AB-INITIO STUDY OF THE RING EXPANSION OF PHENYLNITRENE AND COMPARISONWITH THE RING EXPANSION OF PHENYLCARBENE

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 6-311 G-(2d,p) basis sets. Ring expansion from the (1)A(2) state of 1a is pr edicted to occur in two steps via 7-azabicyclo[4.1.0]-hepta-2,4,6-trie ne (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. Aziri ne 2a is unlikely to be detected, because of the small calculated barr ier (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 kc al/mol less exothermic, but to have a barrier ca. 9 kcal/mol lower tha n the analogous ring expansion of (1)A' phenylcarbene (1b) to cyclohep ta-1,2,4,6-tetraene (3b). Factors which contribute to these and other energetic differences between the ring expansion reactions of la and I b 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 st ate for enantiomerization of 3a. Unlike cycloheptatrienylidene (4b), 4 a 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 betwe en these two states of 4a is probably the reason why triplet 4a has no t been detected by EPR.