Alkylboron derivatives

The mass spectrometry of acetylated alkylboronate derivatives of monosaccharides is referred to in Chapter 24. ... 

The reaction of [ Co"(DMG)2py 2] with B( -C4H,)3 in benzene followed by extraction with CH2Cl2/aq. NaOH produces [ -BuCo(DMG)2py] the reaction is less successful with [pyCo "(DMG)2Cl] 161). This may be a convenient method using the more stable arylboron derivatives, but is probably experimentally more difficult than the Grignard route if a lower alkyl is to be introduced, because the lower alkylboron compounds inflame spontaneously in air, though they do not react with water, while the aryls are stable in air. 

Yet another use for bis(oxazoline) ligands is in the synthesis of (a-chloroalkyl) boronates. " " As shown in Figure 9.71, the alkylboronate 240 was converted to the a-chloro derivative 241 in 86% ee through the use of ytterbium(lll) triflate-complexed phe-box 6. 

A)-Pinanediol ethylboronate with (l,l-dichloroelhyl)lithium yields the (S)-a-chloro boronic ester (89 11 d.r.), which is converted by phenylmagnesium bromide to the (S)-tert-alkylboronate, the same isomer obtained from (S )-pinanediol phenylboronate. The enantiomeric excess of the derived (R)-2-phenyl-2-butanol is 70%. 

Although the reaction of alkylboronic acids is significantly slower than that of trialkylboranes, methylboroxine (MeBO)3 or methylboronic acid exceptionally alkylates bromo- and chloroarenes [125, 126]. Methylation of electron-poor bromoarenes was efficient, whereas prolonged reaction time was required for more electron-rich substrates. The reaction times can be shortened by use of the polar solvent DMF at 115 °C. B-Methyl-9-BBN and its oxy-derivative (48) [127] undergo methylation of aryl and vinyl halides under more mild conditions (Scheme 22). 

Important advances in the field of C-H bond activation have involved the photochemical reactions of boryl complexes such as Cp W(CO)3(Bcat) (98, cat = 1,2-02-3,5-Me2C6H2). Transient species derived from these complexes efficiently activate the C-H bonds of alkanes and arenes (see Alkane Carbon-Hydrogen Bond Activation), and they can convert hydrocarbon solvents into alkylboronate esters (equation 27). Experimental and theoretical studies have shown that these reactions proceed via a boron-assisted, a-bond metathesis see a-Bond Metathesis) pathway involving back donation of electron density from the tungsten atom to a formally unoccupied p orbital centered on the boryl ligand. ... 

At the root of the instability of fluoroaUcylboron compounds is the availability of a vacant orbital on boron it will be seen that when boron is co-ordinately saturated, as in four-covalent boron derivatives (10.26A), or partially saturated by tt bonding with attached oxygen- or nitrogen-containing groups (10.26B), then the stability of perfluoro-alkylboron compounds increases (Figure 10.26). 

Boronic acids (5a) were among the first examples of low-molecular-weight, reversible inhibitors of serine proteinases [151, 152]. Significant inhibition was initially demonstrated against a-chymotrypsin. Unlike the carbonyl-derived reversible inhibitors, which require a polypeptide or peptide-like chain, activity was found with simple alkylboronic acids (e.g. the value for PhCH2CH2B(OH)2 with a-chymotrypsin was = 40 //M) [153], Weak inhibition of elastase (PPE) was first reported for a series of arylboronic acids, for example, (10-1) [123]. Some of the boron-based inhibitors Figure 2.5) were tested as either the difluoroboranes (5b) or as the pinacol boronate esters (5c). These modifications were employed because they were more readily synthesized and/or purified than the boronic acids. For both of these derivatives inhibition was shown to be due to in situ hydrolysis to the parent boronic acid (5a) [154, 155]. 

Alkylboronic acid derivatives retain configuration with Hg salts, e.g. ... 

The procedure represents one of the most convenient methods for the synthesis of optically active alkylboronates. However, the procedure can be of limited use for the hydroboration of styrene derivatives. The optically active organoborates are also obtained by hydroboration of internal alkenes with diisopinocampheylborane, followed by dealkylation of two diisopinocampheyl groups with acetaldehyde [58]. 

Suzuki-Miyaura cross-coupling reactions of alkylboronic acid derivatives or alkyltrifluoroborates widi aryl, alkenyl or alkyl halides and triflates ,... 

Except for trimethylboron, which is a gas at room temperature, the trialkylborons are colorless liquids, but the triarylborons are crystalline. Unlike the alkyl derivatives of other elements of the Third Group, alkylborons are monomeric they are unaffected by water but react with halogens and hydrogen halides, yielding organic boron halides. 

In principle, any difunctional chiral compound, such as a diol, diamine, or amino alcohol, can form a cyclic boron derivative by reaction with borane, haloboronic acid or its esters, alkylboronic acids, trihaloboranes or similar compounds. Several syntheses of this type are described in chapters D.1.3.3.3. (together with applications for allylic additions to carbonyl compounds) and D. 1.1.2.1. (with applications of a-haloboronic acids). 

Suzuki-Miyaura Cross-Coupling Reactions of Alkylboronic Acid Derivatives or Alkyltrifluoroborates with Aryl, Alkenyl or Alkyl Halides and Triflates Doucet, H. Eur. J. Org. Chem. 2008, 2013. 

Alkylboronic acids can be purified by conversion into diethanolamine chelates, which are crystalline air stable materials. Note that intramolecular coordination of nitrogen to boron gives the latter an octet configuration. Similarly dialkyl-borinic acids may be converted into chelate derivatives with 2-aminoethanol. 

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... 

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