Substitution steric classification

With the discovery of the second generation of the ruthenium catalyst, CM has become an invaluable method for the preparation of olefins in numerous natural products. In most cases, high chemo- and -stereoselectivities are achieved under mild conditions. More importantly, the outcome of the CM reaction can be predicted using the empirical classification of olefins proposed by Grubbs et al. Type I-IV olefins are classified according to their relative tendency to undergo a homodimerization. The tendency relies on the steric and electronic properties of olefins, and thus, CM-based syntheses of natural products are described in line with the substitution pattern of olefins. 

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