Nucleophilic Displacement Reactions of cis-Bis((2,2‘-biphenylylene)phosphochloridite ester)tetracarbonylmolybdenum(0). The First Example of an Unusual Hydrolysis Reaction Yielding Unsymmetrically Substituted Products

Abstract

Ligands containing groups derived from bis(aryl)diols are widely used in asymmetric catalysis; however, few studies of the conformations of these ligands in transition-metal complexes have been reported. In this paper, the nucleophilic displacement reactions of cis-Mo(CO)4(2,2‘-C12H8O2PCl)2 (1) have been used to prepare a variety of complexes with [1,3,2]dioxaphosphepin ligands, and the conformations of these ligands have been studied by NMR spectroscopy and X-ray crystallography. The nucleophilic substitution reactions yield both the expected disubstituted complexes cis-Mo(CO)4(2,2‘-C12H8O2PXR)2 (XR = NPrn (2), OMe (4), SC6H4-4-Me (6)) and the unexpected hydrolysis products [R‘3NH][cis-Mo(CO)4(2,2‘-C12H8O2PO)(2,2‘-C12H8O2PXR)] (R‘3 = PrnH2, XR = NPrn, 3; R‘3 = Et3; XR = OMe, 5). NMR studies have demonstrated that the hydrolysis product is the major product when more than a minute amount of water is present, even in the presence of a large excess of the nucleophiles. This reaction is complete in approximately 90 min at 25 °C. A very surprising feature of this reaction is that substitution of one chloride in 1 by the RX- nucleophile greatly enhances the rate of substitution of the second chloride either by water or by another RX- nucleophile. NMR studies of the [1,3,2]dioxaphosphepin complexes in chloroform-d solution suggest that the R* and S* enantiomers of the ligands interconvert via a low-energy pathway. Crystal structures of the complexes demonstrate that both the R*S* diastereomer (1) and racemic mixtures of the R*R* and S*S* diastereomers (2−4) are observed in the solid state. These results suggest that bulkier biaryl groups are needed to prevent the racemization of the [1,3,2]dioxaphosphepin ligands in solution.