9- fluorene deprotonation

Li, Na, or Grignard compounds of cyclopentadiene, indene, or fluorene add to di-substituted cyclopropenium cations forming cyclopentadienyl cyclopropenes, which can be transformed to calicenes by subsequent hydride abstraction and deprotonation, as shown by the following examples 9S91 and 9992 (cf. p. 13) ... 

Just as disulfonium dication 34, diselenonium 113 and ditelluronium dication 114 do not undergo deprotonation. Instead, reaction of dication 113 with fluorenyllithium affords bis-selenide and fluorene dimer 103.96 Softer Lewis base such as ra-tolyl magnesium bromide reacts with diselenonium-dication 113 to give 127, a product of nucleophilic substitution at the onium atom (Scheme 48).129... 

On the contrary, the oxidation of fluorene in a basic solution is not limited by the deprotonation of hydrocarbon [284]. This is in agreement with the oxidation of fluorene and 9,9-dideuterofluorene at the same rate in DMSO and 1,1-dimethylethanol solution. The stoichiometry of fluorene oxidation is close to unity (except oxidation in HMPA) and the main product of the reaction is fluorenone. The stoichiometry and the initial rate of the reaction depends on the solvent (conditions 300 K, [fluorene] = 0.1 mol L 1, [Me3COK] = 0.2mol L 1,p02 = 97kPa). 

The ability of the stable carbene 218 to deprotonate acidic hydrocarbons was examined by NMR in (CD3)2S0.153 Indene (pJta = 20.1) was completely converted to its anion whereas 9-phenylxanthene (pAfa = 27.7) was not measurably deprotonated. The NMR spectra of 1 1 mixtures of 218 with fluorene (pXa = 22.9) and 2,3-benzofluorene (pA"a = 23.5) showed separate absorptions for the hydrocarbons and their anions. From the integration of these spectra, P a = 24.0 for 218 was derived. In THF, 218 failed to deprotonate fluorene but almost completely deprotonated indene. The proton transfer from hydrocarbons to 218 creates ions (ion pairs) from neutral species, which will be less favorable in solvents of lower polarity. 

Although the exact mechanism of the fluorenone formation is not known, it is believed that the monoalkylated fluorene moieties, present as impurities in poly(dialkylfluorenes), are the sites most sensitive to oxidation. The deprotonation of rather acidic C(9)—H protons by residue on Ni(0) catalyst, routinely used in polymerization or by metal (e.g., calcium) cathode in LED devices form a very reactive anion, which can easily react with oxygen to form peroxides (Scheme 2.26) [293], The latter are unstable species and can decompose to give the fluorenone moiety. It should also be noted that the interaction of low work-function metals with films of conjugated polymers in PLED is a more complex phenomenon and the mechanisms of the quenching of PF luminescence by a calcium cathode was studied by Stoessel et al. [300],... 

A similar phenomenon was found by Nakamura and Oki who investigated deprotonation of 9-(2-methoxy-l-naphthyl)fluorene (173) (Scheme 8). The ap... 

Deprotonation energies for 9-substituted fluorenes, calculated using AMI semiem-pirical MO theory, correlate linearly with acidities determined experimentally for these heteroatom-substituted compounds. ... 

Standard organolithium reagents such as butyllithium, ec-butyllithium or tert-butyllithium deprotonate rapidly, if not instantaneously, the relatively acidic hydrocarbons of the 1,4-diene, diaryhnethane, triarylmethane, fluorene, indene and cyclopentadiene families and all terminal acetylenes (1-alkynes) as well. Butyllithium alone is ineffective toward toluene but its coordination complex with A/ ,A/ ,iV, iV-tetramethylethylenediamine does produce benzyllithium in high yield when heated to 80 To introduce metal into less reactive hydrocarbons one has either to rely on neighboring group-assistance or to employ so-called superbases. 

As evidence that a monolithiated compound can form an equilibrium with the corresponding dilithiated compound involved, the group of Linti and coworkers crystallograph-ically examined 9,9-dilithiofluorene (45). The molecule is formed when 9-lithiofluorene (44) is dissolved in a THF/benzene mixture by an intermolecular deprotonation yielding 45 and fluorene (43). In spite of the poor solubility of compound 45, which does not allow a characterization by NMR methods, the equilibrium is still on the side of the monolithiated compound (Scheme 16). The crystal structure of compound 45 has been reported and reveals a coordination polymer in the solid state (Figure 13). 

A second method that has been developed more recently uses the 9-fluorenylmethyl-carboxy (Fmoc) group.148 The Fmoc group is stable to mild acid and to hydrogenation, but it is cleaved by basic reagents through deprotonation at the acidic 9-position of the fluorene ring. 

The increase in the electron acceptor concentration in a solution transforms the reaction of fluorene oxidation into regime when the first stage (deprotonation of fluorene) limits the chain process. For the oxidation of other alkylaromatic hydrocarbons, see Refs. [292 304]. 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

Values of kp emerging from these studies indicate that the (fluoren-9-ylidene)-methane dianions will deprotonate acids in the range pK, 1117 at useful rates. Furthermore there is an indication (Fig. 4) that the kinetic basicity of such EGB s may be roughly correlated with the second reduction potential, E (2). This possible correlation needs further rigorous examination as more reliable kinetic information is obtained the k values used in Fig. 4 are not corrected for the competition from reproportionation. 

Danopoulos and co-workers reported on the preparation of NHC ligands with pendant indenyl and fluorenyl groups.19 Deprotonation of the alkyl -indene or -fluorene imidazolium salts with one equivalent of potassium hexamethyldisilazide leads to NHCs functionalised with neutral indene or fluorene moieties (IndH-NHC and F1H-NHC). Further deprotonation with... 

In the case of a relatively strong acid such as cyclopentadiene, the solution of the acid is vigorously stirred with the KOH for about one hour. Weaker acids, such as fluorene and triphenyl-methane, require 10-15 hours, or even longer, to effect complete deprotonation. The reaction times are not appreciably reduced by increasing the temperature. The rate and, in some cases. 

The Fmoc group can be removed with mild bases such as ammonia, piperidine, or morpholine [281]. The cleavage goes through a rapid deprotonation of the fluorene group to generate an aromatic dibenzocyclopentadienide anion. In a subsequent slower step, elimination generates dibenzofulvene (itself an unstable species that rapidly adds nucleophiles) and a carbonate residue, which then decomposes with loss of carhon dioxide to release the free alcohol (O Scheme 47). 

These are complex reaction systems with a number of side reactions, and it was found that addition of weakly acidic species such as malononitrile and fluorene suppressed competitive solvent deprotonation and side reaction pathways so initiated. The systems were further sophisticated when a redox mediator such as azobenzene was added. Now the cathodic potential in the second step of the procedure could be substantially reduced until only the mediator remained elec-... 

Gold complexes of Cp-related ligands have also been prepared. 9-Fluorenyl (tri-phenylphosphine)gold(I) (2)131 can be obtained via the oxonium salt route, but the reduced C—H acidity of fluorene requires the use of an additional base, such as sodium hydride, to deprotonate the hydrocarbon. 

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