Cyclopropane excited pyrazoline

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

Cyclopropane derivatives are the major products of photoelimination of nitrogen from 1-pyrazolines. Conversion of the fused pyrazoline (10) into the cyclopropane (11) was achieved in this way in 85% yield by irradiation in the presence of acetophenone and constitutes a valuable step in a synthesis of a diquinane alcohol. The short-lived triplet 1,3-cyclopentadiyl biradical, generated by benzophenone-sensitised irradiation of diazabicyclo-(2.2.1]hept-2-ene, has been trapped as a Jbis-alkoxyamine by a nitroxide. The diazabicycloheptenes (12) gave in a similar fashion the bicydo[2.1.0]pentanes (13) on triplet-sensitised photolysis, whereas laser/liquid jet excitation of the same compounds gave in addition the cyclopentenes (14), derived by 1,2-hydrogen shift evidence for a two-photon process is described. The results of a time-resolved spectroscopic study of the photodecomposition of 2,3-diazabicyclo[2.2.l]hept-2-ene in the vapour phase have also been reported.Photolysis of 2,3-diaza-5-methylenebicyclo[2.2.l]hept-2-ene affords the semi-localised... 

This investigation of successive bimolecular reactions is analogous to that carried out on the successive unimolecular processes represented by equations (1.64a) and (1.64b). There an estimate was made of the internal excitation of cyclopropane from photolysis of 1-pyrazoline from their experiments, Endo et al concluded that the average recoil energy from (1.74) amounted to 84 1% of that reaction s exoergicity. 

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