Isomerization barriers for the conrotatory and disrotatory isomerizations of 3-aza-dihydrobenzvalene to 1,2-dihydropyridine and 3,4-diaza-dihydrobenzvalene to 1,2-dihydropyridazine

The isomerizations of 3-aza-dihydrobenzvalene to 1,2-dihydropyridine and 3,4-diaza-dihydrobenzvalene to 1,2-dihydropyridazine have been studied using ab initio methods with a multiconfigurational wavefunction. Transition states for both the allowed, conrotatory and forbidden, disrotatory reaction channels were located. The isomerizations occur through the cleavage of a bond pair in the bicyclobutane moiety. The four allowed pathways for 3-aza-dihydrobenzvalene isomerization are nonconcerted forming intermediates with a trans double bond in a six-membered ring: two isomers of (E, Z)-2,4-dihydro-1,6-pyridine and two isomers of (Z, E)-2,4-dihydro-1,6-pyridine. These intermediates can isomerize to the final 1,2-dihydropyridine product through trans double bond rotation with barriers of only a few kcal mol−1. The allowed pathways have initial activation barriers which range from 35.6 to 42.2 kcal mol−1. The four forbidden pathways lead directly to the final 1,2-dihydropyridine product in a concerted mechanism with transition states of substantial singlet biradical character. Their activation barriers range from 47.0 to 56.1 kcal mol−1. The two allowed pathways for 3,4-diaza-dihydrobenzvalene isomerization are also nonconcerted forming two isomers of (E, Z)-3,5-dihydro-1,2-pyridazine intermediates which then isomerize to 1,2-dihydropyridazine through trans double bond rotation with barriers of only about 2 kcal mol−1. The initial barriers for the two allowed pathways are 32.0 and 41.6 kcal mol−1 with one pathway substantially lower due to steric effects. The two forbidden pathways lead to 3,4-diaza-dihydrobenzvalene through a concerted mechanism and are also highly singlet biradical in character. Their barriers are calculated to be 50.4 and 52.9 kcal mol−1. For both starting structures, there is a substantial lowering of the allowed barriers relative to the analogous dihydrobenzvalene structure due to the presence of the heteroatom(s).