Supramolecular catalysis at work: Diastereoselective synthesis of a molecular bowl with dynamic inner space

Abstract

[GRAPHICS] The supramolecular assistance to Pd(0)/Cu(I)-catalyzed cyclotrimerization of stannylated norbornene 7 has been investigated to give molecular bowl 1(syn) 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 1(syn) 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 synlanti ratio of the isolated cyclotrimer from statistical (1:3) to a more desirable (4.5:1) ratio, in favor of the molecular bowl 1(syn) H-1 NMR spectroscopic studies suggested the coordinating affinity of 1(syn) toward transition metals Cu(I), Ag(I), and Au(I), to account for the observed templation effect. In particular, the tridentate 1(syn) has been shown to bind to one Ag(I) cation in the assembly process that is driven with enthalpy (Delta H-0 = - 19 +/- 2 kcal/mol, Delta S-0 = -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 1(syn) 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 approximate to 8 kcal/mol). Ultimately, Ag(I) has been shown to assist the diastereoselective formation of lending support to the notion, of template-directed synthesis.