Boron acid derivatives esters

However, the application of these classical procedures for 1-alkenylboronic acid or ester synthesis may suffer from the formation of small amounts of the opposite stereoisomers, or from bis-alkenylation leading to the boronic acid derivatives. Also, formation of trialkylboranes may occur. A recent useful variant utilizes organolithium reagents and triisopropyl borate, followed by acidification with HCl to give directly alkyl-, aryl-, 1-alkynyl-, and 1-alkenylboronic esters in high yields, often over 90% (Scheme 2-6) [27]. Triisopropyl borate was shown to be the best of the available alkyl borates to avoid multiple alkylation of the borates. 

Then, the scaffold 9 was reacted with 12 boronic acid pinacol ester derivatives (10a-101), yielding the corresponding 6-arylcoumarins, lla-111. Although the compounds with an electron-withdrawing group, such as nitro (lie), cyano (Hi), or methoxycarbonyl (llj), were obtained in relatively low yield (28%, 45%, and 46%, respectively), most of the reactions proceeded in moderate yield. Some functional groups on the phenyl group were rather easily transformed, as observed in the syntheses of 11m, lln, and llo. 

There are interesting studies, although formally lying beyond the scope of this review, on extraction and membrane transport of saccharides assisted by the formation of covalent bonds. For this purpose Shinbo and co-workers [204] introduced phenylboronic acid. It forms esters with vid-nal-diol compounds, and the resulting anion may be transported across a nonpolar membrane in the presence of, e.g., trioctylmethylammonium counterion. Even ribonucleosides were successfully transported using this technique, with a remarkable and easily understandable 200-fold preference over deoxynucleosides [205]. Some other reports have appeared on the use of boronic acid derivatives in binding [206,207] and transport [208,209] of saccharides and related compounds. 

One of the earliest reports on use of a phosphonium salt as an IL in such a process was that of Kaufmann and co-workers (9). In this work, the use of tri-butyl(hexadecyl)phosphonium bromide as a recyclable medium for the palladium-mediated Heck coupling of aryl halides with acrylate esters was reported (9). While these reactions proceeded without the use of an additive ligand, elevated temperatures (100 °C) were required and the process was most efficient only with more activated aryl halides [Eq. (1)]. More recently, the use of trihexyl(tetradecyl)-phosphonium chloride (Cyphos IL 101) has been reported as a useful medium for the Suzuki cross-coupling of aryl halides with boronic acid derivatives [Eq. (2)] [10]. In this process, a soluble palladium precursor such as Pd2(dba)3-CHCl3 was dissolved in the phosphonium salt, forming a dark orange solution. This solution was stable in the absence of oxygen for an extended period of time and could be... 

The synthesis of dictyodendrins A and F was realized through a sequential C—H functionahzation strategy inclusive of an initial C3 arylation, a site-selective double C—H alkylation with an aryldiazoacetate derivative and a subsequent Suzuki-Miyaura cross-coupHng with indole-3-boronic acid pinacol ester 107 (2015JAC644). A formal 67r-electrocyc-lization of the resultant tetrasubstituted pyrrole 108 fashioned the required pyrrolo-[2,3-c]carbazole core (109) which was further elaborated to the targets. 

The oxazaborolo-benzoxazaborininone derivatives 116 of resorcinar-ene were synthesized in 50—75% yields and in 98% de via a Mannich reaction with L-prohne followed by treatment with a boronic acid or ester (Scheme 38) (2003TA2787). Introduction of the electrophilic boron atom into this structure opens up a potential use for the catalysis of asymmetric reactions. The derivatives 116 were determined to exist as either crown or diamond conformers according to ID and 2D NMR experiments. Similar derivatives can also be prepared from (lS,2R)-ephedrine (2004MI75). 

A fluorescein boronic acid derivative was prepared to function as the fluorescent partner and a series of methyl-red-inspired diols were synthesized as qnencher partners to probe the FOrster resonance energy transfer (FRET) quenching sensing regime based on boronate ester formation (Fignre 11). ... 

A detailed stndy of the combination of flnorescein boronic acid with diol-appended quenchers a-c and comparison with the fluorescence outputs of nonboron or nondiol-containing systems (i.e., fluorescein or methyl red were employed directly) revealed that the boronate ester formation results in enhanced quenching in each case, and that compound c is the best overall quencher. Nncle-osides were also shown to bind to the same fluorescein boronic acid derivative. While the quenching ability of each nucleoside tested was different, the same ratiometric quenching enhancement was observed in each case, sng-gesting similar binding affinities. 

Benzo[l)]furan-6-carbonitrile and 6-cyanobenzo[l>]furan-2-boronic acid pinacol ester were prepared in the presence of Cu. (13SC1974) 2-Substituted benzo[ ]furans were synthesized by employing easily accessible A/-tosylhy-drazones and o-hydroxy or o-amino phenylacetylenes as substrates. (13OBC1490) One-pot access to either 2-naphthols or benzo[l>]furans via the aerobic Wacker-type oxidation/intramolecular aldol cyclization was accomplished. (13T1532 ) 5,5-Disubstituted-2,2 -bisbenzofuran derivatives were produced by treatment of 4-substituted-2-(2-trimethylsilylethynyl) phenyl tert-butyldimethylsilyl ether analogues with CuCl as a catalyst (13TL2655). 

An example for Uquid phase chemistry with solid phase woikup has recently been published. In this case, a quinoline-carboxylic ester was transesterifled with an appropriate bromobenzyl alcohol. The Suzuki coupling with a boronic acid derivative proceeded smoothly and the quinoline handle was cleaved off. The advantage of this method lies in the ease of purification since the quinoline could be sequestered from the reaction mixture by protonation with sulfuric acid and recrystallization. - ... 

For convenience in their purification and characterization, boronic acids are often best handled as ester derivatives, in which the two hydroxyl groups are masked. Likewise, transformation of the hydroxyl groups into other substituents such as halides may also provide the increased reactivity necessary for several synthetic applications. The next sections describe the most popular classes of boronic acid derivatives. 

Compared to aryl- and alkenylboronic adds, alkylboronic acids and esters have found limited use as synthetic intermediates aside for their oxidation into alcohols (Section 1.5.2.1). This is due in part to their inferior shelf-stability. In addition, their trans-metallation with transition metal catalysts such as palladium is presumed to be more difficult than that of the unsaturated and aromatic boronic acid derivatives [296]. For example, alkylboronic adds have long been known to be reluctant substrates in the Suzuki-cross-couphng reaction, and they have become eflfident in this apphcation only very recently with the use of special bases and the advent of new and highly active catalyst systems (Section 1.5.3.1). Perhaps the most synthetically useful class of alkylboronic adds are the a-haloalkyl derivatives popularized by Matteson (Section 1.3.8.4), and their elegant chemistry is described in Chapter 8. 

The treatment of arylboronic acids and esters with alkaline hydrogen peroxide to produce the corresponding phenols was first reported more than 75 years ago [324]. The oxidation of alkyl- and alkenyl- boronic acid derivatives leads to alkanols [40] and alde-hydes/ketones, respectively [85, 257, 279, 316]. With a-chiral alkylboronates, the re-action proceeds by retention of configuration (Equation 53, Figure 1.32) [359, 121]. In fact, the oxidation of boronic acids and esters is a synthetically useful process, mainly in the preparation of chiral aliphatic alcohols via asymmetric hydroboration reactions [300, 302], or from Matteson homologation chemistry [322]. Paradoxically, the... 

---