Lewis acid boron compounds

Chiral boron Lewis-acid complexes have been successfully used in Diels-Alder and aldol reactions. Representative chiral Lewis-acidic boron compounds are shown in Figure 2.297-301... 

Figure 2 Chiral Lewis-acidic boron compounds.

Often Lewis acids are added to the system as a cocatalyst. It could be envisaged that Lewis acids enhance the cationic nature of the nickel species and increase the rate of reductive elimination. Indeed, the Lewis acidity mainly determines the activity of the catalyst. It may influence the regioselectivity of the catalyst in such a way as to give more linear product, but this seems not to be the case. Lewis acids are particularly important in the addition of the second molecule of HCN to molecules 2 and 4. Stoichiometrically, Lewis acids (boron compounds, triethyl aluminium) accelerate reductive elimination of RCN (R=CH2Si(CH3)3) from palladium complexes P2Pd(R)(CN) (P2= e g. dppp) [7], This may involve complexation of the Lewis acid to the cyanide anion, thus decreasing the electron density at the metal and accelerating the reductive elimination. 

In addition to functioning as Lewis acids, boron halides undergo many other types of reactions. As is typical of most compounds containing covalent bonds between a nonmetal and a halogen, the boron halides react vigorously with water to yield boric acid and the corresponding hydrogen halide. 

Compounds containing carbon-carbon double bonds m y also act as Lewis bases. The rather high solubilities of the inorganic Lewis acids, boron trifluoride, aluminum bromide, and silver perchlorate, in benzene derivatives and in olefinic compounds suggest acid-base interactions. Addition compounds formed from silver perchlorate and a number of such hydrocarbons have been isolated it has been shown that the silver ion is not bonded to any single carbon atom but sits astride the two -electron dumbbells as shown in Figure 5-1. Thus, such adducts are... 

Of particular sigiuficance in the chemistry of organoboranes is their behavior as Lewis acids, which is a result of the empty / -orbital of tricoordinate boron. Much recent effort has been devoted to the synthesis and development of highly Lewis-acidic organoboron compounds through introduction of fluorinated organic substituents. Boron can reach the... 

Boron forms strong bonds to fluorine (644kJmol ), usually combining with three or four fluorides in its compounds or salts. Several binary fluorides of boron are known including boron trifluoride, BF3, a stable gas of great industrial importance as a Lewis acid. Boron trifluoride is synthesized industrially by reaction of hydrofluoric acid with borates or boric oxides (equation 42). ... 

Compound 25 has been synthesized by Reetz et al. (82, 83). It has been used to complex ionic pair species such as KF. The potassium ion is complexed by the crown ether and the fluoride ion is held by a combination of orbital overlap with the Lewis acidic boron atom and electrostatic attraction to the positively charged potassium. This receptor provides an elegant example of the use of a combination of intermolecular forces. 

Whereas these and other reports did not indicate the possibility of 1,4-cuprate additions to activated dienes, Yamamoto and co workers" showed in their seminal contributions that this is indeed feasible while the reaction of methyl sorbate with the Gilman cuprate M-Bu2CuLi provided exclusively the 1,6-addition product, the reagent formed from butylcopper and the Lewis acid boron trifluoride led to the 1.4-adduct as the major product (equation 15). The synthetically veiy useful organocopper compounds RCu have been named Yamamoto reagents. In certain cases, the regioselectiv-... 

The boron trihalides, BXj, are Lewis acids (Chapter 6). These compounds are monomeric and planar—unlike diborane, B2H5, and the aluminum halides, AI2X5 (Section 3.1.4). As Lewis acids, boron trihalides can accept an electron pair from a halide to form tetrahalobo-rate ions, BX4. Boron halide catalysts act as halide ion acceptors, as in the Friedel-Crafts alkylation of aromatic hydrocarbons (in margin). 

Catalytic-curing agents (Lewis bases and Lewis acids). Both Lewis bases and Lewis acids in catalytic quantities (2-4 phr) cure epoxy resins rapidly without requiring heat. Lewis bases are compounds that have a free pair of electrons such as tertiary amines while Lewis acids are compounds that can accept a pair of electrons suchasboron trifluoride. Lewis acids such as boron trifluoride are so reactive that, in addition to their being highly... 

Any electron-d cient atom can act as a Lewis acid. Many compounds containing group IIIA elements such as boron and aluminum are Lewis acids because group IIIA atoms have only a sextet of electrons in their outer shell. Many other compounds that have atoms with vacant orbitals also act as Lewis acids. Zinc and iron(III) halides (ferric halides) are frequently used as Lewis acids in organic reactions. 

The trivalent state is the normal state for all the elements, exeept thallium. When the coordination number is also 3, the compounds act as powerful Lewis acids. A number of these compounds serve as important reagents in organic synthesis. As a Lewis acid, boron may even have played a role in the origin of life on Earth (or possibly on Mars). 

Alkyl anion equivalents are still abstracted from zirconocene complexes and related compounds. In contrast the non-aromatic bis(pentafluorophenyl)(N-pyrrolidinyl) borane (2) (J ig. 5, right) does not show this ability. Obviously the less Lewis acidic boron atom in 2 is less attractive to the methanide than that one in 1. 

Metalloids-Based Lewis Acids Boron Catalysis. Metalloid compounds, including boron- and silicon-based organic compounds, have moderate... 

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