Cathleen M. Crudden

Cross Coupling Reactions of Chiral Secondary Organoboronic Esters With Retention of Configuration

We report the first example of a coupling reaction of chiral secondary boronic esters generated by the hydroboration of vinyl arenes. In order for the reaction to take place in high yields, the use of silver oxide as a base and the presence of at least 8 equiv of triphenyl phosphine per Pd are required. The reaction proceeds with >90% retention of configuration in all cases except one. Remarkably, the linear boronate ester does not react under these conditions.

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 1c and 1d are accessed by cooperative Lewis acid-base pair-mediated heterolytic splitting of the B-H bond in pinacolborane by B(C(6)F(5))(3)·DABCO and Ph(3)C(+)/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(C(6)F(5))(3)(-) anion, contrary to typical mechanisms of reduction in FLP systems.

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.

Hydrogenations at Room Temperature and Atmospheric Pressure with Mesoionic Carbene‐Stabilized Borenium Catalysts

1,2,3-Triazolylidene-based mesoionic carbene boranes have been synthesized in a convenient one-pot protocol from the corresponding 1,2,3-triazolium salts, base, and borane. Borenium ions are obtained by hydride abstraction and serve as catalysts in mild hydrogenation reactions of imines and unsaturated N-heterocycles at ambient pressure and temperature.

Double Single-Crystal-to-Single-Crystal Transformation and Small- Molecule Activation in Rhodium NHC Complexes**

Three gases, one crystal: rhodium NHC complexes undergo back-to-back single-crystal-to-single-crystal transformations by selective nonreversible ligand exchange reactions. Slow diffusion of O(2) converts a dinitrogen complex into a dioxygen complex, and CO subsequently replaces O(2).

Modular Synthesis of Triarylmethanes through Palladium-Catalyzed Sequential Arylation of Methyl Phenyl Sulfone†

Triarylmethanes, which are valuable structures in materials, sensing and pharmaceuticals, have been synthesized starting from methyl phenyl sulfone as an inexpensive and readily available template. The three aryl groups were installed through two sequential palladium-catalyzed C-H arylation reactions, followed by an arylative desulfonation. This method provides a new synthetic approach to multisubstituted triarylmethanes using readily available haloarenes and aryl boronic acids, and is also valuable for the preparation of unexplored triarylmethane-based materials and pharmaceuticals.

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(C(6)F(5))(3), the reaction is complete in minutes. Similarly, the reaction of aromatic olefins with HBPin occurs slowly and nonselectively in the absence of B(C(6)F(5))(3), but is accelerated and occurs more selectively in its presence. Preliminary mechanistic studies suggest that the B(C(6)F(5))(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.

N-Heterocyclic Carbene Complexes of Rh: Reaction With Dioxygen Without Oxidation

The reaction of oxygen with rhodium complexes containing N-heterocyclic carbenes was found to give dioxygen complexes with rare square planar geometries and unusually short O-O bond lengths. Analysis of the bonding in these complexes by Rh L-edge X-ray absorption spectroscopy (XAS), Raman spectroscopy, and DFT calculations provides evidence for a bonding model in which singlet oxygen is bound to a Rh(I) d8 metal complex, rather than the more common Rh(III) d6 peroxo species with octahedral geometry and O-O bond lengths in the 1.4-1.5 A range.

Enantiospecific, Regioselective Cross-Coupling Reactions of Secondary Allylic Boronic Esters

Asymmetric cross-coupling reactions in which stereo- chemistry-bearing CC bonds are created are still in their infancy. Considering the wealth of compounds that can be prepared by cross-coupling methodologies, and the impor- tance of synthetic methods leading to enantiomerically en- riched products, this is a surprising reality. Advances in this area have occurred in the past few years, in the realm of enantioselective or enantiospecific cross-couplings of chiral electrophiles. (1) In the area of cross-coupling of stereodefined nucleo- philes, Crudden originally reported the enantiospecific cross-coupling of chiral enantiomerically enriched benzylic boronic esters (Eq. (1)). (2, 3)

Chemical Sensor Based on a Long-Period Fibre Grating Modified by a Functionalized Polydimethylsiloxane Coating

A chemical sensor based on a coated long-period grating has been prepared and characterized. Designer coatings based on polydimethylsiloxane were prepared by the incorporation of diphenylsiloxane and titanium cross-linker in order to provide enhanced sensitivity for a variety of key environmental pollutants and optimal refractive index of the coating. Upon microextraction of the analyte into the polymer matrix, an increase in the refractive index of the coating resulted in a change in the attenuation spectrum of the long-period grating. The grating was interrogated using ring-down detection as a means to amplify the optical loss and to gain stability against misalignment and power fluctuations. Chemical differentiation of cyclohexane and xylene was achieved and a detection limit of 300 ppm of xylene vapour was realized.