Understanding the Correlation of Structure and Selectivity in the Chiral-Phosphoramide-Catalyzed Enantioselective Allylation Reactions:  Solution and Solid-State Structural Studies of Bisphosphoramide·SnCl₄ Complexes

Complexation of bisphosphoramides, linked by various length methylene tethers, with tin tetrachloride was studied by both solution NMR spectroscopy and single-crystal X-ray crystallography. The formation of cis-configured, octahedral 1/1 bisphosphoramide·SnCl₄ complexes was supported by both crystallographic and solution NMR studies. In addition, the formation of such complexes was shown to be highly dependent on the tether length and solution concentration. Single-crystal X-ray analysis of bisphosphoramide·SnCl₄ complexes also provided detailed information on the stereochemical environment relevant to the enantioselectivity of asymmetric allylation reactions.