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Selectivity in an Encapsulated Cycloaddition Reaction

Selectivity in an Encapsulated Cycloaddition Reaction. Jian Chen and Julius Rebek,Jr . Org. L ett . 2002 , 4 , 327-329. Tobe laboratory Shintaro Itano. Contents. Introduction Self-assembly Cage-shaped molecular complexes Previous work Purpose of this work

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Selectivity in an Encapsulated Cycloaddition Reaction

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  1. Selectivity in an Encapsulated Cycloaddition Reaction Jian Chen and Julius Rebek,Jr. Org. Lett. 2002, 4, 327-329 Tobe laboratory ShintaroItano

  2. Contents • Introduction Self-assembly Cage-shaped molecular complexes Previous work Purpose of this work • Results and discussion 1,3-Dipolar cycloaddition 1H NMR Measurement Equilibrium constant and reaction rate • Conclusion

  3. Self-assembly • Self-assembly the spontaneous and reversible organization of molecular units into ordered structures by non-covalent interactions. • Non-covalent interactions • hydrogen-bonding • dipole–dipole interaction • van der Waals interaction • metal–ligand coordination Lackinger, M.; Griessl, S.; Markert, T.; Jamitzky, F.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 13652–13655.

  4. Cage-shaped molecular complexes • Cage-shaped molecular complexes The supramolecules formed by self-assembly viaweak intermolecular interaction and having a cavity encapsulating guest molecules reversibly. 2 Kang, J.; Rebek, J., Jr. Nature1997, 385, 50-52. Yoshizawa, M.; Tamura, M.; Fujita, M. Science2006, 312, 251–254.

  5. Previous work: Capsule complex The authors reported that the compound 1 dimerizeshydrogen bondings between edges. This cylindrical capsule 2 have a large cavity where two aromatic guest molecules can be accommodated. Guest molecule 2 1 Heinz, T.; Rudkevich, D. M.; Rebek, J., Jr. Nature1998, 394, 764-766.

  6. Purpose of this work The interior of molecularcage complexes becomes a spaceto stabilize reactive intermediates and to create new forms of stereoisomerism. The authors investigated the ability of their capsule complex to accelerate a 1,3-dipolar cycloadditionwith regioselectivity.  2 Accelerate? Regioselective?

  7. 1,3-Dipolar cycloaddition • 1,3-Dipole The neutral molecules which have a resonance forms as foll. • 1,3-Dipolar cycloaddition The reaction between a 1,3-dipole and alkenes or alkynes to form a five-membered ring.

  8. Guest molecules They chose phenylacetylene3 and phenylazide4 as the guest molecules.These compounds react to give a mixture of regioisomerictriazoles5 and 6 equally in organic solvent. But, at ambient temperature the reaction rate is very slow. Rate constant k = 4.3 x 10-9 M-1 s-1 half life: several years (at 1 M each component)

  9. 1H NMR measurementAccelerating a 1,3-dipolar cycloaddition t = 0 t = 1540 min 50 mM 25mM t = 4320 min 2 5mM in t = 8500 min Mesitylene-d12

  10. １H NMR measurementRegioselectivity ・Addition of DMF-d7 to A: in mesitylene-d12 B: in mesitylene-d12 C: in mesitylene-d12 (●): 1,4-isomer’s peaks (▼): 1,5-isomer’s peaks Only 1,4-isomer was produced.

  11. Selectivity of guest molecules Triazole compound or 2 3 + 8 + 2 3 + 7 + 2

  12. The various encapsulated species There are four encapsulated species in the solution; homocapsules9 and 10, heterocapsule11 and encapsulated 1,4-isomer 12.

  13. １H NMR measurementAssignment of NH resonance A: At t= 0 incubating in mesitylene-d12 B: At t = 8500 min for incubating in mesitylene-d12 C: in mesitylene-d12 NH resonance (c) NH resonance of 11 (d) NH resonance of 9 (e) NH resonance of 10 (f) NH resonance of 12 (g) ortho-protons of the phenyl rings of encapsulated 1,4-isomer 5 in complex 12

  14. １H NMR measurementAssignment of NH resonance NH resonance (c) NH resonance of 11 (d) NH resonance of 9 (e) NH resonance of 10 (f) NH resonance of 12 (g) ortho-protons of the phenyl rings of encapsulated 1,4-isomer 5 in complex 12

  15. Equilibrium constant KD = [11]2/[9][10] KD: Equilibrium constant [x]: concentrations of x Experimentally observed value is larger than predictedvalue. The space of11 is better occupied or there is a weak attractive force between the occupants. ・Prediction by the statistical distribution KD = 4 ・Experimentally determined value KD = 9  3

  16. Prediction by the statistical distribution 11 10 9 1 : 2 : 1 KD = [11]2/[9][10]= 22/1・1 = 4

  17. Reaction rate v = kcat[3][4] v: reaction rate kcat: reaction rate constant ・Volume of the cavity of 2 ~450 Å ・Reactant concentration in 2 3.7 M ・Estimated reaction rate in 2 ~6 x 10-8 M s-1 ・Observed initial reaction rate in 2 1.3 x 10-9 M s-1 ・Calculated reaction rate outside 2 5.4 x 10-12 M s-1

  18. Reaction rate v = kcat[3][4] v: reaction rate kcat: reaction rate constant ・Estimated rate is larger than initial rate actually observed. ・Reactants’ positions of2 are not ideal for the transition state. ・Reaction rate in 2 is 240 times faster than it outside 2.

  19. Conclusion • 1,3-Dipole cycloaddition was accelerated by the capsule complex 2. • In the capsule complex 2, only 1,4-isomer was formed. • Equilibrium constant KDwas bigger than the prediction. It suggest that the space in 11 is better occupied or there is a weak attractive force between the occupants. • The actual observed reaction rate is slower than the estimated rate, but 240 times faster than the calculated rate outside 2.

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