Of boronic ester

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). 

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

Scheme 32 summarizes Matteson s synthesis of serricornin [(4S,6S,7S)-21], the female sex pheromone of the cigarette beetle (Lasioderma serricorne) via boronic esters [55]. Due to the highly stereoselective nature of boronic ester chemistry, this synthesis of 21 was quite efficient. 

Scheme 11.14 Application of boronic ester 52 to the synthesis of lycopladines F and G...

Severin and coworkers reported (146) the reaction of tris(2-aminoethyl)amine and 4-formylphenylboronic acid with penta-erythritol to give, via multicomponent assembly, the boronic acid based macrobicyclic cage 35 (Fig. 25). The cage has the form of an ellipsoid with a diameter of 20.5 A and binds two Cud) ions in a fashion similar to the smaller tren-based cryptands. The reversible formation of boronic esters has also been employed to build other hollow structures such as nanotubes (147) and porous covalent organic frameworks (148,149). 

Ruthenium-catalyzed arylations of aromatic ketones were reported employing the transmetallation of boronic esters (Equations (127) and (128)).112... 

Homologation, of boronic esters, 23 671 Homologous enzyme structures, 10 337 Homologous promoters, 22 453 Homologous recombination, 22 459-460 Homologous temperature, 23 487 Homology, of proteins, 20 833-834... 

Using 10% of DTBB as electron carrier, 2,5-disubstituted chlorocyclopentanes 6 were lithiated, the corresponding organolithium intermediates having being employed for the preparation of boronic esters 7, after reaction with triethyl borate and the corresponding 1,3-diol (Scheme 4). ... 

During studies toward the synthesis of calyculin C, Karisalmi and Koskinen noted the particular stability of boronate ester 65, although details were not given <200481331 >. 

Syntheses are facilitated by the stability of boronic esters, which generally behave as ordinary organic compounds and can be isolated and handled by ordinary techniques of organic chemistry. Some a-halo boronic esters epimerize easily and are best used quickly without purification, but the other intermediates may generally be chromatographed, and many are distillable1 - 3. 

Halo boronic esters have long been known to undergo anti elimination of boron and halide to form alkenes on treatment with bases as mild as water13, l4. Thus, synthetic schemes involving bases in the presence of boronic esters must avoid any structure incorporating a /(-halogen. 

Results obtained in the conversion of boronic esters of C2 symmetry 1 or 2 to a-chloro boronic esters 4 and on to secondary alkyl boronic esters 6 are summarized in Table 1. For example, the data with R2 = isopropyl, utilized material of only 97% ce38 and significantly underestimated the diastereoselectivity. 

Exploratory diol syntheses included those of (55, 65 )-5,6-decanediol (path ) and (5S,7S)-6-methyl-5,7-undecancdiol (path ( )). These syntheses established the stability of boronic ester intermediates having a benzyloxy substituent a or f to the boron atom15. 

Homologation of boronic esters. The reagent 1 can be formed in situ by reaction of CH2C12 with lithium 2,2, 6,6 -tetramethylpiperidide (LiTMP) or even LDA. The product obtained on reaction of 1 with alkylboronic esters, an a-chlo-roboronic ester, is reduced by potassium triisopropoxyborohydride (KIPBH) to the homologated boronic ester.1... 

SECOND-ORDER RATE COEFFICIENTS (I.mole" .See 1) AND ACTIVATION PARAMETERS FOR THE SUBSTITUTION OF BORONIC ESTERS BY MERCURIC CHLORIDE IN SOLVENT... 

Attachment of the alkene monomers to the template (Fig. 6.5), which must be reversible - readily formed and readily broken - to permit removal of the template after polymerisation, can generally be accomplished in two ways covalently or non-covalently. While the latter interactions (ionic, hydrophobic, n-n, hydrogen bonding) can easily be reversed, there is less scope for reversible covalent linkages. One of these is the formation of boronic esters - from boronic acid units of the monomers and OH groups of sugar templates. 

Reaction of boronic esters, RB(OR )2, with dichloromethyllithium, LiCHCl2, inserts the CHC1 unit into the carbon-boron bond of the boronic ester. This is known as boronic ester homologation. If boronic esters derived from homochiral alcohols are used in this reaction, then new homochiral centres may be generated as will be illustrated below. 

High activity of / -PrMgCI-LiCI in the exchange reaction allows the preparation of highly functionalized aryl- and hetaryl pinacolborates by the exchange reaction in the presence of boronic ester (Scheme 31).85... 

The G-B bond of boronic esters is inert for functionalization of remote sites other than the B-G bond by metal-catalyzed reactions (Equations (98)-(100)). Gross-coupling reactions of 320478 and 321,479 and titanium-Wittig reaction on polymer resin 323480 have been studied. 

Figure 3 [4+2] cycloaddition of boronic esters (4) with cyclopentadiene. 

The re-uptake of template in the imprinted polymer is relatively slow (consider the rates of formation of boronic esters listed above), but more than 90% of the free cavities can be reoccupied. Fortunately, in aqueous alkaline solution or in the presence of certain nitrogen bases (for example NH3 or piperidine) tetragonal boronic esters are formed (equation (b)), which equilibrate extremely rapidly with tetragonal boronic acid and diol [47,55]. In these cases the rate of equilibration... 

Table 1-9. Cross-coupling reaction of boronic esters and halides catalyzed by PdCl2(dppf) in THF at 50 °C for 16 h in the presence of a base (T12C03) (data from [130])...

A nonaqueous workup procedure has been reproted for the preparation of arylboronic esters [ArBCOR a)]-" Uncontrollable polymerization or oxidation of much of the boronic acid occurred during the final stages of the isolation procedure, but could be avoided by in situ conversion to the dibutyl ester by adding the crude product to 1-butanol. The samarium(lll)-catalyzed hydroboration of olefins with catecholborane is a good synthesis of boronate esters." ... 

The reactivity of boranes is dominated by the desire to accept an electron pair into the empty p-orbital. Therefore boranes reduce electron-rich carbonyl groups fastest. In the context of carboxylic acid reduction a triacylborate 35 is formed first. Compared to, for example, ketones, esters are less electrophilic because of conjugation between the carbonyl group and the lone pair of the sp -hybridized oxygen atom. However, in the case of boron esters such as 35, the oxygen next to the boron has to share its lone pair between the carbonyl group and the empty p-orbital of the boron. This fact makes them considerably more reactive than normal esters and allows the chemoselective reduction of carboxylic acids in the presence of esters or acyl chlorides. 

A general synthesis of a-chiral ketones with essentially 100% ee is based on the utilization of boronic esters. These esters can be prepared by asymmetric hydroboration of prostereogenic olefins and subsequent removal of the chiral auxiliary. Two approaches to a-chiral ketone formation are known ... 

---