Credit: © 2010 ACS

Atropisomerism is a type of stereoisomerism that results from severely hindered rotation around a single bond. If the barrier to rotation is sufficiently high then the two atropisomers will be stable at ambient temperature and can be isolated. Certain atropisomeric structures such as those based on a 2,2'-disubstituted 1,1'-binaphthyl structure are popular scaffolds for the construction of asymmetric ligands and catalysts. In this type of structure the rotation around the biaryl bond is so hindered that the isomers can often be heated strongly without any racemization.

Now, Chris Bielawski and co-workers from the University of Texas at Austin and the University of Illinois at Urbana-Champaign have developed1 a conceptually new mechanism for the racemization of these types of structure. Bielawski and co-workers reasoned that the racemization of atropisomeric binaphthyl compounds must proceed through a high-energy transition state in which the two naphthyl rings are coplanar. They suggested that they could gain access to such a transition state by applying a tensile force across the hindered bond. The team achieved this by attaching a polymer chain to both halves of the molecule and then subjecting the compound to sonication while cooling. Starting from either enantiomer, sonication resulted in almost complete racemization in 24 hours.

To confirm that racemization was occurring by their proposed mechanism, Bielawski and co-workers showed that no racemization took place after refluxing in diphenyl ether at over 250 °C. They also found that the polymer chains must be above a threshold length for racemization to occur and that, above this, the rate of racemization was dependent on the polymer chain length.