AZO BRIDGES FROM AZINES .22. [2-LATICYCLIC CONJUGATION BETWEEN PARALLEL AZO GROUPS AND PI-SYSTEMS - THE SYMMETRY CRITERION IN AZO-CARBONYL INTERACTIONS(2] PHOTOCYCLOADDITION AND 1,5)

The known 1,5-lacticyclic interaction between parallel C=C/N=N bridges in 1 is expressed by a bathochromic shift of the azo n-pi transition of +7-10 nm. A C=C-CN bridge (2, 6) increases the bathochromic shift slightly, whereas a C=C-OSiR(3) bridge (3) cases a hypsochromic shift of -6 nm. In a R(3)SiO-C=C-CN bridge (4) the effect of the two substit uents cancel each other. All these compounds (1-6, 13) undergo quantit ative [2 + 2] photocycloaddition by n-pi excitation. A novel transann ular interaction between azo and carbonyl (20, 21: +8 nm) and 1,2-dica rbonyl groups (24: ca. +20 nm) is observed. From photoelectron spectra and MO calculations neighboring group effects on the orbital energies in 20, 21, and 24 are assessed and discussed.