Selective reducing agents: Synthesis of new hydrides of boron, aluminum, and silicon

Developments of the mid 1940's produced the two well known reducing agents sodium borohydride and lithium aluminum hydride (LAH). These two reagents are at opposite ends of the reactivity scale. The first is too selective for many purposes, but the second is too reactive to be selective. The problem is to produce a reducing agent of intermediate selectivity. Hydride agents of intermediate potential may be produced by addition of alkoxides, amide, or thiolates to a borane. The product is not a heteroatom-substituted borohydride but instead a mixture of an "ate" complex and the borohydride. The mixed reagent system is as powerful a reducing agent as would be expected of a heteroatom borohydride. Alternatively, a heteroatom-substituted borohydride may be produced by dehydronation (deprotonation) of an amine-borane complex. The product, an aminoborohydride, is comparable in reactivity to LAH. It readily reduces amides, esters, epoxides, and ketones. In addition, it reduces cyclic ketone systems with a selectivity similar to LAH or sodium borohydride. But, unlike sodium borohydride, lithium aminoborohydrides are ether soluble. They offer a direct replacement for sodium borohydride where a slurry is inconvenient. Also, lithium aminoborohydrides, much like LAH, readily transfer lithium hydride to borane, mono-, di-, and trialkylboranes. The products are the borohydride and an aminoborane. The borohydrides produced by the transfer reaction show the same reactivity as those produced by the classical method. But, if desired, the aminoborane may be removed from the product borohydride. Addition of tetramethylethylenediamine (TMEDA)/pentane forms an insoluble complex with the borohydride freeing it of the aminoborane. Additionally, these reagents may be used to reduce silicon tetrachloride to form silicon tetrahydride (silane). Alternatively, silane may be formed by redistribution of silicon tetrachloride with sodium borohydride. The products of the reaction also include 2 equivalents of diborane. The silane may then be freed of the diborane by use of a Lewis base such as an amine. The by-product of the reaction, an amine-borane, is both air-stable and industrially valuable.