Photolysis of ethyl azidoformate

Surprisingly, in view of the use of hexafluorobenzene as an inert solvent for (ethoxycar-bonyljnitrene insertions into alicyclic C —H bonds,149 the thermolysis, or photolysis, of ethyl azidoformate in an excess of hexafluorobenzene yields hexafluoro-1//-azepine 7.150... 

The ring expansion of arenes by electron-deficient singlet nitrenes is by far the most versatile synthetic route to 1H -azepines. The first l//-azepines were prepared independently in 1963 by Hafner, and by Lwowski, and their coworkers. They found that ethoxycarbonyl-nitrene (Scheme 26, path a R=C02Et), generated by photolysis of ethyl azidoformate, adds to benzene to give initially the unstable azanorcaradiene (227), electrocyclic ring... 

Behavior of Carbethoxynitrene toward Polyisoprene Structures. After studying the reaction of a number of divalent carbon derivatives with polyenes (17, 18), attention was directed to monovalent nitrogen derivatives. Of these, carbethoxynitrene, N—C02Et, gives addition reactions with olefins (12, 13). It can be produced either by photolysis of ethyl azidoformate (12), or by a-elimination (13) (Figure 5). 

A single example of the formation of an oxaziridine by nitrene addition to a carbonyl group was found in the photolysis of ethyl azidoformate in the presence of excess acetone (for about 40 hr), producing oxaziridine 13. The reaction failed to give the corresponding oxaziridine when acetone was substituted by cyclohexanone. ... 

This reaction should be seen in marked contrast to the more difficult photolysis of ethyl azidoformate, which requires low-wavelength light and quartz equipment, suggesting that the ylid is likely to prove a valuable precursor of the nitrene. The photolysis of 60 (R = C02Ph or S02CgH4Me ) proved them to be less efficient nitrene sources (87JCS(P1)1553). 

The 1,3,4-oxadiazole system is formed on cycloaddition of carbo-alkoxy nitrenes to nitriles. The photolysis of ethyl azidoformate in acetonitrile yielded 2-ethoxy-5-methyl-l,3,4-oxadiazole (210) (yields 52-60%) With benzonitriles yields were much lower . 

Proof for the triplet character of the hydrogen abstracting species is obtained in sensitization experiments. Thus, direct photolysis of ethyl azidoformate in cyclohexene results in the formation of only 3% of the hydrogen abstraction product (urethane) and a trace of 6w-cyclo-hexene -. In the sensitized photoreaction, where triplet nitrene is produced directly, the urethane yield increases to 74%, the yield of Aw-cyclohexene to 63%. 

Irradiation of ethyl azidoformate (66) at 2537 A in cyclohexene results in five identifiable products 7-ethoxycarbonyl-7-azabicyclo [4.1.0]-heptane (67) (50%), iV-2-cyclohexenylurethane (68) (9%), JV-3-cyclohexenylurethane (69) (3%), urethane (70) (3%) and bi(cyclohex-2-enyl) (71) (1-7%). Photolysis of ethyl azidoformate... 

All these products are compatible with the formation of a nitrene, yet on their own they do not constitute conclusive proof for the real existence of a discrete intermediate. It was only when Lwowski was able to show that the nitrene produced by a-elimination from N- -nitrobenzenesulphonoxyurethane (74) led to the same products as the photolysis of ethyl azidoformate that the evidence for a photolytic nitrene mechanism became entirely convincing, the more... 

The photolysis of ethyl azidoformate (66) in aromatic solvents leads to the formation of azepines C75). Toluene, xylene, mesitylene... 

Photolysis of ethyl azidoformate (66) in alcohols leads to urethanes as the main products Thus JV-f-butoxyurethane (80) is produced... 

Oxaziridine syntheses by nitrene addition to a carbonyl group, which in the past had failed repeatedly, was described recently by Japanese authors to occur on photolysis of ethyl azidoformate in acetone. It yielded N-ethoxy-carbonyloxaziridine 53. ... 

The asymmetric addition of ethyl azidoformate (N3C02Et) to an optically active enam-ine, prepared from cyclohexanone and (S )-2-pyrrolidinemethyl methyl ether, followed by photolysis produces 2-(ethoxycarbonylamino)cyclohexanone with modest enantiomeric excess (18 %ee) in 40% yield159. Use of (S -pyrrolidinemethyl trimethylsilyl ether as a chiral auxiliary increases the steric hindrance and provides the same product with the highest value of %ee (35%) in 51% yield. 

The standard preparation of 2-oxazolidinones is by treatment of /3-amino alcohols with phosgene or its synthetic equivalents ethyl chloroformate, dialkyl carbonates or even urea (equation 174). Isocyanates react with oxiranes in the presence of amines to yield 2-oxazolidinones (equation 175) (79LA200). The action of isocyanates on cyclic carbonates results in 2-oxazolidinones (equation 176) and photolysis of alkyl azidoformates affords oxazolidinones via intermediate nitrenes (equation 177). 

Sheinker has studied the u.v. and i.r. spectra of azidoformates. The 285 nm band, seen in alkyl azides, was not found perhaps it disappears into the short wavelength band due to a blue shift. The u.v. spectrum of ethyl azidoformate extends to nearly 300 nm (see Table 2) but its photolysis is most conveniently carried out with the mercury resonance line near 254 nm, for which efficient lamps are available . Ethyl azidoformate shows i.r. absorption bands at 2185 and 2137 cm"i (Ng), 1759 and 1730 (C=0) and 1242 (0-0) cm i 235 -pjje y spectrum is given in Table 2. The i.r. and... 

The pyranofurooxazoline 109 can be prepared by a nitrene insertion reaction of the corresponding furan 110 upon treatment with ethyl azidoformate at — 50 °C under photolysis conditions. Compound 109 is moisture sensitive, and upon treatment with wet acidic THF was converted quantitatively to the more polar furanopyran 111. The structure and stereochemistry of 109 were proved unambiguously by X-ray diffraction, showing that the nitrene inserted anti to the bridgehead methyl group <1999JOC736> (Scheme 30). 

Ethoxycarbonyl nitrene is readily formed by thermolysis or photolysis of cthoxycarbonyl azide (ethyl azidoformate), as well as by the base-induced a-elimination of (4-nitrophenylsulfonyloxy)-carbamate9 13> 158> 159. 

As the two components do not react in the dark, a triazoline intermediate is not probable. Other A -carbethoxyaziridines were formed via carbethoxy nitrene when ethyl azidoformate was irradiated in dihydropyran and in enolacetates . The stereospecificity of the addition is high. The photolysis of cis and trans 2-butene at — 20° yielded mainly the cis (130) and trans (131) aziridines respectively This stereospecificity has been found to diminish upon dilution... 

In the photolysis of neat ethyl azidoformate (66) diethyl azodiformate (76) is produced in the early stages of the process, to be later replaced by the triethyl nitrilotriformate (77) which is produced in 58% yield A detailed investigation showed that the nitrilo compound (77) is apparently not formed by the action of a nitrene, and the reaction between an azide and an excited azodiformate was suggested as a possible pathway. Hancock has proposed an alternative mechanism which involves triazacyclopropene (see, however, Huisgen ). 

Photolysis of an acetonitrile solution containing ethyl azidoformate and 1-ethoxy-1-(trimethyl-siloxy)cyclopropane at room temperature for 48 hours gives an aminocyclopropane, which is... 

The insertion of nitrenes, generated via the photolysis or thermolysis of azides, into C-H bonds is well known, although the yields are generally poor. However, when ethyl azidoformate is thermolyzed in the presence of 1-chlorocyclohexene, the allylic insertion product is formed in 49% 3deld (eq 10). In this case it is not clear if the product is formed through direct insertion or by the ring opening of an aziridine intermediate. 

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