C−H Bond Activation in Aqueous Solution:  A Linear Free Energy Relationship Investigation of the Rate-Limiting Step in the H/D Exchange of Alcohols Catalyzed by a Molybdocene

A biphasic linear free energy relationship was found for the H/D exchange reactions of the H atom bonded to the α-carbon atom in substituted benzyl alcohols catalyzed by [Cp‘2MoOH(OH2)]+ in aqueous solution. This observation is consistent with a rate-limiting step involving C−H oxidative addition. Two pathways for the C−H activation are proposed, one involving formation of a π-bonded aldehyde and the other involving a σ-bonded aldehyde. Donor groups favor the σ-bonded intermediate and withdrawing groups the π-bonded intermediate.