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Silanediol hydrogen bonding activation of carbonyl compounds.

Silicon is the second most abundant element on the earth s surface, whereby inorganic silanols (SiOH) make up the reactive hydroxyl groups on the surface of minerals, zeolites, and silica gel. The acidic silanol groups are known to be capable of hydrogen bonding to small molecules for heterogeneous catalysis and separation chemistry, but the discrete surface-molecule interactions are not well understood. Silanediols R2Si(OH)2 are of particular interest as they contain a geminal diol bonding motif that is not commonly accessible for carbon analogues, but they represent a synthetic challenge due to their rapid rates of self-condensation. While condensation is advantageous for the synthesis of siloxane polymers, metallosiloxanes, and silesquioxanes, it creates a barrier that may hinder researchers from exploring the properties and applications of discrete silanediols. Recent studies demonstrate that silanols can function as isosteres and transition-state analogues in drug design, in which the enhanced acidity of the silanol can improve binding to a receptor. Small molecules containing silanol and silanediol groups may be useful as models to understand local surface sites and reactivity of silica materials for catalysis, and also to design new homogeneous small-molecule catalysts. We seek to understand the hydrogen bonding patterns and interactions between organic silanediols with carbonyls for applications to catalysis and molecular recognition. Hydrogen bonding interactions play an important role in molecular recognition and metal-free catalysis, and are frequently used in nature for structural organization and enzyme activity. Previous structural studies have demonstrated the hydrogen bonding networks that silanediols can attain through dual donor and acceptor interactions, and also the ability to hydrogen bond with chloride anions for molecular recognition. Studies of silanetriols have shown that amines can complex with and stabilize silanetriols. Here we describe the synthesis and structural studies of organic silanediols for small-molecule hydrogen bonding activation of carbonyl compounds. We are particularly interested in studying bulky silanediols that do not readily undergo condensation reactions. We have performed crystallization and NMR-binding experiments to investigate the hydrogen bonding interactions in both solid-state and solution for molecular recognition and the design of new catalysts. This is the first study of neutral Lewis basic carbonyl compounds with silanediols. We have synthesized a series of bulky silanediols 2–4 that incorporate electron-withdrawing groups to enhance acidity while incorporating steric effects to overcome condensation reactions. Due to the synthetic challenge, the chemical space