Lead azide, photolysis

The possible formation of intermediates during decomposition of lead azide has been discussed, but results were inconclusive [62]. Shechkov [63] suggested that up to 5% of the product nitrogen may be temporarily retained in the crystal and this may be expected to influence kinetic behaviour. The photolysis of lead azide has also been studied [64]. 

A mechanism for the acceleratory stage based on the catalytic effect of colloidal lead particles would agree with the model used to explain photolysis of alkali azides. Hall and Williams showed that metal films in contact with lead azide alter the photodecomposition efficiency [96]. Alernatively, the acceleratory stage may result from an increased concentration of trapped nitrogen molecules diffusing to the surface. The deceleration is thought to result from a depletion of azide molecules in the near-surface regions [120]. 

As with lead azide, impurities were found to alter photodecomposition rates in AgNa [237]. The addition of Pb " retarded the gas evolution rate while CO2 enhanced it. No explanation for the effect was given, but it may involve changes in stoichiometric charge-compensating defects. The authors claim that photolysis is not concentrated in disordered regions of the crystal as is observed for lead and thallous azides (see Section D). 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide. The reaction has been carried out with R = aryl, cyano, EtOOC, and RSO2, 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 (15-62). Reaction of NsONHC02Et/ CuO [Ns = A(/7-toluenesulfonyl-inimo)] and a conjugated ketone, for example, leads to the A-carboethoxy aziridine derivative.Calcium oxide has also been used to generate the nitrene.Other specialized reagents have also been used." ... 

On the other hand, thermolysis of ferrocenylsulpkonyl azide (14) in aliphatic solvents may lead to the predominant formation of the amide (16) 17>. A 48.4% yield of (16) was obtained from the thermolysis in cyclohexane while an 85.45% yield of 16 was formed in cyclohexene. Photolysis of 14 in these solvents led to lower yields of sulphonamide 32.2% in cyclohexane, 28.2% in cyclohexene. This suggests again that a metal-nitrene complex is an intermediate in the thermolysis of 14 since hydrogen-abstraction appears to be an important made of reaction for such sulphonyl nitrene-metal complexes. Thus, benzenesulphonamide was the main product (37%) in the copper-catalyzed decomposition of the azide in cyclohexane, and the yield was not decreased (in fact, it increased to 49%) in the presence of hydroquinone 34>. On the other hand, no toluene-sulphonamide was reported from the reaction of dichloramine-T and zinc in cyclohexane. 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

Photolysis of sugar oximes produces iminolactones.178,179 These are the compounds proposed to arise from irradiation of glycosyl azides however, the mechanism leading to formation of iminolactones from these two starting-materials (azides and oximes) must be quite different (see Scheme 30). Photolysis of azides is considered to generate a nitrene, whereas photolysis of oximes produces an iminolactone that... 

The addition of nitrenes leads predominantly to the closed [6,6] bridged isomers. The corresponding [5,6] bridged isomer is - if at all - formed only in small amounts, probably via a direct addition to the [5,6] bond [394]. Nitrenes have been generated by thermolysis of azido-formic esters [172,395 00], photolysis of aroyl azides [401] or aryl azide [402], elimination of O-4-nitrophenylsulfonylalkylhydroxamic acid [403] or reaction of amines with Pb(OAc)4 [404]. 

The solution-phase photochemistry of nine alkylazides was studied by Kyba and Abramovitch." ° Photolysis of tertiary alkyl azides leads cleanly to 1,2 alkyl migration. 

Photolysis of alkyl azides bearing pendant aryl groups does not lead to intramolecular trapping of a nitrene by the aryl group. However, intramolecular capture of the putative nitrene is observed upon pyrolysis of the azide precursor. These observations convinced Kyba and Abramovitch" ° that 1,2-migration is concerted with loss of nitrogen from the excited state of the azide, but that a free singlet nitrene is formed upon thermal decomposition. The chemistry of acyl azides will be shown later to exhibit the opposite pattern. 

Two other observations are noteworthy. First, the yield of isocyanate (9) produced on photolysis of 7 in methylene chloride (an inert solvent) is 40%. Photolysis of 7 in cyclohexene leads to a 45% yield of aziridine adduct 10 and a 41% yield of isocyanate 9. Trapping the nitrene does not depress the yield of isocyanate Hence, isocyanate 9 and adduct 10 cannot be derived from the same reactive intermediate. Instead, the isocyanate must be formed from the excited state of the azide, that is, the excited azide (7 ) must partition between the formation of isocyanate and nitrene. 

Photolysis or pyrolysis of most aryl azides in hydrocarbons leads to polymeric tars instead of diagnostic insertion products and aziridines. ... 

Sylwester and Dervan s assignment was supported by the observation that the violet photoproduct decomposes to N2 and H2 upon warming the glass or upon further irradiation of the glass. Furthermore, photolysis of 62 in argon at 10 K leads to the disappearance of the IR spectrum of the azide and the appearance of new bands at 2865, 2808, 2141, 1863, 1574, and 1003 cm. The 2141-cm band is due to the formation of carbon monoxide. The band at 1574 is assigned to the... 

Photolysis of pentacoordinate pentavalent phosphorus azides leads to a Curtius-type reaction36,66. Further investigations into the photolytic behaviour of 25-phosphorus azide derivatives showed that three reaction types can be observed, depending on the nature of the substituents (a) Curtius-type rearrangement (b) tautomeric equilibrium between cyclic and open azides and (c) hydrogen abstraction reaction66. 

The photochemical decomposition of acyl azides, on the other hand, proceeds by an intermediate nitrene whicn, in many cases, leads to typical nitrene reaction products. - 1- 111 -14 This is not unreasonable since a photochemical reaction provides enough energy to break the N-N, bond without alkyl or aryl participation. Homer, for example, has proved the existence of benzoylnitrene as the primary cleavage product of the photolysis of benzoyl azide in the presence of trapping reagents (Scheme II).1 7... 

Thermolysis or sensitized photolysis of the azide (272) leads to the nitrene-inserted product (273). Here electrophilic attack at C-3 by singlet nitrene is prevented by the presence of a substituent. Apparently attack at sulfur is either unfavourable or reversible. 

Photolysis or thermolysis of azides leads to nitrenes. Such nitrenes, attached to the thiophene ring, have been extensively investigated. 

The riboflavin triplet reacts with dGMP acid by ET (k = 6.6 x 109 dm3 mol-1 s 1), and evidence for the formation of the (deprotonated) Gua radical cation has been obtained by laser flash photolysis (Lu et al. 2000). The photosensitized reactions of dGuo by TRP is thought to follow two pathways, the formation of Z has been attributed to an ET reaction (Type I), and the reaction of singlet dioxygen [ChCAg) Type II] leads to 4-OH-8-oxo-G and 8-oxo-G (Ravanat et al. 1998). The effect of D20 and azide on the 4-OH-8-oxo-G yields shows that this... 

Averdung et al. also obtained several aziridinofullerenes by the reaction of C60 with acylnitrenes, generated by photolysis of aroylazides 85a-d, leading to the fullerene adducts 86a-d (Scheme 34) [260], In a typical experiment a solution of C6o and a five-fold excess of azide 85a-d in oxygen free 1,1,2,2-tetra-chloroethane was irradiated for 60 min in Pyrex tubes using a RPR 100 Rayonet... 

Mattay et al. synthesized triazolinofullerenes 180 by the thermal [3 + 2] cycloaddition of aryl and sulfonyl azides 179 with C6o [263,291], Thermal extrusion of N2 predominantly leads to the formation to the opened [5,6]-bridged aza-fulleroids 182, whereas the major products formed by photolysis of triazolinofullerenes are the closed [6,6]-bridged aziridinofullerenes 181 (Scheme 67) [292,293],... 

Thus photolysis of diphenylphosphine azide leads unequivocally to the transient metaphosphonimidate 1 which, by head to tail dimerization, gives rise to the phosphaTJiazetidine 2 and to polymers. 

Two groups have independently observed an interesting product when the vinyl azide (61) is thermolyzed.30 31 Bauer and Hafner have isolated the azirine (62) by low-temperature photolysis of 61 and have shown that thermolysis of this azirine also leads to 170.31 Apparently, elimination of hydrogen cyanide occurs to give 9-fluorenylidene carbene (171), which then reacts with azirine (62) to give the final product. 

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