Carbenes methylene

In this review an attempt is made to discuss all the important interactions of highly reactive divalent carbon derivatives (carbenes, methylenes) and heterocyclic compounds and the accompanying molecular rearrangements. The most widely studied reactions have been those of dihalocarbenes, particularly dichlorocarbene, and the a-ketocarbenes obtained by photolytic or copper-catalyzed decomposition of diazo compounds such as diazoacetic ester or diazoacetone. The reactions of diazomethane with heterocyclic compounds have already been reviewed in this series. ... 

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). 

Carbenes are species which contain a dicoordinated carbon atom formally with two valence electrons. The possible electronic structures of the parent carbene, methylene CH2, are shown in Figure 7.5. The 2p orbital of the dicoordinated C atom is placed above a by about 0.25 fi in order to accommodate the lower electronegativity. 

Rarely has any field in organometallic chemistry encountered the tremendously rapid recent expansion experienced by the synthesis, spectroscopy, structural chemistry, theory, and reactivity of compounds characterized by terminal carbene (methylene, A) and carbyne (methylidyne, B) functionalities. Fischer and his group pioneered the discovery of both these remarkable classes of compounds (/, 2) many other workers have successfully followed their paths, and several comprehensive reviews are eloquent witness to the amazing plethora of compounds this fascinating area of modem research has yielded (3-9). 

Fig. 5.48 The hydrocarbon pyramidane, C5H4, evidently (pyramidane has not been synthesized) has a lone pair of electrons on its pyramidal carbon atom, like carbene (methylene), CH2. While the lone pair on CH2 is no surprise (draw the Lewis structure for the singlet), a cycloalkane with an unshared electron pair is remarkable...

Considering the simplest carbene, methylene, it is clear that it could have a linear structure (D -symmetry) or be bent ((72s-symmetry). In the former case, using the ls-orbitals of hydrogen and the 2s- and 2 -orbitals of carbon, two a molecular orbitals can be constructed and these can clearly accommodate the C—H bond electrons. A degenerate pair of orbitals, similar to carbon atomic p-orbitals, are the ones of next-lowest energy. The linear structure of methylene would thus correspond to a triplet state. 

A1.4 Metal-Carbene, -Methylene, -Carbyne and -Methylidyne Complexes... 

Although the reaction of dihalocarbenes with alkenes gives good yields of halogenated cyclopropanes, this is not usually the case with methylene, tCH2, the simplest carbene. Methylene is readily formed by heating diazomethane, CH2N2, which decomposes and loses N2, but the reaction of CH2 with alkenes often affords a complex mixture of products. Thus, this reaction cannot be reliably used for cyclopropane synthesis. 

By analogy with alkyl radicals and carbenes (methylenes), the species R3Sn are stannyl radicals, and the species R2Sn are stannylenes or stannyldiyls. 

A carbene is an unusual carbon-containing species. It has a carbon with a lone pair of electrons and an empty orbital. The empty orbital makes the carbene highly reactive. The simplest carbene, methylene ( CH2), is generated by heating diazomethane. Propose a mechanism for the following reaction ... 

Carbenes, in general, are very reactive electrophilic species. Their activity depends to some extent on the method and conditions of preparation, on the nature of the substituent groups and also on the presence or absence of metals or metallic salts. Carbenes undergo a variety of reactions, including insertion into C—H bonds, addition to multiple bonds and skeletal rearrangements. The simplest carbene, methylene itself, attacks primary, secondary and tertiary C—H bonds indiscriminantly. However, alkyl carbenes can undergo selective intramolecular C—H, N—H or O—H insertion to provide useful synthetic transformations. In general, no intermolecular reactions are observed when intramolecular insertion is possible. 

The first item in Table 6.9 provides an example of the addition of two equivalents of a carbene, methylene ( CH2), an electrophilic species, to the alkyne, 2-butyne (dimethylacetylene, CHsC CCHs) to provide the bicylobutane (1,3-dimethylbicyclo[1.1.0]butane). With ethyne (acetylene, HC=CH]) itself, as shown in Equation 6.71, the expected cyclopropene cannot be isolated because (apparently) it rearranges to 1,2-propadiene (allene, H2C=C=CH2). 

Full preparative details for lTaCp2MeCH2], the first unsubstituted metal carbene ( methylene complex ) to be isolated, have been published. The paper also deals with some reactions of the complex, including its decomposition. These are summarized in Scheme 37. The complex [M(CH2CMe3)3(CHCMes)]... 

When it is irradiated or exposed to catalytic amounts of copper, diazomethane evolves nitrogen to generate the reactive carbene methylene, H2C Methylene reacts with alkenes by addition to form cyclopropanes stereospecificaUy (Section 12-9). 

Among radicals of the second type, carbenes (methylenes) are well studied. The simplest carbene is methylene CH2, which exists in two forms singlet and triplet. In singlet CH2 the HCH angle is equal to 103°, rc n = 0.112 nm, = 393 kJ/mol. In triplet CH2 all three atoms lie on one line, rc—h = 0.103 nm. 

The polymethylene formation is frequently attributed to a "polymerization" of the carbene (methylene) resulting from C-N bond scission. Such a spontaneous a-elimination seems to occur indeed with trimethylamine butoxymethanide and phenoxymethanide. Unlike methylene itself, the corresponding alkoxy- and aryoxy-carbenes can be trapped in the presence of cyclohexene as the [l+2]-cycloadducts. However, the unsubstituted methylene should be energetically too unfavorable to be accessible in this way. Moreover, the alleged "polymerization" would have to start with a dimerization to ethylene and this would hardly produce a trimethylene. A more plausible fate of such a A-ylide/lithium halide complex is to undergo a direct carbenoid substitution... 

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. 

Di-iodo-, -bromo-, -chloro-. -fluoro-carbene (-methylene) CX2... 

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