N[1,3]-sigmatropic shift in the benzidine rearrangement: experimental and theoretical investigation

Abstract

The N[1,3]-sigmatropic shift in the benzidine rearrangement has been studied in depth experimentally with the aid of density functional theory (DFT) calculations. The designed substituted N,N'-diaryl hydrazines rearrange exclusively to the expected o/p-semidines and diphenylines. Intercrossing experiments support the intramolecular rearrangement process. Radical trapping experiments exclude the intermediacy of biradicals in the rearrangements. Computational results demonstrate that the o-semidine rearrangement involves a novel N[1,3]-sigmatropic shift and the p-semidine rearrangement proceeds via tandem N[1,3]/N[1,3]-sigmatropic shifts, while the diphenyline rearrangement occurs through cascade N[1,3]/[3,3]-sigmatropic shifts. The proposed mechanism involving the key N[1,3]-sigmatropic shift as the rate-limiting step is in good agreement with reported kinetic isotope measurements. The combined methods provide new insight into the formation mechanism of o/p-semidines and diphenylines in the benzidine rearrangement and support the unprecedented suprafacial symmetry allowed N[1,3]-sigmatropic shift with an inversion of the configuration in the migrating nitrogen atom.

An Amine Group Transfer Reaction Driven by Aromaticity

“…51 An alternative higher order N [1,9] sigmatropic shift, in analogy to a N [1,3] sigmatropic shift, reported by Xu and co-worker, and the higher order Claisen rearrangement reported by us can be ruled out. 52 The fact that product formation is dependent on amine concentration and a higher activation energy was computed for this pathway precludes it (see SI for details).…”

Kinetic resolution of substituted amido[2.2]paracyclophanes via asymmetric electrophilic amination

“…The CPAcatalyzed direct addition of the amido group to azodicarboxylate (formation of triazanes 8) was believed to be the stereodetermining step, in which the (S p )-amido-PCP 1 was the kinetically favored enantiomer due to the dual-hydrogen bonding activation between CPA catalyst with substrates and azodicarboxylate. The acidpromoted hydrazine-shift from the amino group to the para-position was proposed to proceed through a para-semidine-type rearrangement, involving either a stepwise double N[1,3]-sigmatropic shift 53,54 or a one-step N [1,5]-sigmatropic shift process 55,56 . In most cases, the rearrangement step is faster than the triazane formation step, and the corresponding triazane intermediate cannot be detected.…”