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

Area of Science:

• Organic Chemistry
• Reaction Mechanisms
• Computational Chemistry

Background:

• The benzidine rearrangement is a classic organic reaction forming C-C bonds.
• Its precise mechanism, particularly the role of sigmatropic shifts, remains debated.

Purpose of the Study:

• To elucidate the detailed mechanism of the benzidine rearrangement.
• To investigate the role of N[1,3]-sigmatropic shifts in forming semidines and diphenylines.

Main Methods:

• Experimental studies using designed substituted N,N'-diaryl hydrazines.
• Density functional theory (DFT) calculations.
• Intercrossing and radical trapping experiments.

Main Results:

• Rearrangement exclusively yields o/p-semidines and diphenylines.
• Intramolecular process confirmed; biradical intermediates excluded.
• DFT reveals distinct sigmatropic shift pathways for o-semidine, p-semidine, and diphenyline formation.
• N[1,3]-sigmatropic shift identified as the rate-limiting step, consistent with kinetic isotope data.

Conclusions:

• The study proposes a detailed mechanism involving novel N[1,3]-sigmatropic shifts.
• Confirms the suprafacial, symmetry-allowed nature of the N[1,3]-sigmatropic shift with nitrogen inversion.
• Provides new mechanistic insights into benzidine rearrangement products.