EPR study of the long-range effect of a β-carbon chiral center on the magnetic equivalence of β-methylene protons

Abstract

Using aqueous TiCl3-based continuous-flow, radical-generating methods at 25 °C we have carried out a comprehensive EPR study of hydroxyl- and aminyl-radical addition to eight alkene substrates that yielded radicals of general formula XCH2—ĊY—C*HR1R2, where C* is a chiral center, to investigate the magnetic inequivalence of β-CH2 protons arising from a nonadjacent β-C chiral center. We attribute the observed nonequivalence to the long-range influence of the chiral center in these radicals, and note that our results are consistent with the reasonable expectation that the presence of bulky groups in the radicals is likely to favor particular conformations, leading to more easily observed inequivalence of the β-CH2 proton splittings.