Triplet alkyl nitrene intermediates: Photolysis of alkyl azides with intermolecular and intramolecular triplet sensitization.

摘要

In this research intermolecular and intramolecular triplet sensitization has been employed to photochemically generate triplet alkyl nitrene intermediates in solution. Photolyzing with light greater than 300 nm assures that only the ketone chromophore is exclusively excited to its triplet excited state. Once generated, the ketone can transfer its triplet energy to the azide moiety which loses molecular nitrogen to give a triplet alkyl nitrene intermediate.;Intermolecular triplet sensitization in solution allowed trapping of triplet alkyl nitrene intermediates via bimolecular reactions. Photolysis of 1-azidoadamantane in solution with acetophenone, benzophenone and acetone led to an azo-dimer as the major product. This product comes from dimerization of two triplet alkyl nitrenes.;Intermolecular sensitized photolysis of benzyl azide led to N-methyleneaniline as the major photoproduct with acetophenone as the triplet sensitizer. With benzophenone as the triplet sensitizer the major product was benzylideneamine, which comes from H-atom abstraction by benzophenone from azide 1. Acetone as a triplet sensitizer gives N-methyleneaniline and benzylideneamine as the major photoproducts. In this case it is not possible to exclusively excite the ketone without directly exciting the azide moiety, thus only singlet reactivity of azide 1 is observed with acetone.;Intramolecular triplet sensitized photolysis of 4-Azido-1,4-diphenyl-butan-1-one gave 2,5-diphenyl-1H-pyrrole and 1,4-Diphenylbutane-1,4-dione as the major photoproducts. These products come from intramolecular gamma-H-atom abstraction. A trace amount of 2,5-diphenyl-3,4-dihydro-2H-pyrrole was also observed. This product comes from triplet alkyl nitrene reactivity. Thus, a gamma-phenyl substituent promotes gamma-H-atom abstraction and the observed reactivity can be controlled.;Intramolecular triplet sensitized photolysis of 3-azido-4,4-dimethyl-1-phenylpentan-l-one yielded 2,2-Dimethyl-1,3-diphenyl-propan-l-one, 4,4-Dimethyl-3,5-diphenyl-isoxazolidine and 2-methyl-1-phenyl-2-{[1-phenyl-meth-(Z)-ylidene]-amino}-propan-l-one as the major photoproducts under ambient photolysis conditions. Low temperature photolysis yielded 2,2-Dimethyl-1,3-diphenyl-3-{[1-phenyl-meth-(Z)-ylidene]-amino-}propan-l-one. Photolysis in an argon matrix appears to yield the triplet alkyl nitrene. Intramolecular triplet sensitized photolysis of 3-azido-1,3-diphenyl-isobutyrophenone gave (E)-4,4-dimethyl-l-phenyl-pent-2-en-l-one and (Z)-3-amino-4,4-dimethyl-l-phenyl-pent-2-2en-l-one as the major photoproducts. Thus, a bulky beta-substituent affects the reactivity of beta-azides. Reactivity from the triplet alkyl nitrene is not observed under ambient photolysis conditions. Instead, we propose either a charge transfer or imidogen mechanism.