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

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

Mesoionic Carbene–Boranes

Mesoionic carbenes (MICs), derived from alkylation and deprotonation of triazoles, are shown to form stable complexes with BH₃. The synthesis of triazoles via Huigsen cycloadditions provides considerable structural diversity. Several routes to MIC–boranes are described, and their structures have been characterized by X-ray crystallography. As predicted on the basis of the increased σ-donor capacity of MICs by comparison to NHCs, the MIC–borane adducts are more reactive reducing agents

Mesoionic Carbene–Boranes

Mesoionic carbenes (MICs), derived from alkylation and deprotonation of triazoles, are shown to form stable complexes with BH₃. The synthesis of triazoles via Huigsen cycloadditions provides considerable structural diversity. Several routes to MIC–boranes are described, and their structures have been characterized by X-ray crystallography. As predicted on the basis of the increased σ-donor capacity of MICs by comparison to NHCs, the MIC–borane adducts are more reactive reducing agents

Preparation of Quaternary Centers via Nickel-Catalyzed Suzuki–Miyaura Cross-Coupling of Tertiary Sulfones

We describe the development of a nickel-catalyzed Suzuki–Miyaura cross-coupling of tertiary benzylic and allylic sulfones with arylboroxines. A variety of tertiary sulfones, which can easily be prepared via a deprotonation–alkylation route, were reacted to afford symmetric and unsymmetric quaternary products in good yields. We highlight the use of either BrettPhos or Doyle’s phosphines as effective ligands for these challenging desulfonative coupling reactions. The utility of this methodology was demonstrated in the concise synthesis of a vitamin D receptor modulator analogue

Synthesis and Structure of Palladium 1,2,3-Triazol-5-ylidene Mesoionic Carbene PEPPSI Complexes and Their Catalytic Applications in the Mizoroki–Heck Reaction

New mono- and bimetallic 1,2,3-triazol-5-ylidene mesoionic carbene (MIC) complexes of Pd have been synthesized, isolated, and characterized. We have described the synthesis of a bis­(MIC) complex via transmetalation from a Ag-MIC complex and two PEPPSI-type complexes which are directly available from their respective triazolium salts by treatment with PdCl₂ in pyridine. The X-ray structures are reported for all complexes described herein. Interestingly, each complex described exhibits various secondary noncovalent interactions in the solid state of the general type C–H···Cl, which appear to be important for the stabilization of the solid-state structure of the complexes. We further demonstrated the utility of the new PEPPSI complexes in the Mizoroki–Heck reaction. In the case of aryl iodides and electron-deficient bromides, high conversion is observed with methyl acrylate. Hg poisoning tests suggest that, even with an easily dissociated ligand, the reaction likely proceeds via Pd nanoparticles

Synthesis and Structure of Silver and Rhodium 1,2,3-Triazol-5-ylidene Mesoionic Carbene Complexes

New mono- and bidentate 1,2,3-triazol-5-ylidene mesoionic carbene (MIC) ligands have been synthesized and reacted with silver oxide, forming cationic 1:2 and 2:2 Ag:MIC ([AgL₂]Br and [Ag₂L₂]­[Ag₂Br₄]) complexes, respectively. These complexes have been isolated and characterized. They were further found to be potent transmetalation agents toward rhodium, resulting in the preparation of mono- and bimetallic rhodium complexes. The reaction of the 2:2 Ag:L complex with [Rh­(COD)₂]­BF₄ results in the first reported example of a cationic bimetallic rhodium MIC complex. This complex was fully characterized, and its X-ray structure is reported. Interestingly, this complex exhibits various secondary noncovalent interactions in the solid state of the general type C–H···X, where X = F, Cl

Synthesis of Enantiomerically Enriched Triarylmethanes by Enantiospecific Suzuki–Miyaura Cross-Coupling Reactions

The Suzuki–Miyaura cross-coupling of chiral, enantio­merically enriched dibenzylic boronic esters is described. The reaction proceeds with almost complete retention of stereo­chemistry, providing access to triaryl­methanes, compounds that have high biological activity and are difficult to prepare in enantio­merically pure form using other methods

Synthesis and Structure of Palladium 1,2,3-Triazol-5-ylidene Mesoionic Carbene PEPPSI Complexes and Their Catalytic Applications in the Mizoroki–Heck Reaction

New mono- and bimetallic 1,2,3-triazol-5-ylidene mesoionic carbene (MIC) complexes of Pd have been synthesized, isolated, and characterized. We have described the synthesis of a bis­(MIC) complex via transmetalation from a Ag-MIC complex and two PEPPSI-type complexes which are directly available from their respective triazolium salts by treatment with PdCl₂ in pyridine. The X-ray structures are reported for all complexes described herein. Interestingly, each complex described exhibits various secondary noncovalent interactions in the solid state of the general type C–H···Cl, which appear to be important for the stabilization of the solid-state structure of the complexes. We further demonstrated the utility of the new PEPPSI complexes in the Mizoroki–Heck reaction. In the case of aryl iodides and electron-deficient bromides, high conversion is observed with methyl acrylate. Hg poisoning tests suggest that, even with an easily dissociated ligand, the reaction likely proceeds via Pd nanoparticles