Olefin Isomerization Promoted by the Grubbs Catalyst

When the allylalcohol contains an additional olefmic part, the reaction competes with RCM (Eq. 12.30) [34b]. In the case of 74, the activation barrier for the RCM pathway toward 75 would become higher because of the sterically hindered trisubsti-tuted C=C double bond of 74. For the RCM of such sterically demanding dienes, complexes 70 and 71 are generally used. 

Migration of the double bond of the cyclic olefin formed by the RCM has also been observed [35]. RCM of the diene 77 by 69 in refluxing dichloromethane resulted in the formation of considerable amounts of the unexpected cyclic olefin 79 in addition to 78 (Eq. 12.31) [35a[. It was also noted that the formation of 79 was effectively suppressed by the addition of amine to the dichloromethane solution or employment of diethylether as solvent, which implies participation of proton in the isomerization reaction. It was also noted that a terminal vinyl group with a free tertiary allylic hydroxyl group accelerates RCM, rather than its methyl ether derivative. These results suggested some interaction between the alkylidene complex 69 with hydroxyl proton in situ. 

Hydrolysis [36], thermolysis [37], and alcoholysis (Eq. 12.32) [38] of the benzyli-dene complex 69 were investigated in relation to decomposition of 69. In each case, formation of the hydride complex was confirmed by the use of NMR spectroscopy. Treatment of 69 with ethanol afforded a hydride complex (PCy3)2(CO)Ru(Cl)(H) (80). Complex 80 has been shown to promote isomerization of 1-octene to 2-octene reaction of 88 000 mol equiv. of 1-octene with 80 at 100 °C for 3 h gave 97% conversion with 92% selectivity for 2-octene [38]. 

Since complexes 70 and 71 have been sho vn to be thermally stable in contrast to 69 [39], and hence their thermal degradation vas negligible. Although reaction of 70 with 1-octene performed at room temperature in ADMET conditions afforded mainly Ci4-olefin, significant amounts of Cy and C9-C13 olefins were observed in the reaction performed at 60 °C [40]. This result suggests that olefin isomerization is promoted to some extent at a higher temperature. 

It has been reported that activities of the ruthenium alkylidene complexes, which contain mesityl groups at the N atoms, are highly influenced by solvent [41]. Reactions in toluene occur substantially faster than those in CH2CI2. While treatment of diene 81 with 1.2 mol% of 71 in toluene led to essentially complete consumption of the starting material in 6.5 h, the same reaction took over 20 h in CH2CI2 by use of 4 mol% of 71 (Eq. 12.33) [42]. 

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