Dimethyl (dimethylaminomethylene)malonate: crystal structure and dynamic behaviour in solution

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Abstract
The structure of the title compound (I) was determined from photographic X-ray data. Crystals are monoclinic, space group P21/a, with Z= 4 in a unit cell of dimensions: a= 6·200 ± 0·002, b= 12·144 ± 0·002, c= 12·859 ± 0·002 Å, and β= 86·41 ± 0·04°. The structure was solved by direct methods and refined by full-matrix least-squares techniques to R 0·055 for 867 observed reflections. The molecule consists of two essentially planar parts: the trans-dimethylaminoacrylate portion, and the remaining methoxycarbonyl group. The nitrogen atom is coplanar with its bonded neighbours. The bond lengths of the group N·C:C are N–C 1·337 and CC, 1·380 Å, indicating extensive delocalisation. Comparison with i.r. results suggests a similarity of the conformations of (I) in the crystalline and liquid phases, and enables the ΔG* value for isomerisation about the CC bond in the similarly delocalised methyl trans-3-dimethylaminoacrylate to be estimated as 30·8 kcal mol–1. The lower value for (I), 15·6 kcal mol–1, cannot be ascribed to its ground-state delocalisation and should be attributed mainly to a stabilisation of the dipolar transition-state.

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