A computational study of the 1,3-dipolar cycloaddition reaction mechanism for nitrilimines

Abstract

The [3+2] and [1+2] cycloaddition pathways between ethene and a series of 13 nitrilimines (R1CNNR2) have been examined by density functional theory [PBE0/6-311++G(2df,pd)] calculations. All reactions have low barriers ranging from 14.14 (R1 = CH3, R2 = H) to 1.01 (R1 = R2 = F) kcal mol–1, and large reaction exothermicities consistent with the transient nature of nitrilimines. The [3+2] and [1+2] transition-state structures are very similar, mainly differing in the relative orientation of their fragments and the newly forming C—C bond distance, and exhibit only minor deviations from the structures of the reactants. Both reaction pathways are concerted and asynchronous, but the [1+2] reaction has a greater degree of asynchronicity. Examination of the frontier molecular orbitals reveals that both the [3+2] and [1+2] barrier heights are related to two sets of orbital interactions, with the interaction between the lowest unoccupied molecular orbital π [Formula: see text] of nitrilimine and the highest occupied molecular orbital of ethene in common. The second interaction in both cases is carbene-like. A relationship between the weights of the 1,3-dipolar resonance contribution in the various nitrilimines and the corresponding [3+2] barrier heights was not found, but a good correlation could be found between the [1+2] barrier heights and both the 1,3-dipolar and carbene contributions. Inspection of the potential energy surface in the vicinity of the two transition states for the reaction between unsubstituted nitrilimine and ethene suggests that the observed [3+2] product is a result of an initial carbene-like approach of the two fragments followed by a ridge bifurcation that leads to the [3+2] product minimum. Key words: nitrilimines, 1,3-dipole, carbene, [3+2] cycloaddition, [1+2] cycloaddition, density functional theory (DFT).

Bifurcations on Potential Energy Surfaces of Organic Reactions

“…For example, Quideau and co-workers have shown that a bis-pericyclic Diels-Alder transition state controls diastereofacial selectivity in the dimerization of ortho-quinols for the synthesis of (+)-aquaticol, a bis-sesquiterpene. [60] Cycloaddition bifurcations were also reported for the reaction of 1,2,4,5-tetrazines and pyridazines with alkynes, the cycloaddition of cycloheptatriene and cyclopentadiene, [61] 1,3-dipolar cycloadditions, [62] and dichlorocarbene addition to cyclopropene.…”

Isothiirane: A Molecular Structure Dilemma Resolved

“…As Figure details, the electronic structure of isothiirane can be tuned through substitution and by modifying their donor/acceptor abilities may result in carbenic and/or 1,3‐dipole reactivity. This make isothiirane a molecule with potential value in [1+2] or [3+2] cycloadditions, similar to substituted cyclopropanes and nitrilimines, and would provide a new means of incorporating sulfur into a ring system.…”

Bifurkationen auf den Potentialenergiehyperflächen organischer Reaktionen

“…berichtet, dass bei der Synthese des Bissesquiterpens (+)‐Aquaticol die diastereofaciale Selektivität der Dimerisierung von ortho ‐Chinolen durch einen bispericyclischen DA‐Übergangszustand gesteuert wird 60. Darüber hinaus sind Bifurkationen bei Cycloadditionen für die Reaktion von 1,2,4,5‐Tetrazinen und Pyridazinen mit Alkinen, Cycloadditionen zwischen Cycloheptatrien und Cyclopentadien,61 1,3‐Dipolare Cycloadditionen62 und die Addition von Dichlorcarben an Cyclopropen beschrieben 63…”