Azoalkane

Other compounds, eg, azoalkanes, acetone, etc, that yield alkyl radicals either thermally or by uv irradiation have been used with molecular oxygen to prepare alkyl hydroperoxides (r56). 

Open-chain and cyclic compounds containing azo groups (-N2 —), such as azoalkanes, azoarenes, pyrazolines, triazolines, etc. may also eliminate N2, but these reactions are called azo-extrusions (IUPAC, 1989 a). The terms denitrogenation and nitrogen extrusion, both used by Adam et al. (1992, 1993) and by Adam and Sengelbach (1993) should not be used. They are superfluous and ambiguous. 

Alkyl radicals generated from azoalkanes as in (7) react with oxygen added to argon matrices giving alkylperoxy radicals. In this manner radicals... 

Use of mild conditions was crucial and the development of diimide reduction of singlet oxygenates, silver-salt-assisted displacement of halide by peroxide nucleophiles, peroxymercuration and demercuration, peroxide transfer from organotin to alkyl triflates, and oxygen trapping of azoalkane-derived diradicals have all played a part in providing the rich harvest of new bicyclic peroxides described herein. 

The influence of the group size on the rate of generation of alkyl radicals has been investigated for the same reactions as mentioned in Table 112a> 21 Most information is available on the thermolysis of t-azoalkanes 20 (R1 -R3 = alkyl 28). 

Table 2. Rate Constants krej and activation parameters for the thermolysis of azoalkanes R1 R2R3C-N=)2 20 in hydrocarbon solvents... 

In comparison with the decomposition of taws-azoalkanes 20 a much larger group size effect has been found for the thermolysis rates of a few c/s-azoalkanes 24. Due to the repulsion of the free electron pairs on the two nitrogen atoms and due to steric interaction between the cis oriented alkyl groups cis azoalkanes 24 decom-... 

Table 3. Steric acceleration of thermolysis of rrans-azoalkanes 20 (180 0, ethylbenzene) and m-azoalkanes 24 (-28 °C, ethanol)...

The rates of homolytic fragmentation of peroxyesters 25 are also enhanced when the size of the side chains R1 —R3 = alkyl is increased. This is shown for several examples in Table 4. The rate enhancing effect is smaller than for the azoalkane thermolyses... 

It should, however, be emphasised that the methods of radical formation we have been discussing all involve the generation of radicals ab initio from neutral molecules, or from ions. In fact, radicals in which we may be interested are often produced via attack on suitable species by pre-formed radicals, Ra-, generated specially for this purpose—with malice aforethought, as it were— from precursors such as peroxides or azoalkanes ... 

Monocyclic 2H-[ 1,2,3 Idiazaphospholes (B) are easily accessible from the condensation of the four-membered chain incorporated in hydrazones or azoalkanes with phosphorus trichloride making available a large number of representatives that have been intensively studied [2, 4, 7], In contrast, their 1//-isomers (A) are less known and are obtained only as second minor product during the synthesis of 2//-[l,2,3]diazaphospholes in some cases. A facile synthesis for pyrido-anellated azaphospholes has been developed in our group by making use of 1,2-disubstituted pyridinium salts for condensation with phosphorus trichloride [8, 13-15], Accordingly, cyclocondensation of 2-alkyl-1-aminopyridinium iodides (1) with phosphorus trichloride in the presence of triethylamine affords pyrido-anellated l//-[l,2,3]diazaphospholes, i.e. l,2,3]diazaphospholo 1,5-a] pyridines (2) (Scheme 1) [16],... 

Electron spin resonance (ESR) studies of the urazole-bridged 1,3 diradicals 64 derived from the azoalkanes 63 confirm a triplet ground state for these species. The nearly zero symmetry parameter, that is, Elhc= 0.0004 0.0001 cm-1, for the triplet diradical 64 of the diphenyl azoalkane 63 establishes a planar conformation <1995JOC308, 1997JA10673>. 

The radical cations of urazole-annelated azoalkanes 65 were generated by pulse radiolysis and the transients characterized spectrally and kinetically by time-resolved optical monitoring. The initial distonic 1,3 radical cations 66 were detected, and the methyl-substituted 66 further deprotonates to radical 67 (Scheme 1) <1997JA10673>. 

The Diels-Alder reaction of isopyrazoles 365 with MTAD gives azoalkanes 366. Direct as well as triplet-sensitized (benzophenone) photolysis of these compounds leads to the corresponding housanes (bicyclo[2.1.0]pentanes) 367. Under acidic conditions, the housanes rearrange to the corresponding bicyclic products 368 <1995JOC308,... 

One of the major uses of ADC compounds as dienophiles is the conversion of the Diels-Alder adducts into cyclic azoalkanes by a sequence of reduction, hydrolysis, decarboxylation, and oxidation reactions as shown in Eq. 

Virtually all bicyclic azoalkanes are made via this route from cyclic 1,3-dienes. (For a recent review on azoalkanes, see Ref. 191.) Azodicarboxylic... 

In recent years ADC compounds have found wide use in the protection of 1,3-dienes. The Diels- Alder adducts, which because of the powerful dienophi-lic properties of ADC compounds are formed under mild conditions, can be reconverted to the diene by a series of hydrolysis, decarboxylation and oxidation reactions, followed by spontaneous loss of N2 (Scheme 18). In the preparation of cyclic azoalkanes (Section IV,D,2) a hydrogenation step is usually included (Eq. 19). The sequence is exemplified in Paquette s use of PTAD to protect the diene system of [4.4.2]propella-2,4,ll-triene,216 and in the protection of the diene system in levopimaric acid.217 However, in the... 

Several relevant papers and review articles have appeared recently. These contain reports on the mechanism and kinetics of the ene reaction of ADC compounds,243-245 examples of four-membered ring formation,246-247 other cycloadditions of ADC compounds,248-252 the synthesis of azoalkanes,253 the use of chiral l,2,4-triazole-3,5-diones,254 and the use of the DEAZD/PI13P reagent in organic synthesis.255... 

The photoelimination of nitrogen from azocycloalkanes is of interest both from the synthetic and mechanistic point of view. Acyclic azoalkanes appear to undergo elimination of nitrogen by a stepwise process involving an intermediate diazenyl radical, but the photoreactions observed in azocycloalkanes are to some extent dependent on ring size. 

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

Photoelimination of nitrogen from 1-pyrazolines has also been employed in the synthesis of tricyclo[3.2.1.02,4]oct-6-ene,338 prismane,339 quadri-cyclane,340 snoutene ,341 and marasmic acid.342 The trimethylenemethanes 414 have been prepared by photolysis of azoalkanes 415 and characterized spectroscopically.343 Dimerization and cycloaddition to alkenes of these biradicals have been reported.344... 

Photodissociation is accompanied by the cis-trans isomerization of azoalkanes. Azoalkanes have the Znmv-configuration. During photodecomposition, they are transformed into the rxs-configuration [66]. The excited molecule of Znmv-asopropane is transformed into cis-asopropane with 0 = 0.31, into the trans-con liguration with 0 = 0.51, and into free radicals with 0 = 0.18 (gas phase, 600 tor C02, room temperature). The following scheme of photophysical stages was proposed [205] ... 

The quantum yield of the selected azoalkanes photodecomposition are given in Table 3.18. The extinction coefficient s depends on the wavelength of light, and for AIBN in benzene solution (room temperature) has the values as given below [205] ... 

Quantum Yields of Azoalkanes Photolysis in Solution at Room Temperature RN2R + hv — N2 + Products ([205])... 

McBride and co-workers have studied extensively the reactions of such free-radical precursors as azoalkanes and diacyl peroxides (246). By employing a variety of techniques, including X-ray structure analysis, electron paramagnetic resonance (EPR), and product studies, and comparing reactions in the crystal and in fluid and rigid solvents, they have been able to obtain extremely detailed pictures of the solid-state processes. We will describe here some of the types of lattice control they have elucidated, and the mechanisms that they suggest limit the efficacy of topochemical control. 

Esr-spectroscopic measurements 145 C—H Bond-dissociation energies 151 C—-C Bond-dissociation energies 154 Rotational barriers 159 Isomerization reactions 163 Addition reactions 170 Azoalkane decomposition 171... 

---