The Boronate Species

This means that although Be2 is unstable, both its 1+ cation and 1- anion are somewhat stable. It also shows that the stability of the boron species is in the order B2+ < B2 < B2. ... 

In order to prove or disprove this hypothesis, you need to find hydrogen in the bulging drum headspace and/or find evidence of sodium borohydride in the EO. The latter might be shown by measuring the element, boron, or by specifically measuring one of the boron species shown in reaction 21.6. 

Better results can be obtained by generating the boronate species with the aid of sodium methoxide. In this case, satisfactory transmetalation occurs on treatment with Cul. Thus, the functionalized copper reagent 55 can be alkynylated with 1-bromo-l-hexyne at —40 °C, furnishing the enyne 56 in 75% yield (Scheme 2.15) [32]. 

Direct aminomethylation at the 5-position of 5-bromopyrimidine 226 to afford 229 has recently been achieved using potassium iV-(trifluoroboratomethyl)piperidine 228 as the boron species <20070L1597>. 

Direct aminomethylation at the S-position of S-bromopyrimidine giving 945 uses potassium iV-(trifluoroborato-methyl)piperidine 944 as the boron species <20070L1597>. 

Methanol distillation serves to remove the majority of the boron species, as B(OMe)3 and MeB(OMe)2... 

A relatively few species are known in which the polyhedron is, at least approximately, a triangulated dodecahedron (Fig. l-4b). These are the boron species BgHl-, B6C2H8, and B8C18. 

In 2007, negative-ion ESI-MS was used to detect the boron species responsible for transmetallation in the Suzuki reaction [PhB(OCH3)3] (Fig. 4D) [46]. Soon after, ESI-MS allowed the in situ observation of small catalytically active palladium clusters during a Suzuki reaction that could be precursors to catalytically active palladium nanoparticles. [(lL)sPd3(H20)], [(lL)3Pd3(H20)7] and the... 

These considerations suggest for the development of an energetically effective procedure, the realization of the direct hydrogenation of the boron species created in the digestion process with molecular hydrogen. Up to now very little work concerning the direct molecular hydrogenation of BX3 species has been carried out. 

On chemical grounds we can formulate the sulfur species as SF3+ and the boron species as BFr" and, furthermore, we can anticipate that the sulfur atom of the SFa group will be the effective center of positive charge. 

B(OH)J. As a corollary, 8 Bb(oh>4 cannot be deduced from the 8 B of the carbonates and vice versa. In the future, some remaining issues of the 8 B-/ H proxy need to be addressed (1) how does temperature, seawater salinity/compo-sition, and total boron concentration affect the results for a(B3-B4) published by Byrne et al. (2006) and (2) what causes the offsets between the 8 B of the boron species in solution and in the carbonates. Meanwhile, neither of these questions compromises the use of 8 B in carbonates as a palaeo-pH indicator. 

Boronation is generally effected by metal-exchange reactions as discussed in Section 6.02.5.6.5. Thus 5-bromopyrimidines have been lithiated, boronated (219), and the boronated species coupled with bromothiophenes and 3-bromoselenophene to form 5-heteroarylpyrimidines (220) (Scheme 35) <86CS305). Using 2,4-di-/-butoxypyrimidine-5-boronic acid a number of heterocycles have been coupled at the 5-position via bromides <90JHC2165). 

The boron species would react with acid to form volatile boric acid and lost during the san le preparatiotL Sulfuric acid was used in wet ashing thus sulfur is not available. 

Though the theoretical calculation on the basis of model 11, it was found that the boron species are mainly existed 6303(011)4- and B(OH)4- while the concentration of 6405(011)42- is very low when the total concentration of boron is low in weak solution. This result demonstrated that the polymerization or depolymerization behaviors of borate are complex. 

Boronic acids bearing strong electron-poor aromatic groups (such as pyridinyl) were reported to lack reactivity in the Petasis-Akritopoulou reaction, using standard conditions (dichloromethane, room temperature) [54]. Boronic esters were also studied [57] and the authors reported that the mechanism of formation of the boronate species was different from that with phenylboronic acid derivatives. Piettre and coworkers considered the use of hexafluoro-iso-propyl alcohol (HFIP), which is an alcohol with higher ionizing power, as the solvent in the Petasis-Akritopoulou reaction with boronic esters (Scheme 6.43). Compared to the use of methanol as solvent and microwave-assisted irradiation (MW), the yields were much higher (a maximum of 99% yield was obtained ) [58]. 

Synthesis of the Diels-Alder precursor began with 56, which was able to react with the Weiler dianion, giving 57 in 64% yield (Scheme 11). Following this, the ketone of 57 was stereoselectively reduced with ammonium boro-hydride due to binding of the alcohol to the boron species at the p-position. Treatment of the resulting alcohol with trifluoroacetic acid converted the ester to the carboxylic acid, which rapidly cyclized to form lactone 58 in 36% yield over two steps. Protection of the alcohol with a triethylsilyl group gave 59,... 

As discussed in Section 3.2 the formation of a boronic acid-saccharide complex acidifies the boron atom, making the resultant boronic ester more acidic than the initial uncomplexed boronic acid. In this instance, a of 8.8 was reported for the neutral 2-anthrylboronic acid, and a p a of 5.9 was reported for the 2-anthrylboronic acid complex formed in saturated fructose solution. Exploiting this phenomenon, the system was bulfered to a pH of 7.4, a value between the corresponding pA a and pKj values reported. With this constraint in place, a high-fluorescence emission intensity was observed from the uncomplexed boronic acid (pH < pKf). However, imder these buffered conditions, addition of a saccharide to the solution formed the boronic ester, lowering the acidity of the boronic species below the pH of the solution pKf < pH). As a direct result, the boronate anion was generated inducing the decrease in fluorescence observed on addition of saccharide. 

On complexation of a saccharide to a boronic acid there is a contraction of the O-B-0 bond angle with a concomitant increase in the addity of the boron species. Boronic acid diol complex formation is heavily pH dependent. Rate and stability constants increase by around four and five orders of magnitude, respectively, at pHs above the pA a of the boronic acid. 

This study was later extended to aryls by the same authors obtaining qualitatively the same overall mechanism and suggesting that results of general validity can be obtained with the less computationally demanding vinyl systems [72]. Besides, this study also supported that the role of the base is to react with the boronate species regardless of which the electrophilic organic substrate is. 

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