Proximate,syn-Periplanar, Rigid Imine(Nitrone)/Ene-, and Diazene(Diazeneoxy)/Ene Systems: Syntheses, Homoconjugate Reactivity and Photochemistry

Abstract

The chances for intramolecular imine/ene ( azetidines), diazene/ene ( 1,2-diazetidines), diazeneoxy/ene ( 1,2-diazetidine oxides) and diazenedioxiene ( 1,2-diazetidine dioxides) [2+2]photocycloadditions and for the isolation of the respective photoproducts, have been probed with specifically designed substrates. Upon direct or sensitized excitation, [2+2]cycloaddition was found to be the exclusive or at least dominant chemical process for the CN/CC, NN/CC and ONN(O)/CC systems featuring very small π,π-distances of 2.8-3.0 A and large π,π-interorbital angles of 160-170° (7 51, 17 55, 33 58 (competing N2 elimination), 22 62). This is not the case, however, in ONNO/CC (23, where electron transfer is a possibility), or in the more flexible, less “proximate” CN/CN (57) and CNO/CN (63) systems (π,π-distances of >3.8 A). While the corseted 1,2-diazetidine photoadducts (55, 58) proved to be thermally stable, their N-oxides (62, 65) were thermally too labile to be directly observable above -65 °C. For the latter's only fleeting existence, electronic rather than strain effects are held responsible (B3LYP/6-31G* calculations). Very facile CNO/CC (12 13) and NNO/CC (22 24) [3+2]cycloadditions, homoconjugate addition of H2 and of dienophiles ([2+2+2]) to the diazene/ene 17 ( 39, 41, 45) are manifestations of “proximity” in these bichromophoric skeletons.