Carbenes triplet, dimerization

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. 

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). 

The IR and UV spectra of the triplet cycloheptatrienylidene [71] were recorded after the UV photolysis (A>574 nm) of diazocycloheptatriene [72] in an argon matrix (McMahon and Chapman, 1986). This carbene interacts with the CO-doped matrix, forming the ketene [73], and it also dimerizes with formation of heptafulvalene [74]. Experiments have shown that [71] cannot be converted into the cycloheptatetraene [48] either photochemically... 

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. 

Photolysis of DAX in a methylcyclohexane glass at 77 K creates a metastable species detected by its optical absorption spectrum (Table 4). This solution does not exhibit an epr spectrum characteristic of a triplet carbene, nor is one observed when DAX is irradiated in Fluorolube (where triplet carbenes are generally more stable). Warming the frozen solution causes the reaction of the metastable transient and the formation of dimeric xanthone azine. 

Flash photolysis of 5-diazo-lO, ll-dihydro-dibenzo[a, d]cycloheptene (75) — which can be regarded as a bridged diphenyl-carbene — at room temperature in hquid paraffin first produced the spectrum of the triplet carbene 16, which then disappeared to give the electronic spectrum of the radical 17. The latter finally gave the dimer 5,5 -bi (10, ll-dihydrodibenzo[a, djcycloheptenyl) 18 2). 

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... 

Bicyclohexyl groups act as an ideal kinetic protector of triplet carbene not only by quenching the intramolecular hydrogen-donating process but also by inhibiting dimerization of the carbene center. 

Benzene is recognized as a very unreactive solvent, especially for triplet carbenes. Therefore, the most reactive reactants under these conditions must be the triplet carbenes themselves. However, the products obtained from the photolysis in benzene consist of a highly complex mixture containing small amounts of car-bene dimers. It is then possible that the simple dimerization of brominated DPCs must suffer from severe steric repulsion and, therefore, the carbene is forced to react at other positions. The most probable reactive sites are the aromatic rings, where spin can be delocalized. 

In spite of those highly favorable stmctural factors, 16a is very ephemeral. Its lifetime in degassed benzene is 0.5 ps, which is shorter even than that of parent triplet DPC. Product analysis smdies have shown that 16a forms a trimer of dianthrylcarbene (116) as the main product (50-60%) (Scheme 9.37). The trimer is the one formed as a result of a threefold coupling at position 10 of the carbene. This observation suggests that delocalization of the unpaired electrons in 16a leads to their leaking out from the carbene center to position 10, where sufficient spin density builds up for the trimerization to take place. At the same time, the lack of formation of olefin-type dimers through coupling of two units of 16a at their carbene centers indicates that the carbene center itself is indeed well shielded and stabilized. 

Many reactive intermediates can decay via self-reactions, giving dimers or disproportionation products, as is the case of free radicals and carbenes. When these self-reactions are not the ones under study, it is desirable to keep the transient concentration low enough to minimize this type of interference. For example, for a radical that dimerizes with fet = 3 x 10 M s and a concentration c of lO M, its first half-life (ti/2 = 1/kc) would be 33 ps. Note that excited triplet states also undergo bimolecular decay by triplet-triplet... 

The mechanism of this reaction is obscure. One suggested mechanism, analogous to the vapor phase reaction, involves concerted decarboxylation of the pyruvic acid to yield a triplet hydroxy carbene which can either dimerize or attack another molecule of pyruvic acid to yield the observed product.91 Dimerization seems to be the less likely process since the carbene can rearrange to acetaldehyde or react with water. Further, this mechanism predicts that acetoin will be formed when pyruvic acid is irradiated in any solvent that does not possess readily abstractable hydrogen atoms, such as benzene, a solvent in which no reaction is observed. One possible explanation of this discrepancy is that the solvation of the pyruvic acid is extremely different in benzene and in water. However, the specific role that the water plays in the reaction has not been determined. 

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. 

Another clear-cut example of abstraction was observed in the reactions of anthronyUdene (3f) S3, 34) th cyclohexane or toluene. AU possible dimerization products 30, 31 eind 32 can be isolated in this case. These abstraction reactions are attributed to triplet carbenes 2c and 3f. All other cycloalkenecarbenes show normal C—H insertion reactions (for the exceptional behaviour of di- and tribenzocycloheptatrienyhdene see p. 137). 

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