The 2,6-dimethylphenylimido 1a therefore offers the best balance between activity and stereoselectivity.
Such performance variations may be observed because catalyst turnover is slower with the more sizeable 1b whereas the methylidene of the relatively unhindered 2 (compare IV, Fig. Consistent with the above scheme, 82% 8a is formed when CM with 1b is allowed to continue for 16 hours; in contrast, conversion with 2 after 10 minutes or two hours is nearly identical (There are several, mechanistically revealing, reasons for use of excess enol ether.
The sphere represents an appropriate size imido substituent.
As the preliminary steps towards the eventual development of an efficient class of Z-selective CM reactions, we investigated two related—but much simpler—versions of the process. 1) represent the catalyst classes used in our studies.
As a result, the stereogenic-at-Mo complexes are generally more effective olefin metathesis catalysts than other Mo-based complexes 3 with strained oxabicyclic alkenes and styrenes.
Homocoupling of terminal alkenes was subsequently shown to proceed with high efficiency and Z selectivity in the presence of members of the same catalyst class.
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.
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