Boronic Bronsted acidity

Boron trioxide is not particularly soluble in water but it slowly dissolves to form both dioxo(HB02)(meta) and trioxo(H3B03) (ortho) boric acids. It is a dimorphous oxide and exists as either a glassy or a crystalline solid. Boron trioxide is an acidic oxide and combines with metal oxides and hydroxides to form borates, some of which have characteristic colours—a fact utilised in analysis as the "borax bead test , cf alumina p. 150. Boric acid. H3BO3. properly called trioxoboric acid, may be prepared by adding excess hydrochloric or sulphuric acid to a hot saturated solution of borax, sodium heptaoxotetraborate, Na2B407, when the only moderately soluble boric acid separates as white flaky crystals on cooling. Boric acid is a very weak monobasic acid it is, in fact, a Lewis acid since its acidity is due to an initial acceptance of a lone pair of electrons from water rather than direct proton donation as in the case of Lowry-Bronsted acids, i.e. 

Diborane reacts with ammonia to form an ionic compound (there are no other products). The cation and anion each contain one boron atom, (a) Predict the identity and formula of each ion. (b) Give the hybridization of each boron atom, (c) Identify the type of reaction that has occurred (redox, Lewis acid-base, or Bronsted acid-base). 

B Boric acid acts as a Lewis acid. The boron atom in B(OH)3 has an incomplete octet and forms a bond by accepting a lone pair of electrons from a water molecule, which is acting as a Lewis base. The complex formed is a weak Bronsted acid in which an acidic proton can be lost from the H20 molecule in the complex. 

Recently, catalytic asymmetric Diels-Alder reactions have been investigated. Yamamoto reported a Bronsted-acid-assistcd chiral (BLA) Lewis acid, prepared from (R)-3-(2-hydroxy-3-phcnylphenyl)-2,2 -dihydroxy-1,1 -binaphthyl and 3,5A(trifluoromethy I) - be nzeneboronic acid, that is effective in catalyzing the enantioselective Diels-Alder reaction between a,(3-enals and various dienes.62 The interesting aspect is the role of water, THF, and MS 4A in the preparation of the catalyst (Eq. 12.19). To prevent the trimerization of the boronic acid during the preparation of the catalyst, the chiral triol and the boronic acid were mixed under aqueous conditions and then dried. Using the catalyst prepared in this manner, a 99% ee was obtained in the Diels-Alder reaction... 

The structures, relative stabilities, and relative Lowry-Bronsted acidities of carboranes and boranes as well as related anions, Lewis base adducts, and heteroelement analogs are rationalized primarily on the basis of rudimentary coordination numbers. The principal factors, in decreasing order of importance, are (a) the various deltahedra and deltahedral fragments, (b) the placement of bridge and endohydrogens, (c) the placement of carbon and other heteroelements, and d) the resulting coordination number of boron. 

Aminosulfides, important building blocks for the synthesis of various bioactive molecules, can be prepared by the thiolysis of aziridines in the presence of Bi(OTf)3 (5 mol%) [36]. Even though Lewis acids such as boron trifluoride etherate and zinc chloride, as well as Bronsted acids such as trifluoromethanesulfonic acid, have been employed as acid catalysts, these methods suffer from disadvantages like the use of stoichiometric amount of catalysts [37—40], harsh conditions, the use of large excess of thiols and necessitate anhydrous conditions to produce the desired products. Furthermore, these reagents cannot be recovered and recycled because they decompose or deactivate under quenching conditions. 

The acylation of unsaturated ketones constitutes one of the earliest routes to pyrylium salts (19CB1195). The reaction is better achieved with acyl halides than by anhydrides, and aliphatic are preferable to aromatic acid derivatives. The presence of a Lewis or Bronsted acid is usually necessary and iron(III) chloride, aluminum chloride, boron trifluoride and perchloric acid have found frequent application. It is considered that these interact with the acid derivative to generate the actual acylating agent. 

Asymmetric Mannich reactions provide useful routes for the synthesis of optically active p-amino ketones or esters, which are versatile chiral building blocks for the preparation of many nitrogen-containing biologically important compounds [1-6]. While several diastereoselective Mannich reactions with chiral auxiliaries have been reported, very little is known about enantioselective versions. In 1991, Corey et al. reported the first example of the enantioselective synthesis of p-amino acid esters using chiral boron enolates [7]. Yamamoto et al. disclosed enantioselective reactions of imines with ketene silyl acetals using a Bronsted acid-assisted chiral Lewis acid [8]. In all cases, however, stoichiometric amounts of chiral sources were needed. Asymmetric Mannich reactions using small amounts of chiral sources were not reported before 1997. This chapter presents an overview of catalytic asymmetric Mannich reactions. 

Piers and co-workers have shown that pyridine-stabilized borabenzenes 76 (R = H, Me, Pr1) react with cationic Bronsted acids such as pyridinium hydrochloride to produce the boronium ions 77 and 78 in 1 1 ratio. Protonation occurs at C-2 and C-4, respectively, followed by addition of pyridine at electrophilic boron <2005CC2480>. 

In the 1920s, Gilbert N. Lewis proposed that bases donate unshared electron pairs to acids, regardless of whether the donation is made to a proton or to another atom. Boron trifluoride is an example of a Lewis acid that is not a Bronsted acid because it is a chemical that accepts an electron pair without involving an H+ ion ... 

We found the acidity of chemically dealuminated mordenites shows a large variation with aluminum content (Figure 9). The alpha values decrease over five orders of magnitude as the aluminum content decreases from four aluminum atoms per unit cell down to 0.8 Al/unit cell. If the Bronsted acid sites associated with boron are of equal strength as those associated with... 

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

E13.24 In a series of boranes, the acidity increases as the size of the borane increases. This is because the negative charge, formed upon deprotonation, can be better delocalized over a large anion with many boron atoms than over a small one. Therefore, the Bronsted acidity increases in the order BjHe < BsH < B10H14. 

Thus, B(0H)3 is a Lewis add rather than a Bronsted acid (see Chapter 1). Because boron adsorbs most effectively in the pH 8 to 9 range on A1 and Fe oxides and silicate minerals, its availability is generally low in coarse-textured, acid-leached soils and in calcareous soils. Deficiency in add soils is the result of boron depletion by leaching, while deficiency in calcareous soils is caused by strong adsorption and predpitation as relatively insoluble Ca borate salts. In contrast, B toxicity is most commonly found in alkaline soils of arid regions these soils often contain high levels of Na which forms quite soluble borate salts. A lack of rainfall allows soluble borate to accumulate to phytotoxic levels. 

Allylations of aldehydes and imines also constitute important carbon-carbon bond-forming reactions [14]. A range of metal-based Lewis acid catalysts have been reported. Hall and co-workers described the Bronsted acid-catalyzed allylation of aldehydes with allyl boronate (Equation 10.8) [15]. 

Zeolites derive their acidity or catalytic activity from the proton associated with the framework aluminum. Replacement of aluminum by the other three-valent elements could also lead to the generation of Bronsted acid sites. However, one needs to exercise caution in proclaiming unusual catalytic properties before one thoroughly understands the nature of these materials. For example, many studies have been reported on the nature of the acidity of boron ZSM-5 (5—6—7-8). Although physical measurements (8-9) indicate that the protonic site associated with the... 

Figure 4.13 Dependence of the concentration of Bronsted acid sites (as measured by the intensity of the band at 1540 cm r) on the catalyst preparation for supported boron trifluoride...

Anderson et al. showed that the active sites involved in the conversion of methanol on zeolites are not Lewis acids. Wolthuizen et al., ° however, presented evidence that the presence of Lewis-acid sites enhances the polymerization of ethylene. This is in agreement with the results obtained with HY zeolites,where reaction of ethylene at 80 °C was observed only after dehydroxylation of the Bronsted acid sites into Lewis acid sites. At higher temperatures, ethylene is well-known to react on catalysts with strong Bronsted acid sites.Sayed and Cooney reported the involvement of aluminum Lewis sites in the formation of dimethylether. Haber and Szybalska observed that, when ethanol is converted on boron aluminum phosphates, only dehydration to ethylene takes place on the Bronsted acid sites, whereas, on Lewis acid-base centers, ethanol is mainly dehydrated to diethylether. 

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