Methylene carbene from diazomethane

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. ... 

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). 

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 ... 

Like CO, CR2 can act not only as a terminal but also as a bridging ligand. On the traditional model, when CO or CR2 bridge, a metal-metal bond is usually present (11.12 and 11.13). In bridging, the carbene carbon moves from tricoordinate sp toward tetracoordinate sp. Fischer methylenes are rare, while the bridged form is better known and less reactive. Bridging carbenes can be made from diazomethane (Eq. 11.35). 

The addition of different types of carbenes onto bicyclopropylidene (1) is a common method for the preparation of [3]triangulane derivatives as well as branched trianguianes and normally proceeds without complications (for a review see [77]). Thus, the cyclopropanation under Gaspar-Roth [60] or modified Simmons-Smith [111] conditions gave dispiro[2.0.2.1]heptane ([3]triangulane, 97) in 80 [105] and 15% yield [5], respectively (Scheme 23). The palladium(II) acetate-catalyzed cycloprop anation of 1 with diazomethane, however, gave a number of products resulting from insertion of one or more than one methylene units into an initially formed palladacyclobutane 115 [112,113] (Scheme 23). 

CARBENE. The name quite generally used for the methylene radical, CH,. It is formed during a number of reactions. Thus the flash photochemical decomposition of ketene (CH2=C=0) has been shown to proceed in two stages. The first yields carbon monoxide and CHj. the latter then reacting with more ketene to form ethylene and carbon monoxide. Carbcne reacts by insertion into a C- H bond to form a C-CH, bond. Thus carbene generated from ketene reacts with propane to form, i-butane and isobutane. Carbene generated by pyrolysis uf diazomethane reacts with diethyl ether to form ethylpropyl ether and ethylisopropyl ether. 

Many other 1,3-dipolar cycloadditions are known, amongst which is the addition of diazomethane (CH2N2). Expulsion of nitrogen from the adduct leads to the formation of a cyclopropane ring. Another way of achieving the same result involves the addition of a carbene such as the Simmons Smith reagent. This is generated from methylene iodide (CHjIj) and a zinc/copper couple. 

In contrast to the wealth of chemistry reported for catalyzed reactions of diazocarbonyl compounds, there are fewer applications of diazomethane as a carbenoid precursor. Catalytic decomposition of diazomethane, CH2N2, has been reported as a general method for the methylenation of chemical compounds [12]. The efficacy of rhodium catalysts for mediating carbene transfer from diazoalkanes is poor. The preparative use of diazomethane in the synthesis of cyclopropane derivatives from olefins is mostly associated with the employment of palladium cat-... 

Diazirine was used as the carbene precursor in an experiment to make diazomethane from methylene and nitrogen in the gas phase. As shown by means of labeled nitrogen, there was 4% diazomethane formation. During the past 10 years thermal as well as photochemical decomposition of 3-chloro- and 3-bromodiazirine has been investigated in detail. Compounds 175 and 176 as well as analogous compounds are easily obtained by reaction of amidines with hypohalites according to Graham. ... 

Diazirine was used as the carbene precursor in an experiment to make diazomethane from methylene and nitrogen in the gas phase. As shown by means of labeled nitrogen, there was 4% diazomethane formation.185... 

Photolysis of diazo compounds (305) generates nitrogen and the carbene 289. Photolysis of diazomethane (311, R = H), for example, generates methylene (H2C ),242 Benzophenone is often added as a sensitizer for the photolysis of diazomethane. Under these conditions, triplet methylene is formed via intersystem crossing (5i Ti)243 (sec. II.IO.B). Energy transfer from triplet benzophenone to triplet diazomethane followed, and triplet diazomethane decomposed to triplet methylene.244 a similar thermal reaction gives the carbene and the usual carbene reactions.245... 

At an early date, Moore and Pimentel (1964 d) carried out photolyses of diazirine in a solid matrix of N2. They detected diazomethane, but concluded from results using a matrix that diazomethane was derived from the reaction of primarily formed methylene with dinitrogen (for later work on reactions of other carbenes with N2 see Sect. 8.1). 

The carbene transfer from bis(trifluoromethyl)diazomethane to octacarbonyl dicobalt gives at room temperature in 28 days the p -bis(trifluoromethyl)methylene complex IS that was isolated in 54% yield (reaction 8.33) [64,65]. 

In Summary Diazomethane is a useful synthetic intermediate as a methylene source for forming cyclopropanes from alkenes. Halogenated carbenes, which are formed by dehydro-halogenation of halomethanes, and the Simmons-Smith reagent, a carbenoid arising from the reaction of diiodomethane with zinc, also convert alkenes into cyclopropanes. Additions of carbenes to alkenes differ from other addition processes because a single carbon atom becomes bonded to both alkene carbons. 

The simplest of all carbenes, methylene (CHj), initially proved difficult to observe in low temperature matrices, owing to its small size and high mobility. One of the first indications that methylene had been generated in matrices was the observation of the supposed chemiluminescence of ethene following photolysis of diazomethane in solid argon or nitrogen. The luminescence exhibited a distinct isotope effect when CDjNj was photolyzed and was presumed to arise from excited ethene molecules formed by the dimerization of methylene. Nevertheless, a reinvestigation of this reaction led to the conclusion that the excited ethene could have arisen either from the dimerization of CHj or from the reaction of CHj with the precursor CHjNj. 

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