Diazene isomers

Focusing on the B3LYP/TZVP results, we found that the N2H2 isomerization energy is increased by 10 kJ/mol in complex 1(N2H2) and does not change appreciably in the case of complex 2(N2H2) as compared to the uncoordinated diazene isomers (82). 

The three diazene isomers are all stabilized significantly upon coordination to Fe by the direct coordination energy as well as by hydrogen bonding. They should all be taken into account as possible stable intermediates in the FeMoco mechanism. Different open FeMoco structures could favor different isomers while traras-diazene needs a larger Fe-Fe distance than cis-diazene, cis-diazene, and especially NNH2 will become important if FeMoco opens only little. 

Figure 7.26 The relative stability of diazene isomers changes upon fluorine introduction.

Dia ene deductions. Olefins, acetylenes, and azo-compounds are reduced by hydrazine in the presence of an oxidizing agent. Stereochemical studies of alkene and alkyne reductions suggest that hydrazine is partially oxidized to the transient diazene [3618-05-1] (diimide, diimine) (9) and that the cis-isomer of diazene is the actual hydrogenating agent, acting by a concerted attack on the unsaturated bond ... 

Diazenes of the type REN=C(R )N=NC(R )=NER have a rich structural chemistry. The selenium derivatives 15.11a,b display a cis,trans,cis conformation with two short l,5-Se N contacts (2.65 A). Several sulfur analogues, e.g., 15.1c, have the same structure, but a different cis,trans,cis conformer 15.12 with two 1,4-S N contacts (2.83 A) has also been characterized. A third type of diazene is the trans,trans,trans isomer 15.13a,b with no intramolecular short contacts. ... 

Meso- and (+ )-azobis[6-(6-cyanododecanoic acid)] were synthesized by Porter et al. (1983) as an amphipathic free radical initiator that could deliver the radical center to a bilayer structure controllably for the study of free radical processes in membranes. The decomposition pathways of the diazenes are illustrated in Fig. 36. When the initiator was decomposed in a DPPC multilamellar vesicle matrix, the diazenes showed stereo-retention yielding unprecedented diastereomeric excesses, as high as 70%, in the recombination of the radicals to form meso- and (+ )-succinodinitriles (Brittain et al., 1984). When the methyl esters of the diazene surfactants were decomposed in a chlorobenzene solution, poor diastereoselectivity was observed, diastereomeric excesses of 2.6% and 7.4% for meso- and ( )-isomers respectively, which is typical of free radical processes in isotropic media (Greene et al, 1970). 

Up to this point, we have not had to address the unexpected photolysis behavior of the meso-compound, namely, that the stereochemical retention in the products resulting from the decomposition of this stereoisomer actually decreased at concentrations above the CMC. Intuitively, one would expect that the restricted environment created by micellization of the diazenes would enhance stereochemical retention. The (+)-diazene behaves as we would have predicted, but the meso-isomer does not. 

A fragmentation reaction which appears to proceed via the generation of 1,4-diradicals is the decomposition of 1,1-tetramethylenediazenes. Unlike the more stable 1,2-diazenes (tetrahy-dropyridazines, see Section 4.2.1.), the 1,1-isomers are not usually isolated or characterized by physical methods but are proposed as intermediates in the thermal decomposition of iV-phenyl-sulfonamidopyrrolidines 1, giving 1,4-diradicals which recombine to yield cyclobutanes 3 and 4. 39 These intermediates are also formed in the photochemical decomposition at low temperature of 1,1-tetramethylenediazenes, prepared in situ from 1-aminopyrrolidines and /er/-butyl hypochlorite.141... 

Although trans-diazene is thermodynamically unstable with respect to its decomposition, the rate of this reaction is surprisingly slow. This is because of orbital symmetry and the sterically unfavorable trans configuration. Thermal conversion to the more reactive cis isomer is necessary for the reaction to occur. It has also been noted that trans-diazene reacts surprisingly slowly with HC1 in the gas phase (167). 

We find that cis-diazene is more strongly bound in both model complexes than irans-diazene. This is apparent in view of the Fe-N bond distances For 1(N2H2) we find a about 4 pm shorter Fe-N distance for the cis-isomer when compared with the trans-form, while this distance is 2-3 pm shorter in cis-2(N2H2) than in trans-2(N2H2). 

Isomerization mechanisms of isolated trans- to cis-diazene were studied and transition states for two possible interconversion routes were found to be more than 200 kJ/mol unstable than traras-diazene (98). The relative stability of trans- to cis-diazene was calculated to be 21 to 29 kJ/mol using different quantum chemical methods and basis sets [see Ref. (98) and references cited therein]. The NNH2 isomer is about 87 kJ/mol (almost independent of the density functional) higher in energy than traras-diazene. 

A different rendering of the relevant core atoms of the dication [Fe, N2H2, S(thiolate) donors] shows that the dicationic diazene complex B and the N2 complex C are potential redox isomers or valence tautomers (Scheme 38). 

Exploration of the scope and limitations of the formation of both types of heterocyclic azomethine imines with an exocyclic terminal nitrogen atom and/or isomers with a ring larger by one nitrogen atom and with all azomethine imine atoms incorporated into the ring requires the extension of the carbon tether (C) between the diazene and the diazo carbonyl functionalities of the co-diazenyl a -diazo ketone 37 (01TH). 

Evidence for both cis- 47) and trans-(4S) diazene shown in Scheme 17 has been found in IR studies. Trani -diazene (48) (d(N N) 124.7(1) pm, d(N-H) 102.9(1) pm, <(H N N) 106.3(2)°) is the dominant structure in the gas phase and has been unambiguously identified. There is evidence that the cis isomer (47) is the reactive species in many reactions, for example, in the hydrogenation of organic compounds. A third isomer, the blue-violet, photolabile 1,1-diazene or isodiazene (49) has been detected in matrix-isolation studies at 12K. It was prepared from aminoisocyanate. Irradiation... 

When the 2,3-diazabicyclo[2.2.2]-2-octene bears an endo-phenyl substituent, only the exo-face of the protonated diazene undergoes the cycloaddition of cyclohexadicnc affording the anti-isomer126. Conversely, an equimolar mixture of the anti- and syw-isomers is produced from the exo-phenyl-substituted diazene. 

Both the cis and trans isomers of diazene are known they are unstable except at very low temperatures. The fluoro derivatives, N2F2, are more stable and have been characterized structurally. Both isomers of N2F2 show N—N distances consistent with double bonds (cis, 120.9 pm trans, 122.4 pm). 

Although the diazene (8) (Scheme 2) decomposes thermally by loss of N2 to give the bicyclobutane (9) and its ring opened isomer (10) as the major products, photolysis with Pyrex filtered UV light gives mainly the rearranged bicyclic compound (11) and an unidentified hydrocarbon mixture of empirical formula C22H22. ... 

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