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

Abstract

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.