Stereoselective Pinacol Coupling in Aqueous Media

摘要

Reductive coupling of aldehydes and ketones to glycols (pinacol coupling) is a standard reaction of organic synthesis, and Mg metal or various low-valent complexes of Ti, Zr, V, Nb, Sn, or Sm can,be used for efficient synthesis of inter- or intramolecular pinacol coupling, often with high diastereoselectivity. However, most conventional reductive-coupling methods are incompatible with aqueous conditions or protic functionality, and reagents possessing aqueous stability might not be stereoselective. For example, although aqueous TiCl_3 can be used for pinacol coupling of aromatic ketones and aldehydes in bask solution, the intermediate aquo titanium complex is apparently sterically undemanding: the "pinacol" coupling of benzaldehyde using aqueous TiCl_3 (pH = 10-12) gives hydrobenzoin in good yield (88%) but with poor diastereoselectivity (dl.meso = 1.3:1).