Showing papers by "Dorothea Fiedler published in 2006"

Molecular recognition and stabilization of iminium ions in water.

Abstract: Iminium ions are known to exist only transiently in aqueous solution due to their high reactivity toward hydrolysis. In this communication, we report on the generation and stabilization of iminium ions in aqueous solution via molecular encapsulation using a K12Ga4L6 host. Our studies revealed that tetrahedral host 1 can encapsulate a variety of iminium cations in a molecular recognition process that is selective based on the charge, hydrophobicity, size, and shape of the guest.

Supramolecular Catalysis of Unimolecular Rearrangements: Substrate Scope and Mechanistic Insights

Abstract: A cavity-containing metal-ligand assembly is employed as a catalytic host for the 3-aza Cope rearrangement of allyl enammonium cations. Upon binding, the rates of rearrangement are accelerated for all substrates studied, up to 850-fold. Activation parameters were measured for three enammonium cations in order to understand the origins of acceleration. Those parameters reveal that the supramolecular structure is able to reduce both the entropic and enthalpic barriers for rearrangement and is highly sensitive to small structural changes of the substrate. The space-restrictive cavity preferentially binds closely packed, preorganized substrate conformations, which resemble the conformations of the transition states. This hypothesis is also supported by quantitative NOE studies of two encapsulated substrates, which place the two reacting carbon atoms in close proximity. The capsule can act as a true catalyst, since release and hydrolysis facilitate catalytic turnover. The question of product hydrolysis was addressed through detailed kinetic studies. We conclude that the iminium product must dissociate from the cavity interior and the assembly exterior before hydroxide-mediated hydrolysis, and propose the intermediacy of a tight ion pair of the polyanionic host with the exiting product.

Guest Exchange Dynamics in an M4L6 Tetrahedral Host

Abstract: Guest exchange in an M4L6 supramolecular assembly was previously demonstrated to proceed through a nonrupture mechanism in which guests squeeze through apertures in the host structure and not through larger portals created by partial assembly dissociation. Focusing on the [Ga4L6]12- assembly {L = 1,5-bis(2‘,3‘-dihydroxybenzamido)naphthalene}, the host−guest kinetic behavior of this supramolecular capsule is defined. Guest self-exchange rates at varied temperatures and pressures were measured to determine activation parameters, revealing negative ΔS‡ and positive ΔV‡ values {PEt4+: ΔH‡ = 74(3) kJ mol-1, ΔS‡ = −46(6) J mol-1 K-1, k298 = 0.003 s-1; NEt4+: ΔH‡ = 69(2) kJ mol-1, ΔS‡ = −52(5) J mol-1 K-1, k298 = 0.009 s-1; NMe2Pr2+: ΔH‡ = 52(2) kJ mol-1, ΔS‡ = −56(7) J mol-1 K-1, ΔV‡ = +13(1) cm3 mol-1, k298 = 4.4 s-1; NPr4+: ΔH‡ = 42(1) kJ mol-1, ΔS‡ = −102(4) J mol-1 K-1, ΔV‡ = +31(2) cm3 mol-1, k298 = 1.4 s-1}. In PEt4+ for NEt4+ exchange reactions, egress of the initial guest (G1) is found to be rate de...