2- boronic ester

3 Boronic Acid-Diol (Sugar) Equilibrium in Water 

---

Both ( )- and (Z)-l-halo-l-alkenes can be prepared by hydroboration of 1-alkynes or 1-halo-l-alkynes followed by halogenation of the intermediate boronic esters (244,245). Differences in the addition—elimination mechanisms operating in these reactions lead to the opposite configurations of iodides as compared to bromides and chlorides. 

An alternative synthesis of (Z)-l-halo-l-alkenes involves hydroboration of 1-halo-l-alkynes, followed by protonolysis (246,247). Disubstituted ( )-and (Z)-a1keny1 bromides can be prepared from ( )- and (Z)-a1keny1 boronic esters, respectively, by treatment with bromine followed by base (248). 

Carbanioas derived from the other above mentioned methane derivatives react with alkylboranes, bornic, and boronic esters, providing rich possibihties for the preparation of siagle-carbon iasertion products. Thus 2-aLkyl-l,3,2-dithiaborolanes are converted iato acids or thioacetals by trichloromethyllithium (335). 

Aldehydes aie conveniendy synthetized by the reaction of boronic esters with dichloromethylhthium or (phenylthio)methoxymethylhthium (336,337). The synthesis of medium-ting boracyclane stmctures by stepwise ring enlargement is based on the reaction of B-methoxyboracycles with chloromethylhthium (338). 

Secondary amines having one oi two chiral groups attached to the nitrogen atom are prepared from boronic esters by their conversion into alkyldichlotobotanes, followed by treatment with organic azides (518). The second chiral group can be derived from an optically active azide. 

Synthesis of OC- and P-Ghiral Ketones, Esters, and Nitriles. Chiral boronic esters are convenient precursors of a-chiral ketones (R COR ), which can be prepared via the dialkylborinic ester or dialkylthexyl route (524,525). 

The conversion of chiral boronic esters iato optically pure B-aIkyl-9-BBN derivatives followed by reaction with a-bromoketones, a-bromoesters, or a-bromonitriles leads to the homologated P-chiral ketones, esters, and nitriles, respectively (526). 

The aryl tin compounds are better substrates for fluorination because they give high yields of fliiorinated aromatics and they may be fluorinated with acetyl hypofluorite, cesium fluoroxysulfate, or fluorine [52, 54 (equation 28). Aryl boronic esters react with cesium fluoroxysulfate to produce fluoroaromatics [55] (equation 29). 

Boronic esters are easily prepared from a diol and the boronic acid with removal of water, either chemically or azeotropically. (See Chapter 2 on the protection of diols.) Sterically hindered boronic esters, such as those of pinacol, can be prepared in the presence of water. Boronic esters of simple unhindered diols are quite sensitive to water and hydrolyze readily. On the other hand, very hindered esters, such as the pinacol and pinanediol derivatives, are exceedingly difficult to hydrolyze and often require rather harsh conditions to achieve cleavage. 

Reaction of methyl a-L-rhamnopyranoside with triphenylboroxole gave a syrupy boronate ester which was characterized as a crystalline phenyl-carbamate. Removal of the phenylboronic acid residue gave a product identified as methyl a-L-rhamnopyranoside 4-N-phenylcarbamate, since it was identical with that resulting from removal of the ketal group from methyl 2,3-O-isopr opylidene-a-L-rhamnopyranoside 4-N-phenylcarbamate (12). This establishes the structure of the original ester as methyl a-L-rhamnopyranoside 2,3-phenylboronate (24). 

The oxazaborolidines are easily prepared by heating ephedrine with borane dimethyl sulfide or the appropriate boronate ester. The aluminum reagent C is obtained by mixing ephedrine and trimethylaluminum. Borolidinc A is superior to its methyl derivative B and to the aluminum analog C. The diastereomeric borolidine obtained from borane and (S,S)-pseu-doephedrine failed to show any cnantioselectivity25. A variety of aromatic aldehydes can be enantioselectively alkylated in the presence of A, however, with heptanal the enantioselectivity is poor25. 

The conversion of arylboronic acids to the corresponding neopentyl glycol arylboronic esters has several advantages The esters are readily soluble in organic solvents, shelf stable, non-hygroscopic and easily characterized as a single entity.9 Furthermore, boronic esters can be utilized in many of the transformations where arylboronic acids usually are employed, making them an attractive alternative from a practical point of view. 

One year later Van der Eycken and Dehaen described the smooth microwave-assisted borylation of 4, 5, 6 and 7-bromo-lff-indole using PdCl2(dppf) as a precatalyst and KOAc as a base (Scheme 30) [48]. With 5, 6, and 7-bromo-lH-indole, DMSO was used as solvent at a temperature of 150 °C (with a set power of 150 W) for 17-27 min, resulting in the corresponding boronate esters in good yields. For 4-bromo-lH-indole, DME gave a better result at the same temperature (with a set power of 250 W). 

Boronic esters, RB(OR )2 react with methoxy(phenylthio)methyllithium, LiCH(OMe)SPh, to give salts, which, after treatment with HgCl2, and then H2O2, yield aldehydes.This synthesis has been made enantioseiective, with high ee values (>99%), by the use of an optically pure boronic ester, for example ... 

The protocol offers a direct and efficient method for the synthesis of the boronic ester in the solid phase, which hitherto met with little success using classical methodology (Scheme 1-42). A solid-phase boronate (113 [155], 114 [156]) is quantitatively obtained by treating a polymer-bound iodoarene with the diboron (82). The subsequent coupling with haloarenes furnishes various biaryls. The robot synthesis or the parallel synthesis on the surface of resin is the topic of further accounts of the research. 

A COMPARISON OF THE VOLATILITY AND CAPTURE DETECTOR OF BORONIC ESTERS... 

Boronic Ester Relative Retention Minimum Detectable Quantity (pg of pinacol) Optimum Detector Temperature ( C)... 

Migration of boron to terminal positions is observed under much milder conditions in the presence of transition metal catalysts. For example, hydroboration of 2-methyl-3-hexene by pinacolborane in the presence of Rh(PPh3)3Cl leads to the terminal boronate ester. 

The usual oxidation conditions then convert this boronate ester to an alcohol.205... 

---