Retention of Configuration andE/Z Stereoselectivity in the Photolysis ofcis- andtrans-Tetraalkyl-4-methylene-1-pyrazolines

Violations to the Principle of Least Motion: the Shortest Path is not Always the Fastest

The reaction between two molecules is usually envisioned as following a least-motion path with both molecules travelling minimum distances to meet each other. However, the reaction path of lowest activation...

Unraveling the computed non-least motion pathway for the homodimerization of superchameleonic isocyanides: the peculiar nonsymmetrical (F-NC)2 reactant complex

Isocyanides are commonly qualified as chameleonic compounds because of their reactions with both nucleophiles and electrophiles. In some instances, their chameleonic behavior changes to superchameleonic when they are involved in homodimerization processes, with the two initially identical isocyanide units adopting different roles along the reaction coordinate. We present here a detailed analysis of the computed non-least motion pathway that two isocyanides, the superchameleonic F-NC and, for the sake of comparison, the standard Me-NC, follow when reacting with themselves by comparing the evolution of a series of representative geometrical and electronic parameters along the respective reaction coordinates. This study shows that the two F-NC units are notoriously distinguishable from each other in all the parameters under scrutiny. Furthermore, we envisage that the superchameleonic character of F-NC seems to be most likely due to a minimal electrostatic interaction between the two entities at the earliest stage of the reaction. We also show that MeO-NC, MeS-NC and Me3P[double bond, length as m-dash]N-NC might be postulated as new examples of superchameleonic isocyanides.

Effects of an electron-withdrawing group on thermal decomposition of 4-alkylidene-1-pyrazolines: a novel stereoselective formation of alkylidenecyclopropane due to participation of pi-electrons on the methylene carbon in decomposition

Thermal decomposition of 4-alkylidenepyrazolines 14 bearing a methoxycarbonyl group at C-3, prepared by 1,3-dipolar cycloaddition between allenecarboxylates 12 and diazoalkanes 13, was carried out. Unlike normal 4-alkylidenepyrazolines, which decompose in stepwise mechanisms at high temperatures, 14 decomposed concertedly at moderately low temperatures (45-110 degrees C), resulting in selective formation of the two isomeric alkylidenecyclopropanes 7 arising from the bond formation between the exo-methylene carbon and the 5-carbon. The selective formation and the configurations of the products are rationalized in terms of the concerted process via the folded conformation of the pyrazolines. Introduction of an electron-withdrawing group at the 3-position of the 4-alkylidenepyrazoline system causes the polarization of the C(3)-N(2) bond inducing the properties of intramolecular diazonium salt 8, in which the pi-electrons on the methylene carbon become more nucleophilic and participate in the cleavage of the C(5)-N(1) bond. The X-ray crystal structure of the typical normal alkylidenepyrazoline 14a with only small steric interactions between the substituents was determined to be a nearly planar ring structure.