For exactly 100 years, chemists have considered double bonds impossible—or nearly so—in organic chemistry under specific circumstances. Known as Bredt’s rule, this axiom was based not on theory, but on decades of previous observations of molecules that lacked such bonds. The belief was high enough that it is widely published in textbooks. New research shows that’s not true and will encourage chemists to look for molecules they previously thought couldn’t exist.
Carbon is such a versatile element that the vast majority of molecules we know contain it. Since we ourselves are mainly composed of molecules built around a carbon structure, the study of what is and is not possible with carbon, i.e. organic chemistry, is particularly crucial for us.
The key to the flexibility of making carbon molecules is that it forms four bonds, which can include single, double or triple bonds with other carbon molecules. However, Julius Bredt claimed to find a limitation in that capacity. When a molecule contains two rings of carbon atoms joined by a bridge, Bredt asserted that the tip of the bridge cannot include a double bond. Now UCLA chemists have shown it can.
Although Bredt reached this conclusion based on the observation of the absence of such double bonds in the corresponding molecules, an explanation subsequently emerged that double bonds under these circumstances would turn the molecule out of a plane. Since then, the rule has been modified twice. Double bonds between larger ring systems are now accepted as existing. Furthermore, other chemists have attempted to make such molecules with smaller rings, but found them unstable, so the rule is now taken to preclude stable molecules with smaller ring systems. In this form the rule is recognized by the International Union of Pure and Applied Chemistry.
However, Professor Neil Garg leads a team that has found molecules that break the rule. These “anti-Bredt olefins” (ABO) may be only the tip of the iceberg, as several types have already been identified.
ABOs were created by applying fluorides to molecules whose names sound like they come from a sketch that mocked chemists: silyl (pseudo)halides. The ABOs produced were initially unstable, showing that the rule wasn’t completely wrong, but the team then used a variety of agents to trap them enough to analyze and potentially use. Perhaps the pseudohalides are not so silyl after all.
Among scientists, as with other people, there are always those who want most to do what they are told is impossible. However, many more have accepted Bredt’s rule, at least in modified form, and haven’t looked back.
“People aren’t exploring anti-Bredt olefins because they think they can’t,” Garg said in a statement. However, they did not ignore ABOs because they thought they would be useless.
“There is a big push in the pharmaceutical industry to develop chemical reactions that yield three-dimensional structures like ours because they can be used to discover new drugs,” Garg said. “What this study shows is that contrary to a hundred years of conventional wisdom, chemists can produce and use anti-Bredt olefins to produce value-added products.”
Findings like this raise questions about how often textbooks are wrong in other ways. Garg sees the problem as treating observational rules as if they were fundamental laws. “We should not have such rules – or if we do, they should exist only with the constant reminder that they are guidelines, not rules. It destroys creativity when we have rules that are supposed to be unbreakable.”
The study is published in Science.