Kai Exner

Proximate, syn‐Periplanar Bisdiazene Skeletons: Syntheses, Structures, Homoconjugate Reactivity and Photochemistry

Rigid NN/NN (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 A (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 A, 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 NN/NN 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 CC/CC and NN/CC 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 NN/NN photocycloaddition would have become observable through metathesis isomerization of the respective tetrazetidines.

Proximate, syn‐Periplanar, Rigid Imine(Nitrone)/Ene‐, and Diazene(Diazeneoxy)/Ene Systems: Syntheses, Homoconjugate Reactivity and Photochemistry

The chances for intramolecular imine/ene ( azetidines), diazene/ene ( 1,2-diazetidines), diazeneoxy/ene ( 1,2-diazetidine oxides) and diazenedioxiene ( 1,2-diazetidine dioxides) [2+2]photocycloadditions and for the isolation of the respective photoproducts, have been probed with specifically designed substrates. Upon direct or sensitized excitation, [2+2]cycloaddition was found to be the exclusive or at least dominant chemical process for the CN/CC, NN/CC and ONN(O)/CC systems featuring very small π,π-distances of 2.8-3.0 A and large π,π-interorbital angles of 160-170° (7 51, 17 55, 33 58 (competing N2 elimination), 22 62). This is not the case, however, in ONNO/CC (23, where electron transfer is a possibility), or in the more flexible, less “proximate” CN/CN (57) and CNO/CN (63) systems (π,π-distances of >3.8 A). While the corseted 1,2-diazetidine photoadducts (55, 58) proved to be thermally stable, their N-oxides (62, 65) were thermally too labile to be directly observable above -65 °C. For the latters only fleeting existence, electronic rather than strain effects are held responsible (B3LYP/6-31G* calculations). Very facile CNO/CC (12 13) and NNO/CC (22 24) [3+2]cycloadditions, homoconjugate addition of H2 and of dienophiles ([2+2+2]) to the diazene/ene 17 ( 39, 41, 45) are manifestations of “proximity” in these bichromophoric skeletons.

Non-pentagonal unsaturated dodecahedranes-in-plane delocalized 4C/3e radical cations and σ-bishomoaromatic 4C/2e dications

Abstract An intriguing route from bissecododecahedrenes to 11,17-disubstituted bishomododecahedrene (7) provides access to X-ray crystallographic data (i.a. pyramidalization angle of ca. 22°). Electrochemical oxidation of the parent olefin (4, Epc=1.46 V) leads via the bishomododecahedradiene (3, dpp=3.0 A, E1/2=0.84 V, Epc=1.67 V) to the in-plane delocalized 4C/3e radical cation (3·+, dpp=2.85 A) and the σ-bishomoaromatic 4C/2e dication (32+, dpp=2.71 A).

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 individual NN/NNO/ONNO chromophores. According to DFT calculations (B3LYP/6-31G*), [2+2]photocycloaddition to the respective oxidized tetrazetidines is significantly to moderately endothermic. The metathesis isomerization of the oxidized tetrazetidines is generally highly exothermic and kinetically increasingly favorable with increasing oxidation state. In practice, four out of the six bichromophoric combinations undergo selectively, in competition with N2 elimination from a DBH unit (13) still partially, metathesis isomerization upon π π* excitation (monochromatic 254 nm light). In the case of the syn-NNO/NNO combinations (5/6, 14), the photoaddition is thermally reversed. For a ONNO/NN 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[diazene mono(di)oxides] undergo relatively fast secondary photoreactions, while the tri(tetra)oxides undergo rapid thermal transformations. For the NN/NNO systems (12), of three potential pathways for its metathesis isomerization, the one that takes place via σ-symmetric intermediates (63, 64) is excluded by virtue of the retention of optical purity in the photometathesis 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 the metathesis reactions, the respective tetrazetidine oxides (increasingly destabilized by interactions between oxygen lone pairs and NNσ* orbitals) function as vibrationally excited transients. That thermal reversion of these transients might be a general, nonproductive competition, is suggested by the experimental verification of a “reversed photometathesis” (51 15) and by the generally low rates in product formation upon irradiation. The question remains to be answered why in structurally analogous molecular skeletons, [2+2]photocycloaddition occurs in the CC/NN and variously oxidized NN/NN, and not, however, in the parent NN/NN combinations.

Cage-delocalised syn-periplanar bishydrazine radical cations and dications—en route to σ-bishomoaromatic 4N2+ systems?

Abstract One- and two-electron oxidation of a caged, proximate, syn -periplanar bishydrazine leads to highly persistent, 4N/(7)6e (radical) cations—with dominant electron delocalisation along the cage bonds (CV, EPR, B3LYP). A NICS value of −20.6 is evidence for strong electron delocalisation in the caged dication.

In-plane homoconjugation/homoaromaticity in preoriented 4N-anions—‘scope and limitations’

Abstract By oxidation of (presumably) bislithiated, unusually proximate, syn-periplanar bishydrazines an alternative access to σ-bishomoaromatic 4N/6e dianions is realized. Formal N-alkylation/arylation of these 4N/6e dianions leads to novel, cyclically delocalized 4N/6e monoanions. Experimental realization is achieved by addition of metal organyls to proximate bisdiazenes as well as by deprotonation of the respective diazene/alkyl(aryl) hydrazines. The nature of the 4N/6e bonding motif is investigated by UV/Vis and NMR spectroscopy as well as by DFT calculations.