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 α-hydrogens in aqueous solutions containing the catalyst precursor [Cp‘2Mo(μ-OH)2MoCp‘2](OTs)2, while some additional β-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‘2Mo(OCR1R2)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 cat...