Photochemical transformations, 84 - Proximate, syn-periplanar diazene/diazene(di)oxy, diazeneoxy/diazene(di)oxy, and diazenedioxy/diazenedioxy skeletons: Syntheses, [2+2]photocycloadditions, metathesis

Of two very proximate syn-periplanar bisdiazenes (1,2) mono-, di-, tri- and tetra-N-oxides were prepared, representing six combinations of the individ ual N=N/N=NO/ON= NO chromophores. According to DFT calculations (B3LYP/6-31 G*), [2+2]photocycloaddition to the respective oxidized tetrazetidines is s ignificantly to moderately endothermic. The metathesis isomerization of the oxidized tetrazetidines is generally highly exothermic and kinetically inc reasingly favorable with increasing oxidation state. In practice, four out of the six bichromophoric combinations undergo selectively, in competition with N-2 elimination from a DBH unit (13) still partially, metathesis isome rization upon pi --> pi* excitation (monochromatic 254 nm Light). In the ca se of the syn-N=NO/N=NO combinations (5/6, 14), the photoaddition is therma lly reversed, For a ON=NO/N=N combination (30), internal electron transfer is responsible for a complex reaction pattern, The preparative value of the metathesis reactions, though, is limited: The metathesis-derived bis[diaze ne mono(di)oxides] undergo relatively fast secondary photoreactions, while the tri(tetra)oxides undergo rapid thermal trans-formations. For the N=N/N= NO systems (12), of three potential pathways for its metathesis isomerizati on, the one that takes place via sigma-symmetric intermediates (63, 64) is excluded by virtue of the retention of optical purity in the photometathesi s of a highly enriched enantiomer [(-)-12]. Matrix irradiation experiments (12 K, IR control) with 12 result in the appearance of a kinetically highly labile transient. Supported by DFT calculations it is concluded that in th e metathesis reactions, the respective tetrazetidine oxides (increasingly d estabilized by interactions between oxygen lone pairs and NN sigma* orbital s) function as vibrationally excited transients. That thermal reversion of these transients might be a general, nonproductive competition, is suggeste d by the experimental verification of a "reversed photometathesis" (51--> 1 5) and by the generally low rates in product formation upon irradiation. Th e question remains to be answered why in structurally analogous molecular s keletons, [2+2]photocycloaddition occurs in the C=C/N=N and variously oxidi zed N=N/N=N, and not, however, in the parent N=N/N=N combinations.