction utilizing x, TiCl3(DME)1.5-Zn(Cu) and TiCl2 * LiCl as coupling reagents

Article abstract of DOI:10.1016/0022-328X(95)05755-E

An investigation of the reaction course and stoichiometry of the McMurry reaction of acetophenone utilizing x (THF=tetrahydrofuran), TiCl3(DME)1.5-Zn(Cu) (DME=1,2-dimethoxyethane) and TiCl2*LiCl as coupling reagents has been undertaken.The detection of 1-phenylethanol (3a) or dideutero-1-phenylethanol (3b) (Schemes 1 and 3) as hydrolysis or deuterolysis products in the early stage of reactions gave the first direct experimental evidence for the occurence of the "side-on" bonded ketones 3 and 3" as possible precursors of the pinacolates 4 and 7.This result supports the nucleophilic rather than the radical mechanism for the C-C coupling step of aromatic ketones.Contrary to the current opinion, upon refluxing TiCl3(DME)1.5-Zn(Cu) mixtures in DME, no reduction of Ti³⁺ to low valence Ti species could be detected.The reduction of Ti³⁺ by Zn (Scheme 2) only starts in the presence of the carbonyl substrate which is coordinated to the Ti (the "instant method"); both the ketone-> pinacolate and the pinacolate-> alkene steps (Scheme 2) apparently involve a transient reduction of Ti³⁺ by Zn.This view is supported by experiments in which TiCl2*LiCl is used as a reagent and in which it behaves as a one-electron reductant (Scheme 3).On the basis of these results, the overall stoichiometry of the McMurry reaction utilizing TiCl3(DME)1.5-Zn(Cu) as a reagent can be represented by Eq. (4).High yields (95-97percent) of the alkene 2 in acceptable reaction times can already be achieved with an acetophenone: TiCl3(DME)1.5:Zn(Cu) molar ratio of 1:2:2.A conclusion which can be drawn from the results is that the McMurry reaction when performed with two of the most commonly applied reagents, namely TiCl3-LiAlH4-THF (in fact HTiCl(THF)0.5!) and TiCl3(DME)1.5-Zn(Cu)-DME, is mainly associated with changes in the (formal) oxidation state of titanium between Ti²⁺ and Ti³⁺.Keywords: McMurry reaction; TiCl3(1,2-dimethoxyethane); Low valent titanium-ketone complexes