Cumulative anomeric effect: a theoretical and x-ray diffraction study of orthocarbonates

Abstract

The combined effect of six anomeric pair interactions at a single carbon atom has been examined by X-ray diffraction and molecular mechanics. MM2 calculations on C(OR)₄ (R = H, Me, Ph) indicate two closely spaced minima with S₄ and D_2d symmetry. An interesting low-energy pathway resembling a three-dimensional domino has been computed for the interconversion of the S₄ conformer to the D_2d form. Single-crystal X-ray structures determined for three C(OAr)₄ derivatives confirm that the energies of the two conformers do not differ significantly. Thus, the two crystallographically nonequivalent molecules in the unit cell of tetraphenoxymethane (4) as well as of tetrakis(3,5-dimethyIphenoxy)methane (5) adopt conformations in which the central C(OC)₄ units have a near D_2d symmetry, while the corresponding fragment has a distorted S₄ symmetry in tetrakis(4-bromophenoxy)methane (6). The experimental C-O bond lengths and C-0-C and 0-C-O angles are consistent with large anomeric interactions.