Isomers of Diazomethane

For the protonation of P2 and PN we refer to the discussion in Section 3.1, as these are purely inorganic compounds. 

Schindler (1987 b) tried to use IGLO correlation effects (IGLO = individual gauge for localized molecular orbitals) to calculate the magnetic susceptibility and NMR chemical shifts of N-containing compounds. For NN multiple bonds, however, the results were not satisfactory. 

The first step of the polymethylene formation was studied with ab initio techniques at RHF/3-21G by Chen and Ning (1993). The results indicate a two-step reaction for the formation of ethene (5-12). The intermediate is calculated to have a structure that is similar to an aziridine. The authors do not discuss whether that intermediate and the ethene will continue polymerization. 

In addition to the various interesting characteristics of diazomethane with respect to structure and reactivity, this smallest organic diazo compound attracted the attention of organic, physical, and theoretical chemists because it is isoelectronic with a variety of other molecules and anions and because of its isomers. 

Diazomethane possesses 16 valence shell electrons. This is also the case for two other molecules of similar structure and reactivity dinitrogen oxide (0 = I = N) and ketene (H2C = C = 0). We have already discussed the similarity to ketene in Section 5.3. Furthermore, diazomethane is also isoelectronic with species that have little in common with its structure and reactivity, namely carbon dioxide, nitryl ion (NO ), hydrogen azide, azide ion (Nb ) and, of course, the structural isomers of diazomethane. 

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It is interesting to note that the existence of an isomer of diazomethane with a diazirine structure was never discussed, so the report on its synthesis 61TL612) simultaneously started and ended the discussion on its existence. 

Diazirine is a cyclic isomer of diazomethane. According to the organometallic literature, scission of both C-N and N-N bonds can occur when diazirines interact with metal complexes. The formation of carbene ligands arises from selective cleavage of the C-N bond, whereas selective N-N bond scission results in the formation... 

This cyclic isomer of diazomethane is also a gas (b.p., — 14°C) which explodes on heating. Several homologues are also thermally unstable [1,2]. 

The cyclic isomer of diazomethane, diazirine, 2, was first synthesized in... 

By three reaction types (5), 4 and (5) in Thble 2-1 of Sect. 2.2, ketones or aldehydes are used as starting materials. Therefore, it is reasonable to treat them together in this section. At the end (Subsection 2.5.4) a related diazomethane synthesis (depicted many decades ago by Staudinger) will be added, although it is not strictly related to this section. From the synthetic point of view, it is not particularly interesting, but mechanistically it is worthy of discussion because nitrilimine, an isomer of diazomethane, is probably formed as a steady-state intermediate in this reaction. 

A mechanistically interesting method for the formation of diazomethane was found by Staudinger and Kupfer (1912). They obtained diazomethane from hydrazine and chloroform in 25% yield. In spite of the ready availability of the reagents, the method is not attractive for the synthesis of diazomethane, even after Sepp et al. (1974) were able to increase the yield to 48% by adding small amounts of 18-crown-6. The mechanism (2-44), which was tentatively proposed by Hegarty (1978, p. 579), is, however, interesting because of the hydrazonyl chloride 2.97 formed primarily elimination of HCl gives the zwitterionic nitrile imine 2.98, which is an isomer of diazomethane (for a discussion of diazomethane isomers, see Sect. 5.4). 

Inorganic diazonium salts have been reviewed by Laali and Olah (1985). Diazosilane (SiH2N2) is an inorganic analog of diazomethane. It has not yet been observed or isolated, but its structure and stability were calculated by Thomson and Glidewell (1983) using MO methods. They are discussed in the section on isomers of diazomethane (Sect. 5.4). 

In analogous fashion, Kroeker and Kass (1990) found that diazomethane is more acidic than diazirine (AG°acid = 1647 13 kJ mol ), but less acidic than cyanam-ide (H2NCN, AG°acid= 1463 13 kJ mol ). This comparison is interesting, because these two compounds are isomers of diazomethane (see Sect. 5.4). 

We add to the discussion of diazoalkanes some remarks on mono- and difluorodi-azomethane. As mentioned above, Moffat (1978 b) calculated extreme values for the total electronic energy and for the NN bond length of difluorodiazomethane. McAllister and Tidwell (1992 b) came to the analogous conclusion that fluorine substituents have an extremely destabilizing influence on diazomethane. Boldyrev et al. (1992) found that mono- and difluorodiazomethane are not viable species as no barrier could be found for these compounds against exothermic dediazoniation. We shall discuss these compounds in the following section on isomers of diazomethane, as difluorodiazirine is known. There we shall also discuss diazosilane (S1H2N2) and its isomers. 

In addition to isocyanamide (5.57) two other isomers of diazomethane have been characterized diazirine (5.59) and cyanamide (5.60). 

For cyanamide, no rearrangement into diazomethane is known for obvious structural reasons. Diazirine, however, is the most interesting isomer of diazomethane in various respects. As discussed briefly before (Sect. 5.1), a cyclic structure was originally proposed by Curtius (1889) for ethyl diazoacetate and other aliphatic diazo compounds. 

It is, therefore, not surprising that all these isomers of diazomethane have attracted the attention of physical chemists interested in the experimental determination of heats of formation and in theoretical calculations of thermodynamic stabilities. From mass spectra and appearence potentials, Paulett and Ettinger (1963a, 1963b) estimated the heat of formation to be 331 kJ mol for diazirine and 206 kJ mol for diazomethane. In other words, diazirine is thermally less stable than diazomethane by 125 kJ mol Shortly after these investigations. Bell (1964) suggested that the heats of formation should be calculated using a different set of assumptions, which resulted in lower absolute values, but still with the same difference between A//f for diazomethane and diazirine. ... 

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