Proximate, syn-periplanar, rigid imine(nitrone)/ene-, and diazene(diazeneoxy)/ene systems: Syntheses, homoconjugate reactivity and photochemistry

The chances for intramolecular imine/ene (--> azetidines), diazene/ene (--> 1,2-diazetidines), diazeneoxy/ene (--> 1,2-diazetidine oxides) and diazene dioxiene (--> 1,2-diazetidine dioxides) [2+2]photocycloadditions and for th e isolation of the respective photoproducts, have been probed with specific ally designed substrates. Upon direct or sensitized excitation, [2+2]cycloa ddition was found to be the exclusive or at least dominant chemical process for the C=N/C=C, N=N/C=C and ON=N(O)/C=C systems featuring very small pi,p i-distances of 2.8-3.0 Angstrom and large pi,pi-interorbital angles of 160- 170 degrees (7 --> 51, 17 --> 55, 33 --> 58 (competing N-2 elimination), 22 --> 62). This is not the case, however, in ON=NO/C=C (23, where electron t ransfer is a possibility), or in the more flexible, less "proximate" C=N/C= N (57) and C=NO/C=N (63) systems (pi,pi-distances of >3.8 Angstrom. While t he corseted 1,2-diazetidine photoadducts (55, 58) proved to be thermally st able, their N-oxides (62, 65) were thermally too labile to be directly obse rvable above -65 degrees C. For the latter's only fleeting existence, elect ronic rather than strain effects are held responsible (B3LYP/6-31G* calcula tions). Very facile C=NO/C=C (12 --> 13) and N=NO/C=C (22 --> 24) [3+2]cycl oadditions, homoconjugate addition of H-2 and of dienophiles ([2+2+2]) to t he diazene/ene 17 (--> 39, 41, 45) are manifestations of "proximity" in the se bichromophoric skeletons.