Hydrophobic chemistry in aqueous solution: Stabilization and stereoselective encapsulation of phosphonium guests in a supramolecular host

Encapsulation of guest molecules inside supramolecular host assemblies provides a way to stabilize reactive species in aqueous solution. The stabilization of reactive phosphonium/ketone adducts of the general formula [(RMeC)-Me-1(OH)PR3](+) by encapsulation as guest molecules within a [Ga4L6](12-) tetrahedral metal-ligand assembly is reported; although these cations decompose in aqueous solution, encapsulation inside the hydrophobic cavity of the assembly lengthens their lifetimes considerably, in some cases up to weeks. By varying the phosphane (PMe3, PEt3, PPhMe2, and PPh2Me) and ketone (acetone, methyl ethyl ketone, 1,1,1-trifluoroacetone, and fluoroacetone) which form these adducts, as well as the pD of the solutions, it was determined that the pH of the solution as well as the size and shape of the guest cations play an important role in the stability of these host-guest complexes. Encapsulation of chiral guests in the chiral [Ga4L6](12-) assembly results in the formation of diastereomers, as characterized by H-1, F-19, and P-31 NMR spectroscopy. Although the [Ga4L6](12-) assembly is formed from non-chiral ligands, the assembly itself has DeltaDeltaDeltaDelta or LambdaLambdaLambdaLambda chirality around the metal centers. Due to the chirality of this assembly, diastereomeric selectivity is observed upon initial guest encapsulation (typical diastereomeric excesses are 30-50%). This initial diastereomeric selectivity decreases over time to reach an equilibrium but does not become 1:1, indicating both kinetic and thermodynamic processes promote selective guest encapsulation. These experiments demonstrate further the applications of nanoscale reaction vessels, self-assembled by design from non-chiral ligands, in providing a chiral and hydrophobic environment for guest molecules in aqueous solution. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).