The Kinetics of the Reaction of RuCl(H)(PPh3)3 with Dienes and Trienes

RuCl(H)(PPh3)3 is a valuable catalyst. It is a very efficient hydrogenation catalyst but has proven difficult to study kinetically. The consumption of hydrogen during the catalytic cycle is very rapid and the kinetics are limited by the rate of hydrogen dissolving into the solvent. We had found that RuCl(H)(PPh3)3 reacts with dienes and trienes to give allyls and dienyls.

The reaction of RuCl(H)(PPh3)3 with cycloheptatriene.

The reaction of RuCl(H)(PPh3)3 with cyclohepta-1,3-diene.

The reaction of RuCl(H)(PPh3)3 with penta-1,4-diene.

These reactions were exploited to determine the mechanism of the key step of the insertion of an alkene into the Ru-H bond of RuCl(H)(PPh3)3. It was shown that the mechanism has two pathways, a direct attack of the diene or triene on RuCl(H)(PPh3)3 and the preferred pathway of PPh3 loss to give RuCl(H)(PPh3)2 which is followed by attack of diene or triene. The mechanism is summarised below.

The mechanism of the reaction of RuCl(H)(PPh3)3 with dienes and trienes.

It is probable that when RuCl(H)(PPh3)3 is used as a hydrogenation catalyst, RuCl(H)(PPh3)2 is the species on the catalytic cycle.


References


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Date:- 6th January, 1997