Diazomethane, 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). 

It will now be shown that the discrepancy can be resolved by considering the possibility of a more energetic methylene in the experiments of Bell and Kistiakowsky, who used diazomethane photolysis to generate methylene. Frey41 has shown that methylene from diazomethane may be more energetic than methylene from ketene by 10-15 kcal. 

The primary process in the near ultraviolet photolysis, in analogy ni> diazomethane photolysis, must be... 

Diazomethane is also used for methylenation under the conditions of diazomethane photolysis. This reaction was first described by Doering et al. [102] and used by Simmons et al. [103] and Dvoretzky et al. [104] to obtain mixtures of hydrocarbons. The methylenation of hydrocarbons is a valuable method for obtaining mbttures of hydrocarbons of specific composition which are used as standard mixtures in GC analysis. The methylenation reaction consists in the homolytic addition of methylene along the... 

Intramolecular attack of the carbenes shown in Scheme 30a provides benzo[6]cyclo-hepta[(5 ]-furans and -thiophenes, but the nitrogen analogue (X = NH) yields 9,10-dihydro-acridine 81AJC1037). Photolysis of 2-biphenyl isocyanide (Scheme 30b) (72JOC3571) and thermolysis of 2-biphenylsulfonyl diazomethane (Scheme 30c) (72CC893) also result in ring expansion. 

The photolysis of a-diazosulfones dissolved in alkenes provides sulfonyl-substituted cyclopropanes in high yields. This is exemplified by the preparation of l-(p-methoxyphenylsulfonyl)-2,2,3,3-tetra-methylcyclopropane in 75% yield from -methoxybenzenesulfonyl-diazomethane and 2,3-dimethyl-2-butene. A similar addition to [Pg.101]

The methyl ester (100, R = CH3), derived from this A-nor acid by treatment with diazomethane, is different from the ester (102) obtained either by Favorskii rearrangement of 2a-bromo-5a-cholestan-3-one (101) or by the action of cyanogen azide on 3-methoxy-5a-cholest-2-ene (103) followed by hydrolysis on alumina. The ketene intermediate involved in photolysis of (99) is expected to be hydrated from the less hindered a-side of the molecule to give the 2j -carboxylic acid. The reactions which afford (102) would be expected to afford the 2a-epimer. These configurational assignments are confirmed by deuteriochloroform-benzene solvent shifts in the NMR spectra of esters (100) and (102). ... 

Preliminary investigations on the formation of carbenes from diazirines have already been made available. Frey and Stevens recently reported the photolysis of cyclic diazomethane. Cyclic diazomethane was irradiated in the gaseous phase with light of wavelength... 

Diazomethane, 56,62 Diazomethyl p-tolylsulfone, 57,100 l-Diazo-l-phenylacetone,59, 69 or-Diazosulfones, 57, 97 photolysis of, 57,101 Diazo transfer by phase transfer catalysis, 59,66... 

Figure 4.2. TRIR difference spectra averaged over the timescales indicated following 266 nm photolysis of (a) diphenyl diazomethane (3.6mM) and (b) bis-(4-methoxyphenyl)-diazomethane (1.1 mM) in C02-saturated dichloromethane. Reprinted with permission from B. M. Showalter and J. R Toscano, J. Phys. Org. Chem. 2004,14,743. Copyright 2004, John Wiley Sons Limited.

The difference in structure between 16 and 17 is readily understood in terms of the addition of strongly electron-donating substituents, but the contrast between 16 and 20 is less easily rationalized. Photolysis of 19 was carried out in HFIP (dielectric constant (e) = 16.75), while TRIR experiments with diphenyl diazomethane (22) were carried out in dichloromethane (e = 9.08), suggesting that a-lactone structure may be dependent on solvent polarity. 

Thus, when acid 76 was crystallized as a salt with (S)-(-)-l-phenylethylamine ([S]-PEA), the X-ray structure showed that the conformational enantiomer 76a was trapped in the crystal, displaying O - H and O - Ht distances of 2.47 A and 3.41 A, respectively. The conformation of 76a placed the carbonyl oxygen and Hj, closer to the ideal values mentioned in Figure 7.26 as compared to H. A significant preference for Hj, was demonstrated after photolysis at 0 °C and diazomethane workup, when ester 77a (B) was obtained in 65% ee after 90% conversion. Figure 7.27 illustrates the minimal atomic displacements required for reaction by comparing the X-ray structure of the reactant with that of the product, and with a structure obtained at 50% conversion. Better chemical results were obtained by photolysis of 76a with (/ )-CEA, which gave 90% ee of ester of 77a (B) after diazomethane workup. 

Photolysis of the salts was followed by diazomethane worknp to yield the corresponding methyl esters. 

Owing to the high reactivity of the intermediates involved, intermolecular carbene insertion reactions are not very selective. The distribution of products from the photolysis of diazomethane in heptane, for example, is almost exactly that expected on a statistical basis.211... 

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... 

The nature of the photochemically produced methylene has been the subject of considerable study. Evidence tends to indicate that this species is produced in its singlet state photochemically. Photolysis of diazomethane in the gas phase or in solution in the presence of excess cis- or trans-2-butene produces cyclopropane products due to methylene insertion in which the... 

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. 

Additional evidence that photolysis of diazomethane leads to singlet methylene was obtained by Kopecky, Hammond, and Leermakers.(21) These workers observed that triplet methylene produced by energy transfer de-... 

The photochemical fragmentation of vinyl-substituted 1,2k5-oxaphosphetanes, representing a step of a photochemical variant of the Wittig reaction with methyl-eneoxophosphoranes, has been examined as a model in the case of 22b20). Photolysis of this compound in methanol affords the 1,3-diene 24b as well as the highly reactive dioxophosphorane 23 which is trapped by the solvent subsequent esterification of the half-ester 62, formed as a primary product, with diazomethane to give the diester 63 was undertaken solely for preparative reasons 20). 

The addition of carbenes is also a stereospecific reaction, each geometrical isomer forms the cis addition product. Thus carbene generated from the photolysis of diazomethane adds in a cis manner to both cis and trans butene-2. 

The photolysis or thermolysis of diazo compounds results in the formation of carbenes. These reactions were identified as early as 1901 by Hantzsch and Lehmann D and Staudinger and Kupfer who decomposed diazomethane photochemically and thermally, respectively. The subsequent work of Hine 3) and Doering ) started the era of carbene chemistry. Excellent reviews of the chemistry of carbenes are available 5-27),... 

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). 

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