Ring Opening Cross-Metathesis ROCM

The olefin cross metathesis (CM) can be described as the intermolecular metathesis of alkylidene fragments between two different olefins [133]. It can be farther divided into three main subtypes cross metathesis, ring opening cross metathesis (ROCM) and enyne cross metathesis (ECM) (Scheme 3.9). 

As one of the earliest demonstrations of the synthetic utility of selective ring-opening cross-metathesis (ROCM) reactions. Snapper and co-workers reported the ROCM of substituted cyclobutene and terminal olefins to yield 1,5-diene products catalyzed by Grubbs first generation catalyst in 1995 (Fig. 20) [46]. Although the selectivity was only moderate (generally 2 1 Z E for six examples reported), the bias towards the formation of Z-alkene represents a rare example. A model was proposed to account for the selectivity, where steric interactions between the substrate and the phosphine ligand are minimized [47]. 

Grubbs and coworkers studied a series of Ru-based catalysts and identified two lead catalysts for alkene cross metathesis that could provide the desired hetero-coupled product with high Z-selectivity for the resulting alkene moiety (Fig. 39) [91-93]. Comparably, Hoveyda and coworkers developed a number of Mo-based catalysts that enabled the selective synthesis of Z-alkenes [94, 95]. They successfully demonstrated that the novel molybdenum adamantylimido complexes (catalysts 154-156) could promote ring opening/cross metathesis (ROCM) of oxabicycle (157 or 158) in good yield and excellent Z-selectivity (Fig. 40) [96]. These elegant studies paved the way to develop further more effective catalysts for Z-selective alkene metathesis and utilize these methods for the synthesis of complex natural products. 

Apart from cross-metathesis, ROMP, and RCM, there are other less common metathesis reactions. These are acyclic diene metathesis polymerization (ADMET), ring-opening cross-metathesis (ROCM), ring-rearrangement metathesis (RRM), and ethenolysis. A general ADMET reaction is shown by reaction 7.3.1.7. ADMET reactions are generally performed on a,well-defined and strictly linear polymers with unsaturated polyethylene backbones. Ethenolysis is the cross-metathesis of ethylene with an internal olefin. 

Scheme 12.5 Total synthesis of baconipyrone C by an enantioselective Ru-catalyzed ring-opening/cross-metathesis (ROCM) reaction. PMB = p-methoxybenzyl Mes = 2,4,6-Me3C6H2.

Grubbs found that (4 a) ring-opened cyclic olefins, and then react with an acrylate to produce end functionalized linear olefins, giving a ring-opened cross metathesis product (ROCM) with two olefins with differing reactivity (equation 19). Key to the distribution of products was the relative rates of ring-opening and cross metathesis with the functionalized olefin. 

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