Fluorenyl Radicals

The fonnation of these substances contradicts common ideas on nucleophilic substitution. The presence of radical traps (oxygen or tetrabromobenzoquinone) decelerates the formation of both unexpected compounds and product of thioarylation. Consequently, the first stage of the reaction depicted in Scheme 4.5 produces phenylthiyl radical and anion-radical of the substrate. Both electron-transfer products undergo further conversions The phenylthiyl radical gives diphenyldi-sulfide, and the anion-radical of the substrate produces 9-fluorenyl radical. The latter reacts in two directions—dimerizing, it forms bifluorenyl reacting with the nucleophile, it gives the anion-radical of the substitution product. The chain continues because the electron from the anion-radical is transferred to the unreacted molecule of the substrate. The latter loses bromine and then reacts with the nucleophile, and so on (Scheme 4.6). 

The mechanism of the reaction depicted in Scheme 4.6 differs from the Sf.,1 or Sf.,2 mechanism in that it involves the stage of one-electron oxidation-reduction. The impetus of this stage may be the easy detachment of the bromine anion followed by the formation of fluorenyl radical. The latter is unsaturated at position 9 near three benzene rings that stabilize the radical center. The radical formed is intercepted by the phenylthiolate ion. This leads to the anion-radical of the substitution product. Further electron exchange produces the substrate anion-radical and final product in its neutral state. The reaction consists of radical (R)-nucleophilic (N) monomolecular (1) substitution (S), with the combined symbol Sj j l. Reactions of Sj j l type can have both branch-chain and nonchain characters. 

Behavior of FL (23) toward the C—H bond is in contrast with that of DPC (14a). Picosecond LFP of diazofluorene (36) in cyclohexane generates a transient absorption band at 470 nm ascribable to FL. As time proceeds, a band at 497 nm appears, while the 470-nm absorption is still present. On a nanosecond time scale, the two bands at 470 and 497 nm are observed and assigned to 9-fluorenyl radical (FLH ) (x = 1.4 ns), indicating that 23 rapidly abstracts a hydrogen atom k = 1.1 X 10 s ) from cyclohexane to give FLH. Although the growth of the... 

Picosecond electronic absorption spectroscopy was used to investigate the formation of the 9-methyl-9-fluorenyl radical following 266-nm, 18-ps pulsed excitation of the peroxyester in either cyclohexane or acetonitrile. The absorption of the 9-methylfluorenyl radical (3) grows with a rise time of 55 15 ps as monitored at a single, unreported wavelength. The proposed events following 266-nm, pulsed excitation is outlined in Scheme 19.1 in terms of a pathway in which the 0—0 bond is... 

The autoxidation of 4-undecanone in ah at 130 °C leads to the formation of hydroperoxides, which decompose at 120-160 °C via different radical pathways to give CO, CO2, and H2 by parallel pseudomonomolecular processes.304 An extremely sterically crowded heptatriene (141) is reported to undergo autoxidation at 25 °C in cyclohexane. The isolated products were rationalized by the dissociation of (141) to the tropyl radical (142) or fluorenyl radical (143) and subsequent attack by molecular oxygen (Scheme 22).305... 

The final radical cation to be discussed in this section is derived from spirofluorenebicyclo[6.1.0]nonatriene (164, R-R = fluorenylidene). This system gives rise to an intermediate (165) in which a fluorenyl radical and a homocyclo-... 

The photochemistry of triphenylmethyl radicals in solution has been of interest for some time because the relatively long lifetimes of these species (some are essentially stable) have made their study accessible with low-intensity light sources. Relatively recently, laser flash photolysis has been used to confirm the tendency of the excited radicals to undergo ring closure to form the 9-phenyldihydro-fluorenyl radicals. 

Two transients have been observed during flash photolysis of 9-fluorenol and both have been assigned the fluorenyl cation structure.A new study has concluded that while one of these transients is indeed the cation, the other corresponds to the fluorenyl radical cation.A transient assigned as the carbocation obtained by loss of hydroxide has also been observed by flash photolysis of arylxanthenol (417) in aqueous ethanol. 

The reduction of perchlorofuchsone with HI/Ij gives the 4-hydroxytetra-decachlorotriphenylmethyl radical (HO—PTM-) (p. 364). Perchloro-9-phenyl-3-fluorenone behaves likewise, although the product is the 9H compound [131] (Ballester et al., 1984c). In fact, the intermediate fluorenyl radical [130] has been detected by esr spectroscopy (Armet, 1978) (176). 

In the fluorenyl radical [130], the shielding of the 9-carbon is obviously not so effective as in HO—PTM- (p. 364), so allowing immediate hydrogen abstraction from HI. The isomeric ketone [132] is not reduced at all, this being explained by the loss of n-electron delocalization between the quino-methane moiety and the fluorene system that would occur in the reduction of cation [133] caused by twisting of the p-hydroxytetrachlorophenyl group around its bond to C(9) of the fluorene system (steric inhibition of resonance) (177) (p. 384). 

S Fluorenyl radicals with oxygen in -position relative to C (9) ... 

Assignment may be reversed. > 3) Assignment may be reversed. HUckel calculation of spin densities.-A radical of similar structure is obtained by addition of phenyl radicals to 6,6-dimethyl-dibenzo-fulvene (well resolved ESR spectrum shown). Erroneously assigned to 9-fluorenyl radical. ... 

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