Boron acid derivatives cyclic

Boronic Acids, Esters, Cyclic and Carbohydrate Derivatives... 

The use of oxidant such as molecular oxygen [60] or di-terf-bntylperoxide [61] with catalytic amount of copper led to eliminate the use of base in the boronic acid oxidative coupling (Scheme 20.28). Fu and coworkers used atmospheric oxygen as oxidant with Cu O for the selective synthesis of primary amines at room temperature using aqueous ammonia as an amine source [62]. Yamamoto and coworkers developed air- and water-stable cyclic triolborate complex to increase the nucleophilicity of the attached group in boronic acid derivative [63]. Copper-mediated selective coupling of methylboronic acids with primary amines has been subsequently developed [64]. 

The Si-O-B fragment can be formed from different starting materials that generally include chlorosilanes or silanols and boron halides or boric acid derivatives. For the preparation of the cyclic six-, eight-, ten- and twelve-membered borasiloxanes 83-89 four synthetic approaches are known so far, starting from either... 

The boron atom dominates the reactivity of the boracyclic compounds because of its inherent Lewis acidity. Consequently, there have been very few reports on the reactivity of substituents attached to the ring carbon atoms in the five-membered boronated cyclic systems. Singaram and co-workers developed a novel catalyst 31 based on dicarboxylic acid derivative of 1,3,2-dioxaborolane for the asymmetric reduction of prochiral ketones 32. This catalyst reduces a wide variety of ketones enantioselectively in the presence of a co-reductant such as LiBH4. The mechanism involves the coordination of ketone 32 with the chiral boronate 31 and the conjugation of borohydride with carboxylic acid to furnish the chiral borohydride complex 34. Subsequent transfer of hydride from the least hindered face of the ketone yields the corresponding alcohol 35 in high ee (Scheme 3) <20060PD949>. 

However, studies on the scope of this sequence revealed that the substrate has to be an N-tosyl sulfonamide and that certain boronic acids are not trans-metallated but rather give rise to the formation of the pyrrole 21 or a pyridine derivative 22 (Scheme 7). The peculiar outcome as a carbopalladation-Suzuki sequence is rationalized by co or dinative stabilization of the insertion intermediate 18 by the sulfonyl oxygen atom, as represented in structure 19, now suppressing the usual /3-hydride elimination. If the transmetallation is rapid the Suzuki pathway is entered leading to product 17. However, if the transmetallation is slow, as for furyl or ferrocenyl boronic acid, either /i-hydride elimination or a subsequent cyclic carbopalladation occurs. The former leads to the formation of the diene 20 that is isomerized to the pyrrole 21. The latter furnishes the cyclopropylmethyl Pd species 23, which rearranges with concomitant ring expansion to furnish piperidyl-Pd intermediate 24 that suffers a -hydride elimination to give the methylene tetrahydro pyridine 22. 

Quach and Batey299 reported the coupling of primary amines with phenylboronic acids catalyzed by Cu(OAc)2 (10 mol%) without base or ligand but in the presence of 4 A molecular sieves and air in CH2C12 solvent (equation 73). Reactions of potassium phenyl-trifluoroborate also occurred, but in lower yields than reactions of boronic acids. They showed that these reactions occur with a variety of functional groups on the amine, including alkenes, esters, ketones and ketals. a-Amino acid derivatives underwent reaction without detectable epimerization. Anilines were poorer cross-coupling partners under these conditions than were primary and secondary cyclic aliphatic amines. 

Boron or tin (II) Z-enolates are generated by reaction with the corresponding triflates with a carbonyl compound in the presence of tertiary amines like r-P NEt or. M-ethylpiperidine (except when using dicyclopentylboron triflate [407]). E-Enolates are prepared by using dicyclohexyl- or other cyclic chloroboranes in the presence of Et3N or Me NEt [407, 685, 686, 1246, 1247, 1248], Because enolization does not take place under such conditions with esters or aliphatic tertiary amides, thiophenyl esters RGH COSPh have been used as ester/amide substitutes. Furthermore, Z-boron enolates of ketones can be prepared by conjugate addition of acid derivatives of dialkylboranes to a-enones [687],... 

The determination of bifunctional compounds was reviewed by Poole and Zlatkis [267]. Only a few reagents capable of forming cyclic derivatives with bifunctional compounds have been described in the literature. Boronic acids are applicable to a wide range of bifunctional compounds. All of these reactions are highly selective, and some reagents have been developed that also have a high detection selectivity. These are of particular interest for the analysis of a few bifunctional components in a complex matrix without the need for a tedious sample clean-up [267]. 

This electron transfer was positively affected by the immobilization of glucose oxidase on glassy carbon modified by aminophenyl boronic acid [193] or adsorption on to metalized carbons [194]. Nevertheless, it was possible to achieve good electron transfer only by modification ( functionalization ) of the enzyme with redox compounds (ferrocene derivatives). The direct electrical communication between the modified enzyme and an electrode has been proved by cyclic voltammetry. Thus attention was paid to the construction of various types of modified glucose oxidase electrodes. For instance, Benneto et al. [195] describe an... 

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