Photochemistry of the Azoalkanes 2,3-Diazabicyclohept-2-ene and Spirodiazabicyclohept-2-ene>: On the Questions of One-Bond vs. Two-Bond Cleavage during the Denitrogenation, Cyclization vs. Rearrangement of the 1,3-Diradicals, and Double Inversion.

Article abstract of DOI:10.1021/jo00218a012

The thermolysis and 350-nm and 185-nm photolyses of the azoalkanes 2,3-diazabicyclo<2.2.1>hept-2-ene (1a) and spirodiazabicyclo<2.2.1>hept-2-ene> (1b) have been investigated.The exo/endo stereochemistry in the bicyclo<2.1.0>pentanes 2a,b and in the rearranged olefin 3b was determined by deuteration experiments using 5,6-exo-dideuterioazoalkanes 1a,b-d₂.Whereas thermal and direct photochemical (350 nm; n, ?^*) denitrogenation of azoalkane 1a-d₂ led exclusively (>99percent) to bicyclo<2.1.0>pentane 2a-d₂ with preferential (1.54,2.94) double inversion, the triplet-sensitized photolysis afforded nearly complete stereoequlibration.In 185-nm denitrogenetion an unexpectedly high exo/endo ratio (3.1) for bicyclo<2.1.0>pentane 2a-d₂ was found, besides isomerization to cyclopentene 2a-d₂.Similar results were obtained in the denitrogenation of spiroazoalkane 1b-d₂, which exhibited exo stereochemical preferences in both photoproducts spiropentane-5,1'-cyclopropane> 2b-d₂ and bicyclo<3.2.0>hept-1-ene 3b-d₂.The 350-nm photolysis of azoalkane 1b-d₂ gave preferential formation of exo-spirobicyclo<2.1.0>pentane 2b-d₂ and exo-olefin 3b-d₂ whereas triplet-sensitized decomposition yielded almost complete loss of stereochemical preference in the olefin 3b-d₂.The 185-nm photolysis of azoalkane 1b-d₂ showed similar behavior compared with the azoalkane 1a, eg., at high exo/endo ratio in spirobicyclo<2.1.0>pentane 2b-d₂.Also olefin 3b was formed with complete stereoequilibration.These diverse experimental results are discussed in terms of one-bond vs. two-bond cleavage processes leading to the diazenyl diradicals D'_?,? and D'_?,? in the case of low-energy activation (350-nm photolysis and thermolysis) and 1,3-cyclopentadiyls D_?,? and D_?,? on high-energy activation (185-nm activation).The relatively high degree of double inversion in the corresponding bicyclopentanes and the formation of rearranged cycloalkenes in the 185-nm photodenitrogenation is presumably a direct consequence of concerted two-bond cleavage via the formation of ¹D_?,? and zwitterionic states of the 1,3-diradical.A Salem diagram for one-bond and two-bond denitrogenation was most helpful in rationalizing these results mechanistically.