Organic Pedagogical Electronic Network Attachment of Molecular Catalysts on Solid Supports - Rh Complex on a Silica Support Jones Group, Georgia Tech Davies Group, Emory University

Introduction to Supported Molecular Catalysts 1) Chepiga, K. M.; Feng, Y.; Brunelli, N. A.; Jones, C. W.; Davies, H. M. L. Org. Lett. 2013, 15, ) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N. Science 1997, 277, ) Zheng, X.; Jones, C. W.; Weck, M. J. Am. Chem. Soc. 2007, 129, Increase catalyst turnover number (TON) by facilitating catalyst recovery and recycle 1 May allow enhanced utilization of expensive metals or ligands 1 Allows for design of experiments to probe key aspects of reaction mechanisms, e.g. prevention/limitation of key steps that are kinetically second order in catalyst concentration 2,3 Porous oxides (e.g. silica, alumina) Pros: tolerant to most organic solvents; high surface areas allow good solid-liquid contacting; broad temperature range; Cons: hydroxyl groups on the surface mildly acidic and surface further covered with oxygen lone pairs (dative bonding to metal complex) - can interact with catalyst or reaction media; Why Attach a Soluble, Molecular Catalyst to a Support? Typical Support Characteristics Polymers [e.g. poly(styrene) resins] Pros: more tolerant to aqueous acidic or basic media compared to oxides; support backbone less likely to bond with metal complex than oxides; Cons: resins swell differently in various solvents, affecting accessibility of catalyst sites; narrowing temperature range;

Catalyst Supporting Methodologies 1) Jones, C. W.; McKittrick, M. W.; Nguyen, J. V.; Yu, K. Top. Catal. 2005, 34, 67. 2) Tada, M.; Muratsugu, S.; Kinoshita, M.; Sasaki, T.; Iwasawa, Y. J. Am. Chem. Soc ) Nakazawa, J.; Smith, B. J.; Stack, T.D.P. J. Am. Chem. Soc Immobilization Method Covalent ligand binding PhysisorptionIon pair formation Encapsulation Applicabilitybroadrestricted Drawbackspreparationcompetition with solvents or substrates competition with polar or ionic substrates substrate size, diffusion General Immobilization Methods 1 Selected Covalent Binding Methods Alkene on Ligand and Surface 2 Olefin coupling Alkyne on Ligand and Azide on Surface 3 Click chemistry

Covalent Attachment of Rhodium Catalyst Chepiga, K. M.; Feng, Y.; Brunelli, N. A.; Jones, C. W.; Davies, H. M. L. Org. Lett. 2013, 15, Supported catalyst (right) depicts product from two AIBN-initiated radicals reacting with each alkene, followed by coupling. Other products also possible. Attaching Rh 2 (S-DOSP) 4 analogue on silica Silica support with functionalized surface Functional group for catalyst grafting by alkene coupling Supported Rh catalyst

Asymmetric Cyclopropanation: Catalyst Recycling Chepiga, K. M.; Feng, Y.; Brunelli, N. A.; Jones, C. W.; Davies, H. M. L. Org. Lett. 2013, 15, cycleyield / %ee / % Rh 2 (S-DOSP) 4 88 Reaction, catalyst recovery via filtration, use in subsequent reaction; 5 cycles with consistent yield and enantiomeric excess (ee); Slightly reduced yield and ee compared to parent Rh 2 (S-DOSP) 4 complex. This reaction was catalyzed by the supported Rh catalyst, shown on the right of the previous slide.

Problems 1) What is the catalyst supporting methodology used in this work adding the Rh 2 (S-DOSP) 4 analogue onto silica? A. Covalent ligand binding; B. Physisorption; C. Ion pair formation; D. Encapsulation. 2) What modification is needed for Rh 2 (S-DOSP) 4 if click chemistry is used to bind the ligand onto solid supports? How might this be achieved?