Diazomethane flash photolysis

Flash photolysis of benzoyl and naphthoyl diazomethane, which should exist in the s-cis conformation, led to ketene intermediates within the duration of the pulse ( 20 ns).241... 

Flash photolysis studies<22) have indicated singlet methylene to be produced from the diazomethane-excited singlet upon loss of nitrogen followed by collisional deactivation to the triplet, the ground state multiplicity for this molecule. 

The term methylene is exclusively reserved for CH2. Spectroscopic investigations demonstrated that photolysis of diazo compounds in fact produces free carbenes. A flash photolysis of diazomethane gave methylene whose spectrum could be recorded 3i>. ESR-spectra were taken of a series of triplet carbenes which had been obtained by direct irradiation of diazo compounds in various matrices at low temperatures (see p. 97). 

Confirmation was provided by the observation that the species produced by the photolysis of two different carbene sources (88 and 89) in acetonitrile and by photolysis of the azirine 92 all had the same strong absorption band at 390 nm and all reacted with acrylonitrile at the same rate (fc=4.6 x 10 Af s" ). Rate constants were also measured for its reaction with a range of substituted alkenes, methanol and ferf-butanol. Laser flash photolysis work on the photolysis of 9-diazothioxan-threne in acetonitrile also produced a new band attributed the nitrile ylide 87 (47). The first alkyl-substituted example, acetonitrilio methylide (95), was produced in a similar way by the photolysis of diazomethane or diazirine in acetonitrile (20,21). This species showed a strong absorption at 280 nm and was trapped with a variety of electron-deficient olefinic and acetylenic dipolarophiles to give the expected cycloadducts (e.g., 96 and 97) in high yields. When diazomethane was used as the precursor, the reaction was carried out at —40 °C to minimize the rate of its cycloaddition to the dipolarophile. In the reactions with unsymmetrical dipolarophiles such as acrylonitrile, methyl acrylate, or methyl propiolate, the ratio of regioisomers was found to be 1 1. 

Absorption Spectrum. Herzberg62-64 has observed two absorption systems of methylene in the flash photolysis of diazomethane in the presence of an inert gas. These features do not appear when the inert gas is absent. (See Sec. III-A for further discussion.)... 

The radical CH2, with one electron less than NH2 (i.e., with no electrons in the lone pair orbital bj), might be expected to have ground and first excited states similar to NH2. Indeed, by using the flash photolysis of diazomethane (CH2N2), an absorption spectrum in the red region of the spectrum very similar to that of NH2 has been found.8,9 The observation of isotope shifts on substituting 13C or D, the presence of an intensity alternation, and the detailed analysis of the... 

Various transitions in the vacuum ultraviolet have been observed by Herzbcrg and Johns (467). The CH molecule dissociates by absorption of light below 3200 A. The CH absorption has been observed in the flash photolysis of diazomethane in the near ultraviolet by Herzberg and Johns (467). 

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-noradamantyl(phenyl)diazomethane. This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with O2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable /er -butylaminoxyl and Ullman s nitronyl nitroxide radicals, e.g. (25), have been prepared by photolysis of the parent diazomethanes. Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected. 

The reactions of [W(N2)2(diphos)2] with RBr are clearly catalyzed by visible light. The homolytic fission of the R— Br bond that takes place at the metal center is preceded by the loss of one N2 molecule. The resulting C—bond is formed by an alkyl radical attack on the remaining coordinated dinitrogen. Product distribution in these photocatalyzed reactions depends on the solvent and the stability of the free radical. This mechanism is strongly supported by flash photolysis experiments. When gem-dibromides are used in the photo-catalyzed reaction, diazoalkanes are produced. With CH2Br2 for example, the diazomethane complex [WBr(diphos)2(N2CH2)]Br is obtained. More recently it has been shown that some of the diazoalkanes do not react with protonic acids, but that the unique carbon atom is attacked by nucleophiles such as MeLi to yield diazenido complexes. ... 

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. 

Leigh, W.J., Kerst, C., Boukherroub, R., Morkin, T.L., Jenkins, S.L, Sung, K., and Tidwell, T.T., Substituent Effects on the Reactivity of the SiUcon-Carbon Double Bond. Substituted 1,1-Dime-thylsilenes from Far-UV Laser Flash Photolysis of a-Silylketenes and (Trimethylsilyl)Diazomethane, J. Am. Chem. Soc., 121, 4744, 1999. 

Braun, W.A., Bass, A.M., and Pelling, M., Flash photolysis of ketene in diazomethane the production and reaction kinetics of triplet and singlet methylene,/. Chem. Phys., 52, 5931, 1970. 

The first carbene ever isolated was la, which was prepared using the most classical route to transient carbenes, namely, the decomposition of diazo compounds. The [bis(diisopropylamino)phosphino](trimethylsilyl)diazomethane precursor (la) was obtained by treatment of the lithium salt of trimethylsilyldiazomethane with 1 equiv of bis(diisopropylamino)chlorophosphine. Dinitrogen elimination occurs by photolysis (300 nm) or thermolysis (250 °C under vacuum) affording carbene la as a red oily material in 80% yield (Scheme 8.1). Carbene la is stable for weeks at room temperature and can even be purified by flash distillation under vacuum (10-2 Torr) at 75-80 °C. 

Two band systems of methylene have been observed by photolysis of diazomethane in presence of excess nitrogen (Herzberg, 1961 Herzberg and Shoosmith, 1959). The radical is short-lived (no absorption is detected 50 psec after the photolysing flash) and appears to be formed in an excited state from which it decomposes unless deactivated by collisions with the inert gas. 

The hitherto elusive nitrilimine (13) (Scheme 5) has been generated in noble gas matrices by photolysis of the tetrazole (12), or 1,2,3- or 1,2,4-triazole, and also by flash pyrolysis. On further photolysis, nitrilimine is transformed into other CH2N2 isomers, including diazomethane, and a novel complex between NH and HCN. The identification of the species involved was aided by isotopic substitution and high-level ab initio calculations. 1,3-Diphenylnitrilimine, generated by photolysis or thermolysis, undergoes 1,3-dipolar cycloaddition to [60]fullerene, forming two or more adducts with A -pyrazoline structures. " ... 

Bis(diisopropylamino)phosphanyl(trimethylsilyl)diazomethane 6, easily available by treatment of chlorobis(diisopropylamino)phosphane with lithiated diazo(trimethylsilyl)methane, provides upon flash thermolysis at 250"C the so-called stable carbene 7.38,39,40 behaves partly as a nucleophilic carbene and reacts only with electron-deficient alkenes such as methyl propenoate or diethyl fumarate under cyclopropanation." In the former case only the Z-isomer 8 is formed. Cyclopropane 9 is thermally unstable and is, therefore, oxidized in situ at the phosphorus atom with elemental sulfur to provide cyclopropane 10, Cyclopropanes 8 and 9 are also generated from the diazo compound 6 and the appropriate alkene by photolysis." ... 

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