Chelate ring geometry selection

The geometry of a chiral 6-membered chelate ring is not conducive to effective asymmetric catalysis. Consider square-planar complexes of d[, l-2,3-diphenyl-l,3 bis (diphenylphosphino) propane (24), which are presumed to exist in chair conformation (Scheme 6) with rapid ring inversion. The close approximation to a-symmetry in the environment of the metal suggests that there will be little discrimination between the diastereomeric modes of binding of a prochiral bidentate ligand, since substituents on the olefin experience similar steric interactions in both isomers. The expectation of low selectivity is borne out in practice, for in some cases enamide complexes derived from this phosphine exist in two diastereomeric forms (Figure 3). 

The results are consistent with the formation of zwitterionic intermediate 35 (See Figure 10). Selective formation of the threO isomer 37 directly from 35 implies a preferred geometry for 35. Although chelation is not possible, electrostatic interactions could lead to a preferred geometry that would lead to the selective formation of 37. This result cannot be rationalized, even a posteriori, from examination of models. An alternative explanation, which is supported by circumstantial evidence, involves the reversible cyclization of 35 to give the more stable /r /w-substituted oxetane 36 followed by ring opening of the acetal with inversion by an Sn2 mechanism. 

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