Borate esters, conjugate

Boron reagents add to conjugated carbonyl compounds. Alkynyl borate esters (p. 815) give conjugate addition in the presence of boron trifluoride ethe-... 

Alkenes (e.g. camphene) are readily prepared by the BF3,Et20-catalysed elimination of secondary borate esters.Terminal conjugated dienes may be prepared by Pd(OAc)2-PPh3-catalysed elimination from allyl phenyl ethers or allylic acetates [e.g. geranyl, neryl, and linalyl acetates to form similar mixtures of myrcene (60—74%), trans-ocimene (8—20%), and c -ocimene (14—20%)], and the complex [(i7-CsH5)Cr(NO)2]2 dehalogenates n/c-dihalides (e.g. limonene tetrabromide) without affecting other halides (except for benzyl halides). 

The element boron forms strong bonds with oxygen. The resultant borates include boric acid, boric oxide, alkali, ammonium, and alkaline earth metal borates. All except for the alkaline earth borates are reasonably water soluble. When dissolved borates release boric acid, a Lewis acid, and its conjugate base, the tetrahydroxyborate anion. Borax, a form of di-sodium tetraborate, releases equimolar amounts of these species and consequently is an excellent mildly alkaline pH buffer. Borates exhibit several important behaviors in solution, including the sequestration of water hardness ions, the ability to form borate esters with polyols, and to deliver the oxidant hydrogen peroxide into solution from the dissolution of sodium perborate. 

Dissolve the toxin to be conjugated in 0.1M sodium phosphate, 0.15 MNaCl, pH 7.5, at a concentration of lOmg/ml. Some protocols use as an SPDP reaction buffer, 50mM sodium borate, 0.3 M NaCl, 0.5 percent n-butanol, pH 9.0. Both buffer systems work well for the NHS ester modification reaction, although the pH 9 buffer is at the higher end of effective derivatization with active esters, since the hydrolysis rate is dramatically increased at this level of alkalinity. 

Method D. The remaining haptens were conjugated by stirring a mixture of 5 mL of chloroform, 5 mL of 0.10 M lithium borate buffer (pH 9), 75 mg of BSA or OVA and 25 mg of the active NHS ester of the corresponding nitroaromatic hapten in a flask stored in a cold room (2°C) overnight. The solutions were centrifuged and the aqueous layers were then treated in the same manner as described in Method C. 

The reversible formation of boronic esters by the interaction of boronic acids and polyols in water was first examined in the seminal study of Lorand and Edwards [49]. This work followed an equally important study on the elucidation of the structure of the borate ion [124]. By measuring the complexation equilibrium between phenylboronic acid and several model diols and monosaccharides using the method of pH depression, ester formation was shown to be more favorable in solutions of high pH where the boronate ion exists in high concentrations (Equation 19, Figure 1.12). This study also confirmed the Lewis acid behavior of boronic acids and the tetracoordinate structure of their conjugate base, i.e., the hydroxyboronate anion (Section 1.2.2.4). Another conclusion is that free boronic adds have lower Lewis add strengths than their neutral complexes with 1,2-diols. For example, the pJC, of PhB(OH)2 decreases... 

Lately, work in this area has focused on olefinic substrates. Sadighi first reported the stoichiometric insertion of an alkene into the Cu-B bond of [(IPr)Cu B(pin) ]. A great deal of work was then performed by the groups of Perez and Fernandez on the p-boration of electron-deficient alkenes e.g. conjugated enals). In the case of a,p-unsaturated esters, the authors examined a number of monodentate chiral NHCs (Figure 11.2) which, despite high... 

Chen, l.-Hon, Yin, L., Itano, W., Kanai, M., Shibasaki, M. 2009. Catalytic asymmetric synthesis of chiral tertiary organoboronic esters through conjugate boration of beta-substituted cyclic enones. J. Am. Chem. Soc. 131 11664-11665. 

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