Remote electronic control in asymmetric cyclopropanation with chiral Ru-pybox catalysts

Abstract

Electronic control by remote substitents far from a catalytically active center was found in an asymmetric cyclopropanation of olefins and diazoacetates with the ruthenium catalysts of chiral 4-substituted bis(4′-isopropyloxazolinyl)pyridine, 4-X-pybox. It was found that electron-withdrawing groups increase the catalytic activity, but electron-donating groups decrease it. The enantiomer ratios of the product cyclopropanes with the electron-withdrawing groups, X = Cl and COOMe, are higher than those with the electron-donating groups, X = OMe and NMe2. The enantiomer ratios of the ethyl and l-menthyl cyclopropanes were corelated toward Hammett's σpara values to give positive ϱ values, 0.365 for trans-ethyl ester, 0.486 for cis-ethyl ester, 0.517 for trans-l-menthyl ester, and 0.517 for cis-l-menthyl ester. However, the trans:cis ratios of the products were not affected by their substitutents, ca. 90:10 for ethyl diazoacetate and ca. 96:4 for l-menthyl diazoacetate. Intramolecular cyclopropanation clearly gave a similar trend.