(113b) 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.).