185-nm Photolysis and Pyrolysis of the Spirocyclopropane-Substituted Azoalkanes of 2,3-Diazatricyclo4,9>non-2-ene and Their Denitrogenated Hydrocarbon Products, the Tricyclo2,7>heptanes

Article abstract of DOI:10.1021/jo00010a026

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 shownthat 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 elctron 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,?^* excited azoalkanes 1a-c, in turn obtained through internal conversion of higher excited states such as ¹?,?^*, ¹n,?^*, 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 states of the azoalkanes 1a-c competes with internal conversion to the lowest excited state, namely the n,?^* state, as it was shown by the formation of norbornenes 3a-c.