Photoelimination of

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) ... 

Azides are known to undergo photoelimination of nitrogen to produce univalent nitrogen intermediates (nitrenes). These electron-deficient intermediates can then undergo a variety of intermolecular and intramolecular... 

Numerous examples of the photoelimination of C02 have been re-ported.al) Some examples are shown below(69 72) ... 

Evidence from nanosecond flash photolysis studies indicates that an oxa-ziridine precursor to the diazoketone is unlikely. Photoelimination of nitrogen is also observed in 4-methyl-1,2,3-benzotriazine 3-oxide and affords 3-methylanthranil a mechanism involving loss of nitrogen from an intermediate oxaziridine has been proposed.74... 

Cyclization arising from the intramolecular photoelimination of HC1, HBr, and HI has been extensively used in the synthesis of heterocycles and alkaloids. The mechanisms of these transformations have not in many cases been thoroughly investigated. Some undoubtedly are initiated by simple homolysis of the carbon-halogen bond whereas others involve photocycliza-tion and subsequent elimination of HX. 

Representative of recent applications of the reaction to the synthesis of heterocycles are the photodehydrochlorination of chlorobenzo[b]thiophen (347) to give the fused pyrimidone 348,287 the photoelimination of HI from iodobenzene derivatives 349 to give the benzazepines 350,288 and the synthesis of the medium ring aza-heterocycle 351 by irradiation of the chloro precursor 352.289 Included among the many other examples of... 

The intramolecular photoelimination of HC1 from JV-chloroacetyl derivatives of suitable amines is a useful and versatile approach to the synthesis of azaheterocycles. The iV-chloroacetyl derivative 357 has been converted in this way to 7-oxodesethylcatharanthine (358) in 55% yield.300 Investigations in this area have been particularly concerned with the A-chloroacetyl derivatives of benzylamines and phenethylamines the N-chloroacetyl-benzylamine 359 on irradiation affords the two 3-oxo-l,2,3,4-tetrahydroiso-quinolines 360 and 361.301 Competing photocyclizations have been observed in the case of l-[3-(chloroacetylamino)propyl]-3-methylindole (362) which is converted into three photoproducts, 363, 364, and 365.302... 

The photoelimination of nitrogen from azocycloalkanes is of interest both from the synthetic and mechanistic point of view. Acyclic azoalkanes appear to undergo elimination of nitrogen by a stepwise process involving an intermediate diazenyl radical, but the photoreactions observed in azocycloalkanes are to some extent dependent on ring size. 

Photoelimination of nitrogen from diazirines, for example, proceeds via carbene intermediates. 3-tert-Butyldiazirine (395) is converted into the cyclopropane 396 and the alkene 397 with the formation of the carbene insertion product being favored from the singlet state.328 3-Cyclopropyl-3-chlorodiazirine (398) has similarly been converted to the carbene 399 which undergoes both rearrangement to l-chlorocyclobutene(400)329 and addition... 

Photoelimination of nitrogen from 3,3,4,4-tetramethyldiazetine has been shown to proceed with a quantum efficiency of 0.52,332 but no systematic study of the photodecomposition of such compounds has been undertaken. The fused diazetine 403 is converted on irradiation into norbornadiene (404).333... 

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

Photoelimination of nitrogen from 1-pyrazolines has also been employed in the synthesis of tricyclo[3.2.1.02,4]oct-6-ene,338 prismane,339 quadri-cyclane,340 snoutene ,341 and marasmic acid.342 The trimethylenemethanes 414 have been prepared by photolysis of azoalkanes 415 and characterized spectroscopically.343 Dimerization and cycloaddition to alkenes of these biradicals have been reported.344... 

Numerous reports of the photoelimination of nitrogen from spiro-3//-pyrazoles have been published.348 The diaza[2,2]spirenes 420 were converted into the benzocyclopropenes 421 by a pathway which has been shown not to involve intermediate indazoles.349 3f/-Indazoles undergo analogous photoreactions.350... 

Evidence for the formation of thiiren by photoelimination of nitrogen from 1,2,3-thiadiazoles has been described,351 and several thiirens, prepared in this way, have been identified in an argon matrix at 8K.352 Phenylthiazirine (422) appears to be an intermediate in the related transformation of 5-phenyl-l,2,3,4-thiatriazole (423) into benzonitrile sulfide (424),353 and further... 

Photoelimination of nitrogen from 1,2,3-triazolines has been widely used as a synthetic route to aziridines the reaction has been reviewed.355 Recent applications include the formation of a new valence isomer (425) of azepine from the triazoline 426,356 and conversion of the triazoline 427 into the aziridine 428, a process with potential as a synthetic route to mitomycins.357... 

Photoelimination of nitrogen from the tetrazole 429 results in the formation of the dihydropyrazole 430, presumably via intramolecular addition of the photochemically generated nitrile imine 431.358 Other examples of this type of behavior have been reported.359... 

The photoelimination of nitrogen from diazo compounds provides a simple and versatile route for the generation of carbenes, and in certain instances, insertion reactions of carbenes can be employed in the synthesis of heterocycles. Carbenes are believed to be involved at least in part in the photochemically induced conversion of N,N-diethyldiazoacetamide (439) into the y-lactam 440 and the /Mactam 441,365 and a similar approach has been successfully employed in the synthesis of a carbapen-2-em366 and of 7-methylcephalosporin analogues.367 Carbene insertion of a different type has been observed on irradiation of the 6-anilino-5-diazouracils 442 to give the indolo[2,3-d]pyrimidines 443.368 Ring contractions in heterocycles... 

Aryl and heteroaryl nitrene insertion processes have also been employed in the synthesis of heterocycles. The azide 459, for example, is converted on irradiation into the imidazole 460,383 and dihydro-10-thiaisoalloxazines are obtained in good yield on photoelimination of nitrogen from 6-(2-azido-phenylthio)uracils.384... 

The photoelimination of carbon dioxide from esters and lactones is a process that has been the subject of detailed investigations. Discussion here is limited to nitrogen containing systems. 3,4-Diphenylsydnone (464), on irradiation in benzene, is converted via the nitrile imine 465 into 2,4,5-triphenyl-1,2,3-triazole (466)388 initial bond formation between N-2 and C-4 followed by loss of carbon dioxide to give the diazirine 467 is proposed to account for the formation of the nitrile imine. Nitrile imines generated in this way have been trapped with alkenes and alkynes to give pyrazoles389... 

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