Grubbs-Hoveyda-type initiators

Figure 19.4 Grubbs- and Grubbs-Hoveyda-type initiators bearing l,3-dimesityltetrahydropyrimin-2-ylidenes.

ROMP with Grubbs and Crubbs-Hoveyda-Type Initiators... 

The mechanisms in operation for some important classes of alkene metathesis catalyst have been studied density functional theory (DFT) studies have also provided useful insights into reaction mechanisms. In particular, the initiation mechanisms of Grubbs-type and Hoveyda-type complexes have been explored. In each case, insights into the effects of structure on initiation rate have been achieved. 

Much effort has been expended recendy to understand how Hoveyda-type precatalysts initiate. Originally, it was simply assumed that the initiation mechanism was dissociative and data to support this was pubHshed by Love et al., in which no dependence of the initiation rate of GH2 on substrate concentration was observed. No entropy change was required to reach the transition state (AH = 19.9(5) kcal moH and AS = 1(2) cal mol for the initiation of GH2 with EVE in toluene). However, activation parameters collected during a later study, also by the Grubbs group, suggested a nondissociative mechanism, due to the negative entropy of activation (AH = 15.2(8) kcal mol and AS = —19(3) cal moH for the... 

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