Carbon-Centered Strong Bases in Phosphonium Ionic Liquids

Phosphonium ionic liquids (PhosILs), most notably tetradecyl(trihexyl)phosphonium decanoate (PhosIL-C9H19COO), are solvents for bases such as Grignard reagents, isocyanides, Wittig reagents (phosphoranes),
and N-heterocyclic carbenes (NHCs). The stability of the organometallic species in PhosIL solution is
anion dependent. Small bases, such as hydroxide, react with the phosphonium ions and promote C−H
exchange as suggested by deuterium-labeling studies. A method to dry and purify the ionic liquids is
described and this step is important for the successful use of basic reagents in PhosIL. NHCs have been
generated in PhosIL, and these persistent solutions catalyze organic transformations such as the benzoin
condensation and the Kumada−Corriu cross-coupling reaction. Phosphoranes were generated in PhosIL,
and their reactivity with various organic reagents was also tested. Inter-ion contacts involving
tetraalkylphosphonium ions have been assessed, and the crystal structure of [(n-C4H9)4P][CH3CO2·CH3CO2H] has been determined to aid the discussion. Decomposition of organometallic compounds may
also proceed through electron-transfer processes that, inter alia, may lead to decomposition of the IL,
and hence the electrochemistry of some representative phosphonium and imidazolium ions has been
studied. A radical derived from the electrochemical reduction of an imidazolium ion has been characterized
by electron paramagnetic resonance spectroscopy.