Bis(oxazolinyl)phenylrhodium(III) Aqua Complexes: Synthesis, Structure, Enantioselective Allylation of Aldehydes, and Mechanistic Studies

The reaction of (Phebox)SnMe3 (4; Phebox = 2,6-bis(oxazolinyl)phenyl) and [(cyclooctene)2RhCl]2 in the presence of CCl4 provided the air-stable and water-tolerant (Phebox)RhCl2(H2O) complexes 5. These neutral (noncationic) aqua complexes 5 acted as asymmetric catalysts for enantioselective allylation of aldehydes with allyltin reagents in the presence of 4 Å molecular sieves (MS 4A). Furthermore, these aqua complexes could be recovered quantitatively from the reaction media. Detailed mechanistic studies of this catalytic system using X-ray and NMR spectroscopy revealed that the (Phebox)RhCl2 fragment, generated by releasing H2O from aqua complex 5, is an active catalyst and the reaction proceeds by a Lewis acid catalyzed mechanism. The relative stereochemistry of the major adduct of the reaction of benzaldehyde with crotyltin reagents was anti (threo). The observed anti diastereoselectivity and si-face attack of allyltins on the carbonyl carbon of aldehydes were explained by the inverse antiperiplanar transition-state model.