An Interesting Period in Lab

I wanted to break up some of the monotony of the total synthesis to present a conundrum in my current research.  As a brief background, I am currently a first year graduate student at UIUC doing research related to drug delivery.  For a part of my project I want to initiate a polymerization of poly(trimethylene carbonate) with an anticancer drug, camptothecin, so that one of the end groups is this compound.  It has been previously reported that this molecule can undergo such a polymerization in the following paper.  However, my molecule I want to initiate is different.  My synthetic scheme is shown below.

Briefly, the mechanism for this reaction (if you are unfamiliar with polymer chemistry) is actually quite simple.  It all revolves around this zinc complex shown above which is named BDI-II.  The zinc metal is the coordinated metal in the center which is the active catalytic center for the polymerization.  Most polymerizations of this monomer (trimethylene carbonate or TMC) are done at high temperatures in excess of 120˚C.  With the zinc catalyst however, room temperature can start the polymerization (as I can see the solution become more viscous).  However, usually a temperature of 50-60˚C is sufficient with the polymerization being done in under 1 hour.  As I was saying, the mechanism is a coordination-insertion mechanism.  When performing this reaction, my drug molecule is mixed with the catalyst for about 20-30 minutes to form a complex which will initiate the polymerization.  The ligands on the catalyst help to control stereochemistry (none on my polymer) and can selectively activate a less hindered alcohol group over other more hindered alcohols and even amines.  The zinc-drug complex then also coordinates with the carbonyl of the TMC monomer and catalyzes a nucleophilic addition of the alcohol from the drug to the monomer.  In this way, the propagation continues.

The Problem: So here's what I really wanted to share, the rest was just background.  I have done this exact same polymerization before with pyrenemethanol acting as the initiator, and my NMR shows that I have incorporation.  However, when I attempted polymerization with this drug derivative that I made, I could get very little incorporation from the NMR.  The peaks from my drug are almost non-existent.  A further study I did actually showed that my drug disappeared with longer reaction times!  So the longer I ran the reaction, the less drug that was incorporated into my polymer.  This struck me as so strange, since the camptothecin molecule and pyrenemethanol had been shown to undergo polymerization perfectly fine.  Other tests I did to try and understand what happened to my polymer involved MALDI-TOF MS, however, my polymer is difficult to ionize, and some of the data I obtained was very confusing and sometimes even contradictory.  I basically came to the conclusion that certain polymer species were ionizing better than others.  I would be able to solve this problem a little easier if we had a working Gel Permeation Chromatography (GPC) system, however we use a DMF solvent in ours, and the refractive index detector has a difficult time differentiating between my polymer and the DMF because the refractive indexes of them are very close (this leads to a low dn/dc value which is used to calculate molecular weights of polymers).

So anyways, I've been stuck on this for a while until I started to think about it a little more.  What was happening and why was my polymerization not working, and also why was my drug disappearing?  I want to show you two pictures below before I break the answer to you.

After mixing catalyst and drug

After terminating polymerization with 1 drop water

These are images during my polymerization which I must perform with no water present so that I do not destroy my catalyst (water sensitive) or initiate my polymerization with water as the nucleophile instead.  As you can see the picture on the left shows a bright orange and clear solution.  My catalyst is a clear while crystalline solid, and my drug derivative is a fluffy slightly yellow crystalline powder.  This orange color is what I see after mixing these both in THF and letting it stir for 30 minutes (the transition takes around 5-10 minutes to appear).  The initiation with pyrenemethanol showed no color change (only a very light yellow clear solution).  On the other picture after I add 1 drop of water to terminate the polymerization, you can see that now the color is a clear yellow (this color change occurs in 1-2 seconds).  So I began to wonder why I was seeing this very intense color in my polymerization.

The Answer: So after toiling and tinkering and thinking, I came to this conclusion, which I am currently testing.  I believe that I am forming a chelate of my drug with the zinc complex!  How is this so?  Well, the only difference between my molecule and the camptothecin drug that worked fine in a previous paper is my linker that is highlighted below.  With this linker, I believe it is now possible for many of the carbonyl groups and alcohol group of the drug to form a chelate (shown below).

Now I know the picture doesn't look pretty, but remember that this is a 3D molecule.  So my hypothesis explained is this.  The zinc metal forms a very strong chelate with this drug, and essentially kicks of the BDI-II catalyst shown earlier (mixing an organic zinc and my compound also gave an identical clear orange color).  In this way, the chelate is so strong that my compound is essentially rendered unactivated as there is little/no room for my monomer to coordinate with the zinc to initiate the polymerization.  This explains why I have very little drug initiating my polymer, but it does not explain why as I increase polymerization time, my drug initiation efficiency becomes less.  My reasoning for this is that if zinc can catalyze a forward addition, certainly it can also catalyze the reverse (definition of a catalyst).  Because of the strong complex that the zinc forms, it can re-chelate with my drug and essentially rip it off the polymer!  This is an amazing example of thermodynamic control.  As my reaction time increases, the zinc (and my drug) and more happy together achieving a lower energy state.  This hypothesis explains everything down to even the color change too though!  for instance, the orange color change comes from the enhanced coordination of the pyridone carbonyl above.  This enhances the resonance structure with the amine forming a more aromatic structure (sound familiar to this post?).  What this essentially does is enhance the aromaticity of the entire molecule.  When this happens (when something becomes more delocalized) the absorbance and emission spectrums do what is called a red-shift (or bathochromic shift).  This means that everything shifts to a longer wavelength in the photo properties of the molecule.  A very well done explanation of this topic is shown on this site(look at section 4).  This shifts my yellow compound toward an orange color.  After I add my water, the zinc forms very strong coordination complexes with water and kicks off my molecule, returning back to a yellow color.

Lessons learned?  Everything has an explanation no matter how "crazy" things in lab can seem.  I always like to troubleshoot things like these.  It feels really good to figure something out, however, I hate the pressure for results in graduate school which makes expeditions such as this seem like nuisances.  So what am I doing now to prove this complex?  I could crystallize some of this compound and do X-Ray Diffraction and solve the crystal structure.  However, that would be going really above and beyond, wasting lots of time getting trained on the instrument, and the final result would not really be something that is publishable.  However, what I can do is a 2-D NMR technique known as NOEsy.  This essentially can tell me what protons couple to one another through space.  This is contrary to what is taught in undergraduate organic where protons can only couple through bonds.  This technique can show me if my linker arm which should initiate the polymer, is close to the protons near the lactone.  If I see this resonance, it is likely that the pyridone carbonyl is involved and would be indirect proof of my complex.  If I don't see this coupling of these two protons on the linker and the lactone, its not to say that I don't have the complex, but I can't really go much further without wasting time and resources.

Hope you found this interesting.