Twisted amides: crystal and optimized structures, and molecular geometry analysis of 1-acetyl-3,4,7,8-tetramethylglycoluril and 1,6-diacetyl-3,4,7,8-tetramethylglycoluril

Abstract

The crystal structures of racemic 1-acetyl-3,4,7,8-tetramethylglycoluril (3) and 1,6-diacetyl-3,4,7,8-tetramethylglycoluril (4) were determined by X-ray diffraction. Compound 3 forms dimers in which two complementary N–H⋯OC hydrogen bonds link a pair of enantiomeric molecules, while 4 crystallizes in two very similar but symmetry unrelated geometries. Unlike the parent 3,4,7,8-tetramethylglycoluril (1), both 3 and 4 are asymmetric: they exhibit a large dihedral angle between the two bridgehead-to-bridgehead substituent bonds (∠Me–C–C–Me∼−23 and −24°, respectively). Further, the plane through one acetyl group in 4 is twisted with respect to the plane through the tetrahydroimidazolone ring to which it is attached, while the other acetyl group is close to coplanar with its corresponding ring, as is the case for the acetyl group of 3. The experimental structures were used as a basis for evaluating optimized geometries at four levels of theory: restricted Hartree–Fock (ab initio) using 3-21G and 6-31G∗ basis sets and semiempirical AM1 and PM3 methods. General and specific geometric criteria of comparison are defined and used to evaluate the optimized structures. Using this scheme for comparison, the 6-31G∗ geometries are closer to the experimental geometries than the 3-21G geometries, and both are found to reproduce the X-ray geometries far better than the two semiempirical methods. The agreement between the conformations of the ab initio and the experimental geometries supports the hypothesis that the asymmetry in these molecules is dominated by intramolecular effects, as opposed to those of crystal packing. The results are consistent with an inherent asymmetry of the electron density between N1 and N6 in 3, and a corresponding induced effect in 4 as a result of the twisting in the amide moiety.