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)(2)RhCl](2) in the presence of CCl4 provided the air-stable an d water-tolerant (Phebox)RhCl2(H2O) complexes 5. These neutral (noncationic ) aqua complexes 5 acted as asymmetric catalysts for enantioselective allyl ation of aldehydes with allyltin reagents in the presence of 4 Angstrom mol ecular sieves (MS 4A). Furthermore, these aqua complexes could be recovered quantitatively from the reaction media. Detailed mechanistic studies of th is catalytic system using X-ray and NMR spectroscopy revealed that the (Phe box) RhCl2 fragment, generated by releasing H2O from aqua complex 5, is an active catalyst and the reaction proceeds by a Lewis acid catalyzed mechani sm. The relative stereochemistry of the major adduct of the reaction of ben zaldehyde with crotyltin reagents was anti (threo). The observed anti diast ereoselectivity and si-face attack of allyltins on the carbonyl carbon of a ldehydes were explained by the inverse antiperiplanar transition-state mode l.