13C-NMR ano theoretical stuoies of internal rotation in methylateo anillnes

Abstract

The conformational properties of ten ring-methylated N-methyl- and N,Ndimethylanilines have been studied using 13C-NMR chemical shifts and spin-Iattice relaxation times in CDCI3, and semi-empírical (AM1) quantum-chemical calculations. The experimental results indicate that, like aryl methyl ethers, N-methylanilines prefer conformations in which the N-methyl carbon lies near the ring plane. Orthosubstitution in these compounds, while forcing the N-methyl group to adopt an anti orientation with regard to the ortho substituent, does not induce any important changes from the vantage point of the electron donor ability of the amine function and therefore does not affect the N-methyl 13C chemical shifts or spin-Iattice relaxation times to any appreciable extent. The preferred conformations of ortho-unsubstituted N,Ndimethylanilines leave the N-methyl carbon atoms oscillating on eíther side of the ring plane, but the conformational space of these compounds is strongly limited by ortho-methylation, so that in these cases one of the N-methyl carbon nuclei is forced to remain c10se to the aromatic ring plane, resulting in much shortened relaxation times and deshielding of that nucleus. The quantum mechanical calculations reproduce these results, allowing the relative stability of the methylated aniline conformers to be discussed in terms of competitíon between the nuclear repulsion energy and electron delocalization. Ortho-methylation of N,N-dimethylanilines leads to an increase of electron density around the nitrogen atom and a change from almost Sp2 to almost Sp3 nitrogen hybridization, in agreement with the experimental results, including the increased basicity of these compounds.