Supramolecular Catalysis at Work: Diastereoselective Synthesis of a Molecular Bowl with Dynamic Inner Space

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Abstract

The supramolecular assistance to Pd(0)/Cu(I)-catalyzed cyclotrimerization of stannylated norbornene 7 has been investigated to give molecular bowl 1syn in a stereoselective fashion. Following a divergent strategy, racemic norbornene 7 was synthesized in satisfactory yield. Self-coupling, promoted by Pd(0)/Cu(I) catalysis acting in synergy with CsF, yielded molecular bowl 1syn in a moderate 30% yield. The reaction diastereoselectivity is affected by the concentration of Cu(I) and Cs+: increasing quantities of the cations enhanced the syn/anti ratio of the isolated cyclotrimer from statistical (1:3) to a more desirable (4.5:1) ratio, in favor of the molecular bowl 1syn. 1H NMR spectroscopic studies suggested the coordinating affinity of 1syn toward transition metals Cu(I), Ag(I), and Au(I), to account for the observed templation effect. In particular, the tridentate 1syn has been shown to bind to one Ag(I) cation in the assembly process that is driven with enthalpy (Delta H degrees = -19 +/- 2 kcal/mol, Delta S degrees = -45 eu). The complete coordination was not cooperative, and was hypothesized to be impeded with the adverse entropy. Accordingly, density functional theory (BP86) calculations of 1syn and its mono-, bis-, and tris-Ag(I) complexes suggested that the coordination of one to three silver cations is highly exothermic. The calculations also revealed that the bowl constriction is necessary for the aromatic arms to become preorganized and bind to a silver cation(s) (Delta E approximately 8 kcal/mol). Ultimately, Ag(I) has been shown to assist the diastereoselective formation of 1syn, lending support to the notion of template-directed synthesis.

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