MECHANISTIC ASPECTS OF THE REACTIONS OF BIS(PENTAFLUOROPHENYL)BORANE WITH THE DIALKYL ZIRCONOCENES CP2ZRR2 (R = CH3, CH2SIME3, AND CH2C6H5)

The reactions of bis(pentafluorophenyl)borane with simple dialkyl zirc onocenes Cp2ZrR2 (R = CH3, CH2SiMe3, CH2Ph) proceed via initial alkyl/ hydride exchange to yield ''Cp2Zr-(H)R'' and RB(C6F5)(2). Two reaction paths are then followed depending on whether further equivalents of H B(C6F5)(2) are present or not. If present, HB(C6F5)(2) reacts with the newly formed Zr-H moiety to form dihydridoborate compounds, ultimatel y yielding Cp2Zr[(mu-H)(2)B(C6F5)(2)](2), 1, and 2 equiv of RB(C6F5)(2 ). Compound 1 was characterized by X-ray crystallography. In the absen ce of more HB(C6F5)(2), the products of alkyl/hydride exchange react t o eliminate RH and produce the borane-stabilized alkylidene compounds Cp2Zr(mu-CH2)[(mu-H)B(C6F5)(2)], 2, and Cp2Zr{eta(3)-CH(C6H5)[(mu-H)B( C6F5)(2)]}, 4. The latter compound is formed cleanly in 92% yield and was characterized by X-ray crystallography. Mechanistic studies on the se reactions involving partially deuterated compounds reveal that the alkyl/hyride exchange process is reversible and takes place via a step wise alkide-abstraction - hydride-replacement sequence rather than a c oncerted, four-centered sigma-bond metathesis type mechanism; This is most convincingly demonstrated by the observed inversion of stereochem istry observed when erythro-Cp2Zr[CH(D)CH(D)-t-C4H9](Cl) ((3)J(HH) = 1 2.82 +/- 0.05 Hz) is treated with excess HB(C6F5)(2), producing threo- (C6F5)(2)B-CH(D)CH(D)-t-C4H9 ((3)J(HH) = 5.00 +/- 0.05 Hz). Further ex periments reveal a H/D scrambling process involving the borane proton and the C-alpha-H positions of the zirconium alkyl groups (R = CH3, CH 2Ph). For example, treatment of Cp2Zr(CD2C6D5)(2) with 1 equiv of HB(C 6F5)(2) leads to a mixture of isotopomers of 4 and toluene, including C6D5CH3 and C6D5CH2D, suggesting a scrambling process in which the bor ane engages in multiple contacts with the metallocene reagent prior to alkane elimination. The H/D scrambling event is proposed to involve h ydridoborate attack of the remaining alkyl group on the forming metall ocene cation as HB(C6F5)(2) abstracts the other alkide ligand. The imp lications of these mechanistic studies within the realms of metallocen e activation and metallocene-catalyzed hydroborations are discussed.