Reductions using Boron Compounds

Reductions using Boron Compounds.—The advantages of lithium triethylboro-hydride as a very powerful reducing agent, and of amine—boranes derived from primary and secondary amines as mild, chemoselective reducing agents for aldehydes and ketones, have been extolled. Several reagents are recom- 

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There are few useful reactions in which new B—H bonds are formed. Although the formation of boranes from the protolysis of borides or the reduction of boron compounds with Hj, either in electrical discharges or in the presence of active metals, have historical importance, these methods have no importance or utility today. Indeed, the preparation of boranes is so dominated by the single common starting material, the tetrahydroborate ion, that the only important reactions in which B—H bonds are formed are those in which hydride ion either reduces species with B—O or B-halogen bonds to form boranes or adds to trifunctional boron compounds to form hydroborates. 

The reactor coolant pH is controlled using lithium-7 hydroxide [72255-97-17, LiOH. Reactor coolant pH at 300°C, as a function of boric acid and lithium hydroxide concentrations, is shown in Figure 3 (4). A pure boric acid solution is only slightly more acidic than pure water, 5.6 at 300°C, because of the relatively low ionisation of boric acid at operating primary temperatures (see Boron COMPOUNDS). Thus the presence of lithium hydroxide, which has a much higher ionisation, increases the pH ca 1—2 units above that of pure water at operating temperatures. This leads to a reduction in corrosion rates of system materials (see Hydrogen-ION activity). 

In addition to being useful reagents for the reductions of carbonyl compounds, boron-based reagents can also be used for the conversion of an alkene to a wide variety of functionalized alkanes. Because the majority of these reagents carry a terpene substituent, they are discussed under these chiral pool materials (Chapter 5). 

Oxidative fluorodesulfuriztuion can also be achieved by the action of nitrosonium tetrafluo-roborate, as oxidant, and hydrogen fluoridc/pyridine, as a source of fluoride ions, on aryl sulfides. The starting compounds are easily prepared from ketones or aldehydes and ben-zenethiol using boron trifluoride monohydratc as catalyst, and subsequent reduction with triethylsihine. - ... 

Elemental boron is a refractory material that is usually isolated either as a shiny black crystalline solid or a softer, browner, more impure amorphous solid. Reduction of readily available boron compounds containing boron oxygen bonds to elemental boron is energy intensive and costly. This has limited the extent of conunercial use of this material. Many related refractory boron compounds have been prepared and characterized including metal borides, boron carbides, boron nitrides, and various boron metal alloys. These refractory materials and elemental boron are also discussed in some detail in the article Borides Solid-state Chemistry. Other general references are available on elemental boron and other refractory boron compounds. " ... 

Chiral modification is not limited to boronate and aluminate complexes. Boranes or alanes are partially decomposed with protic substances such as chiral amines, alcohols or amino alcohols to form useful reagents for enantioselective reduction of carbonyl compounds. For example, reduction of acetophenone with borane modified with the amines (65) to (67) gives (5)-l-phenylethyl alcohol with 3.5-20%... 

E. R. H. Walker, Chem. Soc. Rev., 1976, 23 lists all aluminum and boron hydrides used for the reduction of organic compounds. A table of functional group selectivity is given. 

Many examples have been presented for the generation of olefinic sugar derivatives. Given that olefins provide useful handles for further functionalization, discussion centered on the reduction of these compounds is warranted. In this section, both catalytic and boron-based reductions are addressed. 

However, we know of no report of the reduction of a ketone to a methylene compound using boron trifluoride etherate and triethyl silane. 

Lithium cyanotrihydridoborate reduces aldehydes in hot dioxan, e.g., pyrenecarbaldehyde in 85% yield, but does not affect aliphatic or aromatic ketones.380 Of the alkaline-earth boron hydrides, only calcium hydridotrimethoxyborate has been mentioned it is prepared from calcium hydride and trimethyl borate and its preparative use for reduction of carbonyl compounds has been investigated by Hesse and Jager.381... 

Both enantiomers of binaphthol have found many uses as chiral reagents and catalysts. Thus, they modify reducing agents (e.g., lithium aluminum hydride) for the reduction of ketones to chiral secondary alcohols (Section D.2.3.3.2.) or react with aluminum, titanium or boron compounds to give chiral Lewis acids for asymmetric Diels-Alder reactions (Section D. 1.6.1.1.1.3.) and ene reactions (Section D.I.6.2.). They have also been used as chiral leaving groups in the rearrangement of allyl ethers (Section D.l.1.2.2.) and for the formation of chiral esters with a-oxo acids (Section D. 1.3.1.4.1, and many other purposes. 

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