Diazo compounds matrix isolation

On the contrary. Chapman and McMahon found that H-transfer in o-tolylcar-bene can be directly observed at very low temperatures, through several different spectroscopic methods. Irradiation of Ar matrix isolated diazo compound 50 at 4.2 to 10 K gave triplet carbene 51, which could be characterized by EPR, IR, and UV/VIS spectroscopy. The various spectra of 51 slowly decayed at temperatures as low as 4.2 K (tin ca. 64 h), and o-xylylene (52) could be observed to grow correspondingly... 

The anthronylidenes lp-lr are easily matrix-isolated by photolysis of the corresponding diazo compounds 2 (Scheme 17).99 The reaction of the carbenes with molecular oxygen results in the formation of anthraquinone 0-oxides 7,... 

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). 

TABLE 3. Matrix isolation of silenes R R2Si=CR3R4 by photolysis of (silyl)diazo compounds according to Scheme 2. 

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). 

Photolysis of matrix isolated diazo(2-furyl)methane led to the aldehyde (79) by stereospecific rearrangement of the carbene (80). " The corresponding 3-furyl compound gave the (5-Z)-methylenecyclopropene (81) by ring closure of the initially formed vinylcarbene to give cyclopropene (82) followed by ring opening of the furan. 

Regitz and co-workers have reported the matrix isolation of 1//-azirines generated by the photolysis of diazo compounds, as shown in Scheme 82 (81BSB615). 

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. 

Arynes are novel reaction intermediates that react with dienes or 1,3-dipoles to give the corresponding cycloadducts. Recently, many researchers have reported the reaction of benzyne prepared from 2-(trimethylsilyl)phenyl triflate or benzenedia-zonium carboxylate with imines, aminobenzoate, 2-aminobenzophenones, azides, and diazo compounds, which provides various N-containing cycloadducts, such as acridines, acridones, triazoles, and indazoles. Formally, the reaction proceeds in a [2-1-2], [3-1-2], or [4-1-2] manner. Benzyne is an extremely reactive species because of the presence of a strained triple bond and undergoes polar and pericycHc reactions. The lifetime of benzyne in the gas phase has been estimated to around 20 ns (2 X 10" s) by mass spectroscopic techniques. Some spectroscopic properties of benzyne have been determined by Orville Chapman using matrix isolation techniques (Scheme 7.20). 

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. 

---