Competitive Nitrogen versus Carbon Tunneling

Abstract

Quantum mechanical tunneling (QMT) of heavy atoms like carbon or nitrogen has been considered very unlikely for the longest time, but recent evidence suggests that heavy-atom QMT does occur more frequently than typically assumed. Here we demonstrate that carbon vs nitrogen heavyatom QMT can even be competitive leading to two different products originating from the same starting material. Aminosubstituted benzazirine was generated in solid argon (3−18 K) and found to decay spontaneously in the dark, with a half-life of 210 min, to p-aminophenylnitrene and amino-substituted ketenimine. The reaction rate is independent of the cryogenic temperature, in contradiction to the rules inferred from classical transition state theory. Quantum chemical computations confirm the existence of two competitive carbon vs nitrogen QMT reaction pathways. This discovery emphasizes the quantum nature of atoms and molecules, thereby enabling a much higher level of control and a deeper understanding of the factors that govern chemical reactivity.

Heavy‐Atom Tunneling in Organic Reactions

“…However, benzazirine 16 also rearranged to triplet nitrene 3 15 , presumably via the singlet nitrene and intersystem crossing (Scheme ). During the process 16 3 15 , the nitrogen atom traverses an arc of 0.56 Å according to CASSCF calculations . For comparison, for 16 17 , each of the two carbons of the azirine ring moves about 0.31 to 0.33 Å.…”

Heavy‐Atom Tunneling in Organic Reactions

“…3 15,the nitrogen atom traverses an arc of 0.56 according to CASSCF calculations. [37] Forc omparison, for 16!17,e ach of the two carbons of the azirine ring moves about 0.31 to 0.33 .T hus,this system represents acompetition between two heavy-atom-tunneling processes starting from asingle reactant.…”

Lewis acid catalyzed heavy atom tunneling – the case of 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one

“…Since the tunneling probability is decreasing exponentially with the mass of the tunneling particle, 1,2 many examples are reported for the hydrogen atom, 3 while tunneling reactions with a dominant movement of heavy atoms are rare. [4][5][6][7] Examples involving tunneling of carbon atoms 5 are the Cope rearrangement of semibullvalene 1, 8,9 the ring-opening reactions of benzene oxide 2 to oxepine 3, 10 of 3-(methylthio)-7-azabicyclo[4.1.0]hepta-2,4,6-triene 4 to ketenimine 5, 11 and the related rearrangement of 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one 6 to 4-oxocyclohexa-2,5dienylidene 7 12,13 (Scheme 1). At cryogenic temperatures below 20 K, the thermal energy is not sufficient to overcome the activation barriers for these reactions, strongly indicating that tunneling is involved.…”