Olefin Cross Metathesis

Olefin Cross Metathesis-33
Many alkenes exist as either the E or the higher-energy Z stereoisomer.

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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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.

The utility of this method is demonstrated by its use in syntheses of an anti-oxidant plasmalogen phospholipid, found in electrically active tissues and implicated in Alzheimer’s disease, and the potent immunostimulant KRN7000. 1) represents one of the most attractive approaches to stereoselective preparation of these versatile functional groups.

Through fusion of two terminal alkenes, available in ample quantities as by-products of petroleum purification or readily accessed by a variety of methods, 1,2-disubstituted alkenes can be obtained; the other product generated is gaseous ethylene.

Here we report catalytic Z-selective cross-metathesis reactions of terminal enol ethers, which have not been reported previously, and of allylic amides, used until now only in E-selective processes.

The corresponding disubstituted alkenes are formed in up to 98% Z selectivity and 97% yield.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.Stereogenic-at-Mo 1a, 1b and 2 suggest that these complexes exhibit high activity partly as the result of stereoelectronic effects induced by the electron donor pyrrolide and acceptor monoaryloxide ligands.The fluxional nature of complexes such as 1a, 1b and 2, facilitated by the absence of rigid bidentate ligands, allows the metal alkylidenes to adapt to the structural strains imposed during the catalytic cycle.Through careful consideration of various mechanistic aspects of the process, conditions must be identified where the catalyst promotes CM but fails to react with the product Z alkene to effect equilibration, favouring the lower energy E isomer.Particularly difficult is the development of a process that affords the higher-energy Z alkene predominantly.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 general mechanistic features that engender Z selectivity in the above reactions, and would be expected to do so in a CM process, are depicted in Fig. The preference for Z alkene formation can be attributed to the ability of the large monodentate aryloxide to rotate freely (compare I in Fig.1), causing the incoming alkene to be oriented such that its substituent (R).


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