Insights into the ruthenium-catalysed selective reduction of cardanol derivatives via transfer hydrogenation: a density functional theory study

The detailed mechanism for ruthenium-catalysed selective reductionof cardanol derivatives by transfer hydrogenation has been fully characterisedat the B3PW91-D3/ECP2/PCM//B3PW91/ECP1 level of density functional theory. The explored catalytic cycle involved the hydrogenation of the triene cardanolgiving the diene product through a highly stable η³-allylicintermediate via a kinetic barrier of 29.1 kcal mol⁻¹, which followed further hydrogenation leading to a more stable η³-allylicintermediate. The further reduction to the cardanol monoene product required an overall barrier of 29.2 kcal mol⁻¹, which offers a rationale for the requirement of elevated temperatures (refluxing isopropanol). The computed overall barrier of 46.6 kcal mol⁻¹ to accommodate a fully saturated product is unsurmountable— in good agreement with the experiment, where no such full hydrogenation is observed, and rationalising the 100% selectivity towards the monoene product.