Isopropoxy benzylidene

Symmetrically substituted Al-alkyl derivatives of Grubbs type benzylidenes 32 and 33 reported by Verpoort and coworkers resulted in unstable complexes (Figure 11.6) [56]. However, reducing steric hindrance by the use of a chelating isopropoxy benzylidene resulted in the isolation of complexes 34 and 35 [59]. Although these complexes displayed higher catalytic activity in ROMP compared to the parent catalyst, the RCM and CM activity was lowered, probably due to the increased steric bulk [59]. 

In contrast, catalyst 40 displayed significant improvement in initiation reaching 43% conversion in 1 h, but also a dramatically reduced stability. More interestingly, the phosphine-free complex 43, containing a biphenyl isopropoxy benzylidene moiety, showed surprisingly rapid initiation and enhanced stability giving 90% conversion in just 1.5 h. This result was in accordance with the recent concept of steric activation put forward by Wakamatsu and Blechert [65]. 

Attempting to further enhance activity, various modifications on the naphtho-late and chelating isopropoxy benzylidene moiety were introduced and studied (Figure 11.33) [119]. Indeed, complexes 161-166 were found to be more efficient than 160 at low catalyst loadings for certain asymmetric ring-opening metathesis/cross-metathesis (AROM/CM) reactions. 

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