DF-studies on a ‘stop and go’ rotor: steric and electronic factors determining the regio- and stereochemical position of a η6-Cr(CO)3 metal fragment on a helically distorted biaryl ligand

Ground state structures and relative energies of the regioisomers of 1,3-dialkyl-benzo[b]naphtho[1,2-d]pyranone chromiumtricarbonyl complexes 2 and 4 (R = Me and rBu) have been calculated by means of DF methods. The thermodynamically most stable regioisomers calculated agree with those found experimentally. Additionally, two atropisomeric minimum structures with different rotating positions of the Cr(CO)3 rotor were found per regioisomer. The rotor positions governed by electronic and steric factors are compared with experimental results and theoretical predictions. From the periodically increasing steric interactions during the helimerization process, a ‘stop-and-go’ dynamic of the Cr(CO)3 rotor, depending from the conformation at the biaryl axis, is suggested.