Professor Shawn Collins of the University of Montreal visited us today as the external examiner at Ramsey Beveridge’s PhD defense (Ramsey is one of Rob Batey’s PhD students). The defense went really smoothly and it was great to hear about Ramsey’s accomplishments in the area of total synthesis of macrocyclic natural products. Shawn gave a really nice talk earlier in the day. Some of his lab’s methods tackle the challenge of ring-chain equilibrium in macrocycle synthesis. They bias reactions away from oligomers and polymers without relying on high-dilution. In order to do that, Shawn resorts to biphasic systems that sequester catalysts in a highly polar and/or hydrophilic phase through the use of hydrophilic ligands (http://pubs.acs.org/doi/abs/10.1021/ja208902t). Time and again, it was clear that polyethylene glycol (PEG) has been playing a major role in this research. Shawn’s talk reminded me of a discussion I had with Professor Frey at the University of Mainz last week where we were contemplating some of the mysteries of PEG. Seriously, what is the deal with all these linear PEG-like polyethers? PEG is widely used in areas that range from drug formulation to materials chemistry. The bizarre solvent properties of this and related polether polymers are perhaps best illustrated using a comparison shown below. It is intuitively clear that addition of a methyl group to PEG would increase hydrophobicity, which is the experimentally observed result with PPG (it becomes less water-soluble). However, removal of a methylene group from PEG leads to POM, which is not soluble in water at all (remember those clamps for holding flasks in fume hoods? They are made of POM…). There are models that aim to rationalize this difference in properties and one of them suggests that solvation of water accounts for the observed difference in properties.
