Prospects for the Computational Design of Bipyridine N,N′-Dioxide Catalysts for Asymmetric Propargylation Reactions

Stereoselectivities were predicted for the allylation of benzaldehyde using allyltrichlorosilanes catalyzed by 18 axially chiral bipyridine N,N'-dioxides. This was facilitated by the computational toolkit AARON (Automated Alkylation Reaction Optimizer for N-oxides), which automates the optimization of all of the required transition-state structures for such reactions. Overall, we were able to predict the sense of stereoinduction for all 18 of the catalysts, with predicted ee's in reasonable agreement with experiment for 15 of the 18 catalysts. Curiously, we find that ee's predicted from relative energy barriers are more reliable than those based on either relative enthalpy or free energy barriers. The ability to correctly predict the stereoselectivities for these allylation catalysts in an automated fashion portends the computational screening of potential organocatalysts for this and related reactions. By studying a large number of allylation catalysts, we were also able to gain new insight into the origin of stereoselectivity in these reactions, extending our previous model for bipyridine N-oxide-catalyzed alkylation reactions (Organic Letters 2012, 14, 5310). Finally, we assessed the potential performance of these bipyriciine N,N'-dioxide catalysts for the propargylation of benzaldehyde using allenyltrichlorosilanes, finding that two of these catalysts should provide reasonable stereoselectivities for this transformation. Most importantly, we show that bipyridine N,N'-dioxides constitute an ideal scaffold for the development of asymmetric propargylation catalysts and, along with AARON, should enable the rational design of such catalysts purely through computation.