<i>Ab Initio</i> Study of the Ring Expansion of Phenylnitrene and Comparison with the Ring Expansion of Phenylcarbene

Abstract

The rearrangement of singlet phenylnitrene ( 1a ) to 1-azacyclohepta-1,2,4,6-tetraene ( 3a ) has been studied computationally, using the CASSCF and CASPT2N methods in conjunction with the 6-31G*, cc-pVDZ, and 6-311G(2d,p) basis sets. Ring expansion from the 1 A 2 state of 1a is predicted to occur in two steps via 7-azabicyclo[4.1.0]hepta-2,4,6-triene ( 2a ) as an intermediate. The rearrangement of 1a to 2a is estimated to have a barrier of ca. 6 kcal/mol and to be rate-determining. Azirine 2a is unlikely to be detected, because of the small calculated barrier (ca. 3 kcal/mol) to its rearrangement to 3a . At the CASPT2N/6-311G(2d,p)//CASSCF(8,8)/6-31G* + ZPE level of theory, the reaction 1a → 3a on the lowest singlet potential energy surface is calculated to be exothermic by 1.6 kcal/mol. This reaction is predicted to be ca. 19 kcal/mol less exothermic, but to have a barrier ca. 9 kcal/mol lower than the analogous ring expansion of 1 A‘ phenylcarbene ( 1b ) to cyclohepta-1,2,4,6-tetraene ( 3b ). Factors which contribute to these and other energetic differences between the ring expansion reactions of 1a and 1b are discussed. The lowest singlet state of planar 1-azacyclohepta-1,3,5-trien-7-ylidene ( 4a ) is an open-shell singlet (1 1 A‘‘), which is calculated to be ca. 20 kcal/mol above 3a and to be the transition state for enantiomerization of 3a . Unlike cycloheptatrienylidene ( 4b ), 4a is predicted to have a triplet ground state and an energy difference between 1 1 A‘‘ and the lowest triplet (1 3 A‘‘) of ca. 1 kcal/mol. The similar geometries of and small adiabatic energy difference between these two states of 4a is probably the reason why triplet 4a has not been detected by EPR.