However, the only reported instances of Z-selective CM (65–90% Z) involve substrates with an sp-hybridized substituent (acrylonitrile or enynes).
In an efficient Z-selective CM, it is not only required that reaction between the two substrates proceed selectively (versus homocoupling), it must exhibit a preference for the thermodynamically less favoured stereoisomer (Fig. The inherent reversibility of olefin metathesis (products can re-enter the catalytic cycle) and the higher reactivity of Z alkenes (versus E isomers) further exacerbate the problem.
The more stable alkoxy-substituted alkylidene, generated from reaction of a methylidene complex and an enol ether (I in Fig.
1 with R = On-Bu), can undergo productive CM, giving rise to longer catalyst lifetime and improved turnover numbers.
Such deliberate orchestration is not feasible with catalytic CM, where both alkenes are mono-substituted and, in contrast to ROCM, there is no relief of ring strain to be manipulated.
We began by evaluating the ability of stereogenic-at-Mo complexes to promote transformations of enol ethers, a class of substrates for which a CM reaction has not been previously reported (E-or Z-selective); the resulting products have proven to be of utility in chemical synthesis and can be found in biologically active molecules (see below).
These transformations, promoted by catalysts that contain the highly abundant and inexpensive metal molybdenum, are amenable to gram-scale operations.
Use of reduced pressure is introduced as a simple and effective strategy for achieving high stereoselectivity.
To accomplish a Z-selective CM, a variety of catalysts were considered, such as stereogenic-at-Mo complexes (1, 2) or other previously reported Mo- and Ru-based complexes (3–5).
The structural flexibility of the stereogenic-at-metal complexes 1 and 2 can give rise to exceptional reactivity, and free rotation around the Mo–O bond of these alkylidenes might serve as the basis for development of highly Z-selective olefin metathesis reactions of terminal alkenes.