Azo Bridges from Azines, XXII. [2 + 2] Photocycloaddition and 1,5‐Laticyclic Conjugation Between Parallel Azo Groups and π‐Systems The Symmetry Criterion in Azo‐Carbonyl Interactions

The known 1,5‐lacticyclic interaction between parallel CC/ NN bridges in 1 in expressed by a bathochromic shift of the azo n‐π * transition of +7–10 nm. A CCCN bridge ( 2, 6 ) increases the bathochromic shift slightly, whereas a CC–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 substituents cancel each other. All these compounds ( 1–6, 13 ) undergo quantitative [2 + 2] photocycloaddition by n‐π * excitation. A novel transannular interaction between azo and carbonyl ( 20, 21 : +8 nm) and 1,2‐dicarbonyl 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.