The ability of supramolecular host-guest complexes to catalyse organic reactions collaboratively with an enzyme is an important goal in the research and discovery of synthetic enzyme mimics. Herein we present a variety of catalytic tandem reactions that employ esterases, lipases or alcohol dehydrogenases and gold(I) or ruthenium(II) complexes encapsulated in a Ga(4)L(6) tetrahedral supramolecular cluster. The host-guest complexes are tolerated well by the enzymes and, in the case of the gold(I) host-guest complex, show improved reactivity relative to the free cationic guest. We propose that supramolecular encapsulation of organometallic complexes prevents their diffusion into the bulk solution, where they can bind amino-acid residues on the proteins and potentially compromise their activity. Our observations underline the advantages of the supramolecular approach and suggest that encapsulation of reactive complexes may provide a general strategy for carrying out classic organic reactions in the presence of biocatalysts.

A polyanionic supramolecular assembly (1) is shown to catalytically cyclize the monoterpene citronellal and two homologues. In contrast to cyclization in acidic aqueous solution, the hydrophobic interior of 1 prevents the capture of reactive intermediates by water. This effect was also observed in the gold-catalyzed cycloisomerization of an enyne. Due to the steric confinement of the catalyst's interior, Prins cyclizations in 1 proceed cleanly both for substrates containing and lacking gem-dimethyl substitution. Encapsulation in 1 consequently imposes a degree of mechanistic control that, similar to enzyme catalysis, is not observed in bulk aqueous solution.

Introduction 4499 2. Diels-Alder Cycloadditions 4499 2.1. Unsaturated Sulfones as Dienophiles 4499 2.1.1. Early Studies 4499 2.1.2. Aromatization of Cycloaddition Products of Acetylenic Sulfones 4501 2.1.3. Iterative Diels-Alder and Ramberg-Backlund Reactions of Chloroalkylsulfonyl Allenes 4502 2.1.4. Diels-Alder Cycloadditions of Bis(sulfonyl)acetylenes and Their Equivalents 4502 2.1.5. Diels-Alder Cycloadditions of Acetylenic Sulfones Containing Heteroatom Substituents 4503 2.1.6. Diels-Alder Reactions of Dienyl Sulfones as Dienophiles 4504 2.1.7. Diels-Alder Reactions of Unsaturated Sulfones with Pyrroles 4505 2.1.8. Diels-Alder Reactions of Unsaturated Sulfones with Furans and Isobenzofurans 4507 2.1.9. Miscellaneous Cycloadditions 4508 2.2. Dienyl Sulfones As the Diene Components in Diels-Alder Reactions 4509 2.2.1. 1-Sulfonyl Derivatives † Dedicated to Professor David N. Harpp, for his inspiring contributions as a teacher, mentor and scholar.