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 alcohols selectively exchanged alpha-hydrogens in aqueous solutions containing the catalyst precursor [Cp'Mo-2(mu-OH)(2)MoCp'(2)](OTs)(2), while some additional beta-hydrogen exchange was observed in secondary alcohols. Tertiary alcohols did not undergo H/D exchange. Formation of chelate complexes such as the independently synthesized and crystallographically characterized glycolate complex [Cp'2MoOCH2CH2OH](OTs). inhibited the H/D exchange in multidentate alcohols. The exchange reaction was shown to proceed by formation of ketone hydride molybdocene intermediate [Cp'Mo-2((OCRR2)-R-1)H](+), which can reversibly dissociate the ketone ligand. The molybdocene hydride complex resulting from ketone dissociation was identified by independent synthesis and crystallographic characterization of the hydride complex Cp'2MoH(OTf). 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 degrees C, the exchange of the first methylene hydrogen of PhCH2OH occurs with a rate constant k = 1.16 x 10(-4) (+/-9.88 x 10(-7)) s(-1), The activation 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 primary kinetic isotope effect of k(H,pD6.4)/k(D,pH6.5) = 2 2 was found.