Amphoteric boronic acids

A much higher and more efficient chiral version (>90%) of the cyclopropanation of allylic alcohols is obtained by using the amphoteric bifunctional ligand (R,R)-93 prepared from commercially available (+)-(/ ,7 )-At,TV,A, A -tetramethyltartaric acid diamide and butyl-boronic acid. The dioxyborolane chiral ligand proved to be extremely effective with several types of substituted allylic alcohols 92. The chiral ligand 93 can easily be removed and recovered (> 80%) by a simple aqueous extraction of the organic layer after the reaction. 

Metallic elements with low ionization energies commonly form basic ionic oxides. Elements with intermediate ionization energies, such as beryllium, boron, aluminum, and the metalloids, form amphoteric oxides. These oxides do not react with or dissolve in water, but they do dissolve in both acidic and basic solutions. 

Boron, a metalloid with largely nonmetallic properties, has acidic oxides. Aluminum, its metallic neighbor, has amphoteric oxides (like its diagonal neighbor in Group 2, beryllium). The oxides of both elements are important in their own right, as sources of the elements, and as the starting point for the manufacture of other compounds. 

Metallic elements with low ionization energies commonly form ionic oxides. As remarked in Section 10.1, the oxide ion is a strong base, so the oxides of most of these metals form basic solutions in water. Magnesium is an exception because its oxide, MgO, is insoluble in water. However, even this oxide reacts with acids, so it is regarded as basic. Elements with intermediate ionization energies, such as beryllium, boron, aluminum, and the metalloids, form amphoteric oxides. These oxides do not react with water, but they do dissolve in both acidic and basic solutions. 

The characteristics of this group are that the elements possess a valence of 3, and that the oxides, M2O3, have but a weakly developed basic character. Boron, in fact, shows practically no base-forming properties, but forms rather a weak acid. The oxide of aluminum displays both basic and acidic properties that is, it is amphoteric. The remaining elements are more distinctly base-forming than aluminum, without, however, approaching in any way the alkaline earth metals in this respect. 

This high activity of the catalyst 19 can be explained by its amphoteric properties since it is able to activate simultaneously both ketone and borane. This assumption led to a mechanistic proposal rationalizing the hydride transfer from the borane on the phosphorus atom to a ketone coordinated to the other boron atom acting as a Lewis acid. 

It should be particularly noted that there are in each horizontal period two alkaline elements, two amphoteric elements and four non-alkaline elements, the first three of the latter giving rise to acids and the fourth being inert. All of the non-metals, incidentally, fall to the right of an oblique line drawn from boron (5) to eka-iodine (85). 

As the group is descended, atomic radii increase and ionization energies are all lower than for boron. There is an increase in polar interactions and the formation of distinct M ions. This increase in metaUic character is clearly Ulustrated by the increasing basic character of the hydroxides boron hydroxide is acidic, aluminium and gallium hydroxides are amphoteric, indium hydroxide is basic, and thaUium forms only the oxide. As the elements of group 13 have a vacant p-orbital they display many electron-acceptor properties. For example, many boron compounds form adducts with donors such as... 

The term amphoteric is now taken to mean an oxide or hydroxide that can dissolve in acids to give salts and can also dissolve in alkalis to give metaUates i.e., it can show both basic and acidic properties. Examples include the oxides and hydroxides of boron and aluminum. 

The Group lllA elements clearly show the trend of increasing metallic character in going down any column of elements in the periodic table. Boron, at the top of Group lllA, is a metalloid, and its chemistry is typical of a nonmetal. The compound B(OH)3 is actually acidic (boric acid). The rest of the elements (aluminum, gallium, indium, and thallium) are metals, but their hydroxides change from amphoteric (acidic and basic) for aluminum and gallium to basic for indium and thallium. 

Consistent with the position of the metal-nonmetal line (and the corresponding acid-base character of metal and nonmetal oxides), boron oxide is an acid anhydride, whereas the oxides of the heavier elements progress from amphoteric to basic in behavior. Boron oxide, then, reacts with water, as shown in Equation (14.2), to produce boric acid, B(OH)3 or H3BO3 ... 

It is worth while noting at this point that in equations 5, 7, and 8 the acidi and acid2 compounds are amphoteric in exactly the same sense. The addition compounds of boron trichloride with acetone and with pyridine are amphoteric because they contain both acidic and basic constituents. They can react both ways because stronger bases will displace the weaker bases and stronger acids will displace the boron trichloride. In the same way silver hydroxide in equation 7 and water in equation 8 are also amphoteric. They appear as acids in these equations, but both may act as bases because of the presence of the hydroxyl group. 

When the solvent reacts without ionizing, it reacts with either the acid or the base, but not ordinarily with both. Such solvents are usually not amphoteric. For example, if boron trichloride reacts with triethylamine in ether, the boron trichloride would react with the ether, but the triethylamine would not. The oxygen atom in the ether can donate an electron pair to form a coordinate bond, but the hydrogen atoms in ether have little tendency to form hydrogen bridges ... 

Boron forms a solid acidic oxide, B2O3, like that of silicon, Si02. In contrast AI2O3 is amphoteric and CO2 is acidic. 

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