Triplet ground state carbenes

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. 

Boratetrahedrane (121) that has the triplet A2 ground state and the carbene Cs structure (122) possess higher energies than that of structure (119e). Similar to borabenzene, the stabilization of boracyclobutadiene (119e) can be achieved through the formation of a complex with pyridine. 

Dicyclopropylethylene has been used as a substrate to detect the triplet state in carbene cycloaddition. This depends on the fact that the rearrangement of the intermediate cyclopropylmethyl radical to the allyl-methyl radical is a very fast process. Thus in the addition of thermally generated fluorenylidene to cyclopropylethylene, (557) and (558) are among the products obtained. The ratio of (558) (557) is increased upon dilution, as expected for a carbene decaying to a triplet (biradical) ground state. 

Fig. 2 Singlet and triplet electronic ground state of carbenes...

UV-vis spectra of matrix-isolated intermediates are not so informative as matrix IR spectra. As a rule, an assignment of the UV spectrum to any intermediate follows after the identification of the latter by IR or esr spectroscopy. However, UV-vis spectra may sometimes be especially useful. It is well known, for example, that the energy of electronic transitions in singlet ground-state carbenes differs from that of the triplet species. In this way UV spectroscopy allows one to identify the ground state of the intermediate stabilized in the matrix in particular cases. This will be exemplified below. 

It has been possible to record the IR and UV spectra of several derivatives of the carbene [75] - tetrachlorocyclopentadienylidene [80], indenylidene [81] and fluorenylidene [82] (Bell and Dunkin, 1985). These carbenes were formed by UV photolysis of the corresponding diazo precursors frozen in inert matrices and have a triplet ground state. The carbenes [80]-[82] react with CO in inert matrices at 30 K, but exhibit a lower reactivity than the carbene [75]. Furthermore, they were stabilized in a pure CO matrix at 12 K, whereas the free carbene [75] could not be detected under these conditions. The different reactivity towards CO between [75] and [80]-[82] may be associated with the different steric shielding of the carbene centres and with the different triplet-singlet gap as well. 

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. 

Interest in carbenes in rigid media can be dated to the 1960s when Murray et al. reported diphenylcarbene as the first organic species with a triplet ground state. ... 

The triplet state is usually the ground state for non-conjugated structures, but either species can be involved in reactions. The most common method for generating nitrene intermediates, analogous to formation of carbenes from diazo compounds, is by thermolysis or photolysis of azides.246... 

The stereospecificity of addition suggests a singlet(10) carbene although the ground state of cyclopentadienylidene is known to be a triplet. Attempts to produce a triplet species, which would be expected to react nonstereospecific-ally, in a 4-methyl-cw-2-pentene matrix at 77°K or by dilution of mixtures of the azo compound and olefin with hexafluorobenzene or octafluorocyclo-butane (inert diluents) were unsuccessful. It was concluded that the singlet carbene produced upon photolysis reacts more rapidly with the olefinic... 

EPR studies of diphenylmethylene and a number of other arylmethylenes have indicated that these carbenes have triplet ground states.<30) Photolysis of diphenyldiazomethane in olefin matrices results in the formation of triplet diphenylmethylene, which undergoes primarily abstraction reactions with the olefins. Cyclopropanes are produced as minor products. 

Minima in Ti are usually above the So hypersurface, but in some cases, below it (ground state triplet species). In the latter case, the photochemical process proper is over once relaxation into the minimum occurs, although under most conditions further ground-state chemistry is bound to follow, e.g., intermolecular reactions of triplet carbene. On the other hand, if the molecule ends up in a minimum in Ti which lies above So, radiative or non-radiative return to So occurs similarly as from a minimum in Si. However, both of these modes of return are slowed down considerably in the Ti ->-So process, because of its spin-forbidden nature, at least in molecules containing light atoms, and there will usually be time for vibrational motions to reach thermal equilibrium. One can therefore not expect funnels in the Ti surface, at least not in light-atom molecules. 

Relative rates of some prototypical carbenes, obtained by Stem-Volmer methods, are listed in Table 2. Although many of these carbenes have triplet ground states, reaction with nucleophiles Y occurs prior to spin equilibration. Most often, ylide formation with solvent molecules was analysed in terms of Eq. 3. The pyridine-ylide served as the probe for 154. 

The simplest carbene capable of a 1,2-H shift, methylcarbene (78) is very difficult to study experimentally. Theoretical studies suggest that the carbene is a ground state triplet lying 5 kcal/mol below the singlet.97... 

The reaction kinetics of 2-adamantanylidene (107) have been carefully investigated.118 119 Carbene 107 is computed to have a singlet ground state lying 4.8 kcal/mol below the triplet.120... 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

Cyclopentadienylidene (10) has an even longer history than cyclopropenyli-dene (2).14 Carbene 10 possesses a triplet ground state. Its ESR spectrum indicates a structure with C2V symmetry. One electron is localized at the car-benic center, the other unpaired electron is delocalized over the five-membered ring (structure T-IO ).30 The IR and UV spectra of 10 are also known.31... 

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