Boron-Based Lewis Acids

Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process. The Lewis acid catalysts frequently employed are complexes of boron trifluoride with amines or ethers. 

In reactions between Lewis acids and bases such as amines and boranes or boron halides, bulky substituents on one or both species can affect the stability of the acid-base adduct. Perhaps the most straightforward type of effect is simple steric hindrance between substituents on the nitrogen atom and similar large substituents on the boron atom. Figure 9.3 is a diagrammatic sketch of the adduct between molecules of tripropylamine and triethylborane. This phenomenon is known as front or F-... 

Examples of neutral Lewis acids are halides of group 3A elements, such as BF3. Boron trifluoride, a colorless gas, is an excellent Lewis acid because the boron atom in the trigonal planar BF3 molecule is surrounded by only six valence electrons (Figure 15.12). The boron atom uses three sp2 hybrid orbitals to bond to the three F atoms and has a vacant 2p valence orbital that can accept a share in a pair of electrons from a Lewis base, such as NH3. The Lewis acid and base sites are evident in electrostatic potential maps, which show the electron poor B atom (blue) and the electron rich N atom (red). In the product, called an acid-base adduct, the boron atom has acquired a stable octet of electrons. 

The same considerations are true for Lewis acids and bases. Pure boron trifluoride does not act as a Lewis acid because there is nothing present capable of donating electrons to it. If diethyl ether is added, boron trifluoride etherate, a stable Lewis acid-base complex, is produced. Obviously the electron pairs on the oxygen atom of diethyl ether can be donated to boron trifluoride. When a Lewis base is present, then boron trifluoride functions very effectively as a Lewis acid. 

When you are working on problems with Lewis acids and bases, it is often useful to use Lewis diagrams (or at least modified Lewis diagrams) to determine the behavior and location of the electron pairs. In the reaction shown in Figure 14.2, note how the electron pair from nitrogen (in ammonia) is donated to boron (in BF3). This transfer of the electron pair allows for the formation of a covalent bond. 

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... 

Lewis acids and bases, because of their complexity, shall be examined briefly. Consider the structure of ammonia with its free pair of electrons. If the free pair of electrons were to make a bond with boron trifluoride, which substance is labeled as an acid and which one is the base Because the boron accepted a pair of electrons it is considered to be the Lewis acid. Ammonia is the substance that donated the electron pair and is classified as the Lewis base. (See Figure 9.3.)... 

Most of the mechanistic work on this reaction has been devoted to determining the role of the base. Its most obvious function would be to complex the Lewis-acidic boron reagent, rendering it nucleophihc and thus activating it toward transmetallation. However, Miyaura, Suzuki, and coworkers noted that an electron-rich tetracoordinate boronate complex was less reactive than a bivalent boronic ester. From this, they surmised that the role of the base was not to activate the boron toward transmetallation, but rather to transform the palladium halide intermediate to the hydroxide or alkoxide species, which would then be more reactive toward boron. However, in a mass spectrometry study of a reaction between a pyridyl halide substrate and an aryl boroiuc acid, Aliprantis and Canary saw no evidence of palladium hydroxide or alkoxide intermediates, despite observing signals in the mass spectra assignable to every other palladium intermediate of the proposed catalytic cycle. ... 

The indole unit, which is a -rr-base, is believed to have an attractive interaction with the dienophile, which is a rr-acid. When the dienophile has associated to the Lewis acidic boron, the enal may adopt either the s-cis (8.03) or s-trans (8.04) conformation, as illustrated in Figure 8.1. Complex (8.04) (s-trans) undergoes an unfavourable interaction between the bromine atom and indole ring as the diene approaches, and the bromine atom is forced downwards. The reaction prefers to proceed through the s-cis conformation (8.03) and the model accounts for the very high selectivity that is observed. ... 

Lewis acid or base complexes (boron trifluoride amine complex) Long pot life Tendency to exotherm... 

It is not clear why other Lewis acids like BCI3 require water or other compounds and must be added to the monomer to obtain high conversions [37]. Boron trifluoride can initiate polymerizations of styrene, presumably by itself, in a methylene chloride solvent [38]. One explanation that was offered for the different behaviors of various Lewis acids is based on differences in the electronic configurations arotmd the metal atoms [12]. 

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

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