Methyl/phenyl attraction by CH/π interaction in 1,2-substitution patterns.

In 1,2-Me,Ph substitution patterns of organic compounds the methyl group attracts one of the phenyl sides to establish a CH/π bond with one of the ortho carbon atoms (the C(o) side), leading to a characteristic tilting of the phenyl ring around its C(i)-C(p) axis. This phenyl rotation shortens the C(Me)-C(o) distances to bonding contacts between the methyl hydrogen atoms and the ortho carbon atom C(o) well below the van der Waals distance of 3.70 Å. On the other hand, it elongates the C(Me)-C(o') distances outside of the reach of any CH/π interaction (>3.70 Å). Our study is based on a search in the Cambridge Structural Database for substructures Me-C═C-Ph, Me-C-C-Ph, and Me-C-N-Ph with 1,2-Me,Ph substitution patterns. In the 1,2-Me,Ph substitution motif the torsion angle C(Me)-C-C-C(i) determines the length of the C(Me)-C(i) and C(Me)-C(o) distances. For aromatic compounds these torsion angles are close to 0°, but in five- and six-membered ring compounds and in open-chain compounds the torsion angles vary considerably. Universally, for torsion angles up to 80° CH/π bonds were found, whereas the long C(Me)-C(i) and C(Me)-C(o) distances for torsion angles >80° do not allow a CH/π interaction. The results of the present CSD analysis are supported by calculations.