Synthesis, Photolysis, and Thermolysis of Azoalkanes Derived from Spirocyclopropane-Substituted Norbornenes: Competitive Nitrogen Loss and Diazoalkane Formation

Article abstract of DOI:10.1021/jo00205a008

The azoalkanes 4',5'-diazaspiro(cyclopropane-1,2'-tricyclo<4.3.0.0^3,7>non-4'-ene) (3a), 4',5'-diazaspiro(cyclopropane-1,8'-tricyclo<4.3.0.0^3,7>non-4'-ene) (3b), and 4',5'-diazadispiro(cyclopropane-1,2'-tricyclo<4.3.0.0^3,7>non-4'-ene-8',1"-cyclopropane) (3c) were prepared from the corresponding spirocyclopropane-substituted norbornenes spiro(bicyclo<2.2.1>hept-2-ene-7,1'-cyclopropane) (1a), spiro(bicyclo<2.2.1>hept-5-ene-2,1'-cyclopropane) (1b), and dispiro(cyclopropane-1,2'-bicyclo<2.2.1>hept-5'-ene-7',1"-cyclopropane) (1c) by cycloaddition with 4-phenyl-4H-1,2,4-triazole-3,5-dione (PTAD), affording the respective urazoles 2a-c and subsequent hydrolysis and oxidation.X-ray structures of the urazoles 2b,c confirmed the preferred regioselectivity in the PTAD cycloaddition with the spirocyclopropane-substituted norbornenes 1b,c.Benzophenone triplet-sensitized photolysis of the azoalkanes 3a-c led quantitatively to the respective tricycloalkanes 4a-c via nitrogen loss.While in the thermolysis of the azoalkanes 3a,c the tricycloalkanes 4a,c were formed essentially quantitatively, also traces of methylenecyclohexenes 5a,c were detected as denitrogenation products.In the case of 3b the major thermolysis product was the pyrazole 7b, resulting from Diels-Alder retrocyclization.The direct photolysis (n,?^* excitation) gave the tricycloalkanes 4a-c as major products, together with appreciable amounts of methylene cyclohexenes 5a-c, the latter increasing with increasing temperature of the direct photolysis.On laser irradiation of the azoalkanes 3a-c with the 333.6-nm line in all cases a yellow transient color was observed, which corresponded to the diazoalkanes 6a-c.On prolonged photolysis, the diazoalkanes 6a-c afforded the methylenecyclohexenes 5a-c.Spirocyclopropane substitution affected the relative proporation of tricycloalkane and methylenecyclohexene products.A mechanism is postulated in which the diazenyl diradicals 8a-c figure as common intermediates for the thermal and triplet-sensitized and direct photochemical denitrogenations of the azoalkanes 3a-c.Differences in spin multiplicities (singlet vs. triplet) and electronic configurations (D_?,? vs.D_?,?) allow rationalization of the distinct behavior of the diazenyl diradicals 8.