Accelerating C-C Bond Forming Reactions Through Tuning Amine-Silanol Cooperativity By Controlling the Linker Length and Pore Size

Accelerating C-C bond forming reactions through tuning amine-silanol cooperativity by controlling the linker length and pore size

Nicholas A. Brunelli* and Christopher W. Jones, Georgia Institute of Technology, Atlanta, USA

*Current address: The Ohio State University, Columbus, OH, USA

Catalytic active sites incorporating multiple catalytic units can act cooperatively to achieve higher rates than isolated units. However, a major difficulty with creating cooperative catalytic materials remains in how to design these active sites. Important design concepts can be learned from enzymes. Several enzymes employ a complex 3-D architecture to create well-defined sites, containing three or more functional groups in a structure called the catalytic triad. Consisting of acidic, basic, and hydrogen bonding groups, the catalytic triad can achieve considerable rates due to the precisely positioned groups whose interactions are tuned to avoid strong acid-base interactions that would inhibit catalytic activity. The tuned interactions and precise spatial positioning enable the enzymes to achieve high catalytic rates. Understanding how to incorporate these design concepts into heterogeneous materials would improve catalyst performance.

Previously, we and others have focused on understanding the different aspects of how to tune the acid-base interaction strength to achieve cooperative catalysis.[1-3] In both the aldol and nitroaldol condensation, we have found that utilizing the weakly acidic surface silanols accelerate the catalytic rates more than the stronger, Brznsted acid carboxylic acid. Using the weaker acid limits the acid-base interactions thereby allowing the free acid and free base to catalyze the reaction at a faster rate.

In this presentation, we will demonstrate the effect of controlling the relative spatial positions of amines and silanols and the different catalytic behaviors that we observe for the aldol and nitroaldol condensation.[4-5] For large pore mesoporous silica, we observe for the aldol condensation an asymptotic behavior where catalytic rate increases as the linker length is increased up to a propyl (C3) linker. For the nitroaldol condensation, the trend is different, with catalytic rates increasing monotonically with linker length. The difference in catalytic rates suggests that the surface silanols behave differently in the two reactions. We hypothesize that the aldol condensation requires only one acidic site for efficient acid-base cooperativity whereas the nitroaldol condensation is more efficiently catalyzed with two separate silanols in close proximity to the amine. Overall, the differences in trends of catalytic rates provide insight on designing more efficient, cooperative catalysts.

[1]  R.K. Zeidan, M.E. Davis, J. Catal. 247 (2007), 379.

[2] N.A. Brunelli, K. Venkatasubbaiah, C.W. Jones, Chem. Mater. 24 (2012), 2433.

[3] N. Solin, L. Han, S. Che, O. Terasaki, Catal. Commun. 10 (2009), 1386.

[4] N.A. Brunelli, S.A. Didas, K. Venkatasubbaiah, C.W. Jones, J. Am. Chem. Soc. 134 (2012), 13950.

[5] N.A. Brunelli, C.W. Jones, (submitted to J. Catal.).