The Addition of Fluorenylidene to Olefins in the Presence of Hexafluorobenzene^1,2

Persistent Triplet Carbenes

“…FL reacts with (E)-2-butene to give quite a high yield of cyclopropane in a stereospecific manner. 178 In the reaction of cyclohexane, 9-cyclohexylfluorene is formed as a main product (65%) along with a small amount of escaped products, fluorene (8%) and 9,9′-bifluorenyl (10%). There is no crossover product present in the FL-cyclohexane adduct when generated in an equimolecular mixture of cyclohexane and cyclohexane-d 12 .…”

A Perspective on Physical Organic Chemistry

“…Table 2 reveals that the ISC rates in cyclohexane or benzene, two nonpolar, zero dipole moment solvents are larger than those measured in acetonitrile. Much to our surprise, another slow ISC rate, comparable to acetonitrile, was found 7,11 in a halogenated solvent (hexafluorobenzene) which has zero dipole moment. To explain this result, we posited the formation of an ylide-like complex formed by a bonding interaction between the empty p orbital of the singlet carbene with a nonbonding pair of electrons of the halogen.…”

“…I still find it noteworthy that a curiosity inspired study of the kinetics of hydrolysis of chloroform would lead directly to a now standard synthetic method. Additional mechanistic studies by Doering, 3 Skell, 4 Closs, 5 Moss, 6 and Jones 7 provided a sturdy mechanistic foundation for solution-phase carbene chemistry which has stood the test of time and which has suffered no serious challenge after decades of study by physical and theoretical methods.…”

“…31 This speculation was fueled by classic experiments of Closs and Closs 5 which indicated that in DPC, spin interconversion was faster than bimolecular chemistry. It was also stimulated by the classic work of Jones and Rettig 7 with FL, in which they showed that dilution with an “inert” solvent, hexafluorobenzene, allowed singlet FL (unlike DPC where there was no dilution effect) time to relax to the lower energy triplet.…”

“…The flexibility of the carbene is not a factor in controlling the rate as posited by Salem and Rowland. 31 The difference in the Closs and Closs 5 and Jones and Rettig 7 dilution experiments is based on the difference in bimolecular reactivities of DPC (slow-sterically encumbered) and fluorenylidene (fast, unencumbered) rather than ISC rate differences. It is also clear that there is no violation of the “Golden Rule” of radiationless transitions.…”