A supramolecular approach to combining enzymatic and transition metal catalysis.

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.

Selective monoterpene-like cyclization reactions achieved by water exclusion from reactive intermediates in a supramolecular catalyst.

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.

Rearrangements and intramolecular Diels-Alder reactions of normal and vinylogous aza-Morita-Baylis-Hillman products leading to isoindoline derivatives.

Aza-Morita-Baylis-Hillman (aza-MBH) products derived from arylimines and methyl acrylate or acrylonitrile were N-alkylated with (E)-5-bromopenta-1,3-diene, and the resulting trienes were subjected to intramolecular Diels-Alder (IMDA) cyloadditions to afford the corresponding trans- and cis-fused tetrahydroisoindolines as the major and minor products, respectively. Catalysis with boron trichloride improved the IMDA diastereoselectivities of the nitrile derivatives, while yields were improved in both the nitrile and ester series. Treatment of the nitrile-substituted trienes with DABCO in DMF resulted in unexpected transposition of the N-(pentadienyl)sulfonamide group to the beta-position of the acrylonitrile moiety. Subsequent IMDA cycloadditions produced cis-fused positional isomers of the previous tetrahydroisoindolines. When the products of vinylogous aza-MBH reactions in the nitrile series were N-propargylated, the resulting dienynes underwent a similar transposition and IMDA reaction, producing trans and cis diastereomers of the corresponding dihydroisoindolines as the major and minor products, respectively. In all but one case, only the former products were observed in the presence of methylaluminum dichloride, while the corresponding aromatized isoindolines were obtained when the IMDA reactions were carried out in the presence of DDQ. Thus, a variety of aryl-substituted isoindoline products with different levels of unsaturation and complementary substitution patterns and stereochemistry are readily available through these processes.

Aza-Morita-Baylis-Hillman reactions and cyclizations of conjugated dienes activated by sulfone, ester, and keto groups.

The aza-Morita-Baylis-Hillman reactions of aldimines 2 with several activated conjugated dienes were found to proceed smoothly in DMF in the presence of 3-hydroxyquinuclidine (HQD). Imines 2 reacted with 1-(p-toluenesulfonyl)-1,3-butadiene (3), methyl 2,4-pentadienoate (6), hexa-3,5-dien-2-one (7), and 1-phenylpenta-2,4-dien-1-one (8) to afford adducts 4, 13, 14, and 15, respectively. While products 4, 13, and 15 were formed as E,Z mixtures, adducts 14 were obtained as essentially pure E-isomers. Cyclization of the E-isomers of the products derived from the dienyl sulfone 3 and the dienoate ester 6 occurred via intramolecular conjugate addition under base-catalyzed conditions to afford functionalized piperidines 5 and 16, respectively. The aza-Morita-Baylis-Hillman reaction and subsequent cyclization of the imine 2a with 3 were also carried out as a one-pot reaction, while the reaction mixture was simultaneously irradiated at 300 nm to effect the photoisomerization of the unreactive Z-adduct of the corresponding 4 to the more reactive E-isomer.