Add time:08/22/2019         Source:sciencedirect.com

At low temperatures the benzylidene complexes (C))5WC(R)H (1) [R = C6H5 (a), C6H4Me-p (b)] add PPhCl2 at the carbene carbon to form the ylide complexes (CO)5WC(R)HPPhCl2 (3). These isomerize at room temperature to give the phosphine complexes (CO)5WP(Ph)ClC(R)(H)Cl (4). The rearrangement is stereospecific with only one diastereomer, (RS,SR)-4a and (RS,SR)-4b, respectively, being formed. In a slower succeeding reaction this diastereomer epimerizes until an equilibrium is attained [e.g. (RS,SR)4a/(RR,SS)-4a ca. 3:2].The structure of complex (RS,SR)-4b has been established by X-ray analysis. The epimerization is accelerated by free chloride. However, P-bound, rather than C-bound bromide in (CO)5WP(Ph)BrC(Ph)(H)Br (7) may be exchanged by chloride. Isomerization of the ylide complex 3a to the phosphine complex 4a in the presence of excess bromide leads to partial substitution of the migrating (in 4a C-bound) chloride giving (CO)5WP(Ph)ClC(PH)(H)BrThe ylide complexes obtained via addition of PPh2Cl, PCl3, PPhBr2, PPh2Br, PBr3 and PPh(Me)Cl to 1a also rearrange to give the corresponding phosphine complexes. The rates of isomerization of (RS,SR)- and (RR,SS)-(CO)5WC(Ph)HPPh(Me)Cl into (RS,SR)- and RR,SS)(CO)5WPPh(Me)C(Ph)(H)Cl differ with the (RS,SR)-diastereomer rearranging faster.The stereochemical results of the isomerization studies are discussed on the basis of an intramolecular C,P-migration of (CO)5W, accompanied by dissociation of halide from phosphorus and addition to the carbon atom. The epimerization is proposed to proceed by an associative mechanism with intermediate formation of pentacoordinated phosphorus.

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