Racemization of Secondary Alcohols Catalyzed by Cyclopentadienylruthenium Complexes: Evidence for an Alkoxide Pathway by Fast β‐Hydride Elimination–Readdition

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Abstract
The racemization of sec-alcohols catalyzed by pentaphenylcyclopentadienyl–ruthenium complex 3 a has been investigated. The mechanism involves ruthenium–alkoxide intermediates: reaction of tert-butoxide ruthenium complex 4 with a series of sec-alcohols with different electronic properties gave ruthenium complexes bearing a secondary alkoxide as a ligand. The characterization of these alkoxide complexes by NMR spectroscopy together with a study of the reaction using in situ IR spectroscopy is consistent with a mechanism in which the alkoxide substitution step and the β-hydride elimination step occur without CO dissociation. The alkoxide substitution reaction is proposed to begin with hydrogen bonding of the incoming alcohol to the active ruthenium–alkoxide intermediate. Subsequent alkoxide exchange can occur via two pathways: i) an associative pathway involving a η3-CpRu intermediate; or ii) a dissociative pathway within the solvent cage. Racemization at room temperature of a 1:1 mixture of (S)-1-phenylethanol and (S)-1-phenyl-[D4]-ethanol gave only rac-1-phenylethanol, and rac-1-phenyl-[D4]-ethanol, providing strong support for a mechanism in which the substrate stays coordinated to the metal center throughout the racemization, and does not leave the coordination sphere. Furthermore, racemization of a sec-alcohol bearing a ketone moiety within the same molecule does not result in any reduction of the original ketone, which rules out a mechanism where the intermediate ketone is trapped within the solvent cage. These results are consistent with a mechanism where η3-Ph5C5–ruthenium intermediates are involved. Competitive racemization on nondeuterated and α-deuterated α-phenylethanols was used to determine the kinetic isotope effect kH/kD for the ruthenium-catalyzed racemization. The kinetic isotope effect kH/kD for p- X-C6H4CH(OH)CH3 was 1.08, 1.27 and 1.45 for X=OMe, H, and CF3, respectively.