Diazomethane matrix photolysis

The comparatively small size of the simplest carbene (methylene) ensures that it has a definite mobility in frozen inert matrices, which leads to the formation of dimerization products under these conditions. It became possible only in 1981 to detect in the spectra of the diazomethane photolysis products bands at 1115 cm (Ar matrix) and 1109 cm (Xe matrix) which were attributed to the deformation vibration of methylene in its ground triplet state (Lee and Pimentel, 1981). 

The ESR spectrum of methylene in a xenon matrix (4 °K) was studied by Wasserman Photolysis of diazomethane or diazirin gave anESR-signed which persisted up to 20 °K but disapj)eared above 77 °K. 

In arylcarbenes not only the internal bond angle but also the dihedral angle between the bond plane and the aryl plane can be bent. Photolysis of a- and /S-naphthyl-diazomethanes in a matrix gave two isomeric carbenes, which produced different ESR signals >. 

Photolysis of matrix-isolated (trimethoxysilyl)diazomethane at X > 305 nm produced carbene 3e (equation 1), which was characterized by IR and UV-Vis spectroscopy23. Under these conditions, no other species besides the carbene could be detected spectroscopically. In an 02-doped (1%) argon matrix, the carbene rapidly reacted with oxygen to give carbonyl oxide 4 which was further photoisomerized to formylsilane 5. Again, the fast reaction of 3e with 302 points to a triplet ground state of the carbene. 

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 attempted elimination of five-membered heterocycles from the adducts formal from phenyl azide or diazomethane and hexakis(trifluoromethyl)benzvalene was unsuccessful. The photolysis of the ozonide of the benzvalene at room temperature gives the dimer of tetrakis(trifluoromethyl)cyclobutadiene. When the irradiation is carried out in a matrix at —196 °C, the reaction mixture was coloured yellow. The... 

Direct observation of the carbonyl ylide (49) at 10 K in an argon matrix has also been reported the ylide is obtained by direct photolysis of (o-carbomethoxyphenyl)diazomethane (50) and undergoes reversible photoisomerization to the oxirane (51) as shown in Scheme 4. Competing intramolecular and intermolecular 0-H bond insertion pathways have been observed on analogous photochemically induced generation of [2-(hydroxymethyl)-... 

In order to estimate the stability of triplet carbenes (19) under ambient conditions, laser flash photolysis ( LFP) [26] was carried out on the precursor diazomethanes (18) in solution at room temperature. The transient absorption bands formed upon the flash were recorded by a multi-channel detector. These bands were assigned to the triplet carbenes (19) by comparison with those obtained in matrix at low temperature. The kinetic information was then available by monitoring the decay of the transient absorption with oscillographic tracer. When triplet carbenes decayed unimolecularly, which is often so, lifetime (x) can be determined. However when the decay did not follow a single exponential, which is sometimes the case, x cannot be determined. In this case, a half-life (ti/2) is estimated from the decay curve as a rough measure of the stability. 

The reactive species, trimethylenemethane, has been observed only by photolysis of an adduct of allene and diazomethane in a matrix of hexafluorobenzene at — 185°C. The e.s.r. spectrum shows that it has two unpaired electrons (a triplet ground state) which is consistent with a planar structure with 3-fold symmetry (Djh). This species can, like cyclobutadiene, be stabilized by coordination to tricarbonyliron. The parent compound, a diamagnetic pale yellow solid, m.p. 28°C can be obtained by the two routes shown. 

Methylene (CHj) was generated from diazomethane in some of the earhest matrix studies of organic reactions, and its dimerization to ethene was observed, although its IR spectrum proved somewhat elusive.Various halomethylenes were also observed among the products from the vacuum-UV irradiation of halomethanes in matrices. Examples are CHF from photolysis of methyl fluoride and CClj from dichloromethane. ... 

Matrix investigations of sulfur and phosphorus analogues of acarbonylcarbenes have also been reported. For example, diphenylsulfene (66) and diphenyl-a-sultine (67) were isolated in low-temperature matrices following the thermal reaction of SO2 and diphenylcarbene (65) and the stepwise photolysis of phenyl(phenylsulfonyl)diazomethane (68), as summarized in Scheme 8. The ultimate photoproduct was benzophenone (70). 

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