Unsaturated nitriles: conjugate additions of carbon nucleophiles to a recalcitrant class of acceptors.

Nitrile anion cyclizations

X-Ray structural data, as well as semiempirical and ab initio molecular orbital calculations, reveal no systematic and substantial difference between the C–C bond lengths of cis and trans 1,2-diketones. Additional results on various conformations of 1,2-diimines and 1,2-dithiones follow the same pattern. Therefore, lone-pair repulsions cannot be implicated in the observed lengthening of C–C bonds in isatin and several related molecules. Conjugation in these systems occurs peripherally avoiding the participation of the central C–C bond. Negative hyperconjugative interaction between the oxygen lone pairs and the adjacent C–C σ* orbital is suggested to be the principal reason for the relatively long C–C bond in diketones. This effect is found in both the cis and trans conformations.

The influence of lone-pair repulsions on C–C bond lengths: a critical evaluation of the experimental and theoretical evidence

The captodative substitution effect towards 4π electrocyclic ring-opening of substituted cyclobutenes is investigated using the B3LYP/6-31+G(d) method. For most of the cases, the activation free energy for the ring-opening of substituted cyclobutenes is lower than for the unsubstituted cyclobutenes and for several captodative substitution patterns, it differs by more than 15.0 kcal mol−1. The lowest activation energy barrier is computed for trans-3-C(O)CH3–4-NH2 substituted cyclobutene, which is remarkably 23.7 kcal mol−1 lower than for the unsubstituted cyclobutene. In addition to this, an extra acceleration effect exerted by the captodative substitution is probed through energetic analyses.

Captodative substitution induced acceleration effect towards 4π electrocyclic ring-opening of substituted cyclobutenes

A liquid-phase process is provided for the manufacture from glutaronitriles and related compounds of 2,6-diaminopyridine and related compounds, which are used industrially as compounds and as components in the synthesis of a variety of useful materials. The synthesis proceeds by means of a dehydrogenative aromatization process.

Process for the synthesis of diaminopyridines from glutaronitriles

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.

Shan Wu

Papers

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