Catalysis of a peptidic micellar assembly covalently immobilized within mesoporous silica channels: importance of amphiphilic spatial design.
A mesostructured silica/organic composite 1-MS, constructed from a rodlike micelle of amino acid amphiphile 1 that has a condensable head group and that can be used as a template, was found to be able to catalyze the acetalization of cyclohexanone, in ethanol at 25 degrees C (50% in 12 h), whereas no reaction took place with unfunctionalized mesoporous silica. In sharp contrast, hydrolytic removal of the C16 alkyl tail of immobilized 1 resulted in the complete disappearance of the catalytic activity, which suggests the importance of a hydrophobic inner domain for the admission of cyclohexanone. Unsupported peptide amphiphile 2, under identical conditions to those above, was inefficient for acetalization regardless of the absence (2% in 24 h) or presence of mesoporous silica (7% in 24 h). Reference composite 2-MS, which is a noncovalently immobilized peptidic micelle, was virtually inactive (1% in 24 h). These observations indicate the importance of covalent immobilization of the peptidic rod micelle for catalysis. Mesostructured silicate 3-MS hybridized with a nonpeptidic, ammonium ion amphiphile (3) showed a certain catalytic activity, but the yield (12% in 24 h) of the acetal was much lower than that achieved by using 1-MS as the catalyst. Amorphous silica with immobilized 1 on its surface was much less active than 1-MS for acetalization (5% in 24 h).
Otani
Wataru
W
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
Kinbara
Kazushi
K
Zhang
Qingmin
Q
Ariga
Katsuhiko
K
Aida
Takuzo
T
D000080:Acetals / Q000737:chemistry; D002384:Catalysis; D002412:Cations; D003512:Cyclohexanones / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D008823:Micelles; D008956:Models, Chemical; D036103:Nanotechnology / Q000379:methods*; D010316:Particle Size; D010455:Peptides / Q000737:chemistry*; D016062:Porosity; D000644:Quaternary Ammonium Compounds / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry*; D013997:Time Factors
D000080:Acetals; D002412:Cations; D003512:Cyclohexanones; D008823:Micelles; D010455:Peptides; D000644:Quaternary Ammonium Compounds; C036468:cyclohexanone; D012822:Silicon Dioxide