1.2- Diazocine valence isomerism

TV-Substituted l,4-dihydro-l,4-diazocines 6 can be obtained by [TC2S + 2S + 2S] cycloreversion from. mi-benzene diimine (cA-bisazirinofa. c]benzene, diaza-c-bishomobenzene) derivatives 5 at room temperature or slightly elevated temperatures.2 - 5 The syn-benzene diimines (3,8-dia-zatricyclo[5.1.0.02,4]oet-5-enes), which are required for the valence isomerization, are available by two methods from benzene oxide derivatives. 

In a valence isomerization approach to the 5,6-dihydrodiazocine series, the all-cis-diazabicyclooctadiene (187) was isomerized at 120 °C to the thermodynamically more stable trans- diene (188) (69CB1928). As in the bicyclotriene-tetraene case, ring opening to the diazocine occurs, but is followed by ring flip and recyclization. The monocyclic valence isomer is not detected by NMR. 

Condensations of anthranilic acid derivatives lead to the 6,12-diamino- and 6,12-dioxo-dibenzodiazocines, and the dichloro compound (279) is available from the latter (54JCS3429). Reduction of (279) via the dihydrazino compound was used to prepare the parent dibenzo[6,/][l,5]diazocine (280) (67CC1077). The NMR value (5 8.53 p.p.m.) of the 6(12) proton is indicative of the diazocine structure, presumably in a tub form, rather than the valence isomeric dibenzodiazapentalene (281). 

No studies related to tautomerism of 1,2-diazocines have appeared since the late 1970s. Earlier studies of particular note include spectral studies establishing the positions of the localized double bonds in parent 1,2-diazodne (15) <79JOC1264> and kinetic studies of the rate of tautomerism of 3,8-diaryl-3,4,5,6,7,8-hexahydro-l,2-diazocines (e.g., (13)) to the corresponding 1,4,5,6,7,8-hexahydro analogs, supporting formulation of the former as (Z)-isomers <69JA3226>. The issue of valence isomerization has been particularly important for the fully-unsaturated 1,2-diazodnes, as best... 

Little reaction chemistry has been reported for the comparativeiy rare fuily-unsaturated 1,5-diazocines, although valence isomerization and a handful of other reactions, particularly of the dibenzo[i/][l,5]diazocines, have been reported, as summarized in CHEC-I (Section 5.19.4.5) and in reviews <87H(26)2477, 89AHC(46)1>. These derivatives are discussed here rather than in a separate section devoted to fully-conjugated compounds, as their decidedly nonplanar conformations clearly obviate any possibility of delocalized bonding. 

I, 2-diazocine they synthesized (see Section 11,A,2) had structure 20 (R = R1 = R = H), not 21, and consisted almost exclusively of monocyclic, as opposed to diazabicyclo[4.2.0]octatriene (see 86, Section II,C,3), valence tautomers. This latter observation is also supported by estimated enthalpies of isomerization to bicyclic tautomers, which are less favorable than in the carbocyclic series (79JOC1264). 

Azete 6 dimerizes thermally to the 1,3-diazetidine system 7, which can be isomerized to the 1,5-diazocine 7. Azete 6 shows numerous cycloaddition reactions, especially with activated acetylenes and phosphaalkines for its transformations to the pyridine valence tautomers see p. 364. 

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