C-H bond activation in aqueous solution: Kinetics and mechanism of H/D exchange in alcohols catalyzed by molybdocenes

The mechanism of the catalytic H/D exchange in alcohol substrates promoted by molybdocenes in D2O was shown to occur by C-H bond activation. Primary a lcohols selectively exchanged alpha-hydrogens in aqueous solutions containi ng the catalyst precursor [Cp'Mo-2(mu-OH)(2)MoCp'(2)](OTs)(2), while some a dditional beta-hydrogen exchange was observed in secondary alcohols. Tertia ry alcohols did not undergo H/D exchange. Formation of chelate complexes su ch as the independently synthesized and crystallographically characterized glycolate complex [Cp'2MoOCH2CH2OH](OTs). inhibited the H/D exchange in mul tidentate alcohols. The exchange reaction was shown to proceed by formation of ketone hydride molybdocene intermediate [Cp'Mo-2((OCRR2)-R-1)H](+), whi ch can reversibly dissociate the ketone ligand. The molybdocene hydride com plex resulting from ketone dissociation was identified by independent synth esis and crystallographic characterization of the hydride complex Cp'2MoH(O Tf). The H/D exchange reaction proceeds stepwise, with the active catalyst being derived from the monomeric complex [Cp'Mo-2(OH)(OH2)](+). At T = 90 d egrees C, the exchange of the first methylene hydrogen of PhCH2OH occurs wi th a rate constant k = 1.16 x 10(-4) (+/-9.88 x 10(-7)) s(-1), The activati on parameters were determined as Delta H-double dagger = 19.4 (+/-0.2) kcal mol(-1) and Delta S-double dagger = -22.7 (+/-0.7) cal mol(-1) K-1, A prim ary kinetic isotope effect of k(H,pD6.4)/k(D,pH6.5) = 2 2 was found.