Directing Abilities of Alcohol-Derived Functional Groups in the Hydroformylation of Olefins

The hydroformylation of allylic and homoallylic alcohols and their derivatives using cationic and neutral rhodium complexes has been examined. The highest diastereoselectivity (87:13) was observed in the reaction of 1-methoxymethoxy-2-methylenecyclohexane. Higher yields and similar selectivities were obtained in the reaction of the TBDMS-protected alcohol. The major diastereomer results from hydroformylation syn to the functional group, which would suggest a directing effect. However, hydroformylation of 3-methylene-1-cyclohexanol derivatives occurs on the face opposite to the directing group in the major isomer. These data, in addition to the results of hydroformylation of 1-methyl-2-methylenecyclohexane, suggest that inherent conformational preferences are of significant importance in determining the product distribution and that the directing power of simple alcohols and their derivatives is moderate at best under the conditions examined in this study

Asymmetric Dihydroxylation of Olefins Using Cinchona Alkaloids on Highly Ordered Inorganic Supports

A modified cinchona alkaloid was grafted onto a mesoporous molecular sieve and onto amorphous silica gel. These heterogeneous ligands were employed in the asymmetric dihydroxylation of olefins under Sharpless conditions. The supported ligands yielded equivalent enantioselectivity compared with that of the homogeneous system and were easily recovered and reused

Chiral Hybrid Mesoporous Silicas: Assembly of Uniform Hollow Nanospheres and Helical Nanotubes with Tunable Diameters

Uniform helical silica nanotubes and hollow silica nanostructures with adjustable diameters have been prepared through the self-assembly of sodium dodecyl sulfate (SDS) as the surfactant, <i>N</i>-trimethoxysilylpropyl-<i>N,N,N</i>-trimethylammoniumchloride (TMAPS) as a costructure directing agent (CSDA), a binapthyl-based chiral dopant, and TEOS (Si­(OEt)₄) as the bulk silica constituent. Depending on the ratio of anionic surfactant to cationic costructure directing agent, the morphology can be tuned from hollow spheres to hollow nanotubes. At a 1:1 ratio of TMAPS/SDS, in the presence of the axially chiral dopant molecule, uniformly helical structures are obtained. The chirality of the dopant is shown to affect the sense of helicity. Under identical conditions, a monosilylated chiral dopant only leads to the formation of well dispersed uniform hollow spheres rather than helical nanotubes, which further demonstrates the importance of incorporating the chiral dopant as an integral component of the siloxane network, rather than merely as a surface group

Installing Stable Molecular Chirality within the Walls of Periodic Mesoporous Organosilicas via Self-Assembly

The synthesis of highly ordered chiral periodic mesoporous organosilica (PMO) materials is described using a novel approach. Chiral dopants featuring removable chirality were combined with freely rotating bulk monomers, resulting in a bulk chiral material with handedness related to the chiral dopant. Once incorporated into the PMO, removal of the chiral-linker in the dopant is readily accomplished and occurs with complete preservation of the circular dichroism signal in the PMO material, whereas in the precursor molecule, this transformation would lead to a total loss of chirality. The chirality of the PMO material is retained even after prolonged hydrothermal treatment, indicating stable chirality induction within the walls of solid PMO

Dramatic Effect of Lewis Acids on the Rhodium-Catalyzed Hydroboration of Olefins

The addition of Lewis acids such as trispentafluoroboron as cocatalysts has been found to have a dramatic effect on the Rh-catalyzed hydroboration of olefins with pinacol borane. For example, aliphatic olefins do not react at all in noncoordinating solvents, but with the addition of 2% of B(C6F5)3, the reaction is complete in minutes. Similarly, the reaction of aromatic olefins with HBPin occurs slowly and nonselectively in the absence of B(C6F5)3, but is accelerated and occurs more selectively in its presence. Preliminary mechanistic studies suggest that the B(C6F5)3 needs to be present throughout the course of the reaction, not just at the initiation stage, and implicate this species, along with THF, in the heterolytic cleavage of the B−H bond of HBPin

Benzimidazoles as Ligands in the Ruthenium-Catalyzed Enantioselective Bifunctional Hydrogenation of Ketones

A series of Cl2Ru(diphosphane)L2 (II) complexes in which L = N1-alkylated benzimidazoles, bonding to the metal through nitrogen, have been synthesized and characterized. In the case of 1-methylbenzimidazole, the resulting complexes exist as statistical mixtures of all possible conformational isomers. When the size of the substituent on the benzimidazole was increased to complexes could be prepared that exist as a single diastereomer. All complexes possessing benzimidazole ligands bound to the ruthenium center are active for the mild and chemoselective hydrogenation of ketones in the presence of alkenes. Catalysts that exist as a single diastereomer, prepared with enantiomerically pure diphosphanes, catalyze the hydrogenation of prochiral ketones with moderate levels of enantioselectivity that are significantly improved relative to catalysts existing in several conformations

Mercaptopropyl-Modified Mesoporous Silica: A Remarkable Support for the Preparation of a Reusable, Heterogeneous Palladium Catalyst for Coupling Reactions

The functionalization of SBA-15 with mercaptopropyl trimethoxysilane leads to a material capable of absorbing Pd from organic and aqueous solutions. The resulting Pd-loaded material acts as a catalyst for the Suzuki−Miyaura and Mizoroki−Heck coupling reactions. Leaching studies show that the filtrate contains as little as 3 ppb Pd after reaction. Aminopropylated silica is an effective scavenger, and catalyst for the Mizoroki−Heck reaction, but leaching is significant (35 ppm), illustrating the importance of the thiol ligand to retain Pd on the surface. Heterogeneity tests such as hot filtration experiments and three-phase tests show that the reaction is occurring predominantly via surface-bound Pd

Benzimidazoles as Ligands in the Ruthenium-Catalyzed Enantioselective Bifunctional Hydrogenation of Ketones

A series of Cl2Ru(diphosphane)L2 (II) complexes in which L = N1-alkylated benzimidazoles, bonding to the metal through nitrogen, have been synthesized and characterized. In the case of 1-methylbenzimidazole, the resulting complexes exist as statistical mixtures of all possible conformational isomers. When the size of the substituent on the benzimidazole was increased to complexes could be prepared that exist as a single diastereomer. All complexes possessing benzimidazole ligands bound to the ruthenium center are active for the mild and chemoselective hydrogenation of ketones in the presence of alkenes. Catalysts that exist as a single diastereomer, prepared with enantiomerically pure diphosphanes, catalyze the hydrogenation of prochiral ketones with moderate levels of enantioselectivity that are significantly improved relative to catalysts existing in several conformations

Taking the F out of FLP: Simple Lewis Acid–Base Pairs for Mild Reductions with Neutral Boranes via Borenium Ion Catalysis

Discrete three-coordinate borenium salts <b>1c</b> and <b>1d</b> are accessed by cooperative Lewis acid–base pair-mediated heterolytic splitting of the B–H bond in pinacolborane by B­(C₆F₅)₃·DABCO and Ph₃C⁺/DABCO, respectively. The resulting salts are competent catalysts in the reduction of a broad range of imines and can be generated in situ. Moreover, a mechanistic framework for borenium catalysis based on experimental evidence is proposed. The reaction is suggested to proceed by borenium activation of the imine substrate followed by counterintuitive hydride delivery from HBPin (with the assistance of DABCO) rather than from the HB­(C₆F₅)₃^– anion, contrary to typical mechanisms of reduction in FLP systems

Experimental Demonstration of Base-Catalyzed Interconversion of Isomeric Betaine Intermediates in the Corey−Chaykovsky Epoxidation

The collapse of hydroxysulfonium salts has been examined as a model for the epoxidation of aldehydes. The anti diastereomer reacted with retention of stereochemistry and no crossover, while the syn diastereomer gave crossover products along with cis and trans epoxides. Deprotonation and reprotonation on the carbon of the α-hydroxy sulfonium ylide is presumed responsible for production of the trans epoxide. This reaction pathway has been proposed to explain losses of enantioselectivity but never directly observed