Nitrogen extrusion azoalkanes

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. 

A useful mechanistic probe for the diradical intermediates postulated in the di-ir-methane rearrangement entails generating them via authentic routes and elucidating their chemical behavior. One of the early examples concerns the cyclopropyldicaibinyl diradical postulated in the photoisomerization of bar-relene into semibullvalene by nitrogen extrusion from the appropriate azoalkane. Indeed, as shown in... 

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). ... 

In this last section the preparation of aminocyclopropane carboxylic acid derivatives by nitrogen extrusion from cyclic azoalkanes will be discussed. These amino... 

This contribution is preceded by an earlier chapter, which covered the main topics of the photochemistry of cyclic azoalkanes for over three decades. Herein, we address the following mechanistic aspects of cyclic azoalkane photochemistry (1) the generation of azoalkane triplet states on direct irradiation and their photochemical transformations, and (2) the double inversion of the molecular skeleton in the formation of housane during photochemical nitrogen extrusion. 

Dehnitive for triplet-state photochemistry, the a-CN-bond cleavage has been recently disclosed in the direct irradiation of azoalkanes 5. A prominent feature of the photolysis of azoalkanes 5 (Scheme 4) is the fact that the syn/anti product diastereoselectivity depends on temperature and bridgehead substitution. The temperature dependence of the direct photolysis of azoalkanes 5 stems from the competition between the singlet and triplet reaction channels (Scheme 4), the latter dominating at low temperature. Mechanistic details of this diastereoselective process is considered below, subsequent to a general discussion on the stereochemical inversion phenomenon in the photochemical nitrogen extrusion. 

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