Photoelimination

The photoelimination of nitrogen from azoalkanes provides an easy and convenient method for the generation of alkyl radicals. No new results of any significance have been reported for such systems and attention has been devoted almost entirely to the study of cyclic azoalkanes. 

The pholodecomposition of diazirines is known to proceed via carbene intermediates. Chloromethoxycarbene, for example, has been identified by i.r. spectroscopy on irradiation of 3-chloro-3-methoxydiazirine in an argon matrix at 10 K. Detailed studies of the photoelimination of nitrogen from 3-benzyl-3-chlorodiazirine (1) have been reported. 1,2-Hydrogen migration in the carbene (2) formed on irradiation of (1) affords a mixture of (Z)- and (E)-/3-chlorostyrenes (3) and (4) the stereochemistry of the hydrogen shift is 

In certain 1-pyrazolines in which photoelimination of nitrogen is relatively inefficient, the quantum yield for photodecomposition can be enhanced by an increase in temperature. In this way, the pyrazolines (15) were converted into the cyclopropanes (16) by irradiation in refluxing benzene or acetonitrile.  

The expected tricycloalkane (17) is the major product of irradiation of the azoalkane (18) in benzene.Photolysis using an 18 W argon laser, however, 

Novoselov, S. D. Isaev, and A. G. Yurchenko, Vestn. Kiev. Mitekh. Jnsr., Ser. Mashinostr. Tekhnol, 1983, 20, 17 Chem. Absfr., 1983, 99, 158300). 

The photoelimination of nitrogen from acyclic azoalkanes provides an easy and convenient route to alkyl radicals. Few results of any significance have, however, been reported in this area in the period covered by this report. The photodecomposition of arylazo-phosphonates and arylazoalkylphosphonates has been studied, and 

Much attention has been devoted to the study of the photoelimination of nitrogen from cyclic azoalkanes. The photodecomposition of diazirines is known to proceed via carbene intermediates rather than biradical intermediates, but details of the mechanism of formation of these species are not known with certainty. A theoretical study of the decomposition has been undertaken. Phenylchlorocarbene (1), generated by irradiation of phenylchlorodiazirine (2), has been characterized spectroscopically 

Photoelimination of nitrogen from 1-pyrazolines can be used to generate 1,3-biradicals and products derived therefrom. Triplet 1,3-cyclopentadiyl, obtained in this way by benzophenone-sensitized photodecomposition of 2,3-diazabicyclo[2.2.1]hept-2-ene, 

5- diazatricyclo[4.3.0.0 ]non-4-en-8-one and the corresponding cyclic ethylene acetal has been explained in terms of initial one- 

5- diazatricyclo[4.3.0.0 ]nona-4,8-diene (14), undergoes competing elimination to the tricycloheptene (15) and rearrange- 

A number of papers have appeared which are of general relevance to photoelimination chemistry. These include discussions of spin-orbit coupling in diradicals, and theoretical models for the selectivity of triplet and singlet photoreactions.  

The mechansim of the thermal and photochemical decomposition of azo-alkanes has been the subject of a comprehensive review, and the synthesis of unusual organic molecules by photoelimination of nitrogen from azoalkanes has been surveyed.  

The regioselectivity of cycloaddition of singlet 2-methylenecyclopenta-l,3-diyl (23), generated by photoelimination of nitrogen from 7-methylene-2,3-diazabicyclo[2.2.1]hept-2-ene (24), to alkenes has been explained in terms of 

Photoelimination of nitrogen from triazolines can similarly be employed in the synthesis of aziridines. Thus, the triazolines (41) have been converted in this way 

The major product of irradiation of the. /V-aryltriazolo[4,5-6]pyridine (45) in ethanol is the pyrido[2,3- ]indole (46) the imino-ether (47) is also formed, and 

Carboxylate esters readily undergo photodecomposition with loss of carbon dioxide. Not surprisingly, lactones and related oxygen heterocycles undergo related transformations. A wide variety of lactones behave in this fashion for example, the cyclic dilactone (505) is converted on irradiation to the [2.2]paracyclophane (506). Of particular interest is the use of the jS-lactone (507) as a precursor of matrix-isolated cyclobutadiene (508). Phthaloyl peroxide (509) has similarly been employed as a precursor of benzyne (510). A careful examination in an argon matrix at 8 K has revealed that the / -lactone (511) and the keto ketene (512) are intermediates 

Anhydrides also undergo photodecomposition with loss of carbon dioxide. Tetrafluorocyclobutadiene (520) can be prepared in this way from the anhydride (521) and is isolated as the [,4 +, 2] adduct (522) with furan. In the absence of furan, the dimer (523) and the cyclooctatetraene (524) were obtained. Examples of photoelimination of carbon dioxide from cyclic carbonates have been described.  

A variety of examples of photochemically induced elimination of sulfur dioxide from sulfur heterocycles have been reported, with particular attention to cyclic sulfones. Thietan-3-one 1,1-dioxide (525), however, undergoes fragmentation on irradiation in acetonitrile to ketene (526) and to the sulfene (527), the latter identified by infrared spectroscopy and as the methanol 

2-Benzodithiole 2,2-dioxide (535), however, is converted on irradiation in benzene to the o-thiobenzoquinone methide (536), a species that is readily 

2-trioxides and in 1,2,3-thiadiazole IdJ-trioxides cyclopropane derivatives have been obtained by photoelimination of sulfur dioxide from y-sultines. In a related process, vinylene trithiocarbonates are converted on irradiation in an argon matrix to thiirens by loss of carbon disulfide.  

Phthaloyl peroxide (509) has similarly been employed as a precursor of benzyne (510)446 A careful examination in an argon matrix at 8 K has revealed that the /(-lactone (511) and the keto ketene (512) are intermediates 

Alkyl radicals are readily generated from azoalkanes by photochem-ically induced elimination of nitrogen. 1-Adamantyl radicals can be obtained in this way by photolysis of azo-1,1 -adamantane in acetonitrile the products are derived principally by hydrogen abstraction from the solvent although radical addition to the nitrile group is also important. The effect of solvent viscosity 

More attention has been paid to photoelimination of nitrogen from cyclic azoalkanes. Diazirine is a special case and undergoes decomposition via an identifiable carbene, with evidence in some cases for the intermediacy of a linear diazoalkane. The infrared spectra of phenylbromocarbene and phenyltrifluoromethy1-carbene, generated by photolysis of the corresponding diazirines 

Carbene rearrangement products and insertion products are formed on photoelimination of nitrogen from a phenylmethyl- 

3H-Pyrazoles readily undergo photochemically induced elimination of nitrogen to yield the corresponding cyclopropenes, often by way of detectable vinyl diazo intermediates. The first example of a cyclopropapyridine (29) has been obtained in this 

Photolysis of the bipyrazole (12) in the presence of Rh6(CO)i6 results in smooth loss of N2. The cyclopropene derivative (13) can be isolated after short reaction times, but this loses a second N2 molecule on further irradiation, to yield 2,7-dimethyl-3,6-diphenylocta-2,6 dien 4-yne as the main product. A series of biaryl-5-morpholinotriazolines (14 R = Me, Et, Pr, Ph Y = CH, N) have been synthesized. On direct photolysis they lose N2 and produce the phenanthridines (15 Y = CH) or analogues (15 Y = N). Better yields of these products are obtained, however, by first inducing loss of N2 thermally, followed by photocyclization of the resulting intermediate amidines. 

3 Elimination of Nitrogen from Diazo Compounds and Diazirines 

Reviews on laser flash photolysis studies of arylhalocarbenes and triplet carbenes and on the solution photochemistry of carbenes and biradioals have been published. 

Alkyl radicals are conveniently generated by photodecomposition of azoalkanes. A kinetic study of the rearrangement of cyclopropyl radicals (1), obtained by photolysis of the azoalkanes (2), has been described. Particularly high radical concentrations leading to a dramatic change in the product ratio of tetrakis(pentafluoro-ethyl)hydrazine and perfluorobutane have been achieved in the pulsed laser excitation of perfluoroazoethane. On photolysis, azosulphonates also function as radical traps, with the formation of hydrazyl radicals as reactive intermediates.  

The photochemistry of cyclic azoalkanes continues to attract attention. Diazirines are unique in their behaviour as elimination of nitrogen leads to the formation of carbenes, a process which is increasingly being employed for the generation of such species under mild conditions. 

Adamantylidene, generated by laser flash photolysis of ada-mantyldiazirine (3) in a variety of solvents, reacts much more slowly with typical scavengers than does homocubanylidene. The equilibrium constant for the interconversion of homocub-1(9)-ene 

Cyclopropane derivatives are the major products of photoelimination of nitrogen from 1-pyrazolines. Conversion of the fused pyrazoline (10) into the cyclopropane (11) was achieved in this way in 85% yield by irradiation in the presence of acetophenone and constitutes a valuable step in a synthesis of a diquinane alcohol. The short-lived triplet 1,3-cyclopentadiyl biradical, generated by benzophenone-sensitised irradiation of diazabicyclo-(2.2.1]hept-2-ene, has been trapped as a Jbis-alkoxyamine by a nitroxide. The diazabicycloheptenes (12) gave in a similar fashion the bicydo[2.1.0]pentanes (13) on triplet-sensitised photolysis, whereas laser/liquid jet excitation of the same compounds gave in addition the cyclopentenes (14), derived by 1,2-hydrogen shift evidence for a two-photon process is described. The results of a time-resolved spectroscopic study of the photodecomposition of 2,3-diazabicyclo[2.2.l]hept-2-ene in the vapour phase have also been reported.Photolysis of 2,3-diaza-5-methylenebicyclo[2.2.l]hept-2-ene affords the semi-localised 

AT-Cbz of AT-Fmoc derivatives under neutral conditions by photolysis of mixtures of the amines with the caged protecting groups. 

In a search for a photochemical source of difluorocarbene, 4,4-difluoro-pyra-zolidine-3,4-dione (4) was synthesized as a precursor for the corresponding pyrazolinedione (5). The carbene CF2 was expected to be produced by photoelimination of N2 and CO from (5). Upon oxidation of (4), however, the azo compound (5) turned out to be too unstable to be isolated, although it could be trapped as a cycloadduct with cyclopentadiene. 

Z- and E-isomers, whereas photodecomposition ( 365 nm) occurs only from the Z-isomer and yields products (2)—(5). 

As in previous years, much time and effort have been devoted to the study of the photochemistry of cyclic azo-compounds. Photoreactions of this type offer a unique opportunity to examine the mechanism of this decomposition, and also provide a useful synthetic route to cyclopropane derivatives. Irradiation of several steroidal 4 S,5-dihydro[17a, 16-c]pyrazoles leads exclusively by elimination of nitrogen to cyclopropa[16a,17a] compounds.6 The penicillin derivatives 

The direct and sensitized photolysis of the homopyrazoline (10) and its possible role as an intermediate in the decomposition of bis-1-pyrazolines have been 

Franck-Neumann, D. Martina, and C. Dietrich-Buchecker, Tetrahedron Letters, 1975, 1763. 

8-diene.le A diallylic 1,3-biradical has been proposed as an intermediate in the photodecomposition of a mixture of cis- and trans-3, 5-divinyl-1 -pyrazolines (25) and (26) which gives fram-l, 2-divinylcyclopropane (27) and cyclohepta-1,4-diene (28), the latter derived either from the thermally unstable cw-isomer (29) or 

Department of Pure and Applied Chemistry, University of Strathclyde, Thomas Graham Building, 295 Cathedral Street, G1 IXL, Glasgow, UK 

Two short historical reviews of the development of flash photolysis in George Porter s laboratory at Cambridge have been published, one dealing with applications to gas-phase systems, the other studies of triplet states and free radicals in solution. A comparison of theories of reversible dissociation of molecules following photoexcitation has been made on the basis of integral kinetic equations. A number of different theories were brought to the same integral form. 

For quite a number of years now, the study of photo-induced nitrogen elimination from azoalkanes and analogues has appeared to be the exclusive 

Further experimental corroboration of the intermediacy of diazenyl diradicals, such as (2), has been provided by an investigation of the viscosity 

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The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

QUINONE METHIDES FROM BENZYLIC PHOTOELIMINATION 1.2.1 Photoelimination of Fluoride... 

Application of this technique to a study of the photoelimination of azo compounds has been reported by Porter, Landis, and Marrett.aw Photolysis of the unsymmetrically substituted azo compound (1) in solvents of varying viscosity revealed a dependence of on solvent viscosity as shown in Table 11.2. Photolysis of optically active (1) (40% completion) and examination of the remaining azo compound indicated that 26% of the original optical activity had been lost. This is explained by the following mechanism involving stepwise homolysis ... 

Photoelimination of nitrogen from diazoketones is complicated by Wolff rearrangement of the intermediate carbene, as shown below for diazoaceto-phenone<35) ... 

Other sulfur compounds are known to undergo photoelimination and fragmentation reactions(85 6S) ... 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

This chapter contains discussions of photoelimination, photoaddition, and photosubstitution. Although there may appear to be some degree of overlapping between the first two topics in that the species produced by photo-elimination may undergo addition to another substrate, our approach will be to concentrate on the reactions brought about by light absorption rather than subsequent dark reactions. 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

Compounds containing nitrogen-nitrogen double bonds exhibit extreme photoreactivity due to the stability of the photoelimination product, nitrogen gas. In fact, the photodecomposition of azo compounds occurs so readily that... 

Irradiation of acetone or methanol solutions of this compound at — 50°C, however, yields the cis photoisomer, which upon warming (0°C), decomposes to yield photoelimination products. This is taken as evidence that the major path at room temperature involves photoisomerization of the trans to cis isomer and rapid thermolysis of the latter. Similar behavior was observed for azobis(isobutyronitrile), azobis(cyanocyclohexane), and azobis(2-methyl-propyl acetate).(4>... 

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