Olefin Cross Metathesis

Olefin Cross Metathesis-72
These transformations, promoted by catalysts that contain the highly abundant and inexpensive metal molybdenum, are amenable to gram-scale operations.

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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.

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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 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.CM generates a Mo-methylidene; this unhindered alkylidene can readily react with the Z-alkene product, reverse CM, cause equilibration and lower stereoselectivity.An enol ether reacts with a methylidene complex, circumventing diminution in Z selectivity.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.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).Many alkenes exist as either the E or the higher-energy Z stereoisomer.Catalytic procedures for the stereoselective formation of alkenes are valuable, yet methods enabling the synthesis of 1,2-disubstituted Z alkenes are scarce.


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