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9-Fluorenylidene
9-Fluorenylidene is an arylcarbene (sub-family of carbenes) derived from the bridging methylene group of fluorene. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol lower in energy than the singlet state (grasse et al). For this reason, fluorenylidene has been studied extensively in organic chemistry.

Fluorenylidene is a reactive intermediate, and is short-lived. Reactions involving fluorenylidene proceed through either the triplet or singlet state carbene. The products formed depend on the relative concentration of spin states in solution, as influenced by the solvent’s polarity and its effect on rates of intersystem crossing(2). Solvents that deactivate the higher energy singlet state push the intersystem crossing equilibrium towards the triplet state, giving relatively higher yields of non-stereospecific products (MOODY+WHITAM). Conversely solvents that increase or maintain the energy gap between spin states, favor reaction with the first-formed singlet state, maintaining the stereospecificity of the products formed.

Generation of Fluorenylidene
Fluorenylidene can be produced by photolysis of 9-diazofluorene (DAF).

SHOW REACTION OF (Page 28 Moody+Whitam)

Ultra-fast (300 fs) time resolved laser-flash photolysis of DAF implicates a four-step process in the formation of fluorenylidene by irradiation of 9-diazofluroene. (WANG ET AL 2007) (1) Irradiation of DAF initially yields an excited singlet state diazofluorene molecule ( 1DAF*) (2) 1DAF* decays to form the open shell carbene,1FL*, as the minor product, and the less energetic closed shell carbene, 1FL, as the major product. (3) Any excited singlet 1FL* in solution relaxes to the lower energy singlet state 1FL (20.9 ps) (4) 1FL equilibrates with the ground state triplet 3FL by intersystem crossing. (Grass et al. 1983).

Reaction of Fluorenylidene in Solution
Fluorenylidene effectively exemplifies the chemistry of carbenes, as outlined by the Skell Hypothesis (1965) (1). The stereochemistry of cycloaddition products depends on the relative rates of cycloproponation (or other reactions) and intersystem crossing (Moody Whitam). Intersystem crossing is competitive to chemical reactions (grasse et al). Stabilization of specific spin states, and, by extension, increased stereospecificity can be achieved by using solvents of different polarities (as discussed below) (2).

Triplet Fluorenylidene Reactivity
Triplet fluroenylidene reacts with olefins in a stepwise fashion to produce a racemic mixture, provided that the rate of spin inversion (intersystem crossing) is not significantly faster than rates of diradical bond rotation (Moody + Whitam).

[1] Show cycloaddition

[2] Show insertion reaction BOTH C-H, and X-H insertion

[3]

Singlet Fluorenylidene Reactivity
Singlet fluorenylidene reacts with olefins in a concerted fashion, maintaining the stereochemistry of the reactant olefin. Triplet quenchers such as butadiene solvents can be used to increase stereospecific yields (Moody+Whitam). Halogenated solvents also stabilize the singlet state (2)(Wang et al). For example, dibromomethane and hexafluorobenzene deactivate the higher-energy singlet state (Moody + Whitam), decelerating the rate of intersystem crossing in accordance with earliers studies in diphenylcarbene (2). The mechanism of triplet state stabilization is theorized to occur through halogen-lone pair scomplexation of empty 1Fl P-orbitals (Wang Et Al 2007). SHOW 1Fl stabilization by lone pair solvation

(Turro, N. J., Butcher Jr, J. A., Moss, R. A., Guo, W., Munjal, R. C., & Fedorynski, M. (1980). Absolute rate constants for additions of phenylchlorocarbene to alkenes. Journal of the American Chemical Society,102(25), 7576-7578.) (1) Skell, P. S., & Woodworth, R. C. (1956). Structure of Carbene, CH2. Journal of the American Chemical Society, 78(17), 4496-4497.

(2) Sitzmann, E. V., Langan, J., & Eisenthal, K. B. (1984). Intermolecular effects on intersystem crossing studied on the picosecond timescale: the solvent polarity effect on the rate of singlet-to-triplet intersystem crossing of diphenylcarbene. Journal of the American Chemical Society, 106(6), 1868-1869.