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

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'-C 1 2 H 8 O 2 PCl) 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'-C 1 2 H 8 O 2 PXR) 2 (XR = NPrn (2), OMe (4), SC 6 H 4 -4-Me (6)) and the unexpected hydrolysis products [R'3NH][cis-Mo(CO)4(2,2'-C 1 2 H 8 O 2 PO)(2,2'-C 1 2 H 8 O 2 PXR)] (R' 3 = Pr n H 2 , XR = NPr n , 3; R' 3 = Et 3 ; 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.