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Cyclopropanation

In 2005, Toste and coworkers reported the intermolecular olefin cyclopropanation reaction of propargyl acetate, pivaloate and benzoate with various substituted olefins.10 It was noted that the substitution on the olefin had little impact on the potency of the catalyst. The same observation was noted with the various esters used. Using 5 mol % of (PPh3)AuCl and 5 mol % of AgSBF6 in nitromethane at room temperature, a wide array of cyclopropanes were synthesized in good yields (Scheme 6). Following these encouraging results, Johansson et al. developed an enantioselective gold-catalyzed cyclopropanation.1 Using propargyl acetate with styrene, the best condition and catalyst system were identified. Using 2.5 mol % (R)-DTBM-SEGPHOS(AuCl)2 with 5 mol % of AgSbF6, in nitromethane at room temperature, various chiral cyclopropyl esters were synthesized in good yields with good enantioselectivities (Scheme 7). It was noted that the steric hindrance on the aryl had a positive influence on the enantioselectivity of the reaction. The selection was also extended to trimethylsilyl-olefins to yield the desired product as a 5:1 mixture of cis and trans isomers with 78% yield and 65% ee.

Scheme 6.

Scheme 7.

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Materials

     

References

  1. Johansson, M. J. et al. J. Am. Chem. Soc. 2005, 127, 18002.

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