Boron trifluoride, reactant

Strong acids are able to donate protons to a reactant and to take them back. Into this class fall the common acids, aluminum hahdes, and boron trifluoride. Also acid in nature are silica, alumina, alumi-nosihcates, metal sulfates and phosphates, and sulfonated ion exchange resins. They can transfer protons to hydrocarbons acting as weak bases. Zeolites are dehydrated aluminosilicates with small pores of narrow size distribution, to which is due their highly selective action since only molecules small enough to enter the pores can reacl . 

The product of this reaction, a Lewis acid-Lewis base complex called infonnally boron trifluoride etherate, may look unusual but it is a stable species with properties different from those of the reactants. Its boiling point (126°C) for exanple, is much higher than that of boron trifluoride—a gas with a boiling point of — 100°C—and diethyl ether, a liquid that boils at 34°C. 

If boron trifluoride is omitted as a reactant, the yield falls to about 67%. 

At first glance, neither the reactants nor the product appears to be an acid or base, but the reactants are revealed as a Lewis acid-base pair when drawn as Lewis dot structures as in Figure 16-1. Ammonia donates its lone pair of electrons to the bond with boron trifluoride, making ammonia the Lewis base and boron trifluoride the Lewis acid. 

The formation of a mixture of pyrylium salts when 2-acetylpyridine and chalcone reacted in perchloric acid was overcome by treating the reactants with ethanolic sodium hydroxide. The pentanedione was isolated and subsequently oxidized by reaction with chalcone and boron trifluoride (82JCS(P1)125). 

The Reformatsky reactions of methyl or ethyl bromoacetate with 4-acetoxy-,2,24 4-benzyloxy-,2 4-tetrahydropyranyloxy-,2 4-chloro-,8 and 4,4-dimethoxy-2-butanone1418 have been carried out. The adducts were converted to mevalonolactone by hydrolysis and, in the case of the acetal reactant, by appropriate reduction and oxidation procedures. The same Reformatsky-type syntheses of mevalonolactone have also been performed using the lithium and magnesium carbanions of acetate esters5,19 25 26 and the dianion of acetic acid28,27 instead of the usual zinc reagent. The intramolecular Reformatsky reaction of 4-(bromoacetoxy)-2-butanone gives mevalonolactone directly.28 A related route to mevalonolactone involves boron trifluoride-catalyzed cycloaddition of ketene to 4-acetoxy-2-butanone followed by hydrolysis.183... 

A retro-Westphalen rearrangement occurred when the 10) -fluoro-5) -methyl-7j -ol (396) was treated with boron trifluoride, to abstract fluoride ion. The product was the 4-en-7)3-ol (397). Contrasting behaviour when a 10)8-hydroxy-group was eliminated by acid treatment, reported last year to give a backbone-rearranged A -olefinic compound, requires an explanation. The most obvious difference in the reactants, apart from the different 10)8-leaving... 

Acetylation. A comparative student experiment with salicylic acid and acetic anhydride demonstrates that common acetylation catalysts fall into the following order of relative effectiveness coned. H2SO4 > boron trifluoride etherate > pyridine > sodium acetate. Although it is a relatively weak catalyst, sodium acetate is completely nondestructive and can be employed in much larger than truly catalytic amounts. An example is the acetylation of furylcarbinol. A mixture of the reactants and solvent benzene was heated on the steam bath with stirring to prevent caking... 

Indoles react with epoxides and aziridines in the presence of Lewis acids (see 20.4.1 for reaction of indolyl anions with such reactants) with opening of the three-membered ring and consequent 3-(2-hydroxyethylation) and 3-(2-aminoethylation) of the heterocycle. Both ytterbium triflate and phenylboronic acid are good catalysts for reaction with epoxides under high pressure silica gel is also an effective catalyst, but reactions are slow at normal pressure and temperature. Reaction with aziridines can be catalysed by zinc triflate or boron trifluoride. °... 

T. Mika Cll) has reviewed the chemistry of curing agents and how these influence the properties of the cured resins. Compounds such as phenol and boron trifluoride are effective accelerators for epoxide-amine reactions, while solvents usually slow down this same reaction due the lower concentration of reactants and/or to specific hydrogen bond interactions. 

There are several examples of catalyzed aromatic cycloadditions leading to heterocyclic systems. The rhodium(II) acetate-catalyzed intramolecular Buchner reactions of iV-benzyldiazoacetamides 64a/b afford azabicyclo[5.3.0]decatrienes 66a/b in excellent yields. In contrast, the N-methyl derivative 64c gives 66c in moderate yield. Use of rhodium(II) perfluorobutyrate (Rh2(pfb)4) in place of rhodium(II) acetate increases the yield to 54%. Unlike its carbon counterpart, dihydroazulenone 29a (vide supra), 66a is insensitive to either trifluoroacetic acid or boron trifluoride etherate, even in excess, and the unrearranged reactant is recovered intact even after prolonged treatment at room temperature. 

Friedel-Crafts reaction catalysts like anhydrous aluminum chloride are readily soluble in the nitroalkanes. Solutions containing up to 50% aluminum chloride are easily prepared in nitroalkane solvents. These catalytically active complexes, AICI3-RNO2, can be isolated and used in solvents other than the nitroalkane. The reactants in the Friedel-Crafts reaction are often soluble in the nitroalkane reaction medium. Other catalysts like boron trifluoride (BF3), titanium tetrachloride (TiCl4), and stannic tetrachloride (SnCl4) are also soluble in the nitroalkane solvents. Reaction types which use nitroparaffins as solvents include alkylation of aromatics, acetylation of aromatics, halogenations, nitrations, and the reaction of olefins and hydrogen sulfide to yield mercaptans. 

It was noted in the previous chapter (Section 1.4.2.1.) that the polymerization of isobutene could be accomplished only with cationic initiators. Aluminium chloride and boron trifluoride are the preferred initiators for commercial processes the separate addition of a co-catalyst is not generally necessary and adventitious substances possibly fulfil this role. At ordinary temperatures polymerization is extremely rapid and leads to low molecular weight polymers which are viscous oils or sticky solids. However, at low temperature (—80 to — 100°C) high molecular weight material is produced. Even at these low temperatures the reaction is complete in a few seconds and it is necessary to have particularly efficient means of dissipating the heat evolved. Conventional batch processes are unsuitable and continuous processes are used in which only small quantities of reactants are involved at any one moment. In one process (Badische Anilin- Soda-Fabrik A. G.), solutions of isobutene and boron trifluoride in liquid ethylene are mixed on a moving belt so that the polymerizing system is in the form of a thin film and heat is removed by the vaporization of the solvent. The polymer is then mixed with an alkali or ethanol to deactivate the initiator and treated with steam to remove water-soluble contaminants. Another process for polyisobutene (Standard Oil Co. (N.J.) (U.S.A.)) follows closely the procedure outlined in Section 2.10.2. for the manufacture of butyl rubber. 

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