Methylene, from diazomethane

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. 

CB2527). The latter are formed in a stepwise manner and are not directly formed from the educts by a simple extrusion of N2. In the first step a fragmentation into thioketones and diazomethane occurs, followed by the generation of methylene from diazomethane. Addition of methylene to the thioketones finally leads to the ylides.3... 

The photolysis of diazirine °° at 3130 A yields ethylene and nitrogen methylene is probably formed in the primary process. The long wavelength absorption ( max = 3200 A) had been identified as the allowed (a, n ) transition and semi-empirical Huckel calculations indicate that the methylene produced must be (Ai) or (Bi). There is evidence that methylene produced from the photolysis of diazirine is more selective than methylene from diazomethane, owing to decreased excess translational energy. At low pressures 5-30 torr, diazomethane was identified as an intermediate by its absorption spectrum and the question arises, is methylene formed directly, or does it arise from decomposition of diazomethane The quantum yield of disappearance of diazirine is 2.0+0.5 and the quantum yield of diazomethane formation is about 0.2. The intermediate diazomethane is... 

Early work on the gas-phase interaction of methylene with carbon-hydrogen bonds showed selective reactivity in the order, tertiary C-H > secondary C-H > primary C-H. Methylene from diazomethane was found to be less selective than methylene from ketone . The differences in the relative reactivity of methylene from the two sources have been attributed to excess translational energy of methylene from CH2N2 . Recently, however, it has been shown that the photolysis of ketene produces more triplet CH2 than the photolysis of diazomethane cf. preceeding section). Part of the reported selectivity must be attributed to triplet-methylene effects. 

On the basis of recent work with radical scavengers (Table 12) it appears that there is no real difference in the reactivity of singlet methylene from diazomethane and that from ketene towards C-H bonds. Very probably, methylene is translationally thermalized before reacting. This assumption is consistent with the lack of wavelength dependence for singlet methylene... 

The addition of 2-diazopropane or 3-diazopentane to (112) results in exo-endo mixtures of both the triazoline (113) and the bicyclo[2.1,0]pentane (114). However, with the 1,2,3,4-tetramethyl analogue of (112) no addition occurs, Surprisingly, the imide (115) incorporates two and three molar equivalents of methylene from diazomethane to give products of spirocyclopropanation at C-4. Monocyclopropana-tion of (116) at the least substituted double bond proceeds efficiently when diazo-acetic ester decomposition is catalysed by copperfn) and similar monoaddition to... 

Methylene transfer from diazomethane to olefinic and aromatic double bonds has traditionally been carried out with Cu(I) halides 24 However, other copper salts have occasionally been used. 

Amino-1,2,3-triazoles with a substituent at the 4-position have been prepared (i) from azides and active methylene nitriles (ii) from azides and ynamines (iii) from diazomethane and carbo-diimides (iv) from azides and 1,1-diaminoethenes and (v) from the rearrangement of 3-hydrazono-1,2,4-oxadiazoles. Among these, the first method, a regiospecific process, is the most versatile and convenient although it is suitable only for 5-NH2-substituted triazoles. Other methods are used to prepare 5-NHR , 5-NR R - and 5-NHCOR-substituted triazoles. Intramolecular cyclization of suitable precursors also gives 5-aminotriazoles. For example, a-imino-a-piperidyl phenylhydrazones (838), in the presence of copper acetate, give 5-piperidyl-triazoles (839) (Equation (85)) <94H(38)739>. 

Problem 4.41 Singlet methylene, CH, (Section 3.2). may be generated from diazomethane, CH,N, the other product is N,. It can insert between C—H bonds of alkanes ... 

In the light-induced reactions with diazoalkanes, methylene, the photolytic fragment from diazomethane, may be interposed between the halogen atom in the organic halide and the carbon atom to which it is attached,289 290 or may add to the double bonds in the aromatic ring.58 69... 

The methylene generated from diazomethane reacts with alkenes to form cyclopropanes, but diazomethane is very toxic and explosive, and the methylene generated is so reactive that it forms many side products. A safer and more reliable way to make cyclopropanes is with the Simmons-Smith reagent. 

Palladium-catalyzed methylene transfer from diazomethane has proved effective for the cyclopropanation of 1-alkenylboronic acid esters allylic alcohols and amines 1-oxy-l,3-butadienes and allenes " Readily accessible iron complex (CO)2FeCH2S Me2 BF4 35 undergoes direct reaction with a range of alkenes to give cyclopropanes (equation 67) The salt is sensitive to steric effects and the reaction proceeds... 

Wittig and Jauterat 547) prepared a complex (XV) from diazomethane and zinc benzoate, which undergoes the methylene transfer reaction with olefins. Reactions (2), (5), (8), and (13) can be understood as transfer... 

Carbenes are known intermediates in the photodecomposition of diazo-compounds. Insertions of singlet methylene, generated by photoelimination of nitrogen from diazomethane, into 2,3-dimethyl-2,3-epoxybutane and into acetonitrile have been described. Methylene and other photochemically generated carbenes (43) add efficiently to 2-acetoxyacrylonitrile to give the corresponding 1-acetoxy-l-cyanocyclopropanes (44). The formation of methylidyne radicals by irradiation of diazomethane in the presence of hydrogen, however, does not seem to involve an intermediate carbene. ... 

Assuming that reaction (4) proceeds with unit collisional deactivation efficiency, then the value of is computed as 3 X 10 sec. . This value is close to the value obtained when diazomethane is used as the methylene precursor and implies that the total energy of the methylene produced by the photolysis of diazomethane and diazirine is the same. These results were rationalized on the basis that methylene from diazirine contained less translational energy and was more vibrationally excited than methylene from diazirine. 

There is much evidence that the photolysis of ketene, diazomethane, and diazirine produces methylene predominantly in the singlet state. It is, however, possible, by using mercury photosensitization or by photosensitization using benzophenone to produce triplet methylene. Since the triplet is the ground state for methylene, it is also possible to produce triplet methylene from singlet methylene by carrying out reactions in the presence of a large excess of inert gas. Recently, much evidence has accumulated to indicate that even in the normal photolysis of all three of the methylene precursors, some of the methylene is produced in the triplet state. ... 

Methylene ( CH2) generated photochemically or thermally from diazomethane is highly reactive and is prone to incur side reactions to a substantial extent. In order to avoid these undesirable complexities, the cyclopropanation of multiple bonds with diazomethane has usually been carried out under catalytic conditions The catalysts most frequently employed are copper salts and copper complexes as well as palladium acetate. The intermediate produced in the copper salt-catalyzed reactions behaves as a weak electrophile and exhibits a preference to attack an electron-rich double bond. It is also reactive enough to attack aromatic nuclei. In contrast, the palladium acetate-catalyzed decomposition of diazomethane cyclopropanates a,a- or a,jS-disubstituted a,jS-unsaturated carbonyl compounds in high yields (equation 47). The trisubstituted derivatives, however, do not react. The palladium acetate-catalyzed reaction has been applied also for the cyclopropanations of some strained cyclic alkenesstyrene derivatives and terminal double bondsHowever, the cyclopropanation of non-activated, internal double bonds occurs only with difficulty. The difference, thereby. 

The methylene group transfer from diazomethane to alkenes... 

In hydrocarbon solvents, the reaction of methylene, generated photochemically from diazomethane, is faster than intersystem crossing to the triplet ground state, permitting the study of exclusively singlet carbene chemistry. "... 

Methylene is produced from diazomethane by the photodissociation reaction. This carbene reacts with benzene to form toluene and cylohepta-triene (Scheme 4). 

On the basis of eqn. (36), the yields of triplet methylene from ketene are found to be roughly 15% at 2800 A, 15% at 3130 A, 30% at 3340 A and 40% at 3660 A . Equation (36) neglects the formation of singlet-type products from triplet methylene via radical recombination (35). If the reduction in yield of rra/ti-2-pentene and 2-methyl-2-butene by added oxygen is also considered to be the result of triplet CH2 reactions, a recalculation of triplet methylene fractions yields 20% triplet at 2800 A, 55% at 3340 A and 60% at 3660 A for ketene photolysis . Assuming that 10% added oxygen suppresses all triplet products and measuring product yields relative to an internal standard, Eder and Carr " arrived at 29 3% triplet CH2 at 3130 A, and 87 2% at 3660 A. For the diazomethane-troni-2-butene system, the fraction of triplet methylene was found to be 12% at 3550-4000 A . 

Methylene generated from diazomethane is so reactive that it inserts into C-H bonds as well as C=C bonds. For example, in the reaction of propene with diazomethane-generated methylene, several side products are obtained (Scheme 5.44). Methylene inserts into both primary and tertiary C-H bonds whereas carbomethoxymethylene reacts almost solely at the tertiary position (Scheme 5.45). 

Cyclopropanation of alkenes, in both inter- and intramolecular modes, constitutes an attractive route to polycycloalkanes. These cyclopropanations of alkenes are carried out through car bene or carbenoid intermediates . For the direct cyclopropanation of alkenes, the methylene iodide zinc-copper couple (Simmons-Smith reaction)reagent is commonly used. Several modifications of this procedure, including acceleration with ultrasound, are known °°. A somewhat less frequently used procedure for cyclopropanation is through methylene addition from diazomethane, which can be carried out either thermally or photochemically or in the presence of metal salts, e.g. Pd(OAc)2 or Rh2(OAc)4. In Table some examples of the preparation of cyclopropane bearing... 

The simplest polyhydrocarbon, with structural element —CHi—, can be produced by the polymerization of ethylene (CH2=CH2), diazomethane (CH2N2), or a mixture of CO and H2. The polymer produced from diazomethane is called poly(methylene). It is only lightly branched. The polymerization mechanisms has not been established. A carbene mechanism has been discussed for gold as initiator. With boron trifluoride-water as initiator, on the other hand, a regular cationic polymerization with proton addition and subsequent propagation is considered ... 

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