UC Santa Cruz

Abstract

THE DEVELOPMENT OF ORGANIC REDUCTION REACTIONS BY BINARY HYDRIDES AND BINARY METAL CATALYSTS

Gabriella A. AmberchanThe synthesis of a binary hydride, diisobutylaluminum borohydride [(iBu)2AlBH4], was achieved from the equimolar addition of diisobutylaluminum hydride and borane dimethyl sulfide. Exploration into the reductive scope of (iBu)2AlBH4 showcased its versatility as a reducing agent. It was found that (iBu)2AlBH4 can convert aldehydes, ketones, and epoxides into alcohols (up to 98% yield), nitriles and tertiary amides to amines (up to 99% yield), and carboxylic acids and esters into alcohols (up to 93%). These reactions occurred at ambient conditions and the product was isolated with simple acid-base extractions, without the need for further purification via column chromatography. Further investigation showed that (iBu)2AlBH4 is effective as a selective hydride, as shown through a series of competitive reactions. Borohydrides are not limited to reducing organic functional groups, as shown in the synthesis of an amorphous nickel and boron composite (NBC) catalyst. The NBC catalyst was synthesized from the reduction of a nickel salt by sodium borohydride and supported on mesoporous aluminosilicate nanoparticles (MASN). The NBC-MASN catalyst demonstrated excellent catalytic activity for the selective reduction of the nitro group on a variety of substituted nitroarenes, using hydrazine hydrate (N2H4•H2O) as the H2 source. Reuse and regeneration of NBC-MASN for the reduction of para-nitrotoluene demonstrated that the catalyst could be recycled up to nine times (67-99% yield). The structure and composition of NBC-MASN were fully characterized by electron microscopy, various X-ray spectroscopy techniques, thermogravimetric analysis, and inductively coupled plasma optical emission spectroscopy. By using Raman spectroscopy in a non-invasive, ex situ style, the reduction of nitroarenes in aqueous media was monitored. The catalytic reduction of nitroarenes was observed by the disappearance of the distinct nitro stretch (1350 cm-1). Spectroscopic data determined that the aqueous reaction proceeded with pseudo first-order kinetics. Similarly, the reduction of cyclic ketones by dimethylamine borane (DMAB) in aqueous medium was also monitored using Raman spectroscopy. In this biphasic system, consisting of a carbonyl compound layer and an aqueous DMAB layer, the ex situ probe monitored the loss of the carbonyl stretch (1713 cm-1). Density functional theory analysis established that the reaction occurred through a concerted hydride transfer. As hydrogen cylinders are no longer available at UC Santa Cruz for academic research, it was of interest to develop a safe and efficient method of generating on-demand hydrogen gas. A binary gallium:aluminum (3:1) alloy was synthesized as a reagent to efficiently produce hydrogen gas from water. Gallium acts to remove the aluminum oxide coating. This allows pristine aluminum sites to be available for the water splitting reaction. Any source of water, including wastewater, commercial beverages, or ocean water were found to produce H2. Similarly, commercial and waste aluminum were used to rapidly generate H2. Gallium in the spent alloy was isolated and reused indefinitely. Characterization of the alloy showed aluminum particles within gallium were responsible for this room temperature water splitting activity, which likely proceeded through the Grotthuss mechanism. This work illustrates a possible method of generating H2 without the need of fossil fuels and at ambient conditions.