Triazolines, photolysis

Numerous aziridines have been prepared by triazoline photolysis it is the principal route to the synthesis of N-arylaziridines. Photolysis of triazolines derived from simple79,113,144 and cyclic79,87,113 olefins as well as norbomene... 

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

Intramolecular dipolar azide-olefin cycloaddition of 723 took place upon heating in benzene to afford 724 (83JA3273). An alternative rearrangement process can take place upon photolysis of 724 to give 725. Mesylation of 4-(3-hydroxypropyl)-2,4,6-trimethyl-2,5-cyclohexadiene-l-one (78JA4618) and subsequent treatment with sodium azide in DMF afforded the respective azide 726 which underwent intramolecular cycloaddition to afford the triazoline 727 (83JOC2432). Irradiation of 727 gave the triazole derivative 728 (Scheme 126). 

Pyrazolines (51) can be converted to cyclopropane and N2 on photolysis""" or pyroiysis. The tautomeric 2-pyrazolines (52), which are more stable than 51 also give the reaction, but in this case an acidic or basic catalyst is required, the function of which is to convert 52 to 51." In the absence of such catalysts, 52 do not react/ In a similar manner, triazolines (53) are converted to aziridines." Side reactions are frequent with both 51 and 53, and some substrates do not give the reaction at all. However, the reaction has proved synthetically useful in many cases. In general, photolysis gives better yields and fewer side reactions than pyrolysis with both 51 and 53. S/Z-Pyrazoles" " (54) are stable to heat, but in some cases can be converted to... 

Photolysis of the azirines 68 in the presence of DEAZD gives 1,2,4-triazolines (69, R = Et) via cycloaddition to the nitrile ylid.114 The nitrile ylid generated thermally from 70 gives 1,2,4-triazolines (69, R = Me, R1 = R2 = CF3) (Scheme 7).11S The cycloadditions proceed in good yield, and the triazolines 69 are readily converted into aromatic 1,2,4-triazoles. 

In molecules containing two cyclobutenes, addition of organic azides 26 yielded two adducts, e.g. reaction with 50 with benzyl azide 56 produced the cr-isomer 57a in which the (V-benzyl substituents were sy -aligned and the C2-isomer 57b in which they were anti-orientated (Scheme 7). The structure of syn-isomer 57a was confirmed by X-ray (Figure 2). The fact that both isomers yielded the same fns-aziridine 58 upon photolysis made separation of the individual triazoline isomers unnecessary. 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide.812 The reaction has been carried out with R = aryl, cyano, EtOOC, and RS02, as well as other groups. The reaction can take place by at least two pathways. In one, the azide is converted to a nitrene, which adds to the double bond in a manner analogous to that of carbene addition (5-50). In the other pathway a 1,3 dipolar addition (5-46) takes place to give a triazoline (which can be isolated), followed by extrusion of nitrogen (7-46). Evidence for the nitrene pathway is most compelling for... 

The loss of CO, S, SO, S02 and N2 by thermolysis or photolysis has been used to make three- and four-membered rings for example, thiiranes (67) are obtained from (66) (CHEC 5.06.4.4). A2-1,2,3-Triazolines give aziridines and Wolff rearrangement of (68) gives (69). 

Scheiner has shown that triazolines undergo both direct and sensitized photoelimination of molecular nitrogen to yield aziridines.463 With the triazolines prepared from phenyl azide and cis- or trans-fi-methylstyrene, direct photolysis produces a mixture of aziridines in which the starting geometry has been largely retained. Sensitized photolysis, however, is totally nonstereospecific. 

Photolysis and thermolysis of 4-aryl-l,2,4-triazoline-3,5-diones (70, R = aryl) have been studied by Wamhoff and Wald (77CB1699). Photolysis produces the same products as were found in FVP. The thermal conversion of 70, R = Ph, into the corresponding s-triazolo[l,2-a]-s-triazole derivative, which takes place below the decomposition temperature of the educt, is assumed to proceed via a radical chain reaction (Scheme 8). 

The scope of this reaction has been extended to include a variety of 1,2,3-triazolines [see, for example, Eqs. (23)91 and (24)92]. The preparation of 2-vinylaziridines from 5-vinyltriazolines has been reported,93 and the synthesis of the first stable benzoazetine achieved by photolysis of 3-phenyl-4fl-benzo-l,2,3-triazine [Eq. (25)].94... 

The triazoline diazo compound equilibrium may be considered to play a role in the cis-trans isomerization332 and oxidation (Scheme 119)284 of certain triazolines. Triazole formation by elimination of amine214 or alcohol405 is also postulated to involve intermediate diazo compounds (Scheme 123). Unlike normal triazolines, the inability of 1-fluorotriazolines to undergo thermolysis or photolysis reactions to yield aziridines is ascribed to the fact that the triazoline always exists in equilibrium with the diazo compound (Scheme 98).356 Spontaneous isomerizations of triazolines not... 

Triazoline ring cleavage with nitrogen expulsion can be achieved by photolysis or thermolysis and leads to aziridines, imines, or the isomeric enamines.12 In contrast to thermolysis where product mixtures are obtained, photolysis is more selective and aziridine is the major product.27,79,2 2-290... 

Irradiation of triazolines with UV light of k > 240 nm, the region of maximum absorption, constitutes one of the principal methods for the synthesis of various substituted aziridines.79 "3 The photolysis reaction is essentially independent of substituent and solvent effects in some cases aziridines are accompanied by imines.79... 

Photolysis reactions of 5-amino- or 5-alkoxy-substituted triazolines have not been studied extensively usually they yield triazoles (Section IV,A,3). 

When electron-withdrawing groups are present on the 4-carbon, triazoline thermolysis is insensitive to solvent polarity a homolytic decomposition to a singlet diradical similar to that formed in photolysis is proposed to account for the selective formation of aziridine in these cases (Scheme 162).67,172,454 The lower thermolysis temperatures of N-aryltriazolines ( 25°C)6 7,322 as compared to those for the N-alkyl compounds ( 90° C)6 7 are consistent with the stabilization of the diradical by aryl groups. 

The influence of both anti and syn substitution in the 10-position on the thermolysis of triazoline adducts from a number of 7-substituted norbornenes (Scheme 14) has been investigated.128 Both the anti- and syn-substituted triazolines give only the exo-aziridine upon photolysis but pyrolysis yields endo-aziridines from the syn- and exo-aziridines from the anti compounds. The syn substituents have a strong endo directing influence and no exo-aziridine is detected in any case. Ketone formation in some cases apparently results from hydrolysis of the imines during workup.128... 

Schemer s observations (1964) on spin inversion and bond rotation in nitrogen-containing diradicals lends support to Skell s view. Direct photolysis of the triazolines 42 and 43 gives the related aziridines, 44 and 45 with predominant retention of the geometric arrangement of the methyl and adjacent phenyl substituents. Photolysis in the presence of a triplet sensitizer, benzophenone, results in a product distribution showing much lower stereoselectivity. The inference is that...

Diethylphosphoryl azide gives an Mo-triazoline with noibomene subsequent photolysis gives the phosphorylated aziridine, but pyrolysis gives the imine isomer. Maikovnikov addition of diethyl dibro-mophosphoramidate to alkenes (BF3 catalysis) occurs in high yield the initial adducts were converted to IV-unsubstituted products but would be readily convertible to phosphorylated aziridines ty the Gabriel method.In any case, IV-unsubstituted aziridines are readily phosphoiylated. > ... 

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