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 1A2 state of 1a is predicted to occur in two steps via 7-azabicyclo[410]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 16 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 1A‘ phenylcarbene (1b) to cyclohepta-1,2,4,6-tetraene (3b) Factors which contribute

Ab Initio Study of the Ring Expansion of Phenylnitrene and Comparison with the Ring Expansion of Phenylcarbene

The rearrangement of phenylcarbene (1) to 1,2,4,6-cycloheptatetraene (3) has been studied theoretically, using SCF, CASSCF, CASPT2N, DFT (B3LYP), CISD, CCSD, and CCSD(T) methods in conjunction with the 6-31G*, 6-311+G*, 6-311G(2d,p), cc-pVDZ, and DZd basis sets. Stationary points were characterized by vibrational frequency analyses at CASSCF(8,8)/6-31G* and B3LYP/6-31G*. Phenylcarbene (1) has a triplet ground state (3A‘‘) with a singlet−triplet separation (ΔEST) of 3−5 kcal mol-1. In agreement with experiment, chiral 3 is the lowest lying structure on this part of the C7H6 potential energy surface. Bicyclo[4.1.0]hepta-2,4,6-triene (2) is an intermediate in the rearrangement of 1 into 3, but it is unlikely to be observable experimentally due to a barrier height of only 1−2 kcal mol-1. The enantiomers of 3 interconvert via the 1A2 state of cycloheptatrienylidene (4) with an activation energy of 20 kcal mol-1. The “aromatic” 1A1 state, previously believed to be the lowest singlet state of 4, is roughly 10 kc...

Carbene Rearrangements Unsurpassed: Details of the C7H6 Potential Energy Surface Revealed

Ab initio calculations at the G2(MP2,SVP) and B-LYP/6-311+G(3df,2p)+ZPVE levels have been used to examine the potential energy surface of C7H6. Fulvenallene (6) is the most stable C7H6 isomer considered in this study. 1-Ethynylcyclopentadiene (9A), benzocyclopropene (10), and 1,2,4,6-cycloheptatetraene (4) lie 12, 29, and 49 kJ mol-1, respectively, above 6. Phenylcarbene (1) is calculated is to have a triplet (3A‘‘) ground state, 16 kJ mol-1 more stable than the singlet state (1A‘). Interconversion of 1 and 4 is predicted to have a moderate activation barrier, with the involvement of a stable bicyclic intermediate (bicyclo[4.1.0]hepta-2,4,6-triene, 2). However, 2 is found to lie in a shallow potential energy well with a small barrier (8 kJ mol-1) to rearrangement to 4. At the G2(RMP2,SVP)//QCI level, the 3A2 and 3B1 triplet states of cycloheptatrienylidene (33) are predicted to lie very close in energy. The singlet “aromatic” cycloheptatrienylidene (13, 1A1) is found to be a transition structure interconv...

Interconversions of Phenylcarbene, Cycloheptatetraene, Fulvenallene, and Benzocyclopropene. A Theoretical Study of the C7H6 Energy Surface

The structures and reactivities of nitrile imines are subjects of continuing debate. Several nitrile imines were generated photochemically or thermally and investigated by IR spectroscopy in Ar matrices at cryogenic temperatures (Ph-CNN-H 6, Ph-CNN-CH3 17, Ph-CNN-SiMe3 23, Ph-CNN-Ph 29, Ph3C-CNN-CPh3 34, and the boryl-CNN-boryl derivative 39). The effect of substituents on the structures and IR absorptions of nitrile imines was investigated computationally at the B3LYP/6-31G* level. IR spectra were analyzed in terms of calculated anharmonic vibrational spectra and were generally in very good agreement with the calculated spectra. Infrared spectra were found to reflect the structures of nitrile imines accurately. Nitrile imines with IR absorptions above 2200 cm–1 have essentially propargylic structures, possessing a CN triple bond (typically PhCNNSiMe3 23, PhCNNPh 29, and boryl-CNN-boryl 39). Nitrile imines with IR absorptions below ca. 2200 cm–1 are more likely to be allenic (e.g., HCNNH 1, PhCNNH 6, HCNN...

Nitrile Imines: Matrix Isolation, IR Spectra, Structures, and Rearrangement to Carbodiimides

Rearrangements and interconversions of carbenes and nitrenes.

Phenylcarbene and tropylidene interconvert in the gas-phase, as evidenced by the formation of stilbene and heptafulvalene (5) from both at 300°. Heptafulvalene itself rearranges to stilbene and anthracene above 500°. Phenylcarbene, but not tropylidene, undergoes ring contraction to fulvenallene (9) at 900°. Formation of fluorene from diphenylcarbene involves a complete ring expansion to 2-phenyltropylidene (14), and subsequent ring contraction to 2-biphenylylcarbene (15).

K. Wilczek

Papers

Thermal Interconversion of Phenylcarbene and Tropylidene. Preliminary communication