Surfactants as Catalysts for Organic Reactions in Water

1. Surfactants as Catalysts for Organic Reactions in Water Atefeh Garzan 11/07/07

2. Homogeneous catalysts: • Brønsted acid catalysis: • Lewis acid catalysis: - Advantages: high activity, selectivity - Disadvantages: separation and recycling problems • Homogeneous catalysis: catalyst is in the same phase as reactants.

3. Heterogeneous catalysts: • Heterogeneous catalysis: catalyst is in a different phase than reactants. - Metals alone - Metals plus other component - Advantages: easy separation and recovery - Disadvantages: less activity and selectivity

4. Other catalysts: • New Methods to combine the benefits: - high activity, selectivity - easy separation and recovery • Transfer a homogeneous catalyst into a multi phase system: - surfactant - phase transfer system - organic or inorganic support: H. Turkt, W. Ford, J. Org. Chem.1991, 56, 1253. C. Starks, J. Am. Chem. Soc.1971, 93, 195.

5. Surfactant: surfactant Surface Active Agent • Surfactants have amphiphilic structure: Hydrophilic Hydrophobic

6. Classification of surfactant: • Anionic • Cationic • Amphoteric • Nonionic Sodium dodecylsulfate (SDS) Cetylpyridinium bromide Dipalmitoylphosphatidylcholine (lecithin) Polyoxyethylene 4 lauryl ether

7. Behavior of surfactants: • When a molecule with amphiphilic structure is dissolved in aqueous medium, the hydrophobic group distorts the structure of the water. • As a result of this distortion, some of the surfactant molecules are expelled to the surfaces of the system with their hydrophobic groups orientedto minimize contact with the water molecules. Nonpolar tail Polar head

8. Micelle formation: • When the water surface is or begin to be saturated, the overall energy reduction may continue through another mechanism: • Micelle formation Hunter, Foundations of Colloid Science, p. 572, 1993.

9. Driving force: Hydrophobic effect Electrostatic repulsion Formation of the micelle No formation of the micelle

10. Critical Micelle Concentration (CMC): • CMC decreases with increasing alkyl chain length • CMC increases as the polar head becomes larger. • CMC of neutral surfactants lower than ionic CMC Hydrophobic effect Electrostatic repulsion

11. Krafft temperature: • For surfactants there exists a critical temperature above which solubility rapidly increases (equals CMC) and micelles form  Krafft point or Krafft temperature (TK ) • Krafft point strongly depends on the size of head group and counterion. Surfactant Tk (oC) C12H25SO3-Na+ C12H25OSO3-Na+ n-C8F17SO3-Na+ n-C8F17SO3-K+ 38 16 75 80 D. Myers, surfactant science and technology, p.111, 2006.

12. Surfactant aggregates:

13. Surfactant in organic reaction: • Easy separation and recovery, high activity, selectivity. • In this system, we can use water as solvent; in water, surfactants can: - Act as a catalyst - Help to solubilize the organic compounds in water • In comparison to organic solvents, water is: - Cheap - Safe - Less harmful

14. Micellar catalysis: • Electrostatic interaction: Additive Conc. (M) 103Kobs (s-1) - - 1.83 0.01 1.28 0.01 1.78 0.01 2.05 M. N. Khan, N. H. Lajis, J. Phys. Org. Chem.1998, 11, 209.

15. Micellar catalysis:

16. Micellar catalysis: • Electrostatic interaction: Additive Conc. (M) 103 Kobs (s-1) - - 1.83 0.01 1.28 0.01 1.78 0.01 2.05 M. N. Khan, N. H. Lajis, J. Phys. Org. Chem.1998, 11, 209.

17. Micellar catalysis: • A catalyst is a substance that increases the rate of a chemical reaction without itself being changed in the process. [A---B] ≠ • A + B A_B • Rate= k [A][B] energy activation energy activation energy • The reactants are concentrated through insertion to the micelle. • The TS≠ can be stabilized by interaction of polar head group. A+B uncatalyzed reaction catalyzed reaction A_B time

18. Lewis acid catalysis: • Lewis acid catalysis is generally carried out under strictly anhydrous conditions because of the water-labile nature of most Lewis acids. • Some metal salts such as rare earth metal triflates can be used as water-stable Lewis acids. S. Kobayashi, T. Wakabayashi, S. Nagayama, H. Oyamada, Tetrahedron Lett. 1997, 38, 4559.

19. Lewis Acid Surfactant Catalyst: Lewis acid surfactant • “Lewis acid-surfactant-combined catalyst (LASC)”, acts: -as a Lewis acid to activate the substrate molecules -as a surfactant to help to solubilize the organic compounds in water K. Manabe, Y. Mori, T. Wakabayashi, S. Nagayama, S. Kobayashi, J. Am. Chem. Soc. 2000, 122, 7202.

20. Lewis Acid Surfactant Catalyst: Colloidal Dispersion : K. Manabe, Y. Mori, T. Wakabayashi, S. Nagayama, S. Kobayashi, J. Am. Chem. Soc. 2000, 122, 7202.

21. Size of surfactant aggregates: Solutions Micelles Microemulsion Solubility Colloidal dispersion Emulsion 0.1 1.0 10.0 100.0 1000 10000 size (nm)

22. Aldol reaction: 92% 76% 83% 19%

23. Stability of colloidal dispersion: • CMC decreases as the polar head becomes smaller • CMC decreases with increasing alkyl chain length High stability >1.0 µm 0.5-1.0 µm <0.5 µm (92%)(83%) (~1.5 µm) (1.1 µm) Medium stability (76%) (0.7 µm) Low stability (19%) (0.4 µm)

24. Effect of solvents: solvent yield (%) H2O 92 DMF 14 DMSO 9 CH2Cl2 3 K. Manabe, Y. Mori, T. Wakabayashi, S. Nagayama, S. Kobayashi, J. Am. Chem. Soc. 2000, 122, 7202.

25. Kinetics for aldol reaction: • Aldol reaction in water was found to be 130 times higher than that in CH2Cl2. in water in CH2Cl2 K. Manabe, Y. Mori, T. Wakabayashi, S. Nagayama, S. Kobayashi, J. Am. Chem. Soc. 2000, 122, 7202.

26. Necessity to use water: solvent yield (%) none 10 DMF 21 pyridine 23 Et2O14 H2O 80 S. Kobayashi, I. Hachiya, J. Org. Chem.,1994, 59, 3590.

27. Mechanism of catalytic reaction: H2O H2O

28. Role of water: • Hydrophobic interactions in water lead to increase the local concentration of substrates, resulting in the higher reaction rate in water. • Hydration of Sc(III) ion and the counterion by water leads to dissociation of the LASC salt to form highly Lewis acidic species such as [Sc(H2O)n]+3. .Manabe, Y. Mori, T. Wakabayashi, S. Nagayama, S. Kobayashi, J. Am. Chem. Soc. 2000, 122, 7202.

29. Mechanism of catalytic reaction: H2O H2O

30. Interface: • The rate of the reaction depends on the total area of the interface. • Stirring of the reaction would increase the total area of the interface.

31. LASC-catalyzed Aldol reactions: R1 R2 R3 yield (%) K. Manabe, Y. Mori, T. Wakabayashi, S. Nagayama, S. Kobayashi, J. Am. Chem. Soc. 2000, 122, 7202.

32. Workup: After centrifugation at 3500 rpm for 20 min, the colloidal mixture became a tri-phasic system. water LASC Mixture of organic compounds

33. Friedlander synthesis of Quinolines: LASC (catalyst) yield (%) Sm(O3SOC12H25)3 82 Ce(O3SOC12H25)3 91 Sc(O3SOC12H25)3 90 L. Zhanga, J. Wua, Adv. Synth. Catal.2007, 349, 1047. M. Zolfigol, P. Salehi, A. Ghaderi, M. Shiri, Z. Tanbakouchian, J. Mol. Cat. A2006, 259, 253.

34. Rhodium catalyst: • Cationic rhodium catalysts are frequently employed as homogeneous catalysts for: - hydrogenation - hydrosilylation - hydride transfer - cycloaddition B. Wang, P. Cao, X. Zhang, Tetrahedron Lett.2000, 42, 8041.

35. Add surfactant D. Motoda, H. Kinoshita, H. Shinokubo, K. Oshima, Angew. Chem. Int.Ed.2004, 43, 1860.

36. [4+2] annulation of dienynes: Decreasing temperature • The Krafft temperature is strongly dependent on the head group and counterion and increases by increasing the size of counterion. D. Motoda, H. Kinoshita, H. Shinokubo, K. Oshima, Angew. Chem. Int.Ed.2004, 43, 1860.

37. [4+2] annulation of dienynes: No ligand D. Motoda, H. Kinoshita, H. Shinokubo, K. Oshima, Angew. Chem. Int.Ed.2004, 43, 1860.

38. Decreasing the amount of catalyst

39. Formation of micellar catalyst: Ion-electrode analysis: - concentration of Cl- (obs.): 2.54 × 10-3 molL-1 - concentration of Cl- (cal.): 2.50 × 10-3 molL-1 Formation of micelle: D. Motoda, H. Kinoshita, H. Shinokubo, K. Oshima, Angew. Chem. Int.Ed.2004, 43, 1860.

40. [4+2] annulation in water: Dienyne t[min] Product Yield (%) D. Motoda, H. Kinoshita, H. Shinokubo, K. Oshima, Angew. Chem. Int.Ed.2004, 43, 1860.

41. Brønsted acid catalyst: • The use of a Brønsted acid is one of the more convenient and environmentally benign methods of catalyzing organic reactions in water. • The advantage of water over organic solvents in Brønsted-catalyzed reactions is that the: - nucleophilicity of the corresponding base may be of less concern due to extensive solvation of charge by hydrogen-bonding water molecules. • Brønsted acid surfactant combined catalyst

42. Dehydration reactions in water: Remove Water Add excess amount of substrates

43. Brønsted acid surfactant Catalyst: : Brønsted acid surfactant catalyst K. Manabe, S. Iimura, X. Sun, S. Kobayashi, J. Am. Chem. Soc.2002, 124, 11971.

44. Esterification with various catalysts: Catalyst Yield (%) K. Manabe, S. Iimura, X. Sun, S. Kobayashi, J. Am. Chem. Soc.2002, 124, 11971.

45. Initial rate of esterification in water: DBSA catalyzed the reaction 2.3 times faster than OBSA and 59 times faster than TsOH

46. Various amounts of DBSA: amount of DBSA (mol %) yield (%) K. Manabe, S. Iimura, X. Sun, S. Kobayashi, J. Am. Chem. Soc.2002, 124, 11971.

47. Size of particles: 10 (mol%) 200 (mol%) 10 µm K. Manabe, S. Iimura, X. Sun, S. Kobayashi, J. Am. Chem. Soc.2002, 124, 11971.

48. Effect of substrates: n yield (%) a: ethanol was used.

49. Esterification of various substrates: R R` yield (%) K. Manabe, S. Iimura, X. Sun, S. Kobayashi, J. Am. Chem. Soc.2002, 124, 11971.

50. Etherification: • Williamson ether synthesis: • Lewis acid catalyze: G. V. M. Sharma, T. Rajendra Prasad, A. K. Mahalingam, Tetrahedron Lett.2001, 42, 759.