Researchers have produced the first “naked” aryl anion in solution—a carbanion not linked to counterions or stabilized by hydrogen bonding. Until now, many chemists believed such a species couldn’t exist.
Although transient, the naked carbanion persists long enough for the scientists to have characterized some of its properties. The work was carried out by Charles L. Perrin and Gabriel J. Reyes-Rodríguez of the University of California, San Diego (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja507517g).
Carbanions in solution usually take on one of two forms. Either they exist as large ions, ion pairs, or ion aggregates whose negative charge is spread out, or they exist as small ions surrounded by oriented dipolar solvent molecules or hydrogen bonds that help stabilize their negative charge. The carbanions in the new study are small but are not stabilized, says emeritus UC Berkeley physical organic chemist Andrew Streitwieser, “and that’s what makes them significant.” Small naked carbanions have been created and studied in the gas phase before but never in solution.
Naked aryl anions in solution “are unlikely to form new materials or to be important intermediates in organic synthesis,” Streitwieser notes. “Their importance lies in their physical organic chemistry—providing a previously unknown type of species in solution for testing our concepts and understanding of the relation between structure and reactivity.”
Perrin and Reyes-Rodríguez “were able to test some of the properties of these novel ions and compare them with theoretical expectations,” he adds. “This is the real significance of the work.”
“This is an elegant and important contribution that provides new insights on the reactivity of reactive intermediates,” says Igor Alabugin of Florida State University, who has studied similar reaction systems.
In the study, the UCSD researchers cyclized an enediyne to form a p-benzyne diradical. This reaction, the Bergman cyclization of enediynes, is responsible for the DNA damage and anticancer activity of natural enediynes. They then added iodide to the p-benzyne diradical. Perrin and coworkers discovered this halide addition reaction earlier (J. Am. Chem. Soc. 2007, DOI: 10.1021/ja070023e) and have now found that it produces the naked aryl anion as an intermediate.
After the naked aryl anion forms, Perrin estimates that it persists for about one nanosecond before picking up a neutralizing proton or deuteron from water, acetonitrile, or dimethylsulfoxide.
“It is brilliant work,” comments Robert G. Bergman of UC Berkeley, who discovered the Bergman cyclization. “Almost no one in the field would have predicted that p-benzyne diradicals could be trapped by nucleophiles such as iodide ion, and the case Charlie makes for the intermediacy of free aryl anions when this trapping occurs is very convincing.”