Carbenes singlet, dimerization

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. 

In the Carter and Goddard formulation, the strength of the C=C double bond resulting from the dimerization of singlet carbenes should correspond to that of a canonical C=C double bond (usually that of ethene) minus twice the singlet-triplet... 

Similarly to imidazol-2-ylidene (IV), the calculated value for the energy of dimerization of Enders-type carbene VII is only 9.5 kcal/mol. " These remarkably small values, at least partially due to the loss of aromaticity in the carbene dimers IV 2 and VII 2, highlight the difficulty of dimerization of such carbenes. In contrast, in the case of the parent acyclic diaminocarbene IX, Heinemann and Thiel found a dimerization energy of 45 kcal/mol. This poses another question What is the value of the energy barrier for the dimerization Nowadays, the dimerization of singlet carbenes is believed to follow a nonleast motion pathway that involves the attack of the occupied in-plane ct lone pair of one singlet carbene center on the out-of-plane vacant orbital of a second carbene (Fig. 8.13). Calculations... 

Figure 8.13. Schematic representation of the mechanism for the dimerization of singlet carbenes.

The formation of olehnic dimerization products as the main product is rare in the decomposition of diazo compounds, whereas formation of ketazine is virtually omnipresent. The authors explained these data by assuming that the hindered diarylcarbenes do not have accessible singlet counterparts, because the singlet would require a smaller carbene angle and incur severe aryl-aryl repulsion. As a result of severe steric hindrance and consequent resistance to external attack by solvent, the... 

Like many singlet carbenes, nucleogenic, arc generated and chemically generated C atoms react with aliphatic C—H bonds by insertion. In the simplest case, reaction of chemically generated C atoms with methane yields ethylene and acetylene. When a mixture of CH4 and CD4 is used, product analysis indicates that the acetylene results from H abstraction followed by dimerization of the CH, while the ethylene results from C—H insertion followed by H migration in the carbene (Eq. 15). It seems probable that CH is formed in all reactions of carbon with hydrocarbons as acetylene is invariably produced in these reactions. 

The so-called least-motion path has been popular for many years. In the original formulation, the elementary reactions that involve the least change in the atomic and electronic configurations are favored (160,161). This hypothesis has found numerous applications in organic and inorganic chemistry (162-167). However, it is necessary to admit that there exist rather numerous exceptions, primarily due to the fact that the non-least-motion pathway is symmetry allowed while the least-motion path is symmetry forbidden (168,169). Dimerization of singlet carbenes is a typical example. 

The destabilisation of the carbene by the effect of annulation results in a smaller singlet-triplet gap. l,3-Dineopentyl-lff-benzimidazol-2-ylidene, an annulated carbene with intermediate singlet-triplet gap, shows an interesting equilibrium between the monomeric free carbene and the dimer form, a tetraaminoethylene [103] (see Figure 1.17). 

Carbenes are highly reactive and undergo insertion into a-bonds, cycloaddition reactions, dimerization, complex formation and intramolecular reactions. The singlet carbene, which often acts as an electrophile, gives different products than the triplet carbene, which behaves as a radical. Despite their very different nature, they manage to produce the same product in some reactions. 

Carbene adducts of germanium and tin have also been reported, as shown in Scheme 9.56 58 model for the dimerization of singlet carbenes along a non-least-motion pathway was provided by the reaction of Gel2 with carbene (1) (R = Mes, R = H). The formation of a polarized Ge-C bond, rather than a true double bond, was indicated by the C NMR chemical shift of the carbene atom (5 = 60.88) in the resulting germene-carbene adduct (81). The solid-state... 

The triplet dimer diradical DR2(Ti) finally will relax into thermal equilibrium (kT) with its singlet ground state DR2(So). As we have seen from the ESR spectra (see Fig, 10) the energy separation between the singlet and triplet diradical states is very low and thermally activated transitions occur even at low temperatures. Furthermore the ESR spectra have revealed an admixture of about 10% carbene character with the diradical intermediates. This carbene character may be important in determining the probability x of the side reactions (see Eq. (19)) for the DR -+ AC chain termination reaction. It surely is not, however, the only essential factor, otherwise there should be no difference in the optical and thermal termination reaction steps. Up to now a direct observation of the metastable triplet state Ti(M) has been possible only in two specific crystals where the polymerization reactions are very weak. 

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