Reactions of Superelectrophiles
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1 Reactions of Superelectrophiles CHEM 535 Jonathan Lehmann
2 Superelectrophiles Friedel- Crafts Diffusion Control Electrophilicity No reaction Michael Additions Nucleophilicity Adapted from: R. Lucius, R. Loos, H. Mayr, Angew. Chem. Int. Ed. 2002, 41(1), 91.
3 Superelectrophiles Highly-reactive, densely-charged cations that are activated by superacids. High site selectivity Application: Polymerizations High molecular weight, narrow polydispersity 100% Degree of branching
4 Classifying Superelectrophiles Consider both quantity and separation of positive charge. Superelectrophiles Monocations Dications Trications and higher Gitonic (positive charges in close proximity) Distonic (positive charges separated by two or more atoms) Geminal Vicinal 1,3-dicationic Superelectrophile strength depends on positive charge density. G.A. Olah, D.A. Klumpp Superelectrophiles and Their Chemistry, Wiley 2007
5 Quantifying Electrophilicity Experimentally Kinetic measurements (Herbert Mayr) log k 20 C = s N + E N = nucleophilicity parameter E = electrophilicity parameter s = slope parameter ( 1) k = 1 Lmol -1 s -1 Diffusion Control 983 nucleophiles N E electrophiles E 8.02 Limitation: Dications not measured experimentally No reaction at 20 C N R. Lucius, R. Loos, H. Mayr Angew. Chem. Int. Ed. 2002, 41(1), 91 J. Ammer, C. Nolte, H. Mayr J. Am. Chem. Soc. 2012, 134, 13902
6 Molecular Orbital Approach to Electrophilicity No reaction because of HOMO / LUMO energy mismatch Protonation can generate a superelectrophile by lowering the LUMO energy. H. Shi, Y. Wang, Z. Zhang, J. Mol. Catal. A-Chem., 2006, 258, 35. DFT quantum calculations at B3LYP/6-311G(d,P)
7 Higher Protonation State, Lower LUMO Superelectrophile LUMO Diprotonation can convert larger molecules into superelectrophiles. D. N. Zakusilo, D. S. Ryabukin, I. A. Boyarskaya, O. S. Yuzikhin, A. V. Vasilyev Tetrahedron 2015, 71, 102. B3LYP/6-311+G(2d,2p)
8 Effect of Positive Charge on pka First Protonation pka Second Protonation pka Diprotonation is unfavorable and requires superacids. P. J. Brignell, C. D. Johnson, A. R. Katritzky, N. Shakir, H. O. Tarhan, G. Walker J. Chem. Soc. B 1967, 11, 1233
9 Hammett Acidity and Superacids Hammett Acidity Function (H 0 )- an extension of the ph scale for acid too strong and/or concentrated for conventional measuring methods. H 0 = pk BH + + log B [BH + ] CF 3 CO 2 H TFA H 2 SO 4 pure CF 3 SO 3 H TfOH FSO 3 H Carborane Acid H(CHB 11 Cl 11 ) Lewis acid basicity suppression HF-BF 3 H 0 = -15 to -17 HF-SbF 5 H 0 = -15 to -28 G.A. Olah, G. K. S. Prakash, A. Mólnar, J. Sommer. Superacid Chemistry, 2 nd ed. Wiley, E. S. Stoyanov, S. P. Hoffmann, M. Juhasz, C. A. Reed, J. Am. Chem. Soc., 2006, 128, 3160
10 Reactivity Explained by Protonation State TfOH / TFA H 0 1 : % TfOH : % TfOH : % TfOH : % TfOH : 100 Reaction outcome depends on acid strength. Can superelectrophilic activation occur by partial protonation? G.A. Olah, et. al. J. Org. Chem. 1997, 62, 6666
11 Superelectrophilic Activation of Small Cations δ + Unreactive to Alkanes δ + Partial protonation enables reactions with very weak nucleophiles. G. A. Olah, A. Germain, H. C. Lin, D. A. Forsyth J. Am. Chem. Soc. 1975, 97(10), 2928 G. A. Olah, A. Orlinkov, A. B. Oxyzoglou, G. K. S. Prakask J. Org. Chem. 1995, 60, 7348
12 G. A. Olah, N. Hartz, G. Rasul, G. K. S. Prakash J. Am. Chem. Soc. 1993, 115, 6985 G. A. Olah, G. Rasul, A. Burrichter, G. K. S. Prakash Proc. Natl. Acad. Sci. USA 1998, 95, 4099 Partial Protonation of Cations MP2(FU)/6-31G* Can hydrogen bonded cations be directly observed?
13 Partial Protonation of an Oxatriquinane δ + H-bonding vibrations (1075 and 795 cm -1 ) Different from vibrations in carborane acid Graphical representation Two CH2 stretches (3026 and 2905 cm - ) Retention of symmetry. Higher frequency due to positive increase E. S. Stoyanov, G. Gunbas, N. Hafezi, M. Mascal, I. V. Stoyanova, F. S. Tham, C. A. Reed J. Am. Chem. Soc. 2012, 134, 707
14 Halomethyl Cations Highly Reactive Aprotic Superelectrophiles Better orbital overlap, greater stability Stability CCl 3 + > CBr 3 + > CI 3 + Observed order of reactivity not consistent with resonance stabilization Reactivity CCl 3 + > CHCl 2 + > CH 2 Cl + δ + δ + δ + Additional Lewis acid increases electrophilicity by superelectrophilic solvation of the cation. J. Sommer, J. Bukala Acc. Chem. Res. 1993, 26, 370 G. A. Olah, G. Rasul, L. Heiliger, G. K. S. Prakash J. Am. Chem. Soc. 1996, 118, 3580
15 The Effect of Charge Repulsion Can charge repulsion be used to promote selectivity? G.A. Olah, G. K. S. Prakash, T. N. Rawdah, J. Am. Chem. Soc. 1980; D. A. Klumpp, R. Rendy, A. McElrea Tet Lett, 2004, 45, 7959
16 Charge Migration Through Resonance Dication Trication Does charge migration play a role in site-selectivity? D.A. Klumpp, J. Am. Chem. Soc. 2011, 133,
17 NMR Observation of Charge Migration External standard methine Not Tested External standard methine para To what extent would charge repulsion occur based on 13 C- NMR chemical shifts? Positive charge migration in the trication can be observed by 13 C-NMR chemical shifts. R. R. Naredla, C. Zheng, S. O. N. Lill, D.A. Klumpp, J. Am. Chem. Soc. 2011, 133,
18 Site-Selectivity Via Charge Repulsion Reactivity of secondary C-H bonds depends on distance from positive charge center. A. V. Orlinkov, N. D. Kagramanov, P. V. Petrovskii, I. S. Akhrem Tet. Lett 2010, 51, 259
19 Isomerization Depends on Alkane Length Isomerization only occurs when there is enough distance between positive charge centers. A. V. Orlinkov, N. D. Kagramanov, P. V. Petrovskii, I. S. Akhrem Tet. Lett 2010, 51, 259
20 Superelectrophilic C-H Functionalization Substrate Scope Esters Ketones Amides Substrate scope includes esters, ketones, and amides with a variety of nucleophiles A. V. Orlinkov, N. D. Kagramanov, P. V. Petrovskii, I. S. Akhrem Tet. Lett 2010, 51, 259 I. S. Akhrem, L. V. Afanas eva, N. D. Kagramanov, P. V. Petrovskii, Tet. Lett., 2012, 53, 4221 I. S. Akhrem, D. V. Avetisyan, L. V. Afanas eva, O. I. Artyushin, N. D. Kagramanov, Tet. Lett., 2015, 56, 562 I. S. Akhrem J. Organomet. Chem. 2014,
21 Complementary C-H Functionalization Strategies Transition metal catalyzed- coordinating directing group Most proximal C-H bond activated Superelectrophilic C-H Functionalization- repelling directing group Most distant C-H bond activated Esters Ketones Amides N. Hasegawa, V. Charra, S. Inoue, Y. Fukumoto, N. Chatani J. Am. Chem. Soc. 2011, 133, 8070 K. S. L. Chan, M. Wasa, L. Chu, B. N. Laforteza, M. Miura, J-Q. Yu Nat. Chem. 2014, 6, 146 I. S. Akhrem J. Organomet. Chem. 2014, I. S. Akhrem, D. V. Avetisyan, L. V. Afanas eva, O. I. Artyushin, N. D. Kagramanov Tet. Lett. 2015, 56, 562
22 Future Directions for C-H Functionalization Treatments for Influenza Could these reaction conditions also be applied to linear alkane functionalization? I.S. Akhrem, Tet. Lett. 2010, 51, 907; D Shi. et. al. Tetrahedron 2014, 70, 1395
23 Future Directions for C-H Functionalization New substrates Limitations Low functional group tolerance to weak nucleophiles. If directing group is too close, no reaction occurs. If directing group is too far away, isomerization occurs No way of controlling enantioselectivity
24 Partial Summary High site selectivity Application: Polymerizations High molecular weight, narrow polydispersity 100% Degree of branching
25 Polymerizations of Superelectrophiles Hydroxyalkylation CF 3 SO 3 H : CF 3 CO 2 H H 0 1 : 2 0 : : 0 5 : : : : : : 100 This acidity dependence is consistent with formation of dicationic intermediates. No monophenylated product obtained, indicating that k 2 > k 1 D. A. Klumpp, K. Y. Yeung, G. K. S. Prakash, G. A. Olah J. Org. Chem. 1998, 63, 4481
26 Linear Polymers from 1,2-diketones Reactivity < Does these reactions really involve fully diprotonated superelectrophiles? H. M. Colquhoun, M. G. Zolotukhin Macromolecules 2001, 34, 1122 M. G. Zolotukhin, L. Fomina, R. Salcedo, L. E. Sansores Macromolecules 2004, 37, 5140
27 Monocations or Dicationic Superelectrophiles? ΔG p = 1.79 kcal/mol ΔG a = kcal/mol ΔG p+a = kcal/mol ΔG p = kcal/mol ΔG a = kcal/mol ΔG p+a = kcal/mol Partial protonation is another possible method of superelectrophilic activation. D. R. Nieto, S. Fomine, M. G. Zolotukhin, L. Fomina, M. d. C. G. Hernandez Macromol. Theory. Simul. 2009, 18, 138
28 Synthesis of Ultrahigh MW Linear Polymers Chain growth (intermolecular) is favored over macrocyclization (intramolecular) because of an excess of aryl terminated oligomers A. R. Cruz, M. C. G. Hernandez, M. T. Guzmán-Gutiérrez, M. G. Zolotukhin, S. Fomine, et. al. Macromolecules 2012, 45, 6774
29 Step-selective Growth Polydispersity Index (PDI): a measure of molecular mass distribution Unfavored Unfavored Conversion of monomers to dimers Conversion of dimers to tetramers Longer oligomers that possess more positive charge exhibit lower nucleophilicity, enabling step-selective polymer growth. A. R. Cruz, M. C. G. Hernandez, M. T. Guzmán-Gutiérrez, M. G. Zolotukhin, S. Fomine, et. al. Macromolecules 2012, 45, 6774
30 Molecular Orbitals of Oligomers How to compare the nucleophilicity of growing oligomers? Nucleophilicity of aromatic compounds correlates with HOMO energy HOMO (ev) PM Number of Repeated Units Not Tested: Could the reaction be monitored over time by NMR or mass spectrometry to validate the step-selective growth mechanism? A. R. Cruz, M. C. G. Hernandez, M. T. Guzmán-Gutiérrez, M. G. Zolotukhin, S. Fomine, et. al. Macromolecules 2012, 45, 6774
31 High MW, Low PDI Linear Polymers A 2 / B 2 (mol / mol) M w (kilodaltons) PDI Characterization NMR Gel Permeating Chromatography Static Light Scattering Viscosity Measurements Could superelectrophilic reactions also be applied to the synthesis of branched polymers? A. R. Cruz, M. C. G. Hernandez, M. T. Guzmán-Gutiérrez, M. G. Zolotukhin, S. Fomine, et. al. Macromolecules 2012, 45, 6774
32 Branching Polymers Possess unique properties- low viscosity, high solubility, and high degree of functionality. Synthesized via one-pot polycondensation of an AB X monomer. If both reactions occur at the same rate, DB = 50% DB = 2D 2D + L D = Dendritic Unit AB 2 Monomer L = Linear Unit Low Degree of Branching 100% Branching How can this be done? Y. Segawa, T. Higashihara, M. Ueda, Polym. Chem., 2013, 4, 1746
33 Hyperbranching Via Different Reaction Rates Linear Growth Slow Branching Fast Because the second reaction is faster than the first, polymers are 100% branched. J-Y. Chen, M. Smet, J-C. Zhang, W-K. Shao, X. Li, et. al. Polym. Chem. 2014, 5, 2401
34 Hyperbranched Polymer with a Core Monomers react with the core faster than other monomers because of differences in reaction rates between acenaphthenequinone and isatin. J-Y. Chen, M. Smet, J-C. Zhang, W-K. Shao, X. Li, et. al. Polym. Chem. 2014, 5, 2401
35 One-pot Synthesis of a 100% Branched Polymer AB 2 / Core (mol/mol) PDI Evidence for 100% branching 5.11 (D) 4.99 (T) (D) (T) No comparison to a standard for linear unit Variable molecular weights (high PDI) due to unselective growth mechanism. J-Y. Chen, M. Smet, J-C. Zhang, W-K. Shao, X. Li, et. al. Polym. Chem. 2014, 5, 2401
36 Future Directions for Superelectrophilic Polymerizations Step-selective growth of hyperbranched polymers via differences in nucleophilicity among growing oligomers. Limitations Requires large amounts of triflic acid Acid-sensitive functional groups not tolerated
37 C-H Functionalization Diffusion Control Superelectrophilicity Superelectrophilic Polymerizations Electrophilicity Michael Additions Nucleophilicity
38 Acknowledgements Professor Jeffrey S. Moore Professor Martin D. Burke Burke group CHEM 535 class and Kevin Cheng
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