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Abstract:
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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. |