Fluorenyl carbanion

The electron transfer reaction from fluorenyl carbanions adjacent to sulfoxide or sulfone (9) to fluorenone (FIO) has been studied by means of flash photolysis37. For n — 1 as well as n — 2 the transient F10T, M + (contact ion pair) appeared in THF and F10T(free ion)... 

Bordwell and coworkers63 87 have studied the reaction of 9-fluorenyl carbanions (9-RFP) with a series of electron acceptors and in particular a-halosulfones and sulfoxides, in dimethyl sulfoxide solution. The overall reaction is characterized by the formation of the 9,9 -bis-fluorenyl derivative and the reduction of the halogenated acceptor. A family of 9-substituted fluorenyl carbanions covering a basicity range of 9.1 pKa units was used and... 

TABLE 8. Second-order rate constants for reactions of 9-R-fluorenyl carbanions with a-halosulfones and sulfoxides in Me2SO solution at 25 °c63a-87... 

Fluorenyl carbanions 1055, 1065, 1066 Fluoroalkylsulphinyl groups, sigma values of 507-509... 

Fluorenyl carbanions, as initiators, 14 258 (+) -1 -(9-Fluorenyl)ethylchloroformate, chiral derivatizing reagent, 6 76t 9-Fluorenylmethyl chloroformate, molecular formula, 6 29It Fluorescamine, chemiluminescence reagent, 5 850... 

Wong, Konizer, and Smid (42) find by a proton nuclear magnetic resonance technique that in the solvent tetrahydrofuran, dicyclohexyl-18-crown-6 binds the ion pair Na+, F (F = fluorenyl carbanion) more strongly than it does the K+, F ion pair. Although no K values are given they estimate log K to be larger than 10 for the reaction Na+, F + L = Na+, F L (L = dicyclohexyl-18-crown-6 or dimethyldibenzo-18-crown-... 

Background and Possible Intermediates. Accepting the premise of carbanion formation in the basic media, the mode of reaction with molecular oxygen can now be considered. Sprinzak (8) reported that the autoxidation of fluorene in basic media proceeds by direct reaction of the fluorenyl carbanion with oxygen to form initially the hydroperoxide, which decomposes to yield 9-fluorenone, as depicted below. 

Addition of a second crown produces the loose ion pair A, Cr,K, Cr. However, the complexation constant for adding the second crown is 1800 M 1 for the fluorenyl carbanion and only 200 M 1 for the picrate salt. The lower value for picrate may in part be due to less charge delocalization, e.g., the free ion dissociation constant for potassium fluorenyl in TEF is 1.6 x 10 7M (18) as compared to 9.2 x 10 M for potassium picrate (17). The two N02 substituents close to the 0 bond in picrate may also hinder the enlargement of this ionic bond and the insertion of a crown ether molecule because of electronic or sterlc effects. 

Intramolecular triple Ion formation was studied by Collins and Smld using cesium bolaform salts of fluorenyl carbanions (29). The cesium salts of a, oe-bis (9-fluorenyl) polymethylenes (VI, n 2,3,4 and 6), when dissolved In THF,... 

A typical cyclic voltammetric trace for the anodic oxidation of the fluorenyl anion 2 at platinum is shown in Figure 1. The oxidation potential for this and several other resonance stabilized carbanions lies conveniently within the band gap of n-type Ti02 in the non-aqueous solvents, and hence in a range susceptible to photoinduced charge transfer. Furthermore, dimeric products (e. g., bifluorenyl) can be isolated in good yield (55-80%) after a one Faraday/mole controlled potential (+1.0 eV vs Ag quasireference) oxidation at platinum. 

Fluorenyl Carbanions. Salts of fiuorene (pAa = 22.6) (6) are more hindered and less reactive than many other organometallic initiators. These carbanions can be readily formed by reaction with alkali metal derivatives as shown in equation 19 for 9-methylfiuorene (99). Carbanion salts of 9-methylfiuorene are preferable to fiuorene, since the latter generate chain ends which retain reactive, acidic fluorenyl hydrogens which can participate in chain-transfer reactions (100,101). Fluorenyl salts are useful initiators for the polymerization of alkyl methacrylates, epoxide, and thiirane monomers. 

Activated nitro and halo substituents have been efficiently replaced by a variety of alkyl groups via SsAx reaction with carbanions. Examples include the displacement of the nitro group in compounds (10 X = 4-PhCO, 4-MeOCO, 4-CN, 4-N02, 4-PhS02, 3,5-(CF3>2) by the anion of 2-nitropropane in HMPA at room temperature (equation 2),83 and the reaction of p-dinitrobenzene with several ketones, esters and nitriles (RH equation 3) in Bu OK/liquid NH3 at -70 C.84 Interestingly, under the latter reaction conditions, p-chloronitrobenzene gave the product of alkylation rather than of SNAr displacement of chloride, as in equation (4).85 Further examples include the dehalogenation of p-halonitrobenzenes by 9-fluorenyl anions in DMSO at room temperature,34 and dehalogenation and denitration reactions by the carbanions of phenyl- and diphenyl-acetonitrile in DMSO or under PTC conditions.86... 

These rearrangement reactions are interpretable in terms of [2.3] sigmatropic shifts of the intermediate ylides. A number of such rearrangements of open-chain systems have been described, involving sulfonium ylides [43] [44] [45], ammonium ylides [46] [57], anions in a-position to oxygen (Wittig rearrangement) [48] [49], and fluorenyl carbanions [50]. 

The ion pair status is not predictable only on the basis of the cation used. For highly delocalizable carbanions like 9-fluorenyl in THF, the CIP fraction was found to increase rapidly in the order Li < Na < K < Cs [22] and the caesium salt was found to be the most bathochromic in the series. The degree of association of carbanion ion pairs in non polar solvents is highly dependent upon the extent of charge delocalization [23]. With highly delocalizable anions SSIP will be formed with Li cation. A completely different sequence of ion pairs was shown in the case of naphtholate anions the CIP fraction increases in the order K < Na < Li [24], These anions are thus to be considered as more localized anions in order to agree with the preceding observations. 

The solvent effects on the absorption spectra of ion pairs were studied by many authors and the direction of the observed shift depends on the change (increase or decrease) of dipole moment upon the electronic transition [25]. Generally a bathochromic shift is observed with an increase of solvent polarity. When going from a polar solvent to a less polar one, the association in the ground state increases more strongly than in the excited state this may be understood if the ion pair switches progressively from SSIP to CIP status. Observations of this type were often made, together with cation effects, as for instance in the case of alkali phenolates and enolates [7], fluorenyl and other carbanion salts [22] or even for aromatic radical anions [26, 27],... 

The formation of the polymeric carbanions 81 of the fluorenyl-type is successful starting from the poly(para-phenylene) ladder polymer 71 with butyl-lithium as metallating agent. The degree of lithiation lies in the range of 90-95% (NMR). The UV/VIS absorption spectrum of these polymeric anions (81) is comparable with that of the 9-phenylfluorenyl anion and indicates the presence of mostly localized (anionic) sub-structures [101]. 

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