255876-66-5Relevant articles and documents
Preferred conformation and barriers to internal rotation of ortho-disubstituted cyclopropylbenzenes
Ernst, Ludger,Rieck, Thomas,Soliven, Mark
, p. 1697 - 1706 (2007/10/03)
Schaefer's "J method" was employed to show that 2-cyclopropyl-1,3-dimethylbenzene (5) in solution prefers the perpendicular conformation in which the torsional angle Θ between the C(1)-H bond of the cyclopropyl group and the plane of the benzene ring is 90°. This is opposed to the situation in cyclopropylbenzene (3) where the bisected conformer (Θ = 0°) prevails. From the value of -0.85 ± 0.01 Hz for 6J(H-α,H-para) in 5 (for solutions in CS2 and in acetone) a barrier to rotation about the cyclopropyl-aryl bond of 6.4 kJ/mol can be derived if a predominantly two-fold potential and a vanishing 6J(H,H) for Θ = 0° are assumed. The introduction of the two orrho-methyl groups into 3 thus effectively interchanges the ground and transition state conformations of the internal rotation. This effect is well reproduced by ab initio (STO-3G and 6-31G*) and semiempirical (AMI) molecular orbital computations. The preference for the perpendicular conformation of an ortho-disubstituted cyclopropyl substituent was also demonstrated by a dynamic NMR study of 2-cyclopropyl-4-isopropyl-1,3,5-trimethylbenzene (10). Exchange-broadened 1H spectra due to slow rotation of the cyclopropyl group were only obtained near the low-temperature limit of the spectrometer (-140°C), and the barrier to rotation is estimated to lie near 28 kJ/mol (ΔG?). MM3 molecular mechanics computations suggest that the rather large increase of the rotational barrier in 10 relative to 5 is caused by the combined buttressing effect of the isopropyl and the 5-methyl groups. The present findings explain why an earlier attempt (in 1970) to determine the rotational barrier in 2-cyclopropyl-1,3,5-trimethylbenzene by dynamic NMR was bound to fail.
