The 185-nm photolysis and pyrolysis of the spirocyclopropane-substituted azoalkanes of 2,3-diazatricyclo[4.3.0.04,9]non-2-ene and their denitrogenated hydrocarbon products, the tricyclo[3.2.0.02,7]heptanes
作者:Waldemar Adam、Gerald Zang
DOI:10.1021/jo00010a026
日期:1991.5
The 185-nm photolysis and pyrolysis of the spirocyclopropane derivatives of the azoalkanes 2,3-diazatricyclo[4.3.0.0(4,9)]non-2-ene (1a), 4',5'-diazaspiro(cyclopropane-1,8'-tricyclo[4.3.0.0(3,7)]non-4'-ene) (1b), and 4'5'-diazadispiro(cyclopropane-1,2'-tricyclo[4.3.0.0(3,7)]non-4'-ene-8',1"-cyclopropane) (1c) and their denitrogenated hydrocarbon derivatives tricyclo[3.2.0.0(2,7)]heptane (2a), spiro(cyclopropane-1,3'-tricyclo[3.2.0.0(2,7)]heptane) (2b), and dispiro(cyclopropane-1,3'-tricyclo[3.2.0.0(2,7)]heptane-6'1"-cyclopropane) (2c) were investigated. It was shown that the 185-nm photochemical behavior of these substrates does not depend on the degree of spirocyclopropane substitution. As common products in the 185-nm photolysis of the azoalkanes 1a-c were obtained the tricycloalkanes 2a-c (major products), the norbornenes 3a-c, the vinylcyclopentenes 5a-c (minor products), and the exo-methylenecyclohexenes 6a-c (traces). In the case of the parent azoalkane 1a additionally bicyclo[3.2.0]hept-2-ene (4) and bicyclo[4.1.0]hept-2-ene (7a) were detected. The major products in the photolysis of the tricycloheptanes 2a-c were the vinylcyclopentenes 5a-c, but also the norbornenes 3a-c and the methylenecyclohexenes 6a-c were formed in considerable amounts. Although the norbornenes 3a-c and the bicyclo[3.2.0]heptene 4a are logical Wagner-Meerwein rearrangement products, attempts to trap the suspected radical-cationic and zwitterionic intermediates with CF3CH2OH failed. Efforts to generate the authentic radical-cationic species by means of photosensitized electron transfer (PET) by using sensitizers such as cyanoarenes, quinones, and pyrylium salts were unproductive. Vibrationally excited bicyclo[2.2.1]hepta-2,7-diyls, generated by the pyrolyses of 2a-c, are not precursors to the norbornenes 3a-c because, instead of such rearrangement products, cyclobutane cleavage of the bicyclo[2.1.0]pentane moiety takes place to afford the isomeric vinylcyclopentenes 5'a-c. Carbene intermediates, produced either from the 1,3-diyl-type species through fragmentation or from the photodenitrogenation of diazoalkanes, which are generated by retro-cleavage of n,pi* excited azoalkanes 1a-c, in turn obtained through internal conversion of higher excited states such as 1-pi,pi*, 1n,sigma*, and R(y), are proposed as the most likely precursors to either the vinylcyclopentenes 5a-c or methylenecyclohexenes 6a-c, respectively. In violation of Kasha's rule, photochemistry directly from the higher excited states of the azoalkanes 1a-c competes with internal conversion to the lowest excited state, namely the n,pi* state, as it was shown by the formation of norbornenes 3a-c.