Captodative substitution and cyclopropane geometry. Part 5. X-Ray structure of five new compounds and asymmetry parameters of the substituents

Abstract

Five new captodative (cd) substituted cyclopropanes have been synthesized and their structures determined from X-ray diffraction data. These are cis-1,2-dicyano-1,2-dimethyl-(1), trans-1,2-dicyano-1,2-bisethoxycarbonyl-(2), cis-1,2-dicyano-1,2-dimethoxy-(3), trans-1,2-dicyano-1,2-bismethylthio-(4), and trans-1,2-dicyano-1,2-bis(dimethylamino)-cyclopropane (5). These structures have been solved by direct methods and refined by least-squares using 865, 1 332, 766, 1 075, and 1 341 reflections respectively, to R 0.062, 0.041, 0.044, 0.031, and 0.049. This work completes former results and permits the discussion of the influence of polar substituents on cyclopropane geometry in relation to their reported ease of cis–trans isomerization. The effect on ring bond lengths is small but significant: in both captodative and dicapto-substitution, the distal ring bond is shortened and vicinal bonds are lengthened. Mean distal-bond shortenings are proposed for SR, OR, and NR2; the values calculated for C=N, C=O, and phenyl agree with published data. The particular facile isomerization of cd cyclopropanes appears best explained, not as the consequence of a destabilized ground state but rather as increased spin delocalization in the transition state.