STEREOSPECIFIC REARRANGEMENT OF P-HALOGEN SUBSTITUTED PHOSPHORYLIDE COMPLEXES INTO PHOSPHANE COMPLEXES

At low temperatures the benzylidene complexes (CO)(5)W=C(R)H (1) [R = C6H5 (a), C(6)H(4)Me-p (b)] add PPhCl(2) at the carbene carbon to form the ylide complexes (CO)(5)W-C(R)H-PPhCl(2) (3). These isomerize at r oom temperature to give the phosphine complexes (CO)(5)W-P(Ph)Cl-C(R)( H)Cl (4). The rearrangement is stereospecific with only one diastereom er, (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 co mplex (RS,SR)-4b has been established by X-ray analysis. The epimeriza tion is accelerated by free chloride. However, P-bound, rather than C- bound bromide in (CO)(5)W-P(Ph)Br-C(Ph)(H)Br (7) may be exchanged by c hloride. Isomerization of the ylide complex 3a to the phosphine comple x 4a in the presence of excess bromide leads to partial substitution o f the migrating (in 4a C-bound) chloride giving (CO)(5)W-P(Ph)Cl-C(Ph) (H)Br. The ylide complexes obtained via addition of PPh(2)Cl, PCl3, PP hBr(2), PPh(2)Br, PBr3 and PPh(Me)Cl to la also rearrange to give the corresponding phosphine complexes. The rates of isomerization of (RS,S R)- and (RR,SS)-(CO)(5)W-C(Ph)H-PPh(Me)Cl into (RS,SR)- and (RR,SS)-(C O)(5)W-PPh(Me)-C(Ph)(H)Cl differ with the (RS,SR)-diastereomer rearran ging faster. The stereochemical results of the isomerization studies a re discussed on the basis of an intramolecular C,P-migration of (CO)(5 )W, accompanied by dissociation of halide from phosphorus and addition to the carbon atom. The epimerization is proposed to proceed by an as sociative mechanism with intermediate formation of pentacoordinated ph osphorus.