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Kinetically controlled E-selective catalytic olefin metathesis

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Science  29 Apr 2016:
Vol. 352, Issue 6285, pp. 569-575
DOI: 10.1126/science.aaf4622

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EZ catalyst control in olefin metathesis

A decade has passed since the partner-swapping chemical dance known as olefin metathesis garnered a Nobel Prize, and distinct routines continue to emerge. In general, olefins are most stable in an E configuration, with the two largest substituents diametrically opposed. However, chlorine and fluorine substituents often invert this trend, favoring the alternate Z geometry. Nguyen et al. report a molybdenum metathesis catalyst with ligands carefully optimized to produce Cl- and F-substituted E olefins more quickly than the more stable Z isomers.

Science, this issue p. 569

Abstract

A major shortcoming in olefin metathesis, a chemical process that is central to research in several branches of chemistry, is the lack of efficient methods that kinetically favor E isomers in the product distribution. Here we show that kinetically E-selective cross-metathesis reactions may be designed to generate thermodynamically disfavored alkenyl chlorides and fluorides in high yield and with exceptional stereoselectivity. With 1.0 to 5.0 mole % of a molybdenum-based catalyst, which may be delivered in the form of air- and moisture-stable paraffin pellets, reactions typically proceed to completion within 4 hours at ambient temperature. Many isomerically pure E-alkenyl chlorides, applicable to catalytic cross-coupling transformations and found in biologically active entities, thus become easily and directly accessible. Similarly, E-alkenyl fluorides can be synthesized from simpler compounds or more complex molecules.

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