Frustrated Lewis Pair Starts A New Radical Family

Chemists have plotted a synthetic route to a new class of persistent nitroxide radicals by taking advantage of the synergistic action of an acid-base species known as a frustrated Lewis pair (FLP) to capture nitric oxide (NO). The new radicals facilitate selective oxidation reactions and controlled radical polymerizations, adding diversity to the pool of nitroxide radicals chemists have at their disposal for chemical reactions, the researchers say in a report (J. Am. Chem. Soc., DOI: 10.1021/ja302652a).

FLPs are complexes in which the Lewis acid and Lewis base have bulky substituents that prevent them from drawing close enough to each other to form a neutral adduct—the unquenched pair is said to be “frustrated.” This frustration can be used for a synthetic advantage, however: FLPs harbor pent-up reactivity, comparable with that of an organometallic catalyst or an N-heterocyclic carbene.

In the report, the researchers show that FLPs constructed from dimesitylphosphanes and bis(pentafluorophenyl)borane capture NO to form a set of unprecedented FLP-bound nitroxide radicals. The work was carried out by an interdisciplinary German-American research team led by Gerhard Erker and Armido Studer of the University of Münster, in Germany; Timothy H. Warren of Georgetown University; and Stefan Grimme of the University of Bonn, in Germany.

In testing the reactivity of the new FLP-NO species, the team found that the radicals abstract hydrogen from a variety of molecules, such as cyclohexadiene and toluene, leading to O-substituted alkoxyamine derivatives. In addition, the researchers used one of the new radicals to control the radical polymerization of styrene. The chemists observed that their FLP-NO radicals are significantly more reactive than TEMPO, the popular tetramethylpiperidine-based nitroxide radical used as a structural probe for NO in biological systems, selective chemical oxidations, and nitroxide-mediated radical polymerizations.

“By designing a totally new nitroxide structure, the authors have opened up a unique family of stable radicals whose properties can be tuned by the structure of the frustrated Lewis pair,” says polymer chemist Craig J. Hawker, a former IBM research scientist who is now at the University of California, Santa Barbara. “I can see this added structural handle breathing new life into the field of nitroxide-mediated polymerizations.”

Douglas W. Stephan of the University of Toronto, who created the first FLPs, says he is impressed by the diverse team of organic, inorganic, theoretical, and radical chemists that came together on the project “to take FLPs in a completely new direction.”

FLPs are edging toward commercial use in a variety of “green” applications, Stephan adds, including CO₂ and N₂O capture, aromatic reductions, and metal-free hydrogenations (C&EN, Feb. 27, page 8). “This highly creative and insightful strategy for FLP-based radicals presents new capabilities,” he says.