Carbenes, generation triplet

The related [1,2]-H shift in the isomeric triplet 1-phenylethylidene (53) was also investigated. The triplet carbene, generated from irradiation of the corresponding diazo compound, was characterized in low temperature inert matrices by EPR, IR, and UV/VIS spectroscopy. In this case, the carbene was stable in Ar up to the temperature limits of the matrix (36 K). Irradiation, however, readily converted the carbene to styrene. 

Bis(2,4,6-tribromophenyl)carbene (106f) was easily generated by photolysis of the precursor diazomethane (105f) and was characterized by EPR spectroscopy (Scheme 9.14). The triplet carbene generated in a degassed solution at room temperature decayed very slowly, persisting for at least 30 s. The decay was found to be second order Ik/el = 8.9 s ). The parameter fj/2, is estimated to be Is (Table 9.14). 

These results are quite distinct from the chemistry of phenyl carbene (PC) [100]. The chemistry of PC in hydrocarbons is independent of initial singlet or triplet carbene generation. Equilibration of singlet and triplet PC is faster than their intermolecular reactions. 

Triplet carbene generated by photolysis of bis[9-(10-phenyl)anthryl]-diazomethane (56) in MTHF glass at 77 K gave EPR signals very similar to those... 

In the case of the photochemical reaction, irradiation of diazomethane in the presence of cw-2-butene provides cw-l,2-di-methylcyclopropane with no detectable amount of the tram isomer (eq 45). This reaction is thought to proceed via a singlet carbene. However, if the same reaction is carried out via a triplet carbene, generated via triplet sensitization, then a 1.3 1 mixture of tram to cis dimethylcyclopropane is observed (eq 46). The yields in the photochemical reaction are typically lower than the metal-mediated processes, and are usually accoii5)anied by more side products. 

As to other carbenes, some react as triplets, some as singlets, and others as singlets or triplets, depending on how they are generated. There are, however, molecules that generate persistent triplet carbenes. 

There is a limitation to the use of stereospecificity of addition as a diagnostic test for singlet or triplet carbenes.When carbenes are generated by photolytic methods, they are often in a highly excited singlet state. When they add to the double bond, the addition is stereospecific but the cyclopropane formed carries excess... 

Direct irradiation or thermolysis of a diazo-compound (2) is believed to generate the carbene initially in its singlet spin state. Triplet sensitization (5) is presumed to give the triplet carbene directly without first forming its singlet state via the triplet diazo-compound. In some cases, careful comparison of the results of direct irradiation experiments with those from triplet sensitization can provide useful information to identify the spin state initiating a reaction. 

A second process that has a central position in the analysis of the chemical properties of carbenes is their reaction with hydrocarbons. As is the case for alcohols, singlet and triplet carbenes react with hydrocarbons in distinctive ways. It has long been held that very electrophilic singlet carbenes can insert directly into carbon-hydrogen bonds (11) (Kirmse, 1971). On the other hand, triplet carbenes are believed to abstract hydrogen atoms to generate radicals that go on to combine and disproportionate in subsequent steps (12)... 

Cyclohexane and cyclohexane-d12 have been used as the probe for crossover and, hence, the reactive multiplicity of the subject carbenes. By combining direct and triplet-sensitized generation of the carbene with kinetic analysis from laser spectroscopy and the results of the crossover experiments, a rather complete picture of the reaction of aromatic carbenes with hydrocarbons emerges. 

A complicating factor associated with experimental application of the Skell Hypothesis is that triplet carbenes abstract hydrogen atoms from many olefins more rapidly than they add to them. Also, in general, the two cyclopropanes that can be formed are diastereomers, and thus there is no reason to expect that they will be formed from an intermediate with equal efficiency. To allay these problems, stereospecifically deuteriated a-methyl-styrene has been employed as a probe for the multiplicity of the reacting carbene. In this case, one bond formation from the triplet carbene is expected to be rapid since it generates a particularly well-stabilized 1,3-biradical. Also, the two cyclopropane isomers differ only in isotopic substitution and this is anticipated to have only a small effect on the efficiencies of their formation. The expected non-stereospecific reaction of the triplet carbene is shown in (15) and its stereospecific counterpart in (16). 

Examination of DABA photolysis in cyclohexane instead of benzene solution leads to predictably different results. The laser spectroscopy shows that 3BA is formed but, in this solvent, the triplet carbene undergoes an additional rapid reaction to generate the mesitylbora-anthryl radical (BAH ). This radical is identified by comparison of its spectrum with that of an authentic sample prepared from dihydrobora-anthracene. The half-life of... 

Another interesting allenic intermediate is generated when di(9-anthryl)diazo-methane (256) is deazotized by irradiation in benzene (Scheme 5.39). The carbene produced, 257, belongs to the long-sought persistent triplet carbenes, although its... 

Chapman et al. [229] observed the IR spectrum of 10 in an argon matrix at -258 °C in 1988. Photolysing (7 > 470 nm) p-tolyldiazomethane, they generated the respective triplet carbene, which was converted to 10 on further irradiation (7 >416 nm). Analogously, from m-tolyldiazomethane, a mixture of 10 and 4-methyl-... 

This technique is most useful in studying triplet carbenes since it responds to triplet states only. The carbenes are mostly generated at low temperatures (4 or 77 °K) by photolysis of diazoalkanes in a) solid solutions in single crystals or b) in randomly oriented glasses. 

The triplet carbenes fluorenylidene 20 and anthronylidene 24, which can be generated from the diazoalkanes by photolysis, show a similar behaviour. Fluorenylidene in cyclohexane yields 9-cyclohexenyl-fluorene 27, and 9,9 -difluo-renyl 22 which are clearly formed by an abstraction-recombination process Another example is anthronylidene 24 in cyclohexane or toluene, which yields the products 25,26,27 resulting from an abstraction-recombination process Benzene, on the contrary, failed to give the radical pair product 20... 

Fluorenylidene — which was photochemically generated from diazofluorene — indicated a larger content of triplet carbene (see Table 7) when diluted. 

In the sensitized generation of ethoxy-carbonyl-carbene the fight capture by the diazocompound is only 25% A number of photosensitized reactions of triplet carbenes with cis- and trans-olefins are fisted in Table 10. The sensitizers used, as well as their various triplet energies Et, are also given in the table. 

Laser flash photolysis of phenylchlorodiazirine was used to measure the absolute rate constants for intermolecular insertion of phenylchlorocarbene into CH bonds of a variety of co-reactants. Selective stabilization of the carbene ground state by r-complexation to benzene was proposed to explain the slower insertions observed in this solvent in comparison with those in pentane. Insertion into the secondary CH bond of cyclohexane showed a primary kinetic isotope effect k ikY) of 3.8. l-Hydroxymethyl-9-fluorenylidene (79), generated by photolysis of the corresponding diazo compound, gave aldehyde (80) in benzene or acetonitrile via intramolecular H-transfer. In methanol, the major product was the ether, formed by insertion of the carbene into the MeO-H bond, and the aldehyde (80) was formed in minor amounts through H-transfer from the triplet carbene to give a triplet diradical which can relax to the enol. 

Take the reaction of carbenes generated by photolysis of diazo compounds (Scheme 9.1), for example. Direct irradiation of a diazo compound (12) is believed to generate the carbene initially in singlet state ( 3) via the singlet excited state of the precursor. Triplet sensization, on the other hand, is presumed to give the triplet... 

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