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 °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). A mesostructured silica/organic composite, constructed from a rodlike micelle of an amino acid amphiphile as a template, catalyzed the acetalization of cyclohexanone in ethanol at 25 °C. As a result of the amphiphilic core–shell architecture of the immobilized micelle, reactants can be incorporated into the silica channels (see figure) and activated through hydrogen‐bonding interactions with the peptidic functionalities.