Fluorenone radical anion

Early ESR studies demonstrated that the hyperfine coupling constant (ac 13) for 13C(car-bonyl)-substituted fluorenone radical anion is counterion-dependent. For the free ion, ac 13 = 2.75 Gauss. In contrast, when the counterion is Li+, ac 13 = 6.2 Gauss23. Consider Scheme 4 For the free ion, canonical structure 1 and 2 are contributors to the resonance hybrid. For the >C=0 / Li+ ion pair, association of Li+ with oxygen increases the relative contribution of canonical structure 1 to the resonance hybrid, resulting in greater spin density at carbon. The fact that spin (and charge density) varies as a function of counterion (and presumably solvent) will certainly affect the reactivity of the radical ion. However, very few quantitative studies exist which directly address this point. 

Since the lifetime of fluorenone radical anions lies in the ps-time region, conventional flash photolysis has been used to measure the transient absorption spectra [32]. The effect of solvent and added salts on the reactions of triplet fluorenone and electron rich alkenes is shown in Figs. 1 and 2, respectively. 

This corresponds to results reported by Peters [35] who observed the fluorenone radical anion in the presence of diazabicyclo[2.2.2]octane (DABCO) with a halflife of ca. 60 ps. The unusually long lifetime of the transients observed by us can be explained with the formation of radical ion pairs in the triplet state. 

The 02 -Sc + complex can be stabilized by complexation with 3 equiv. of HMPA ligand as reported for the case of the fluorenone radical anion-Sm +... 

The interaction between alkali metal fluorenone radical anions and phosphorus ligands has been investigated using NMR spectroscopy." ... 

The samarium-lV-bromosuccinimide combination reductively dimerizes carbonyl compounds.163 This pinacol-type coupling gives diols in 60-80% yield, with some diastereoselectivity the by-product from simple reduction (i.e. alcohol) is typically 5-10%. The conditions suggest a single electron transfer to give carbonyl radical anion, which then self-couples. Even congested ketones such as benzophenone and fluorenone worked well. 

In this account, we will focus on the transient analysis of these systems, which has strongly contributed to a deeper understanding of the diverse reaction modes (Patemo-Buchi, proton abstraction, cycloaddition). In general, aromatic ketones were selected as electron acceptors for reasons of suitable excitation and long wavelength absorption of the radical anion intermediates. Among them, fluorenone 3 is particularly well suited since the concentration, solvent, temperature, and cation radius dependence of the absorption spectra of pairs formed with metal cations are already known [29]. Hogen-Esch and Smid [30, 10] pointed out that a differentiation between CIP and SSIP is possible for fluorenone systems. On the other hand, FRI s and SSIP s cannot be differentiated simply by their UV/Vis absorption spectra, whereas for instance conductance measurements may be successful. However, the portion of free radical ions in fluorenyl salt solutions was shown to be less important [9, 31]... 

Benzocyclobutadienequinone (199) S78 fluorenone.benzil, 4,5-phenanthryl-eneketone (200), o-phenanthrenequinone and acenaphthenequinone 579 yield stable ketyls on cathodic reduction in DMF or acetonitrile, and these have been characterized by ESR. An interesting electrocyclic isomerization of the anion radical 201 (Eq. (252) ) to the 2,2 -bisallyl radical anion 202 was observed by ESR, when 1,2-dimethylenecyclobutane (203) was reduced in THF at-90°C 581 ... 

In some cases the radical anions of azines in DMF are stable enough to allow their electron spin resonance (esr) spectra to be obtained. The radical anion of fluorenone azine is thus stable for months in DMF [52,53]. In the absence of a proton donor, the radical anion is reduced to the dianion, which is stable on the time scale of CV. In the presence of a relatively strong proton donor, such as hexafluoro-2-propanol, fluorenone fluorenylhy-drazine is obtained [53] this is analogous to the reduction of benzalazine in aqueous alkaline solution [1]. 

Table 9 Percentages of Radical Anions Formed in Reactions of Dibenzylmagnesium (0.375 M) with Fluorenone (0.345 M) in Various Solvents [35]...

One of the strongest pieces of evidence against the formation of the carbene radical anion comes from an experiment using a nitrogen-15 labelled diazofluorene. The latter was prepared by reacting fluorenone oxime labelled 88% with nitrogen-15 with chloramine (equation 24). 

Section 15.2 contains hydrocarbon and substituted hydrocarbon anion radicals. It is divided into two main parts Section 15.2.1 deals with hydrocarbon radical anions whilst Sect. 15.2.2 deals with substituted compounds and is subdivided into 3 subsections. Section 15.2.2.1 is concerned with the substituted radical anions, Sect. 15.2.2.2 deals with perfluoro and perchloro substituted radical anions, and Sect. 15.2.2.3 contains substituted fluorenones. These have been included for completeness and because we feel that they may not be included elsewhere. Mono-, di- and trianions are also included. 

From ketones containing a carbonyl group in the ring radical anions have been obtained in the case of 9-fluorenone [58, 136], 2,7-difluorofluorenone [136], 4,5-phenanthryleneketone [136], and an-throne [58]. Radical anions have also been obtained and investigated for ketones in the heterocyclic series — 4-acetylpyridine [138] and 2-, 3-, and 4-benzoylpyridine [68]. 

The stereoselective 1,4-addition of lithium diorganocuprates (R2CuLi) to unsaturated carbonyl acceptors is a valuable synthetic tool for creating a new C—C bond.181 As early as in 1972, House and Umen noted that the reactivity of diorganocuprates directly correlates with the reduction potentials of a series of a,/ -unsaturated carbonyl compounds.182 Moreover, the ESR detection of 9-fluorenone anion radical in the reaction with Me2CuLi, coupled with the observation of pinacols as byproducts in equation (40) provides the experimental evidence for an electron-transfer mechanism of the reaction between carbonyl acceptors and organocuprates.183... 

Catalysis by various low-valent metalloporphyrins of the type already depicted in Section 3.7.2 (see reference lb for a precise list) is represented in Figures 4.3 and 4.4 for several cyclic and acyclic 1,2-dibromides. A striking example of the contrast between redox and chemical catalyses is shown in Figure 4.3a, with fluorenone anion radical on the one hand and iron(I) octaethylporphyrin on the other. Starting with the oxidized, inactive form of the catalyst, in each case—the active form is produced at a reversible wave. Addition of the same amount of 1,2-dibromocyclohexane triggers a catalytic increase in the current that is considerably less in the first... 

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