Diazirines matrix isolation

The great majority of matrix isolation studies of carbenes and nitrenes have employed their formal adducts with molecular nitrogen, that is, diazo compounds or diazirines in the case of carbenes, azides in the case of nitrenes, as precursors for their in situ generation. Usually, these compounds will readily release N2 on irradiation with a low-pressure mercury lamp (254 nm), and this fragment has the advantage that it will usually not react with or perturb the targeted reactive intermediate (see Scheme 17.2). 

Fluorodiazirines have also been used extensively as carbene photoprecursors for cryogenic matrix isolation " and laser flash photolysis studies. Photolysis of fluorophenoxydiazirine has made observation of the corresponding carbene directly observable in yet another fashion encapsulated in a hemicarcer-and. Diazirines have been used for studies in solution of carbene philicities as well as for synthesis of cyclopropanes and cyclopropenes.Of course, diazo compounds can also serve as carbene photoprecursors, but for carbenes such as difluorocarbene, the diazo precursor is either nonexistent or extremely unstable. 

The only route to carbonyl O oxides 1 which is suitable for matrix isolation is the oxidation of free carbenes 2 with molecular oxygen 02 [5-7]. The carbenes are generated by photolysis of the corresponding diazo compounds or diazirines. Prerequisite for this method is that the carbenes are stable under the conditions of matrix isolation (solid rare gas at 10 K). This excludes simple alkyl substituents, because these carbenes rapidly undergo [1,2]-H shifts to yield alkenes in essentially quantitative yields. During the last years we were able to study the spectroscopy and chemistry of a number of carbonyl oxides (R and R were H, Ph, HC=C, CF3, cyclic systems etc.) [8-12] by IR and UV/vis spectroscopy. 

A prominent strategy in matrix isolation photochemistry has been the expulsion of N2 from stable precursors. Both diazo/diazirine precursors and 1,2-diazene precursors have been useful. The former generate carbenes and related species, while the latter are useful precursors to biradicals. 

Irradiation of diazoketones (reactions 8.7 and 8.8) [17-19], diazoesters (reaction 8.9) [20], and various other diazoalkanes (reactions 8.10 and 8.11) [21,22] as well as diazirines (reactions 8.12 and 8.13) [23,24] under matrix isolation conditions also led to the detection of the corresponding carbene intermediate, which could be trapped with carbon monoxide in the form of ketenes. For example. 

Recently, Moore and Pimentel have photolyzed diazirine isolated in a solid nitrogen matrix. They found that the photolysis did produce diazomethane. In order to decide whether the diazomethane arose by a photoisomerization reaction or by the reaction of methylene with the nitrogen of the matrix, experiments were carried out in a nitrogen matrix enriched in N. On the basis of these experiments they conclude that the photolysis of the diazirine produces methylene and nitrogen, and that the diazomethane arises from the reaction of the methylene with the matrix and not by photoisomerization. Photolysis of diazomethane does not... 

The photochemistry of 2-furaldehyde isolated in cryogenic matrix has been studied both experimentally and computationally, identifying the role of various conformers. A versatile assembly of photoaffinity scaffolds well suited for small molecules probes has been reported and is based on a four-component Ugi reaction between diazirine, arylazides and benzophenone. ... 

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