Aryl aldehydes, pinacol coupling

Cross pinacol coupling of aldehydes. Pedersen et al.2 have reported the first general method for stereoselective pinacol coupling of aldehydes by use of this vanadium(II) reagent. Aryl aldehydes couple in the presence of 1 to pinacols in >90% yield and with high diastereoselectivity (dllmeso = 12-100 1). Aliphatic aldehydes under similar conditions couple only slowly, but depending on the struc-... 

Normally, reasonably high diastereoselectivity for the erythro isomer is observed in the pinacolic coupling of aryl aldehydes, and thus enantiomerically pure threo isomers are difficult to access by this method. Uemura and coworkers have, however, discovered that the coupling of planar chiral organometallic aldehydes leads to threo-didis in high yields [34], often as single diastereomers (Eqs. 23, 24). 

Diols.—Pinacolic coupling of some aryl aldehydes or ketones to give 1,2-diols, using aqueous TiClg in basic media, has been reported and various alicyclic ketones and aromatic ketones or aldehydes have been coupled by low-valent cerium reagents. ... 

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