Thermolysis of azo compounds

Eq. 15). On the other hand, thermolysis of azo compound (45) is known to generate the same radical pair in the singlet state and gives the same products in a different ratio (Eq. 16). The NMR spectrum of the coupling product 31 shows that the sense (AE) of polarization observed for the carbene reaction is opposite to that observed (EA) for the thermolysis of azo compound. This result implies that the coupling product 31 in the photolysis is formed by recombination of benzyl and diphenylmethyl generated from a precursor believed to be triplet DPC. 

In order to determine the appropriate conditions for incorporating the desired azo monomer into a polymer, and indeed for deciding on the best azo compound for the intended application, it is essential to know its thermal characteristics. Numerous experiments on the thermolysis of azo compounds and azo monomers have been reported in the literature 28-37,39-52). jnvarjabiy the thermal decomposition follows first-order kinetics (Tables 3.2 and 3.3). 

Although the effect of changing the solvent on the thermolysis of azo compounds is small (see Table 3.2), as it is typical for radical reactions, the effect of varying the substituents, and especially R2 (Table 3.3) on the decomposition temperature is relatively large. As a consequence, simply by varying the substituents, it is possible to control the temperature at which an azo compound will decompose over a very wide temperature range. 

Table 3.3. The effect of varying the substituents on the thermolysis of azo compounds of the type R1—QH —N=N—R2...

Isopropylidenecyclopentane-l,3-diyl, as generated by the thermolysis of azo compound 145 can be trapped efficiently with such acceptors as fumaronitrile, cis-and trans-l, 2-dichloroethylene, dimethyl maleate, and dimethyl fumarate.222 ... 

Diaryl disulfides have also proven to be a clean source of thiyl radicals by photolysis, and this system was used to study addition reactions using time-resolved spectroscopies [22]. However, the radical chain addition of disulfides to olefins is quite inefficient, due to the effective competition of -elimination with the attack of the )9-thioalkyl radical on the disulfide. An alternative approach is represented by the thioselenation of olefins [23] achieved with a diphenyl disul-fide/diphenyl diselenide mixed system (equation (6)). In this case, the j8-thioalkyl radical adduct is trapped more efficiently by diselenide than disulfide. 5-Benzoyl phenylselenosulfide adds across the double bond of olefins like thioacids to give selenothiocarbonylated products regiospecifically (equation (7)) [24]. The reaction can be initiated either by thermolysis of azo compounds (AIBN at 80°C) or by UV irradiation at room temperature. 

The thermolysis and photolysis of azo-compounds (R.N N.R) are well documented sources of radicals (for a review, see Strausz et al., 1972). As such they have been extensively used as radical sources in CIDNP studies (Fischer and Bargon, 1969 Gloss and Trifunac, 1969, 1970b, c Iwamura and Iwamura, 1970 Iwamura et al., 1970b Kasukhin et al., 1970). The application of CIDNP to unravel some of the mechanistic complexities of the decomposition of azo-compounds has now begun. 

The yield of 3//-azepine appears to be influenced by substituents on the azide ring although no in-depth study has been reported. 4-Halophenyl azides, in most cases, give rise to complex mixtures of products.177 4-Tolyl azide, on thermolysis in aniline, yields 5-methyl-iV-phenyl-3//-azepin-2-amine (18% mp 157-158°C), whereas the thermolyses of 4-methoxy-, 4-methyl-, 4-chloro-, and 4-nitrophenyl azides in a range of arylamines (4-chloro-, 4-methoxy-, and pen-tafluoroaniline) yield only azo compounds and uncharacterized tarry products.178... 

Fluorinated 1,2-diazepines (23) can be prepared by the thermolysis of 2,4,6-trimethylphenyl azo compounds with elimination of HF from the Me and F ortho to the azo linkage [84CC832 88JFC(41)439]. The oxidation of these unsymmetrical diareno-1,2-diazepines gave N-oxides and diazepi-nones, depending on the oxidant [89JCS(P1)1117]. 

Thermolysis rates are enhanced substantially by the presence of certain Lewis acids (e.g. boron and aluminum halides), and transition metal salts (e.g. Cu ", Ag1).46 There is also evidence that complexes formed between azo-compounds and Lewis acids (e.g. ethyl aluminum scsquichloridc) undergo thermolysis or photolysis to give complexed radicals which have different specificity to uncomplexed radicals.81 83... 

When mesitylene-2-sulphonyl azide (3) is heated to 150 °C in n-dodecane, a Curtius-type rearrangement of the nitrene (4) occurs as discussed in Section 2.1 i to give 2,4,6-trimethylaniline and the hexa-methylazobenzene 14>. A similar result has now been observed by a careful analysis of the thermolysis products of durene-3-sulphonyl azide in w-dodecane at 150 °C. The amine is definitely formed but the azo-compound could barely be detected 13>. 

This stems from the weakness, i.e. ease of thermal fission, of the Pb—R bond, and radicals may be generated in solution in inert solvents, as well as in the vapour phase, through such thermolysis of weak enough bonds, e.g. those with a bond dissociation energy of < w 165 kJ (40kcal)mol 1. Such bonds very often involve elements other than carbon, and the major sources of radicals in solution are the thermolysis of suitable peroxides (O+O) and azo compounds (C+N). Relatively vigorous conditions may, however, be necessary if the substrate does not contain substituents capable of stabilising the product radical, or... 

Qualitatively the saipe reactivity pattern was observed for the decomposition of m. azonitriles 20 (R = CN, R, R = alkyl) and several symmetrically and un-symmetrically substituted azo compounds A selection of these results is found in Table 2. It is apparent from these data that the thermal stability of 20 decreases as the size of the groups R —R increases. Riichardt et al. have observed that a linear relationship exists between the thermolysis rates of Table 2 and the 1-solvolysis rates of corresponding f-alkyl-p-nitrobenzoates 27 in 80% acetone-water The... 

Table 22. Cyclorcversion of Tricyclo[4.2.0.0 2,5]octanes, - Octadienes and Related Azo Compounds by Thermolysis...

Synthesis of the prepolymer by partial thermolysis of the polymeric azo compound in the presence of monomer A. 

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