Phosphine Dissociation Related to Initiation and Metathesis Efficiency

Phosphine Dissociation Related to Initiation and Metathesis Efficiency 

35°C was 4.6 E-04s , or 6.52 E-05s at 20°C (our temperature correction using the Eyring equation). This illustrates that phosphine dissociation is 250 times faster for 1. The associated activation parameters for this step show that each precatalyst follows a dissociative pathway, but the higher enthalpy of activation for 2 increases its AG by about 3kcalmol . This result was initially counterintuitive because 2 is a far better catalyst than 1. 

The next critical step was to measure the alkene binding step. To investigate this, the partitioning of the 14-electron reactive intermediate between productive alkene metathesis (Step 2) reversion back to precatalyst (A .i) was probed. Unfortunately, the rate of alkene binding could not be directly determined instead, the metathesis efficiency ratio k /t2 was kinetically determined from a plot of 1//Toi,s versus [CygP]/[alkene]. 

In all cases, the aryhdenes showed metathesis efficiency quotients larger than that of 2 (Table 9.3) [18]. In each, the relative rate of return to the phosphine complex was faster than that for 2, which is consistent with the principle of microscopic reversibility. However, to make these catalysts truly superior to 2 would require increased stabilization of the Ru(IV) intermediate through alternative ligand modifications. 

Increasing the size of the halide was found to increase the rate of initiation for both the first- and second-generation catalyst motifs (Table 9.4). The bromide was made from 1 dibromide by the addition of the l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (H2lMes) ligand. The iodides can be made by salt metathesis or exchange via the pyridyl solvates [5]. The larger iodide resulted in a significant increase in the rate of phosphine dissociation, presumably due to increased steric pressure in the 16-electron precatalyst. The iodide complexes 

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