Carbenes, fluorenylidene

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... 

ESR spectra of the three cyclic carbenes fluorenylidene, indenyl-idene, and cyclopentadienylidene have been studied.40,42 They all display D values around 0.4 cm-1, while their E parameters and the observed 13C hyperfine splittings indicate that the angles between the bonding orbitals of the divalent carbons are 140-145°. Such a value is obviously too large for a five-membered ring, leading to the suggestion that the bonds may be bent in the sense proposed by Coulson and Moffitt.43... 

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. 

Final resolution of these problems, particularly the complications from multiple matrix sites, came from investigations using spectroscopic methods with higher time resolution, viz. laser flash photolysis. Short laser pulse irradiation of diazofluorene (36) in cold organic glasses produced the corresponding fluorenylidene (37), which could be detected by UV/VIS spectroscopy. Now, in contrast to the results from EPR spectroscopy, single exponential decays of the carbene could be observed in matrices... 

An important conclusion, independent of the actual magnitude of AGSX, is that the electron donating methoxy-substituents on the fluorenylidene nucleus lower the energy of the singlet state of the carbene more than that... 

The synthesis of 9-diazofluorene 19 is easily accomplished from commercially available 9-fluorenonehydrazone (HgO, Et20 followed by KOH in EtOH yield = 98%) [72], Diazo compounds on photolytic decomposition tend to generate carbenes [73,74], Fluorenylidene, generated by the photolysis of 9-diazofluorene adds to olefins with negligible amounts of hydrogen abstraction [75,76]. Copper and its salts, however, have been shown to lead to... 

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

These data demonstrate that a triplet carbene 80 is produced, which gives the same ratio of products 81182 with the cis- and with the trans-olefin A dear differentiation of the products formed is observed in the reaction of fluorenylidene 69 with dicyclopropylethylene. 

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. 

The formation of reactive carbenes from alkylidene Meldrum s acids has also been observed. Thus pyrolysis of 1-indanylidene Meldrum s acid at 640 °C gave the corresponding carbene which further rearranged to benzofulvene and naphthalene (Scheme 12) <1998JA8315>. Similarly, EVP of 9-fluorenylidene Meldrum s acid at 1100°C provided a mixture of phenanthrene and biphenylene <1996TL6819>. 

However, be aware that the ratio of E/D is not always a reliable guide to the structure of triplet carbenes. For example, diphenylcarbene and fluorenylidene have very different geometries but almost exactly the same D and E values.If one really wants to learn about the geometry, one must label the carbene carbon with and measure the hyperfine coupling constants. 

Fluorenylidene and its substituted derivatives are typical examples of the other class. Other carbenes showing similar behavior are monoarylcarbenes such as mononaphthylcarbenes (12) and phenyl(methoxycarbonyl)carbene (53, AGst = 0.2 kcal/mol). 

A number of minima corresponding to oxonium ylides and H-bonded structures were found on the potential-energy surface for reaction of singlet carbenes with water and alcohols." Laser flash photolysis revealed that the rates of reaction between cyclopentadienylidene or fluorenylidene and alcohols increased with alcohol acidity and had linear Bronsted plots with slopes of 0.061 and 0.082, respectively.100 These results point to protonation with a very early transition state or to concerted OH insertion. For tetrachlorocyclopentadienylidene, the results showed that ylide formation (100) is predominant. 

Triplet diphenylcarbene has a Tl/2 of 2 ps in the presence of 1.0 M isoprene (Eisenthal et al., 1980). The lifetime of singlet phenylchlorocar-bene ranges from 5 to 500 ns in the presence of 1.0 M alkenes with various substituents (Turroetal., 1980). In 1.0M methanol singlet fluorenylidene and singlet diphenylcarbene have half-lives of 0.77 ns and 0.02 ns respectively (Zupancic and Schuster, 1980 Eisenthal et al., 1980). Triplet carbenes react very rapidly with 02. 

The potential of laser flash photolysis in the study of carbene reactions with heteroatoms has come to be recognized in recent years. A number of kinetic studies using this technique have been carried out with carbene precursors in nitrile solvents.122-127 An absorption band at 470 nm was observed in the laser flash photolysis of diazofluorene (246) in inert solvents. This band was assigned to triplet fluorenylidene (247). In acetonitrile, however, a second band was also detected at 400 nm and whose buildup is concurrent with the decay at 470 nm.122 Laser flash experiments in other nitrile solvents (i.e., benzonitrile and pivalonitrile) also produced a transient absorption band which is very similar to that observed in acetonitrile. The band at 400 nm was assigned to an intermediate nitrile ylide (248). This absorption could be quenched on addition of an electron-deficient olefin providing good support for its... 

Two groups have independently observed an interesting product when the vinyl azide (61) is thermolyzed.30 31 Bauer and Hafner have isolated the azirine (62) by low-temperature photolysis of 61 and have shown that thermolysis of this azirine also leads to 170.31 Apparently, elimination of hydrogen cyanide occurs to give 9-fluorenylidene carbene (171), which then reacts with azirine (62) to give the final product. 

In contrast with fluorenylidene, singlet-triplet equilibrium in the carbene derived by photoelimination of nitrogen from the diazodihydroboraanthracene (57) is slow. Direct irradiation affords singlet carbene which reacts with propan-2-ol to give the ether (58), whereas 2-acetonaphthone-sensitized irradiation yields triplet carbene which undergoes non-stereospecific addition to alkenes. Products obtained by irradiation of 3-diazo-2,5-diphenylpyrrole can arise via the intermediacy of singlet and triplet carbenes. Decomposition of the pyrrole (59) in the presence of benzene, for example, yields the 2H-cycloocta[c]pyiTole (60) as the major product. 

Laser flash-photolysis investigations have been carried out for benzylchloro-carbene, phenylchlorocarbene, methoxyphenylcarbene and fluorenylidene, and some other carbenes, generated from diazirine, diazo and other precursors, in the presence of oxiranes and thiiranes. These carbenes abstract oxygen or sulfur atoms with bimolecular rate constants in the range 10 to 10 s ... 

The majority of reactions using polycyclic aromatic diazo compounds as carbene precursors involve fluorenylidene itself, substituted fluorenylidenes and diazafluorenylidenes, but a handful of studies have also reported investigation of the reactivity of benzocycloheptenylidenes. Furthermore, there are some reports on the addition of other three-ring diarylcarbenes to C-C double bonds. In most cases, the decomposition of the diazo compounds has been carried out photochemically. 

The chlorine substituents on the 2- and 7-positions of the fluorenylidene nucleus have a meta relationship to the carbene-carbon atom. Halogen substituents in these positions are electron withdrawing inductively. Since these substituents increase ACjj, it is clear that electron donating and electron withdrawing groups have opposite influence on the magnitude of AGst and thereby, in a predictable way, the substituents control the chemical properties of these carbenes. 

Singlet and triplet fluorenylidenes have been generated and observed spectroscopically as transient products of irradiation of diazofluorene in acetonitrile. As a result, it has proved possible to calculate the rate of intersystem crossing and the rate constants for reaction of both carbenes with alkenes and alcohols. Reaction of singlet fluorenylidene (67) with alkenes affords cyclopropanes as the major product, as shown for methyl methacrylate in Scheme 9. Surprisingly,... 

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