Diazo-compounds, photoelimination

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... 

The photoelimination of nitrogen from azo compounds, diazo compounds, and azides is a topic that has been thoroughly reviewed elsewhere. These transformations are found in oxygen and sulfur heterocycles, but as they are not unique to these systems, they will not be discussed in this article. Examples can be found in a review on the photochemistry of nitrogen-containing heterocycles.1... 

Photochemically produced small atoms (H, C, N, O, F), co-deposited with matrix-isolated substrates, led to a wide variety of unstable molecules and radicals. This synthetic route is very efficient if the substrate itself acts as a matrix material (O2, CO, CH4). Apart from these reactions, photo-induced decomposition and isomerization processes of matrix-isolated molecules can be studied by vibrational spectroscopy. Photoelimination of small, thermodynamically stable molecules (N2, CO2, CF4, HF,...) from a precursor is the most important photoprocess for generating unstable molecules in low-temperature matrices. Photochemical loss of dinitrogen from azido and diazo compounds is frequently observed in matrices. One example is that of azido halides (Milligan and Jacox, 1964). 

Carbenes are known intermediates in the photodecomposition of diazo-compounds. Insertions of singlet methylene, generated by photoelimination of nitrogen from diazomethane, into 2,3-dimethyl-2,3-epoxybutane and into acetonitrile have been described. Methylene and other photochemically generated carbenes (43) add efficiently to 2-acetoxyacrylonitrile to give the corresponding 1-acetoxy-l-cyanocyclopropanes (44). The formation of methylidyne radicals by irradiation of diazomethane in the presence of hydrogen, however, does not seem to involve an intermediate carbene. ... 

The photoelimination of nitrogen from diazo-compounds provides a... 

Diazo-compounds can in turn be generated photochemically from sodium salts of toluenesulphonyl hydrazones. Irradiation of the cyclobutane tosylhydrazone (44) gave trans-tricyclo[5.1.0.0 ]-octane (45 R=Me) by the pathway shown in Scheme 4 44 the tricycle (45 R=H) can also be obtained by photoelimination of nitrogen from the diazatricyclo[ 5.2.1.0 4]decene (46). The unexpected conversion of the anthracenocycloheptatriene derivative (47) into triptycene with loss of one carbon atom was observed on irradiation in tetrahydrofuran 4 the mechanism of this unusual reaction remains obscure and merits further investigation, but neither 1-triptycenyl nor 2-triptycenyl carbene appears to be an intermediate in this transformation. 

Carbenes can conveniently be prepared by photoelimination of nitrogen from a wide range of diazo-compounds. Investigations have been reported on the effect of solvent on the selectivity... 

The photodecompositions of 3,3-bis(l,l-difluorohexyl)diazirine and 3-(l,l-difluoro-octyl)-3//-diazirine have been examined. On irradiation in cyclohexane or methanol, competing photoisomerization to linear diazo-compounds and photofragmentation via carbene intermediates to cyclopropanes and alkenes has been observed and is illustrated for 3,3-bis(l,l-difluorohexyl)diazirine (6) in Scheme 1. The majority of investigations of nitrogen photoelimination in cyclic... 

Photoelimination of nitrogen from diazo-compounds is a simple and versatile process for the generation of carbenes. The diazoadamantane (65) is a surprisingly stable compound which on irradiation yields the hydrocarbon (66), the azine (67), and the alcohol (68) via the carbene. The origin of the oxygen atom... 

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. 

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