Vinyl boronate esters

Another useful method for generating various IV-allylhydroxylamines is the reaction between vinyl boronic ester of pinacol and nitrone in the presence of dimethylzinc (655). 

Scheme 10 Asymmetric hydrogenation of vinyl boronic esters...

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

The boronate esters used for Suzuki reactions can be synthesized from commercially available alkyl-, vinyl-, and arylboronic acids. Alkyl and vinyl boronate esters are also synthesized by the hydroboration of double and triple bonds, similar to the hydrobora-tion of alkenes and alkynes in Chapters 8 and 9. Note that the boron atom generally adds to the less substituted end of a double or triple bond. Also, the B and H add to the same side of a triple bond (syn addition) to give a trans alkenylboronate ester. 

Benzylic boronic esters [52]can be obtained from bromomethyl boronic esters and aryl or vinyl stannanes [53]. Alkyl and vinyl boronic esters are accessible through hydroboration of alkynes with dialkylboranes followed by oxidation with acetaldehyde [43]. Alternatively, vinylboronates can be obtained directly from alkynes in a reaction -with catecholborane without a subsequent oxidation step [53]. Further diversification in the boronic acid derivatization is achieved by binding the boronic acids bearing another derivatizable functional group (for instance an amine) to suitable polymeric supports. The modified boronic acid may then be... 

Furthermore, this regio- and stereoselective bond formation between unsaturated carbon atoms was applied to the synthesis of functionalized dienes under extremely mild conditions. Thus, even vinylic boronic esters containing an allylic acetal moiety and alkenylboronate having a chiral protected allylic alcohol were successfully accomplished with vinylic iodides under aqueous conditions in 60-90% yield [30]. In addition, an exceptionally simple and efficient synthesis of a prostaglandin (PGEj) precursor was reported by Johnson, applying a DMF/THF/ water solvent mixture with a bis(diphenylphosphino)ferrocene palladium catalyst [31]. It is curious that the presence of water is an absolute necessity in order to succeed in this approach (Scheme 3). 

Cataiyst Cu(I)Cl, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos ligand), NaO Bu Keywords Alkynes, methyl iodide, bis(pinacolato)diboron [B2(pin)2], Cu(I)Cl, xantphos, tetrahydrofuran (THF), room temperature, regioselective carboboration of alkynes, tri-and tetra-substituted vinyl boronic esters... 

The hydroboration of alkenes and alkynes to highly chemo-, regio-, and stereoselective alkyl and vinyl boronic esters with pinacolborane is catalysed by bis(imino)pyridine Fe(II) complex with activating agent tolylMgBr in THF. Preliminary mechanistic experiments suggest that an Fe(I) catalyst may be formed under the reaction conditions. ... 

Bicyclo[2,2,2]octanes.—Extensions of the Diels-Alder reaction, still the most important route to this skeleton, concern the use of keten equivalents/ including the novel dienophiles vinyl boronate esters, the better established a-acetoxyacrylonitrile, and the highly regiospecific a-chloroacrylonitrile, the... 

Danishefsky has applied the cross-metathesis reaction for the synthesis of highly functionalized alkenyl boronate precursor 118 from the terminal olefin 119 and vinyl pinacol boronate in the presence of first generation Grubbs-catalyst 121 [95]. The metathesis reaction was driven to completion by increasing the amount of vinyl boronate ester 120 affording the polyfunctional boronic ester 118 in 93% yield and with exclusive trans-stereochemistry. The alkenyl boronic ester 118 was subsequently used in the synthesis of epothilone 490 via intramolecular Suzuki macrocydization (Scheme 3.62). 

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