Influence of ring annelation on the mode selectivity of the ionized diazabicycloheptenes and corresponding housanes: An ESR and ENDOR study

The tricyclic azoalkanes, (1 alpha ,4 alpha ,4a alpha ,7a alpha)-4,4a,5,6,7 ,7a-hexahydro-1,4,8,8-tetramethyl-1,4-methano-1H-cyclopenta[d]pyridazine (1 c), (1 alpha ,4 alpha ,4a alpha ,6a alpha)-4,4a,5,6,6a-pentahydro-1,4,7,7-t etramethyl-1,4-methano-1H-cyclobuta[d]pyridazine (1d), (1 alpha ,4 alpha ,4 a alpha ,6a alpha)-4,4a,6a-trihydro-1,4,7,7-tetramethyl-1,4-methano-1H-cycl obuta[d]pyridazine (1e). and (1 alpha ,4 alpha ,4a alpha ,5a alpha)-4,4a,5, 5a-tetrahydro-1,4,6,6-tetramethyl-1,4-methano-1H-cyclopropa[d]pyridazine (1 f), as well as the corresponding housanes, the 2,3,3,4-tetramethyl-substitu ted tricyclo[3.3.0.0(2,4)]octane (2c). tricyclo[3.2.0.0(2,4)]heptane (2d), and tricyclo[ 3.2.0.0(2,4) ]hept-6-ene (2e), were subjected to gamma -irrad iation in Freon matrices. The reaction products were identified with the us e of ESR and. in part, ENDOR spectroscopy. As expected, the strain on the C -framework increases on going from the cyclopentane-annelated azoalkanes an d housanes (1c and 2c) to those annelated by cyclobutane (1d and 2d), by cy clobutene (1e and 2e), and by cyclopropane (1f). Accordingly, the products obtained from Ic and 2c in all three Freons used, CFCl3, CF3CCl3, and CF2Cl CFCl2, were the radical cations 3c(.+) and 2c(.+) of 2,3,4,4-tetramethylbic yclo[3.3.0]oct-2-ene and 2,3,3,4-tetramethylbicyclo[3,3,0]octane-2,4-diyl, respectively. In CFCl3, and CF3CCl3, matrices, Id and 2d yielded analogous products, namely the radical cations 3d(.+) and 2d(.+) of 2,3,4,4-tetrameth ylbicyclo[3.2.0]hept-2-ene and 2,3,3,4-tetramethylbicyclo[3.2.0]heptane-2,4 -diyl. The radical cations 3c(.+) and 3d(.+) and 2c(.+) and 2d(.+) correspo nd to their non-annelated counterparts 3a(.+) and 3b(.+) and 2a(.+) and 2b( .+) generated previously under the same conditions from 2,3-diazabicyclo[2. 2.1]hept-2-ene (1a) and bicyclo[2.1.0]pentane (2a), as well as from their 1 ,4-dimethyl derivatives (lb and 2b). However, in a CF2ClCFCl2 matrix, both ld and 2d gave the radical cation 4d(.+) of 2,3,3,4-tetramethylcyclohepta-1 ,4-diene. Starting from le and 2e, the radical cations 4e(.+) and 4e ' (.+) of the isomeric 1,2,7,7- and 1,6,7,7-tetramethylcyclohepta-1,3,5-trienes a ppeared as the corresponding products, while 1f was converted into the radi cal cation 4f(.+) of 1,5,6,6-tetramethylcyclohexa-1,4-diene which readily l ost a proton to yield the corresponding cyclohexadienyl radical 4f(.). Reac tion mechanisms leading to the pertinent radical cations are discussed.