Adventures in organic chemistry

​Before entering the Winterthur/University of Delaware Program in Art Conservation, Chun (Tracy) Liu earned her PhD in organic chemistry from Princeton University, where she worked as part of the MacMillan Group. Tracy graduated from WUDPAC in 2020 with a specialization in paintings conservation and held internships and fellowships at the Rijksmuseum in Amsterdam. She now works at Janssen Pharmaceuticals in Belgium and serves as WUDPAC adjunct faculty in Fundamentals of Science for Conservation.​ Below, Tracy Liu shares her thoughts on working with David MacMillan, co-recipient for the 2021 Nobel Prize in Chemistry.​

Wednesday October 7th was an exciting day for me and many other alumni and current members of the MacMillan group as Dave MacMillan was awarded this year's Nobel Prize in Chemistry, shared with Prof. Ben List from the Max-Planck Institute in Mülheim.  They were awarded the Nobel Prize for developing the field of Asymmetric Organocatalysis, specifically using chiral enamine and iminium intermediates.  

Unless you have studied synthetic organic chemistry, these words are likely meaningless. Thus, in  thinking about what I could write about as an alumna of the MacMillan group, I thought it would be appropriate for me to try to describe the significance of Asymmetric Organocatalysis for the WUDPAC family.  

Firstly, a quick contextualization of the field or Synthetic Chemistry.  This is a sub-branch of Organic Chemistry that focuses on developing new reactions to build bonds between various organic molecules (i.e. new C–C, C–O, C–N bonds).  Since the Industrial Revolution, society has relied more and more on making all the products we use and consume rather than sourcing them from Nature, and these products all require methods of making the final molecule.  

Certain molecules we need and use in society have a special property called 'chirality', meaning that these molecules have specific 3-dimensional geometries that make their mirror images non-superimposable on each other. These symmetric mirror image pairs, while seemingly identical, can have widely different chemical properties, making synthetic technologies that can synthesize only one of the two symmetries (hence the term 'Asymmetric' catalysis) important. Chiral molecules are highly prevalent in modern day medicines (i.e. penicillin), but setting a chiral center is synthetically very challenging.

Traditional methods of setting chiral centers have used transition metal catalysts with chiral ligands, which are less green (due to the use of transition metals), costly depending on the chiral ligands used, and often require special inert atmospheres for their synthesis (which can exponentially increase production costs). Asymmetric Organocatalysis offers a greener, cheaper, and operationally simple (reactions are neither moisture nor air-sensitive) method of setting chiral centers through a simple reversible condensation reaction of an small, chiral organic molecule catalyst.

In the MacMillan group, the catalysts we used were chiral amine-based catalysts. Using these inexpensive, robust catalysts we were able to access reactive intermediates called enamines or iminiums, and demonstrate many new useful asymmetric C–C, C–O, and C–N bond formations.

Personally, my 5 years in the group were the hardest of my life, but definitely prepared me well for life's challenges – chemical or not. I'm very grateful for those years, for Dave's vision and leadership, and the extraordinary group members he brought together.

— Chun (Tracy) Liu, WUDPAC Class of 2020​