Azoalkanes, thermolysis

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)...

Azo compounds can exist in either the cis or trans form. It is reasonable to assume that the azoalkanes in Table 5-8 exhibit the trans configuration. Contrary to the small solvent effects obtained in the decomposition of trans -azoalkanes, the thermolysis of definite cu-azoalkanes reveals a significant solvent influence on rate. Thermolysis of ah-phatic symmetrical cw-tert-azoalkanes can lead either to the corresponding trans-tert-azoalkanes, presumably via an inversion mechanism, or to tert-alkyl radicals and nitrogen by decomposition via a free-radical transition state [192]. An example of the first type of reaction is the Z)I E) isomerization of [1,1 jazonorbornane. Its rate is virtually solvent-independent, which is consistent with a simple inversion mechanism [565, 566], The second reaction type is represented by the thermal decomposition of cis-2,2 -dimethyl-[2,2 ]azopropane, for which a substantial decrease in rate with increasing solvent polarity has been found [193] cf Eq. (5-60). 

Analogous to m-azoalkanes (1,2-diazenes), the thermolysis of 1,1-diazenes is also solvent-sensitive. The monomolecular decomposition rate of A-(2,2,5,5-tetramethyl-pyrrolidyl)nitrene decreases with increasing solvent polarity [568]. 

The generatiphotolysis resulted in nitrogen elimination. The formation of 4-(3-butenyl)-l,2-pyrazole (214) occurred via rDA reaction of adduct (213) (equation 93). Apparently the rigidity of the tricyclic azoalkane (213) prevented nitrogen extrusion under the usual thermolysis OHiditions. Again, photolysis resulted in nitrogen elimination rather than cycloreversion. 

Intramolecular rDA processes have been used to produce a variety of cyclic compounds. Thermolysis of azoalkane (223) followed by tautomerization gives 4-(2 -styryl)pyrazole (224) as shown in equation (100). Apparently the C-1—C-6 and C-4—C-5 bonds are sufficiently weakened compared with the C-1—N-2 and N-3—C-4 bonds in strained (223) that the rDA process is preferred over expected molecular nitrogen extrusion. An investigation of the thermal stability of (225) led to production of 2-vinylindene (227) via an intramolecular cycloreversion to intermediate (226), which underwent a l,S-hydrogen shift to yield the more stable aromatized product (equation 101). ... 

The thermolysis of tetramethyl and tetraethyl tetrazene in the gas phase, of 1,4-dimethyl-l,4-diphenyl tetrazene in hydrocarbon solvents and of dibenzyl, di-/-butyl hyponitrite, azocamphene and ethyl 2,2 -azobisisobutyrate in solution have been shown to proceed by homogeneous first order processes. Their rate parameters are collected in Table 9. Unlike the azoalkanes they all seem to have normal frequency factors ... 

Evidently further kinetic data on the thermolysis of cw-azoalkanes would be extremely helpful, since they could provide valuable information on the mechanism of the thermal decomposition of the trans isomers and could perhaps make it possible to assess the rote of trans - cis isomerization in thermolysis. The thermal decomposition of c -azoalkanes could also be advantageously exploited as low temperature thermal sources of alkyl free radicals. 

Photolysis of the azoalkane, 2,3-diazatricyclo[4.3.0.0 ]non-2-ene in pentane at 185 nm gave a mixture of products, among which the tricyclo[3.2.0.0 ]heptane (12) was the major product (20%). At 350 nm in benzene this major compound 12 (93%) was formed together with 7% of methylenecyclohexene. Thermolysis of such compounds has also been reported. It has been postulated that deazetization of the higher excited states of this... 

---