Chemistry Seminar: Xiaotai Wang

CSM was honored to hear from Dr. Xiaotai Wang on Friday in his lecture on computational mechanistic studies of transitional metal-catalyzed synthetically useful organic reactions. Dr. Xiaotai Wang has been a professor at the University of Colorado, Denver, since 1997. His last lecture at Mines was over 10 years ago on the synthesis of metal frameworks. Since then, Dr. Wang has been attracted to the field of computational chemistry. Dr. Wang said that he was drawn to this field because of its utility in providing insights into the designing of new molecules. He is currently researching the synthesis and of metal-organic frameworks (MOFs) with newer structures and a whole host of useful properties. Dr. Wang worked in a field called computational chemistry, a branch of chemistry involving computer simulations of chemical structures to assist in solving different chemical questions. Dr. Wang noted that there are two main divisions of computational quantum chemistry: wave function based and density function based.

Dr. Wang’s research looked at how metal catalysts could be improved to obtain different arrangements, or stereospecific molecules. Specifically, Dr. Wang and other researchers wanted to synthesize “Z” molecules as opposed to “E” molecules. These two letters are used to describe to position of different larger groups of molecules in relation to a carbon-to-carbon double bond. If the molecule is Z, then the two groups are on the same side across the double bond. If the molecule is E, then the molecules are diagonal from one another across the double bond. This stereospecificity might seem quite nit-picky, especially when the product being examined may contain hundreds of other molecules, but the two different orientations can immensely affect how a catalyst works. If a catalyst is meant to adhere to a binding sight, the slightest difference can cause the whole reaction to take much longer or not occur at all. This is especially important when looking at catalysts used to synthesize functional polymers or pharmaceutical molecules.

It is well known among chemistry students that transition metals make excellent catalysts for organic chemistry reactions. There are several reasons why this is. The first is that they have d orbitals, giving them the ability to form complexes and activate organic molecules. They also have different oxidation states, which facilitate the transfer of electrons. This is the basis of redox catalysts. Dr. Wang looked at several different catalytic reactions of interest. They each involved a metal catalyst (such as Ru or Pd) resulting in carbon double bonds, single bonds, or bonds to hydrogen.

These catalysts can be used in what is called “olefin metathesis.” This involves olefins, or carbon chains containing double bonds, being broken apart and swapping pieces with another olefin. Dr. Wang calls it a “change your partners dance.” This reaction was discovered in the 1950’s but not fully understood or explained until the 1970’s when a man named Yves Chauvin proposed the first plausible mechanism of the reaction. Because the direct addition of two alkenes has such a high activation energy, it is forbidden and cannot occur just on its own. Chauvin proposed instead that the d orbitals in the double bond of the catalyst could interact with those of the double bond of the carbon to form a lower energy intermediate. This means that the activation energy is lowered significantly and the reaction can occur. Piggybacking on this research, in the 1980’s two men named Robert Grubbs and Richard Schrock made well defined catalysts that supported Chauvin’s mechanism. For their research and discoveries, Chauvin, Grubbs, and Schrock were awarded a Nobel Prize in chemistry in 2005.

Prior to 2011, there were only three generations of Grubbs catalysts that had been discovered and produced. These were Grubbs I, Grubbs II, and Grubbs-Hoveyda. Each catalyst was unique and modified slightly each time to fit different needs. All 3 are still commercially available today. All of them work by the same mechanism and produce no stereoselectivity, meaning that E and Z compounds appear in relatively the same amounts. However many researchers were looking to find a catalyst that would result in just Z isomers for the synthesis of natural products and functional polymers.

This is where Dr. Wang and his fellow researchers come in. Through much experiment, they were able to make a breakthrough in 2011 and create a catalyst with 95% conversion to the Z isomer. Through much trial and error and many failed attempts at deducing the transition states, Dr. Wang made several discoveries on how these catalysts work. He was also involved in the development of several other new catalysts that can give mostly Z isomers. Dr. Wang’s research and development has been published in several scientific journals for the past couple of years. He had a paper published in JACS earlier this year. Now he is pushing onward to the next ongoing project: ligand promoted carbon hydrogen activation and functionalization and isolating intermediates in this process.