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 Luwdin bond order) was studied computationally. Benzo[b]oxirene (4ox), naphtho[2,3-b]o xirene (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 a s implemented in the program Spartan. This method was chosen because with 4 ox it gave results similar to those from the much slower QCISD(T)/6-31G*//M P2(full)/6-31G* and MP4SDTQ/6-31G*//MP2(full)/6-31G* methods. The calculate d 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 mo l(-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, t he 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 serie s. In several cases the oxirene isomerized to a ketene without going throug h an oxo carbene (ketocarbene). Benzoannelation is only the second kind of substitution (after dimethyl substitution) that has been indicated, computa tionally, to be able to stabilize oxirenes.