Boronic acids, acidity esters

In a similar way, Carreaux and coworkers [53] used 1-oxa-l,3-butadienes 4-155 carrying a boronic acid ester moiety as heterodienes [54], enol ethers and saturated as well as aromatic aldehydes. Thus, reaction of 4-155 and ethyl vinyl ether was carried out for 24 h in the presence of catalytic amounts of the Lewis acid Yb(fod)3 (Scheme 4.33). Without work-up, the mixture was treated with an excess of an aldehyde 4-156 to give the desired a-hydroxyalkyl dihydropyran 4-157. Although this is not a domino reaction, it is nonetheless a simple and useful one-pot procedure. 

The boronic acid ester B was synthesized by transesterification of the corresponding pinacolester A with (lR,2R)-l,2-dicyclohexyl-l,2-dihydroxyethane. Stereoselective chlorination of B was carried out with (dichloromethyl) lithium and zinc chloride. Reaction of the obtained chloroboronic ester C with lithio 1-decyne followed by oxidation of the intermediate D with alkaline hydrogen peroxide afforded the propargylic alcohol E. Treatment with acid to saponify the tert-butyl ester moiety and to achieve ring closure, produced lactone F. Finally, Lindlar-hydrogenation provided japonilure 70 in an excellent yield and high enantiomeric purity. 

Pinanediol esters cannot be readily displaced by treating with diethanolamine. A convenient method of removing either pinanediol esters or pinacol esters for boronic acids that are insoluble in ethers and are readily soluble in water has been developed. 34 The boronic acid ester, e.g. 21, is incubated with a hydrophobic boronic acid such as phenylboronic acid 34 in a rapidly stirred mixture of water and ether (Scheme 8). After 3 hours, the phases are separated and the aqueous phase is concentrated to give the free boronic acid 22. In this case the reaction is driven to completion by the greater solubility of the free boronic acid in the aqueous phase and the greater solubility of the pinacol ester of phenylboronate in the ether phase. This procedure has also been used to remove the pinanediol ester from Ac-D-Phe-Pro-boroArg-pinanediol 34 (see Section 15.1.7.5). 

Boranes and, to a lesser extent, boronic acids can undergo slow hydrolysis (protode-boration) in the presence of protic solvents. This unwanted reaction can become predominant if a cross-coupling reaction only proceeds slowly (e.g. with electron-rich, sterically demanding, or unreactive halides Scheme 8.20 see also Scheme 8.14) or if the boron derivative is particularly sensitive, for example 2-formylphenylboronic acid. In such instances the reaction should be performed under anhydrous conditions in an aprotic solvent with a boronic acid ester [151] or a stannane. 

Thus, for our present purposes a similar approach was followed using Suzuki cross-coupling reactions as the key steps in the synthesis of our target compounds. Symmetrically substituted compounds were synthesized in a twofold Suzuki crosscoupling reaction from commercially available p-substituted phenylboronic acids or esters and 4,4 -dibromobiphenyl or 4,4 -biphenyl-bis-boronic acid ester and a p-substituted arylhalide, respectively, using tetrakis (triphenylphosphino) palladium as catalyst together with cesium fluoride as base in dry tetrahydrofurane as shown in Scheme 8.1. The desired products were obtained in respectable yields after heating at reflux for 50 h. 

Boronates have been used in a variety of linker types either as linkers for diols [42] or as precursors for metal-mediated cleavage. A boronic acid ester, which contains an aryl iodide moiety attached by an appropriate tether, can act as an intramolecular arylation agent. A polymer-bound precursor furnished a macrocydic constrained / -tum peptide mimic via biaryl coupling, leading to cleavage [43] (Scheme 6.1.10). 

Alkenylboronic acid esters B react with bromine to give the trans-addition product initially. This primary product is not isolated but is immediately reacted with a solution of NaOMe in MeOH. Addition of MeO ion to the B atom of the bromine adduct forms the borate complex D. This borate complex is converted into a cis-configured bromoalkene and a mixed boronic acid ester by way of a / elimination. 

Fig. 16.15. Stereoselective preparations of trans-alkenyl-boronic acid esters (B) and trans-alkenylboronic acids (C) and their stereoselective conversion into c/s-bromoalkenes and trans-iodoalkenes, respectively.

In 2005, Renaud and co-workers reported a novel procedure for the formal hydrogenation of alkenes via hydroboration with an excess of catecholborane, followed by treatment of the intermediate boronic acid esters with methanol in the presence of air as a radical initiator [88]. A typical example, the reduction of 43 to 44, is shown in Scheme 38. Similar results were obtained for a wide range of primary, secondary, and tertiary alkylcatecholboranes. 

For the synthesis of a cavitand functionalized with terpyridyl groups via rigid linkages, transition metal catalyzed cross-coupling reactions are especially well suited. Starting with the boronic acid ester 48 [65], attachment of the terpyridyl groups to the cavitand was realized by Suzuki-Miyaura reaction with the tetraiodo-cavitand 47 (Fig. 15). 

In cooperation with the group of Dirk Volkmer (Ulm University), the synthetic procedure established for the preparation of the tetra-(4-(2,2 6, 2"-terpyridyl)-phenyl)-cavitand 49 was adapted for the synthesis of terpyridyl-substituted calix [n]arenes [77]. The tetra-(4-(2,2 6, 2"-terpyridyl)-phenyl) calix[4]arenes 54a,b and the penta-(4-(2, 2 6, 2"-terpyridyl)-phenyl)calix[5]arene 55 can be prepared from the boronic acid ester 48 and tetrabromocalix[4]arene 52a or b and pentabromocalix[5]arene 53, respectively (Fig. 21). 

These boronic esters are easily hydrolyzed to give frara-alkenylboronic acids with complete retention of their stereochemistry (C in Figure 13.10). Alkenylboronic esters and alkenylboronic acids are organometallic compounds that can be alkenylated and arylated in Pd-catalyzed reactions (Section 13.3.2). Aside from this, the fram-alkenyl-boronic acid esters as well as the frara-alkenylboronic acids are valuable precursors of haloalkenes (Figure 3.10). 

A/ "-Hexamethylguanidiniuin chloride (542 Scheme 100) is readily reduced by means of NaH in the presence of sodium alkoxyaluminates, boronic acid esters or aluminum alkoxides J Organo-metallic derivatives such as phenyllithium, 2-thienyllithium or 2-furanyllithium or sodium acetylides add to the guanidinium salt to give the corresponding orthoamides. From tetrakis(dimethylamino)methane and phenylacetylene l-phenyltris(dimethylamino)-l-propyne (543 equation 242) was prepared. ... 

Various carbon-carbon cross-coupling reactions are employed to arrive at oligomers from fluorene monomers, including nickel-cataylzed Yamamoto coupling between aryl halides [48], palladium(0)-catalyzed Suzuki coupling between boronic acids/esters and halides [49], and ferric-chloride-catalyzed Scholl reaction, also called Friedel-Crafts arylation, of bare fluorenes [50,51]. 

In a dried Nj-filled two-neck, round-bottom flask fitted with a rubber septum and a bubbler, 1-alkenyl-boronic acid ester (2 mmol) was dissolved in toluene (2mL). 1.1 M EtjZn in toluene (1.82 mL, 2 mmol) and CHjIj (0.75 g, 2.8 mmol) were added successively. The mixture was stirred at 50 C for 10 h. The same amount as before of Et Zn and CHjIj was added again, and the mixture was kept at 50 °C for another 10 h, then poured into HCl (5 mL). After addition of sat. aq NaHCOj (20 mL), the mixture was filtered and the filtrate was extracted with Et20. The combined organic layers were washed with H2O, dried (MgS04) and concentrated. The residual oil was subjected to Kugelrohr distillation. 

Aryl and alkyl boron compounds have received significant research attention as building blocks in organic synthesis. One of the most straightforward methods for making such compounds is the reaction of a boronic acid ester with a Grignard reagent. However, the reaction... 

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