Selectivity in an Encapsulated Cycloaddition Reaction Jian Chen and Julius Rebek,Jr. Org. Lett. 2002, 4, Tobe laboratory Shintaro Itano

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

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.

Cage-shaped molecular complexes The supramolecules formed by self-assembly via weak intermolecular interaction and having a cavity encapsulating guest molecules reversibly. Yoshizawa, M.; Tamura, M.; Fujita, M. Science 2006, 312, 251– Kang, J.; Rebek, J., Jr. Nature 1997, 385,

Previous work: Capsule complex The authors reported that the compound 1 dimerizes hydrogen bondings between edges. This cylindrical capsule 2 have a large cavity where two aromatic guest molecules can be accommodated. 1 2 Guest molecule Heinz, T.; Rudkevich, D. M.; Rebek, J., Jr. Nature 1998, 394,

Purpose of this work The interior of molecular cage complexes becomes a space to 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 cycloaddition with regioselectivity.  2 Accelerate? Regioselective?

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.

Guest molecules They chose phenylacetylene 3 and phenylazide 4 as the guest molecules. These compounds react to give a mixture of regioisomeric triazoles 5 and 6 equally in organic solvent. But, at ambient temperature the reaction rate is very slow. Rate constant k = 4.3 x M -1 s -1 half life: several years (at 1 M each component)

1 H NMR measurement Accelerating a 1,3-dipolar cycloaddition t = 0 t = 1540 min t = 4320 min t = 8500 min 50 mM25 mM 5 mM 2 in Mesitylene-d 12

１ H NMR measurement Regioselectivity ・ Addition of DMF-d 7 to A: in mesitylene-d 12 B: in mesitylene-d 12 C: in mesitylene-d 12 Only 1,4-isomer was produced. (●): 1,4-isomer’s peaks (▼): 1,5-isomer’s peaks

Selectivity of guest molecules or 2 Triazole compound

The various encapsulated species There are four encapsulated species in the solution; homocapsules 9 and 10, heterocapsule 11 and encapsulated 1,4-isomer 12.

NH resonance １ H NMR measurement Assignment of NH resonance A: At t = 0 incubating in mesitylene-d 12 B: At t = 8500 min for incubating in mesitylene-d 12 C: in mesitylene-d 12 (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

NH resonance １ H NMR measurement Assignment of 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

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

Prediction by the statistical distribution : 2 : 1 K D = [11] 2 /[9][10] = 2 2 /1 ・ 1 = 4

Reaction rate v = k cat [3][4] v: reaction rate k cat : reaction rate constant ・ Observed initial reaction rate in x M s -1 ・ Calculated reaction rate outside x M s -1 ・ Volume of the cavity of 2 ~450 Å ・ Reactant concentration in M ・ Estimated reaction rate in 2 ~6 x M s -1

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

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 K D was 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.