The lowest excited state of many azo compounds, like that of ketones, is an (n,n*) state. Photolytic cleavage of a C N bond analogous to the a cleavage of ketones is therefore to be expected:
Another conceivable route would be the concerted elimination of nitrogen:
Experimental data and theoretical arguments indicate that the concerted path is energetically unfavorable, s o in general the two-step mechanism is involved (Engel, 1980).
As a model for this reaction the orbital correlation diagram for the cleav- age of one N H bond of cis-diimide is shown in Figure 7.17a, and the state correlation diagram derived therefrom is displayed in Figure 7.17b. The sim-
7.2 PHC)'f( )L)lSSOCIAI IONS
Figure 7.17. Cleavage of one cis-diimide NH bond; a) orbital correlation diagram, b) state correlation diagram (adapted from Bigot et al., 1978).
ilarity to the corresponding diagram for the a cleavage of ketones is appar- ent. The singlet state of the a,o biradical correlates with the ground state, and the triplet state with one of the higher excited (n,dc) states of the azo compound. The'conversion of the '.'(n,llc) states of the azo compound to the 'v3B,, biradical states is electronically allowed but is expected to be weakly endothermic. The results of quantum chemical calculations shown in Figure 7.18 confirm the expectations based on the correlation diagram.
In comparing these results with experimental data it has to be remem- bered that in contrast to ketones, azo compounds can also undergo photo- chemical trans-cis isomerizations. (Cf. Section 7.1.7.) In the gas phase n+n? excitation results in photodissociation with nearly unit quantum effi- ciency. At higher pressures, however, and especially in solution, this reac- tion almost completely disappears and photoisomerization dominates. The latter is observed even at liquid nitrogen temperatures. This is understand- able if it is accepted that photodissociation proceeds in the gas phase as a hot ground-state reaction. According to Figure 7.18, it has to overcome a barrier in the excited state and is therefore not observed in solution. For the trans-cis isomerization, on the other hand, no excited-state barrier is to be expected from the results in Section 7.1.7.
For most acyclic azo compounds photodissociation in solution is an in- direct process, that is, the photochemical reaction proper is a trans-cis iso- merization, and the cis-azoalkane formed undergoes thermal decomposition to nitrogen and radicals. This occurs especially if the cis isomer is suffi-
Figure 7.18. Calculated potential energy curves of the ground and low-lying excited states of cis-diimide in the one-bond cleavage (by permission from Bigot et al., 1978).
ciently unstable due to the presence of sterically demanding alkyl groups.
The use of triplet sensitizers has shown that the major part of direct pho- tolysis does not involve the triplet state; the extrusion of nitrogen from the triplet state requires an activation energy Ea (Engel, 1980).
Cis-trans isomerization of cyclic azo compounds is only possible for six- membered or larger rings. Otherwise only N, loss is observed, and direct irradiation and triplet sensitization can yield different products, as is the case for the cyclic azo compound 17 (Bartlett and Porter, 1968). This is due to the fact that the biradical R f .T R formed from the triplet-excited reactant cannot recombine to form a cyclobutane until spin inversion has occurred;
the triplet biradical can live long enough to be diverted to the rotamer R t f R'.
According to Scheme 5 cis and trans products are formed; direct photo- lysis produces 18a and 18b in a ratio 10: 1, whereas triplet sensitization yields a product ratio of 1.4: 1, which is nearly equal to what would be expected for an equilibrium distribution between the different rotameric biradicals. A bir- adical such as this, which gives different products depending upon its mul- tiplicity, is said to exhibit a spin-correlarion effect.
Photochemical elimination of Nz from bicyclic azo compounds produces cyclic cis-1,n-biradicals followed by stereospecific ring cleavage or cycliza-
Scheme 5
tion as exemplified in Scheme 6, whereas disproportionation would require a change in conformation and is therefore in general not observed (Cohen and Zand, 1962).
Sl+Tl intersystem crossing and therefore enhances the formation of diazacy- clooctatetraene on direct irradiation.
(Mlproport.1 Scheme 6
-
Example 7.5:
Compound 19, which yields the valence isomers of benzene on direct irradia- tion, and diazacyclooctatraene (20) as the major product upon triplet sensiti- zation, is an interesting example of the different reactivity of the S, and TI states of azo compounds (Turro et al.. 1977a):
The photochemical parameters for 19 are summarized in Figure 7.19. From these data it is apparent that at low temperatures diazacyclooctatetraene be- comes the exclusive product, since the loss of N, from the '(n,lr*) state requires an activation energy of - 5-6 kcal/mol. Oxygen has a catalytic effect on the
6 = 6 kcallrnol
€1 @,,>- SY 00 75k[r\ q; 0.2. kSy lo7 S-'
isomers 56 kcallrnd . I 3. 0.06
Figure 7.19. Jablonski diagram and photochemical parameters of 7,8-diaza- tetracycl0[3.3.0.0~4.0'.~]oct-7-ene (by permission from Turro, 1978).
The photoextrusion of N, from cyclic azo compounds is a very useful way of producing strained ring systems such as 21 (Snyder and Dougherty, 1985) or 22 (Liittke and Schabacker, 1966). Unstable species such as the o-quinodimethanes 23 (Flynn and Michl, 1974) and 24 (Gisin and Wirz, 1976), and biradicals such as 25 (Gisin and Wirz, 1976), 26 (Watson et al., 1976), 27 (Platz and Berson, 1977), 28 (Dowd, 1966), and 29 (Roth and Erker, 1973), can also be generated in a matrix by this route and spectroscopically identified.
392 ORGANIC PHOTOCHEMISTRY 7.2 PHOTODISSOCIAI'IONS
Some azo' compounds undergo the usual photolysis (A > 300 nm) only to a minor degree or not at all and are therefore dubbed "reluctant azoal- kanes." These are cyclic azo compounds such as 30,31, and 32.
Photolysis of such compounds can be accelerated by employing elevated temperatures or by introducing substituents that stabilize the radicals formed. (Cf. Engel et al., 1985.) Short-wavelength irradiation (A = 185 nm) also enhances photodissociation. Bridgehead azoalkanes such as 33 are also reluctant compounds and undergo photochemical trans-cis isomerization (Chae et al., 1981). Loss of nitrogen and formation of bridgehead radicals are observed upon excitation to the second singlet state (S,) of the trans or the cis isomer, with quantum yields of cD, = 0.3 and @, = 0.16, respectively (Adam et al., 1983).