The crystal and molecular structure of (Z)-2-(p-nitrobenzoyloxy)-3-phenyl-2-penten-4-one

Abstract

The enol-ester derivative of acetylacetone, (Z)-2-(p-nitrobenzoyloxy)-3-phenyl-2-penten-4-one, is orthorhombic, space group Pbca. At ca - 150°C, a=7.213 (1), b= 32.882 (3) and c= 13-055 (2)/~, Z=8. Diffraction intensities were measured with monochromated Mo K~ (2=0.71069 /~) radiation, for a crystal cooled to ca -150°C, to a resolution of sin 0]2_<0.904/~-1. Block-diagonal least-squares refinement of 277 variables and 7235 reflexions resulted in R=0-082 and Rw=0.100. C18H15NO5 is representative of a series of enol-esters for which an intramolecular O-acyl migration has been demonstrated to occur in solution. The molecular conformation determined from this analysis displays a short intramolecular non-bonded contact between the two carbonyl groups, 0(4)...C(1)=2.862 ~, which is consistent with the interaction of these two atoms in the proposed rearrangement mechanism. The geometry displayed by the interacting groups in the known examples of short intramolecular contacts between carbonyl groups is highly constrained by the nature of the chemical linkage between the carbonyl groups. Previous examples of such interactions display geometry with the molecular backbone linking the carbonyl groups in an eclipsed conformation; however, the title compound, which has somewhat more conformational freedom, displays a staggered geometry that is well suited to O-acyl migration through an intramolecular anti addition intermediate; the dihedral angle C(4)O(4)C(1)C(11) is 176.7 °.