A COMPARATIVE-STUDY OF THE MCMURRY REACTION UTILIZING [HTICL(THF)(APPROXIMATE-TO-0.5)](X), TICL3(DME)(1.5)-ZN(CU) AND TICL2-CENTER-DOT-LICL AS COUPLING REAGENTS

An investigation of the reaction course and stoichiometry of the McMurry reaction of acetophenone utilizing [HTiCl(THF)(approximate to 0.5)](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 occurrence 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 law 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+ to Ti2+ 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 (95-97%) 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+.