Benzooxirene, naphthooxirenes, and anthraceneoxirenes: the stabilization of the oxirene system by bond fixation in acenes. A comparison of ab initio and DFT in the investigation of annelated oxirenes

The effect on the stabilities of benzoannelated oxirenes (oxacyclopropenes) of variations in the C-C bond electron density (as measured by Löwdin bond order) was studied computationally. Benzo[b]oxirene (4ox), naphtho[2,3-b]oxirene (5ox), naphtho[1,2-b]oxirene (6ox), anthro[2,3-b]oxirene (7ox), and anthro[1,2-b]oxirene (8ox) were investigated using the DFT pBP/DN* method as implemented in the program Spartan. This method was chosen because with 4ox it gave results similar to those from the much slower QCISD(T)/6-31G*//MP2(full)/6-31G* and MP4SDTQ/6-31G*//MP2(full)/6-31G* methods. The calculated barriers to oxirene ring opening for the "linear" 4ox, 5ox, and 7ox (bond orders 1.35, 1.24, and 1.20, respectively) were 23.6, 39.9, and 46.6 kJ mol-1, in contrast to the "angular" 6ox and 8ox (bond orders 1.50 and 1.59, respectively), with barriers of ca. 0 kJ mol-1. The high-barrier vs. low-barrier series corresponds to the stabilities expected from the resonance structures of the oxirenes or their formal precursor acenes. Empirically, the ring-opening barriers were linearly related to the oxirene bond orders, and extrapolation to tetracene using this relationship gave a barrier of ca. 50 kJ mol-1, which seems to be the limiting barrier in the linear series. In several cases the oxirene isomerized to a ketene without going through an oxo carbene (ketocarbene). Benzoannelation is only the second kind of substitution (after dimethyl substitution) that has been indicated, computationally, to be able to stabilize oxirenes.Key words: oxirenes, ketocarbenes, benzooxirenes, AM1, ab initio, DFT.