Azoalkanes denitrogenation

A is exothermic. An additional advantage is that one has a wider margin to select spin-state specific photochemical transformations. Representative examples include ds-trans-olefin isomerization (Eq. 49), olefin dimerization (Eq. 50), oxetane formation (Eq. 51), dienone rearrangement (Eq. 52), di-vr-methane rearrangement (Eq. 53), azoalkane denitrogenation (Eq. 54a, b), and photocyclization (Eq. 55). 

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. 

In a similar way, the 1,3-diradicals postulated in the di-ir-methane rearrangement of several benzonor-bomadienes and related species have been produced via denitrogenation of the appropriate azoalkanes (27)-(30). The products obtained in the thermal and photochemical denitrogenation of (27), for example, are the norbomadiene and the tricyclic di- ir-methane product (occasionally other products not derived from denitrogenation have been observed). The latter hydrocarbon, which derives from direct collapse of the 1,3-diradical, is formed almost exclusively (equation 37). 

Unlike deazatation (denitrogenation) of azoalkanes, loss of N2O from aliphatic azoxy compounds is rarely observed. Thus they are neither free radical initiators nor thermally labile, contrary to published comments. For example, the common free radical initiator azoisobutyronitrile (AIBN) decomposes readily at 80 C but the azoxy analog 1 is stable up to 180 Thermolysis of azoxycumene 2 is very slow and does not yield N2O, leading instead... 

The first investigation on the stereoselectivity of the stepwise thermal denitrogena-tion of the diethoxy-substituted azoalkane has been reported. The double inversion product was the major isomer of the ring closure product. In photochemical denitro-genation, the inverted azoalkane was also the major isomer at 70 °C, although the stereoselectivity of the reaction was temperature dependent. An inversion process was proposed as the mechanism that determines the stereochemistry of the denitrogenation reaction products. The inversion product may alternatively be formed as a result of dynamic effects. 

In contrast to DBH, the so-called reluctant azoalkanes are inert to photochemical denitrogenation. One representative system is the 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO, O Scheme 39-7). DBO exhibits a long-lived excited n,7t -singlet state (on the order of f[Pg.1379]

Numerous reviews on the photochemistry and photophysics of ketones [18,23,25,78,79] and azoalkanes [38,43,80,81] are already available. This chapter focuses on intermolecular photoreactions of azoalkanes, which are compared with known data for ketones (see Structure 3.1). Unimolecular reactions such as the Norrish type-1 a-cleavage reaction of ketones [17,82-87] and their Norrish type-II reactions [20,24,73,83,88-91] as well as denitrogenation [8,9,43,68,92,93] and cis-trans isomerization of azoalkanes [43,92,93] are not discussed. The anphasis lies, besides data compilation, on mechanistic understanding, such that classical applications of azo compounds as dyes [94] or more recent apphcations of azo compounds in photochromic materials [10-12], or ketones as radical initiators in polymerization [95], are omitted as well. 

Disregarding inconsistencies in the natural fluorescence rate constants, it is important to note that aliphatic ketones (biacetyl and acetone) as well as several cis-azoalkanes show fluorescence in solution [81], This includes not only the examples of DBH-T [47,98] and DBO [56,103,138] (see Structnre 3.1), but also the smaller derivatives (see Structure 3.2) 2,3-diazabicyclo[2.1.1]hex-2-ene [139], diazirines, e.g., adamantyldiazirine [140], and even some sterically hindered azobenzenes [141], In contrast, DBH is not flnorescent with the exception of a report in siliceous zeohtes [66] it undergoes denitrogenation with unit quantum efficiency from the... 

Subsequent studies exposed a pronounced medium effect on the product distribution. In the hquid-phase photolysis of the azoalkanes 3a,b, for which the triplet channel competes effectively with the singlet-state deazetation, the product distribution depends significantly on the type of solvent. In contrast, the azoalkanes 3c,d, with efficient denitrogenation in the singlet state, do not display solvent-dependent photobehavior. Thus, the aziranes 11 are produced almost exclusively in polar protic solvents, while housane formation is predominant in nonpolar solvents. It should be noted that the observed solvent dependence derives from bulk medium interactions, as similar product distributions are exhibited by the hydroxy-substituted derivative 3f and azoalkane 3b in benzene. Evidently, the intramolecular hydroxy functionahty in the azoalkane 3f does not influence the photochemistry of the triplet azo chromophore. 

Computational and experimental evidence exist in favor of the stepwise nitrogen elimination in the photolysis of azoalkanes, which implicate the intervention of a diazenyl radical species. Thus, a recent theoretical study on the parent DBH discloses the singlet diazenyl biradical DZ as the lowest-energy transient on n,7i excitation. Similarly, computational results on the photolysis of azomethane suggest a stepwise mechanism for the denitrogenation. Indeed, simple symmetry considerations in terms of the Dauben-Salem-Turro theory on photochemical transformations predict a stepwise process. " ... 

---