Nov 21 2017
Chemists have created another catalyst that can selectively trigger a carbon-hydrogen bond, part of a continuing strategy to transform the field of organic synthesis and pave the way for new chemical space.
The dirhodium (shown in blue) drives the reaction, while the shape of the scaffold around it controls which C-H bond the catalyst works on. (CREDIT - Graphic images by Kuangbiao Liao, Emory University.)
The Nature journal is publishing the research by chemists at Emory University, following on their development of a similar catalyst in the previous year. Both of the catalysts are able to selectively functionalize the unreactive carbon-hydrogen (C-H) bonds of an alkane without employing a directing group, while also keeping nearly full control of site selectivity and the 3D shape of the molecules created.
Alkanes have a lot of C-H bonds and we showed last year that we can bring in one of our catalysts and pluck out a particular one of these bonds and make it reactive. Now we are reporting a second catalyst that can do the same thing with another C-H bond. We're building up the toolbox, and we've got more catalysts in the pipeline that will continue to expand the toolbox for this new way of doing chemistry.
Huw Davies, Emory Professor of Organic Chemistry
Selective C-H functionalization holds particular potential for the pharmaceutical sector, Davies adds. "It's such a new strategy for making chemical compounds that it will opens up new chemical space and the possibility of making new classes of drugs that have never been made before."
Alkanes are the simplest of molecules, comprising only of carbon and hydrogen atoms. They are inexpensive and abundant. Until the latest development of the catalysts by the Davies lab, however, alkanes were thought to be non-functional, or unreactive, except in uncontainable situations such as when they were burning.
Emory Chemistry Graduate Student Kuangbiao Liao is the first author of the Nature paper.
Davies is the director of the National Science Foundation's Center for Selective C-H Functionalization (CCHF), which is located at Emory and includes 15 key research Universities from across the country, as well as industrial partners. The NSF in recent times awarded the CCHF renewed funding of $20 million spread across the next five years.
The CCHF is leading a paradigm change in organic synthesis, which has traditionally concentrated on adjusting reactive, or functional, groups in a molecule. C-H functionalization flouts this rule for how to create compounds: It sidesteps the reactive groups and does synthesis at what would usually be considered inert carbon-hydrogen bonds, plentiful in organic compounds.
Twenty years ago, many chemists were calling the idea of selectively functionalizing C-H bonds outrageous and impossible. Now, with all of the results coming out of the CCHF and other research groups across the world they're saying, 'That's amazing!' We're beginning to see some real breakthroughs in this field.
Huw Davies
Many other methods under progress for C-H functionalization use a directing group - a chemical entity that integrates with a catalyst and then directs the catalyst to a specific C-H bond.
The Davies lab is creating a group of dirhodium catalysts that sidestep the requirement for a directing group to regulate the C-H functionalization. The dirhodium catalysts are encased within a 3D scaffold.
The dirhodium is the engine that makes the chemistry work. The shape of the scaffold around the dirhodium is what controls which C-H bond the catalyst works on.
Huw Davies