Methyl/Phenyl Attraction by CH/pi Interaction in 1,2-Substitution Patterns

摘要

In 1,2-Me,Ph substitution patterns of organic compounds the methyl group attracts one of the phenyl sides to establish a CH/pi bond with one of the ortho carbon atoms (the Co side), leading to a characteristic tilting of the phenyl ring around its Ci-Cp axis. This phenyl rotation shortens the CMe-Co distances to bonding contacts between the methyl hydrogen atoms and the ortho carbon atom Co well below the van der Waals distance of 3.70 angstrom. On the other hand, it elongates the C-Me-C-o' distances outside of the reach of any CH/pi interaction (>3.70 angstrom). 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-Ci determines the length of the CMe-Ci and CMe-Co distances. For aromatic compounds these torsion angles are close to 0 degrees, 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 degrees CH/p bonds were found, whereas the long CMe-Ci and C-Me-C-o distances for torsion angles >80 degrees do not allow a CH/pi interaction. The results of the present CSD analysis are supported by calculations.