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 N2‐elimination took place. It is argued that unproductive NN/NN photocycloaddition would have become observable through metathesis isomerization of the respective tetrazetidines.

Synthetic details are presented for a series of more or less rigid, “parallel‐in‐plane” preoriented bis(diazenes), with N=N/N=N distances (d) of 3.3−2.9 Å and interorbital angles (ψ) of 142−164° (X‐ray crystal structures). DFT calculations (B3LYP/6−31G*) and one‐/two‐electron reduction experiments with the two least preoriented, most “distant” bis(diazenes) (dN=N/N=N ca. 3.3 Å; ψ 142−146°) provide more insight into the structural prerequisites for bis(homoconjugative) in‐plane electron delocalization in 4N/5e radical anions and 4N/6e dianions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

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Preparatively useful synthesis for the novel, relatively mobile polycyclic syn1‐periplanar bisazo polycycles 4 and 5 are based on hydrazine condensation with readily available dienedione 6 and acid‐catalyzed rearrangement of bisurazole 12, respectively. In the preferred “closed” and “open” conformations of 4 and 5 the NN bonds are 314 and 496 pm, respectively, apart (MMX). From the reactions of 4 and 5 with carbonyliron and ‐ruthenium compounds, complexes are derived which are typical of monoazo ligands [4 · Fe(CO)4  20, 4 · Fe2(CO)721, 4 · Fe3(CO)922, 4[Fe2(CO)6]223, 4 · Ru3(CO)9 · 24, 5 · [Fe2(CO)6]2  25, 5 · Fe2(CO)6 · Fe3(CO)9  26]. For 21, 23 and 26 crystal structure analyses were performed, confirming i.a. in 26 the unique presence of a Fe2(CO)6 and a Fe3(CO)9 fragment attached to two azo units of the same molecule.

The as yet unknown route to triaziridines by the addition of nitrenes to the n side of the N = N bond is achieved intramolecularly by the photolysis (thermolysis) of syn-azo azide substrates with high proximity; the yields reflect the sterical (and possibly inductive) influences of the skeleton. The product composition is rather independent of the exciting light; intramolecular azo + azide energy transfer is assumed. The kinetic skeleton stabilization of the triaziridines permits the synthesis of 17 (18) by the thermolysis of 7 (8) at 200°C with the product composition deviating only marginally from that of the photolysis. Attempts towards enlargement of the triaziridines to N3X rings (X = CHR, 0, NR) lead exclusively to products of intramolecular fragmentation.The route to cis,cis-trialkyltriaziridines C as delineated in the preceding paper lb) encompasses the intramolecular cycloaddition of photochemically or thermally generated, unstabilized "discrete" secondary alkyl nitrenes B to unstabilized (dialkyl) azo units A. Yet, this route -with its obviousanalogy to the intramolecular three-membered ring construction by carbene') and nitrene3' addition to C = C bonds4) -has its intrinsic drawbacks: (i) Upon photoexcitation of secondary alkyl azides imine formation is generally very fast and may take place via discrete nitrenes or electronically excited a~ides*,~); (ii) N = N bonds are recognized as poor K~ components in cycloaddition reactions') with the consequence of even more severe stereoelectronic requirements for nitrene/N = N than e. g. for nitrene (carbene)/C = C additions; (iii) there is an omnipresent competitive process due to N2 elimination upon photoexcitation of azo systems, even though bi-and polycyclic azo compounds with skeletons similar to that of the azo azides considered for this study (Table 1) are found to be rather "reluctant" 7), the elimination of N2 being not generally a (very) fast process. Consequently, irrespective of the actual course of the nitrene generation, proper and as proximate and rigid as possible placement of the potential nitrene nitrogen atom with respect to the TC plane of the azo unit is mandatory for triaziridine formation. Thus, conformational and strain implications which often hamper intramolecular nitrene (carbene) additions to C = C bonds should be minimized'); (iv) intramolecular cycloaddition reactions between azide and N = N units yielding pentazolines should cause no complications, since the steric prerequisites for this ring formation are only met in severely distored excited states. In fact, such N5 rings have only been postulated as rate-determining transients in the unusually rapid thermolysis of o-azidoazobenzene~~). In this paper we present (i) the MMX-derived structural and energetic details of representative syn-azo azide substrates 1 -7 and of the respective triaziridines 11 -17; (ii) the results of the photolysis study on eight such syn-azo azide substrates (1,2,5,6-10; 3 and 4 are not yet available); (iii) some properties of the derived cis,cis-tria...