Boron boration with

Boron does not occur free in nature in minerals, it occurs as borates, for example, kernite. Na2B407.4H2O. and borax. Na2B407. IOH2O there are extensive deposits of these in the USA. Boron can be obtained by heating boron trioxide with magnesium ... 

Treatment of the borates with iodine leads to boron- C2 migration of an alkyl group[9]. This reaction has not been widely applied synthetically but it might be more applicable for introduction of branched alkyl groups than direct alkylation of an indol-2-yllithium intermediate. 

Boron also has a high affinity for oxygen-forming borates, polyborates, borosiUcates, peroxoborates, etc. Boron reacts with water at temperatures above 100°C to form boric acid and other boron compounds (qv). 

From Boron Halides. Using boron haUdes is not economically desirable because boron haUdes are made from boric acid. However, this method does provide a convenient laboratory synthesis of boric acid esters. The esterification of boron haUdes with alcohol is analogous to the classical conversion of carboxyUc acid haUdes to carboxyUc esters. Simple mixing of the reactants at room temperature or below ia a solvent such as methylene chloride, chloroform, pentane, etc, yields hydrogen haUde and the borate ia high yield. 

Boron-crosslinked galactomannan fracturing fluids have an increased temperature stability. The temperature stability of fracturing fluids containing galactomannan polymers is increased by adding a sparingly soluble borate with... 

The use of other crosslinking metals developed simultaneously with the use of antimony, chromium, and boron(borate). Tiner, et al.(242) introduced titanium (IV) crosslinkers in 1975 as ammonium tetralactonate or bis(triethanolamine)bis(isopropyl)titanium(IV). Upon contact with water soluble titanium (IV) derivatives ordinarily form orthotitanic acid, Ti(0H)4, which rapidly forms oligimeric metatitanic acid, [Ti(0H)2] and titanium dioxide. Electron donors such as the hydroxyl groupsxof polysaccharides, if properly oriented, can participate in the sequence of titania reactions and a crosslinked gel network results. Various titanium metal crosslinkers remain in common use today. More will be said about titanium crosslinked gels later. 

Inverting the orientation of the C4-N3 imine unit of a 2,3,1-diheterabotine gives a boron heterocycle with a markedly different chemical reactivity. In effect, the weakly basic oxime- or hydrazone-type imine nitrogen in the 2,3,1-diheteraborine is replaced by a much more basic imidate- or amidine-type imine nitrogen in the 2,4,1-diheteraborine. Likely, the Lewis acid tendency of the boron is enhanced by the ready protonation of this basic N4, and the formation of a stable borate-based zwitterion becomes thermodynamically favored. 

If the pKa of the corresponding acid R1 - H from the stabilized carbanion is smaller than 35, the migration of R1 fails in (dichloromethyl)borate complexes. Failure to convert pinanediol [(phenylthio)methyl]boronate to an a-chloro boronic ester has been reported15. Reaction of (dichloromethyl)lithium with an acetylenic boronic ester resulted in loss of the acetylenic group to form the (dichloromethyl)boronate, and various attempts to react (dichloromethyl)boronic esters with lithium enolates have failed17. Dissociation of the carbanion is suspected as the cause, but in most cases the products have not been rigorously identified. 

Chiral cyclic boronic esters with (dichloroniethyl)lithium at —100 C form borate complexes4. Borate complexes cart also be formed by generation of (dichloromethyl)lithium from dichloro-methane and lithium diisopropylamide in the presence of a boronic ester at —78 C to — 5 C (Section 1.1.2.1.2.2,)28,19. In situ generation of (dibromomethyl)lilhium is required for preparing a-bromo boronic esters (see Sections 1.1.2.1.1.2. and 1.1.2.1.3.2.). 

An important consequence of the C2 symmetry of the cyclic boronic esters is that both faces of the trigonal boron atom are equivalent. Consequently, there is only one possible intermediate borate, which can also be generated by reaction of a (dichloromelhyl)boronic ester with an organolithium or organomagnesium reagent at — 78 C2126. 

Chloromethyl)lithium, generated by addition of butyllithium to chloroiodomethane in the presence of a boronic ester, is efficiently captured to form a (chloromethyl)borate, which rearranges to the homologous boronic ester with full retention of the configuration of the stereocenter as shown by oxidation to a known alcohol48. For a related synthesis of (chloromethyl)boronic esters, see Section 1.1.2.1.3.2. 

B NMR was used to quantify the concentrations of the various boron species in solution. Borate diesters exhibit signals distinct from those of borate esters, and both are distinct from unbound boron. Therefore, with knowledge of the solution pH — to partition the unbound boron signal into boric acid and the active borate anion — as well as total boron concentration, the actual species concentrations can be determined. 

Nitride Boron nitride, BN, white solid, insoluble, reacts wiLh steam Lo form NHj and boric acid, formed by heating anhydrous sodium borate with ammonium chloride, or by burning boron in air. 

Aryl and heteroaryl (furyl, thienyl) boronic acids are especially suitable for the preparation of their iodonium salts, having the added advantage of better yields and lack of toxicity [108]. Tetraarylborates (sodium or potassium) reacted with (diacetoxyiodo)arenes in acetic acid to afford diaryliodonium salts in excellent yield (Scheme 37). It appears that triarylboranes formed upon reaction of the borates with acetic acid serve actually as the real arylating agents [109]. 

Aryl borates such as catechol butylborate and triphenyl borate undergo a syn -stereoselective reaction with arylazirines.41 The syn-stereoselectivity was attributed to a transition state where the phenoxide tethered to the boron is transferred intramolec-ularly to the most substituted carbon. The reaction is completed when the phenoxide anion tethered to the boron reacts with the most substituted carbon of the substrate in an intramolecular reaction. 

Until recently, very little had been reported on the important area of metal borate complexation in aqueous solution. The effect of salts on the ionization of boric acid (358, 375) has been mentioned above, and subsequent research suggests that complexation of borate with, for example, calcium ions can account for the enhanced acidity of H O Literature on cationic complexes of boron was reviewed in 1970 (376). 

Diastereotopic differentiation of two leaving groups has been achieved for the first time by the reaction of (dichloromethyl)borates with BuLi in the presence of Yb(OTf)3 and the chiral bisoxazoline ligand 203.108 As shown in Figure 51, pinacol dichloromethyl boronate (204) gave 205 in up to 88% ee. The reaction can be said to be catalytic, as the use of 0.5 equiv. of chiral ligand 203 and 0.2 equiv. of Yb(OTf )3 provided 205 in 55% ee and 86% yield. 

The further new nitroethyl compounds based on boron esters are tris-(2-nitro-ethyl) borate and tris-(2,2,2-trinitroethyl) borate. Especially the trinitroethyl derivative is a suitable candidate for high energy density oxidizers and for smoke-free, green coloring agents in pyrotechnic compositions. Tris-(2-nitroethyl) borate and tris-(2,2,2-trinitroethyl) borate can be obtained from boron oxide with 2-nitroetha-nol and 2,2,2-trinitroethanol, respectively ... 

In general, the crystal chemistry of borates is similar to that of silicates differences arise from the fact that boron combines with oxygen not only in four-fold (tetrahedral) but also in three-fold (trigonal planar = triangular) coordination. As a result, silicate chemistry is considerably less complicated than borate chemistry. 

Although B(0H)3 and B(OH)4 are monomeric in dilute solutions, at concentrations above about O.lmolL, condensed borate species that are often referred to as polyborates form. Titration of a boric acid solution with one molar equivalent of a strong base leads to formation of the tetrahydroxyborate anion, B(OH)4, as the principal species in solution. Mixtures of boric acid and its conjugate base, the tetrahydroxyborate anion, form what appears to be a classical buffer system where the pH is determined primarily by the acid salt ratio with [H+] = K[B(OH)3]/[B(OH)4 ]. This relationship is approximately correct for sodium and potassium borates with a sodium boron ratio of 1 2. Here the B(0H)3 B(0H)4 ratio equals one, and the solution pH remains near 9 over a wide range. However, for borate solutions with pH values significantly above or below 9,... 

The interaction of borates with carbohydrates and other polyhydroxy componnds, inclnding perhaps proteins and glycoproteins, provides a basis for the biological role of boron. It has been shown that at least one essential role of boron in plants involves ester crosslinking of the complex carbohydrate rhamnogalactnr-onan II (RG II) as part of an intricate control mechanism vital to the maintenance of proper plant cell wall fhnction. ... 

The palladium-catalyzed coupling of boronic acids (as well as other boron derivatives) with aryl and vinyl halides and psendohalides is known as the Suzuki or Suzuki-Miyaura reaction. Because boron is nontoxic, this reaction has been used in pharmaceutical syntheses. In addition, hydroboration or borate substitution allows for the synthesis of virtually any desired coupling partner. For these reasons, as well as the high yields and functional group compatibility, the Suzuki reaction is the first reaction to consider for carrying out a cross coupling. Representative substrates and catalysts are shown in Scheme 17. The various bases are used to generate four-coordinate boron ate complexes that are more reactive in transmetalation. 

Allylic alcohols may be derived from alkenes by metallation to give the allylpotassium species, followed by treatment with fluorodimethoxyborane. Oxidation of the resultant boronic ester with hydrogen peroxide gives the allylic alcohol (Scheme 15). ° - ° Some allylic rearrangement may be observed for example, metallation of a-pinene with potassium r-butoxide in petroleum ether solution and subsequent boration and oxidation gave myrtenol (42%) and /rons-pinocarveol (1%) (equation 24), while treatment of the allylpotassium with oxirane gave the alkylated prc ucts in a ratio of ca. 2 1. ° ... 

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