Friday, July 12, 2013

Tunneling in the Bergman Cyclization

Greer, E. M.; Cosgriff, C. V.; Doubleday, C. J. Am. Chem. Soc. DOI: 10.1021/ja402445a
Contributed by Dean Tantillo

Edyta Greer and Christopher Cosgriff, in collaboration with Chuck Doubleday, have reported a quantum chemical study of the Bergman reaction of cyclodec-3-en-1,5-diyne (below). Using DFT and CASSCF calculations, they found evidence for a large contribution from heavy atom tunneling to the rate of this reaction even above room temperature.  

The performance of several levels of theory was examined, including a modified BLYP functional (mBLYP) and CCSD(T) on CASSCF geometries; the mBLYP/CASSCF method performed best.  Multidimensional tunneling effects were treated using the small curvature tunneling (SCT) approach in POLYRATE.  

It was predicted that at 200K, 60% of the rate is due to tunneling.  Moreover, the bulk of the tunneling was predicted to originate from energy levels within 2.3 kcal/mol of the transition state.  At temperatures between 310-350K a smaller energy range is predicted (~1.5 kcal/mol), which corresponds to only small changes in the length of the forming C-C bond (<0.2 Å) from its value at the transition state (this energy range is also smaller for wider barriers).  At these temperatures, rate enhancements of >30% due to tunneling were predicted.  

Experimentally testable predictions of the 12-C/13-C kinetic isotope effect at various temperatures were also made. This work not only puts some meat on the bones of various concepts associated with heavy atom tunneling, it also issues a challenge to experimentalists.