Compound 1 undergoes a thermal elimination of nitrogen at C to form nitrile 4 (Org. Lett. 1999, 1, 537–539). One proposed subsequently refuted) mechanism for this transformation involves intermediates 2 and 3:
(b) To determine whether or not the proposed mechanism operates, one of the nitrogen atoms was isotopically labeled as 15N, and its location in the product was determined. The absence of 4b in the product mixture demonstrates that the proposed mechanism is not operating. Using resonance structures, explain why the proposed mechanism predicts that 4b should be formed.