Diastereoselectivity McMurry reaction

As reflux temperatures are usually required to effect the deoxygenation step of the McMurry reaction, pinacol intermediates can be isolated if the McMurry reaction is run below room temperature. Though a mixture of diol diastereomers is formed in intermolecular reactions, intramolecular reactions can be highly diastereoselective. ... 

Wynberg studied stereochemistry of the McMurry reductive dimerization of camphor in detail (64). In Scheme 37, A and B are homochiral dimerization products derived by the low-valence Ti-promoted reduction, while C and D are achiral heterochiral dimers. The reaction of racemic camphor prefers homochiral dimerization (total 64.9%) over the diastereomeric heterochiral coupling (total 35.1 %). Similarly, as illustrated in Scheme 38, oxidative dimerization of the chiral phenol A can afford the chiral dimers B and C (and the enantiomers) or the meso dimer D. In fact, a significant difference is seen in diastereoselectivity between the enaritiomerically pure and racemic phenol as starting materials. The enantiomerically pure S substrate produces (S,S)-B exclusively, while the dimerization of the racemic substrate is not stereoselective. In the latter case, some indirect enantiomer effect assists the production of C, which is absent in the former reaction. Thus, it appears that, even though the reagents and reaction conditions are identical, the chirality of the substrate profoundly affects the stability of the transition state. 

The original Mukaiyama and Tyrhk reagents, TiCl4-Zn and TiCb-Mg in THF, have been modified in order to favor the intermolecular McMurry pinacol reaction and to enhance its diastereoselectivity. The effects of solvent, additives, and/or modified ligands can be illustrated by the reaction of benzaldehyde, and are summarized in Table 6.12. 

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