Carbenes cyclopentadienylidene

In 1967, Lloyd et al. claimed the first synthesis of a bismuthonium ylide by heating triphenylbismuthine and diazotetraphenylcyclopentadiene at 140°C. However, experimental details confirming the product are not available [67CC1042, 88JCS(P2)1829], Thermogravimetric analysis of the reaction shows that the carbene, cyclopentadienylidene, is formed at the initial step [72CC912]. 

Spectroscopic evidence for the carbene cyclopentadienylidene (71) has been observed in the photodecomposition of diazocyclopentadiene (72) in a low-temperature matrix.41 Dimerization of the carbene to afford fulvalene (73) occurs... 

The cyclic voltammogram of ferrocenyl-alkynyl monomer shows two quasi-reversible processes, and the redox potentials for the metal centers are considerably less anodic than those for MCl2(dppm)2. This is interpreted as electron donation to the metal center from the ferrocenyl group occms through the =C linker. In contrast, the redox potentials of the ferrocenyl moieties are more positive. These data are consistent with contributions of the resonance effect from the Mvene-carbene (cyclopentadienylidene) form ([M] C C Fc<->[M]+=C=C=Fv, Fv (t) -cyclopentadienyl-Ti -2,4-cyclopentadiene)iron(I)) (Fig. 4). 

Figure 4 Contributions of the resonance effect from the Mvene-carbene (cyclopentadienylidene) form in the metal-acetylide-ferrocenyl complex.

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... 

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... 

Similar results were obtained for cyclopentadienylidene 58). Cyclohepta-trienyhdene, which belongs to carbenes of group 2b), reacts as singlet and also clearly shows a nucleophilic character 59), in good agreement with the earlier calculations 5 ). 

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... 

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. 

Common to these molecules with their cyclopentadiene moieties is the so-called fulvene subunit 27. The first fulvenes, 6,6-dialkylfulvenes, were prepared as early as 1906 by Thiele et al. from sodium cydopentadienide and ketones [16]. The parent hydrocarbon 27 and many other derivatives have been thoroughly studied since the 1960s [17-19]. Diazocyclo-pentadiene (28), which is also easily prepared from cydopentadienide, is a heteroanalogue of fulvene. It has frequently been used as a precursor to other theoretically interesting molecules containing annelated cydopentadiene moieties, because its irradiation readily generates the cyclopentadienylidene 29. This carbene has, for example, been trapped with alkynes to form spiro-annelated cydopentadiene derivatives 30 (Scheme 5) [20]. It has been proved by UV spectroscopy [21] and supported by calculations [22] that these spiro[2.4]heptatrienes (so-called [1.2]spirenes) 30 experience a spedal kind of electronic... 

Cyclopentadienylidene (2) shows a very small splitting of the carbene energy levels, AE =0.13 eV. The energy of the new px and a is very close, and therefore a triplet ground state for cyclopentadienylidene is expected. This is in fact borne out by ESR measurements ) (see p. 109). 

From Table 1 we may conclude that the singlet carbenes 7,4, 7a and T should be nucleophilic. Carbene 2 shows only a weak net negative charge for the (t2 form and a slightly positive one for the configuration. Thus no great electrophilicity or nucleophilicity is to be expected for either the or or the carbene (seep. 125). For greater clearness, the orbital schemes of the (T , and op carbenes of cyclopentadienylidene are reproduced. 

The apparent lack of an electrophilic order for the addition of cyclopentadienylidene (see p. 126) may result from a product like transition state of a highly reactive carbene 2 a. Thus little charge would suggest a very high energy species with a very low activation for the addition. The reason could be a major contribution from the antiaromatic (T -carbene oi 2 a which should show some destabilisation in contrast to the aromatic />2-carbene (see also... 

As can be seen from Table 11 all carbena-cyclopentadienes (2) add stereospecifically as singlets (S). The only exceptions are tetrachloro-cyclopentadienylidene (2g) and the fluorenyhdenes 2c, 2d). The nonstereospecific addition — which is of the same order as for diphenyl-carbene — in 2g is most probably due to a heavy-atom effect of chlorine favouring intersystem crossing to the triplet (T). An aromatic ring as in 2c and 2d (as well as Br substituents) also facilitates the S—T inter con version step. 

We add to the purely organic carbene rearrangements the reactions of carbenes with molecular oxygen (8-29), because they involve an oxygen rearrangement from the primary O-oxide (8.49) to the dioxirane (8.50), as shown by Dunkin and Shields (1986) for cyclopentadienylidene (8.48). 

What about a nucleophilic carbene, for which negative charge should build up on the olefinic carbon atoms during the carbene addition cf. 5 With ArCH=CH2 substrates, electron-withdrawing aryl substituents would stabilize such a transition state and the p value should be positive. There are several examples of this phenomenon. For example, cycloheptatrienylidene, 10, adds to / -substituted styrenes with p = -t-1.02 - 1.05 (vs. a) consistent with a nucleophilic selectivity that seems to implicate the aromatic resonance form 10a as an important feature of the carbene. [45] It is satisfying to compare this result with p = -0.76 (vs. a) or -0.46 (vs. a" ") for additions to styrenes of cyclopentadienylidene, 11, where contributions of the cyclopentadienide form (11a) would render the carbene electrophilic. [46] However, these conclusions are too facile. There is reason to believe that the chemistry attributed to 10 might in fact be due to its allenic isomer 12. [47] And the electronic structure of 11 is also more complicated than the simple depiction above. [48]... 

Unsaturated carbenes are classified (Figure 10) into methylene carbenes (e.g., vinylidene, H2C=C ), vinylidene carbenes (e.g., propadienylidene, H2C=C=C ), vinylcarbenes and related a, p unsaturated carbenes (e.g., aryl carbenes and cyclopentadienylidenes), and propargylenes (e.g., propynyli-dene). ... 

One of the first unequivocal examples of carbene dimerization in matrices seems to have been that of cyclopentadienylidene (44) (Scheme 11). ° Photolysis of diazocyclopentadiene (43) in Nj matrices produced the carbene 44, which could be detected directly through both its UV and IR absorptions, provided that very dilute matrices were employed. When matrices containing 44, but little or no 43, were warmed to 20-25 K, fulvalene (45) was produced, which was recognized by its highly structured and distinctive UV absorption spectrum. Several carbenes analogous to 44, e.g., tetrachloro- and tetrabro-mocyclopentadienylidene and fluorenylidene, - failed to undergo similar dimerizations even at 30-35 K. 

Matrix IR Spectra of Radicals Photochemistry of Matrix-Isolated Radicals Carbenes and Their Reactions Cyclopentadienylidene and Related CarbenesAryl Carbenes ... 

---