Boron trifluoride Lewis acid/base complex with

Neutral compounds such as boron trifluoride and aluminum chloride form Lewis acid-base complexes by accepting an electron pair from the donor molecule. The same functional groups that act as lone-pair donors to metal cations can form complexes with boron trifluoride, aluminum chloride, and related compounds. 

Boron trifluoride reacts with NH3to produce the Lewis acid-base complex, F3BNH3 F3B(g)+ NH3(g)- F3B-NH3(s)... 

Anions formed from group 6 and manganese Fischer carbene complexes undergo aldol condensations with aldehydes and ketones. Allylic carbenes exclusively react in the y position with aldehydes affording dienyl-substituted carbenes. For alkoxy-substituted carbenes, the presence of an excess Lewis acid see Lewis Acids Bases), such as boron trifluoride etherate, titanium tetrachloride, or tin tetrachloride is required for the reaction to proceed in reasonable yield. The initial aldol product can be isolated without elimination (Scheme 12). ... 

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. 

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. 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

Being an electron deficient compound, boron trifluoride forms complexes with Lewis bases and compounds that have unshared pair(s) of electrons. With ammonia, it forms boron trifluoride ammonia. Similar coordination compounds are formed with monoethylamine, BF3-NH2C2H5 diethyl ether, CH3CH20(BF3)CH2CH3 and methanol, BF3—OHCH3. It forms a sohd complex HNO3-2BF3 with concentrated nitric acid. 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Thus, for example, in the case of boron trihalides, boron trifluoride coordinates best with fluorides, but not with chlorides, bromides, or iodides. In coordination with Lewis bases such as amines and phosphines, BF3 shows preference to the former (as determined by equilibrium constant measurements).66 The same set of bases behaves differently with the Ag+ ion. The Ag+ ion complexes phosphines much more strongly than amines. In the case of halides (F, CP, Br, and P), fluoride is the most effective base in protic acid solution. However, the order... 

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.)... 

Lewis acids like boron trifluoride form Lewis acid Lewis base complexes such as H3N BF3 with ammonia. Ammonia also forms metal complexes, the aminocomplexes (see Ammonia N-donor Ligands). Deprotonation leads to amido, imido, and nitridocomplexes (see Scheme 14). 

When halogen atoms are present in the epoxide such as in epichlorohydrin, 3,3,3-trichloropropylene oxide (TCPO) or 4,4,4-trichloro-l,2-butylene oxide (TCBO), or in the initiator, acid catalysts, e.g. boron trifluor-ide etherate, may be used (13-18). Vogt cind Davis (16) found that, if the concentration of catalyst/ini-tiator (polyol) complex is decreased with respect to TCPO in order to obtain higher molecular weight products, side reactions such as cyclization reactions become increasingly important. Boron trifluoride also promotes dimerization of alkylene oxides to dioxane or alkyl derivatives of dioxane as described by Fife and Roberts ( ). The use of acid catalysts, e.g. Lewis acids, promotes formation of a greater amount of terminal primary alcohol groups when compared to base catalysis of epoxides. 

O vs. N Protonation.—Controversy over the site of protonation of amides has been raised anew by Liler. Using u.v. spectroscopy, she has suggested that benzamide is 50 50 O N protonated in 80 % sulphuric acid. However, a considerable weight of evidence has been produced to indicate that protonation or Lewis acid complexation of amides consistently occurs by co-ordination with oxygen. Studies have included the acidity-dependent changes in the tt-tt and n-it u.v. absorption bands of aliphatic amides kinetic evidence based on rate data for acid-catalysed amide hydrolysis in both dilute and concentrated acid an n.m.r. study of proton exchange rates, where for A -methylacetamide the molar ratio of 0 N protonated species exceeds 10 a n.m.r. study of adducts of boron trifluoride and antimony pentachloride with N-labelled ureas, where the hybridization-dependent —H coupling constants were inconsistent with N-co-ordination ... 

Since the discovery of the complex H3N-BF3 by Gay-Lussac in 1809 [1], a great number of complexes of boron trifluoride with Lewis bases have been smdied. Indeed, BF3 is an archetypical Lewis acid. It is clear that the central boron is electron deficient since it has only six electrons in its outer shell and the highly electronegative fluorine atoms further decrease its o electron density. 

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