Acceptorless dehydrogenative coupling of alcohols catalysed by ruthenium PNP complexes: Influence of catalyst structure and of hydrogen mass transfer
Résumé
Base-free catalytic acceptorless dehydrogenative homo-coupling of alcohols to esters under neat conditions was investigated using a combined organometallic synthesis and kinetic modelling approach. The considered bifunctional ruthenium aliphatic PNP complexes are very active, affording TONs up to 15,000. Notably, gas mass transfer issues were identified, which allowed us to rationalize previous observations. Indeed, the reaction kinetics are limited by the rate of transfer from the liquid phase to the gas phase of the hydrogen co-produced in the reaction. Mechanistically speaking, this relates to the interconverting couple amido monohydride/amino bishydride. Overcoming this by switching into the chemical regime leads to an initial turnover frequency increase from about 2000 up to 6100 h(-1). This has a significant impact when considering assessment of novel or reported catalytic systems in this type of reaction, as overlooking of these engineering aspects can be misleading. (C) 2016 Elsevier Inc. All rights reserved.