Proximate, <BI>syn</BI>-Periplanar Bisdiazene Skeletons: Syntheses, Structures, Homoconjugate Reactivity and Photochemistry

Rigid N=N/N=N (diazene/diazene) systems (F) consisting of more or less alkylated DBH and DBO chromophoric units (1, 2, X-ray structures), with very short π,π distances [d = 2.849 (1a, av.), 2.822 Å (2)] and almost perfect syn-periplanar π,π alignments [ω = 168.6 (1a), 174.2° (2)] as well as the more flexible, less "proximate" metathesis isomers (3a,c, 27a,c, d >4.6 Å, ω = 90-100°) have been synthesized. Homoconjugate π,π interaction (in 1, 2, not in 3, 27) is deduced from UV spectroscopic measurements [π → π* maxima at 239 (234) nm (sh, 260)], while PE analyses furnished only small interaction parameters (1a: <0.3 eV). The potential of the novel syn-periplanar N=N/N=N motif in 1 and 2 for the synthesis of somewhat exotic polyheterocycles has been explored by calculation (B3LYP) as well as experimentally: i.a. kinetically stabilized, all-cis-peralkylated tetrazolidines (38, 44) and perhydro-1,2,4,5-tetrazines (41, 47) have become accessible (i.a. via novel azomethine/diazene and azomethine/azomethine cycloadditions). In 1a with its unreactive DBO chromophoric subunits, in the "buttressed" derivatives 1b-d, as well as in the DBH/DBO combination 2, and likewise in more "distant" 27 (differently from the analogous C=C/C=C and N=N/C=C systems), irrespective of the excitation conditions employed (light of λ ≥≥ 280, 254 nm, low temperature matrix irradiation, acetone sensitization) no [2+2]photocycloaddition was observed. Instead exclusively N 2 -elimination took place. It is argued that unproductive N=N/N=N photocycloaddition would have become observable through metathesis isomerization of the respective tetrazetidines.