Add time:07/24/2019         Source:sciencedirect.com

The mechanism of photochemical isomerization of 3-phenylpropene to 1-phenylpropene in the presence of hydridotetrakis(diethyl phenylphosphonite)cobalt(I) (HCoP4; P = P(OEt)2Ph) has been claimed to be a ‘photoassisted reaction’, on the basis of the fact that the isomerization stopped immediately when the light was turned off. On the other hand, by means of laser flash photolysis, we have identified a transient as a coordinatively unsaturated species (HCoP3) which maintains the catalytic cycle of the double-bond migration, and have noticed that the species may be regenerated in the dark catalytic cycle, namely, the reaction may proceed via a ‘photoinduced catalytic’ mechanism.In order to clarify the role of HCoP3, we have measured quantum yields and examined deuterium incorporation using DCoP4. Quantum yields (366 nm) of both the double-bond migration and the geometrical isomerization were found to exceed unity, e.g., 2.0 and 5.5 at 0.2 M of the olefins, respectively. This is consistent with a ‘photoinduced catalytic reaction’ rather than a ‘photoassisted reaction’. Indeed, even in such a diluted concentration of 3-phenylpropene (2.7 mM) as to keep Φ as low as 0.44, the double-bond migration using DCoP4 gave only 10% deuterated 1-phenylpropene, which means that DCoP3(HCoP3) maintained 18 turns of catalytic cycle.

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