ISSN:
1434-193X
Keywords:
Chromophores
;
Photochemistry
;
Heterocycles
;
Diazenes
;
Imines
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
---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 C=N/C=C, N=N/C=C and ON=N(O)/C=C systems featuring very small π,π-distances of 2.8-3.0 Å 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 ON=NO/C=C (23, where electron transfer is a possibility), or in the more flexible, less “proximate” C=N/C=N (57) and C=NO/C=N (63) systems (π,π-distances of 〉3.8 Å). 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 C=NO/C=C (12 → 13) and N=NO/C=C (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.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
