A heterogeneous Ru–hydroxyapatite catalyst for mild racemization of alcohols

Racemization of chirally stable secondary alcohols was performed using a truly heterogeneous catalyst under mild reaction conditions in the absence of any additive or cocatalyst. The catalyst is based on Ru3+ immobilized on calcium hydroxyapatite (Ru–HAP). The preparation of the calcium hydroxyapatite (HAP) support and the Ru content and immobilization were optimized leading to maximal catalytic performance. Via combined XRD, Raman, SEM, XPS, EPR, N2 adsorption, and ICP analysis, information on both the compositional and the structural properties of the material was obtained. The Ru–HAP catalyst is able to racemize benzylic as well as aliphatic secondary alcohols. The catalyst scope was further investigated for racemization of several mono- and polyfunctional secondary alcohols. Performing standard racemization of a reference substrate in the presence of a functional additive demonstrated the inhibitory effect of some of these functions. Finally the heterogeneous nature of the catalyst was verified by filtrate tests.