1392 Chem. Commun., 2013, 49, 1392--1394 This journal is c The Royal Society of Chemistry 2013 Cite this: Chem. Commun., 2013, 49, 1392 Sugar complexation to silicone boronic acids† Michael A. Brook,* Laura Dodge, Yang Chen, Ferdinand Gonzaga and Hazem Amarne A new class of surface-active compounds based on the combi- nation of silicones and boronic acids is described. The properties of the compounds can be tuned by manipulation of both the hydro- phobic (silicone size and 3D structure) and hydrophilic components (by binding different saccharides to the boronic acid). Stabilization of the four-coordinate boron structure is provided by Tris buffer that also maintains neutral pH to suppress silicone hydrolysis. Silicones are both highly mobile (Tg o 123 1C) 1 and hydrophobic entities (surface energy 23 mN m 1 ) 2 and, as a consequence, are widely used – depending on formulation – as both foaming and defoaming agents. 3,4 When combined with hydrophilic entities, 5 particularly poly(ethylene glycol) and other polyether oligomers, they exhibit surface activities that cannot be matched by either fluoro- carbon- or hydrocarbon-based surfactants. Silicone surfactants are used in applications ranging from polyurethane foam stabilization 3 to delivery of agricultural bioactives. 6 It would be of interest to develop responsive surfactants, whose properties could be manipulated by external stimuli including pH, temperature, etc. Silicones are exceptionally stable near neutrality, but are subject to hydrolytic cleavage and depolymerization away from pH 7. 7 Amino- or carboxylic acid-modified silicones are readily available, but are not normally used as surfactants due to the associated pH sensitivity. As part of an examination of other potentially responsive organic functional groups, we chose to establish if boronic acids could be incorporated on a silicone backbone and used to mediate surface activity of the silicone. Boronic acids (RB(OH) 2 , BA) provide a highly flexible functiona- lity that can be manipulated using a number of well-studied procedures. Not only can they be used in carbon–carbon bond forming reactions such as the Suzuki–Miyaura cross-coupling, 8 but the boronic acid hydroxyl groups provide pH-sensitive binding sites for appropriate diol-containing substrates. The stability of boronate complexes is affected by the pH of the solution and the presence of any Lewis base. For example, the equilibrium between tri- 1 and tetracoordinate 2 boron compound favours the latter when Lewis bases are present, including hydroxide at higher pH. 9 The equilibrium is also affected by the R group on the boronic acid: arylboronic acids are more acidic than alkylboronic acids; and bulky substituents surrounding boron can cause a decrease in acidity due to restricted access of water to the p-orbital on boron. 10 Many applications of boronic acids rely on binding selectivity for specific 1,2- or 1,3-diol sites 3, which are particularly prevalent on saccharides (Fig. 1). This specificity has led to the use of boronic acids as tunable sensors for saccharides: the structure of a given BA will determine to which sugar it will preferentially bind. Based on selective binding several in vivo applications for BAs, such as drug delivery devices 11 and artificial lectin (sugar binding protein) mimics, 12 have been proposed. Key to the utility of boronic acids in such applications is the very large range of binding constants – over three orders of magnitude – for sugars with different diol stereostructures. 13,14 Stability of the cyclic products is enhanced when good Lewis bases are present 4. Silicone-modified boronic acids should be interesting, tuneable surfactants. If the presence of the hydrophobic silicone does not significantly affect the sugar binding properties of the boronic acid, it should be possible to modify the surface activity by controlling the size (length of linear polydimethylsiloxane chains) and 3D structure (the presence of branching in the silicone moiety) of the silicone hydrophobe, and also the hydrophilicity of the boron head group by manipulating the type and size of hydrophilic sugar to which the boron is temporarily grafted. We report the first synthesis of silicone Fig. 1 Boronate-diol binding equilibria. 15 Department of Chemistry and Chemical Biology, McMaster University, 1280 Main St. W., Hamilton ON, Canada L8S 4M1. E-mail: mabrook@mcmaster.ca † Electronic supplementary information (ESI) available: NMR of 7 before and after hydrolysis to 13 in the absence and presence of Tris; full experimental protocols and characterization for compounds 7–12 (Table 1); hydrolysis and sugar binding protocols; and complex viscosity from rheolgoical data for 17 and 18. DOI: 10.1039/c2cc37438b Received 11th October 2012, Accepted 20th December 2012 DOI: 10.1039/c2cc37438b www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 21 December 2012. Downloaded by University of Windsor on 11/05/2014 07:36:06. View Article Online View Journal | View Issue