Intramolecular reactions of nitro-olefin–cyclohexane-1,3-dione Michael adducts. Crystal structure of 3-(4-bromophenyl)-6,7-dihydro-2-hydroxyiminobenzofuran-4(5H)-one

Abstract

Cyclohexane-1,3-dione and its 5,5-dimethyl derivative (dimedone), in contrast to other β-diketones, behave abnormally in Michael additions to nitro-olefins. From nitro-olefins RCHCH·NO₂(R = alkyl or aryl, not H) 3-substituted 6,7-dihydro-2-hydroxyiminobenzofuran-4(5H)-ones can be prepared. These compounds are the first known butenolide derivatives having a 2-hydroxyimino-substituent. 3-(4-Bromophenyl)-6,7-dihydro-2-hydroxyiminobenzofuran-4(5H)-one has been prepared from cyclohexane-1,3-dione and 4-bromo-β-nitrostyrene and resolved into pure syn- and anti-forms. The structures of both forms have been established from n.m.r. spectra, and by X-ray crystallography from diffractometer data. Single crystals of a 1 : 2 mixture of syn- and anti-forms (VIIA and B; anti: syn= 2 : 1) are orthorhombic with space-group Pbca, cell dimensions a= 17·37, b= 7·89, and c= 19·76 Å. The crystals were found to be disordered because they contain a mixture of syn- and anti-forms of the oxime. Three possible ways of describing the mixture of syn- and anti-forms were refined by full-matrix least-squares of 805 statistically significant reflections to weighted R values of 0·051, 0·056, and 0·073. None of the three refinements gave good agreement with bond distances previously reported for CN an N–OH in oxime structures (although mean values for the 0·051 and 0·057 refinements did). The observed apparent planarity of the cyclohexenone ring is attributed to the presence of equal amounts of both ring-inversion isomers in the crystalline state. Short intermolecular distances of 2·63 and 2·84 Å between the oxygen of both syn- and anti-oxime groups and the ketonic oxygens give strong evidence for intermolecular hydrogen bonding.