Esterification boronates

Figure 10.15 (Upper) The synthesis of the nickel phthalocyanine covalent organic framework (NiPc COF) by a boronate esterification reaction, eclipsed stack of phthalocyanine 2D sheets and microporous channels in NiPc COF (a 2 X 2 grid is shown). Colors used for identification phthalocyanine unit sky blue Ni green N violet C gray O red B orange H white. (Middle, from left to right) (First) Absorption profiles of NiPc COF (solid line) and [(MeO)gPcNi] (dotted line) (Second) I-V curves of NiPc COF (red curve) and [(MeO)aPcNi] (black curve) sandwiched between AI/Au electrodes. (Third) Transient conductivity profiles upon irradiation with a 355 nm pulse laser at different photon densities 4.51014 (red curve) to...

Figure 12.1 FE-SEM image of submicrospheres formed by pyridine-assisted boronate esterification of BDBA and THB in THF.

Figure 12.3 Basicity dependence of polymeric boronate esterification. FE-SEM images and PXRD patterns of solid isolated from mixtures of BDBA and THB in THF at room temperature after addition of (a) pyridine, (b) 3-picoline, (c) 4-methoxypyridine, (d) 4-dimethylaminopyridine, and (e) triethylamine. The use of 3-cyanopyridine induced no aggregation. Conditions [BDBA] = [THB] = [base] = 1.0 x lo" M. The solids were isolated from the mixture by filtration at 15 min reaction time. (Reproduced with permission of Wiley-VCH Verlag GmbH from ref. 17.)...

Kubo and Nishiyabu describe the use of reversible boronate esterification to build well-ordered microparticles through supramolecular polymerisation of benzene-l,4-diboronic acid with tetraols. When pentaerythritol is used as the tetraol component for self-assembly, thermodynamically stable flower-like microparticles are produced. Surface functionalisation enables formation of nanometal-deposited heterogeneous catalysts and white-light-emissive chemosensors. 

Kataoka K, Okuyama S, Minami T, James TD, Kubo Y (2009) Amine-triggered molecular capsules using dynamic boronate esterification. Chem Commun 1682-1684... 

Kataoka K, James TD, Kubo Y (2007) Ion ptiir-driven heterodimeric capsule based on boronate esterification construction and the dynamic behavior. J Am Chem Soc 129 15126-15127... 

Simultaneous, multiple ester formation was employed to assemble larger multitopic assemblies. For example, self-assembled molecular capsules via boronate esterification have been demonstrated. Two calix[3]arenes, one having the upper rim functionalized with diols, the other functionalized with boronic acids, were combined in the presence of a templating agent to form the heterodimeric capsule (Fig. 25). Extensive NMR analysis was performed to show that the two calixarenes in solution by themselves do not interact. It is only when tetramethyl ammonium acetate is added that the capsule forms. NMR diffusion studies have further demonstrated that the ammonium cation is included within the cavity. In addition, reversible decomposition-reconstruction of the capsule was observed in response to pH. 

In each case the configuration around the boron changes from trigonal planar to tetrahedral on adduct formation. Because of this ability to form additional compounds, boron trifluoride is an important catalyst and is used in many organic reactions, notably polymerisation, esterification, and Friedel-Crafts acylation and alkylations. 

As a dibasic acid, malic acid forms the usual salts, esters, amides, and acyl chlorides. Monoesters can be prepared easily by refluxing malic acid, an alcohol, and boron trifluoride as a catalyst (9). With polyhydric alcohols and polycarboxyUc aromatic acids, malic acid yields alkyd polyester resins (10) (see Alcohols, polyhydric Alkyd resins). Complete esterification results from the reaction of the diester of maUc acid with an acid chloride, eg, acetyl or stearoyl chloride (11). 

The esterification of boric oxide does not require the removal of water. However, if high yields based on boron are desired, six or more moles of alcohol must be used and the water must be removed. 

From Boron Halides. Using boron haUdes is not economically desirable because boron haUdes are made from boric acid. However, this method does provide a convenient laboratory synthesis of boric acid esters. The esterification of boron haUdes with alcohol is analogous to the classical conversion of carboxyUc acid haUdes to carboxyUc esters. Simple mixing of the reactants at room temperature or below ia a solvent such as methylene chloride, chloroform, pentane, etc, yields hydrogen haUde and the borate ia high yield. 

Catalysts. The choice of the proper catalyst for an esterification reaction is dependent on several factors (43—46). The most common catalysts used are strong mineral acids such as sulfuric and hydrochloric acids. Lewis acids such as boron trifluoride, tin and zinc salts, aluminum haHdes, and organo—titanates have been used. Cation-exchange resins and zeoHtes are often employed also. 

In the case of ethylene, it is necessary to use high temperatures and pressures as well as active catalyst to effect esterification (82). Yields of 40—50% based on ethylene were obtained with boron trifluoride—hydrogen fluoride mixtures as catalysts at 150°C. 2-Butene under pressure at 115—120°C with an excess of glacial acetic acid containing 10% H2SO4 gave as much as a 60% yield of I -butyl acetate (83). 

The reaction is reversible and reaches equilibrium slowly. Generally, acidic catalysts ate used, such as strong sulfuric acid, hydrochloric acid, boron triduoride, and i)-toluenesulfonic acid (27). Batchwise and continuous processes ate used for the esterification reaction. 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

Esterification is readily achieved in THF in the presence of a dehydrating agent. The boronate is stable to chromatography, has good stability to 2 M TEA/ CH2CI2, but is not stable to aqueous 1 M NaOH. Cleavage is also achieved by hydrogenolysis. ... 

When the related saccharin derived sultam (R)-29 is converted into the (Z)-boron enolate and subsequently treated with aldehydes,. vy -diastereomers 30 result almost exclusively. Thus, the diasteromeric ratios, defined as the ratio of the major product to the sum of all other stereoisomers, surpass 99 1. Hydroperoxide assisted saponification followed by esterification provides carboxylic esters 31 with recovery of sultam 32106a. 

The first example of a stable 1,1-bidentate Lewis acid based on boron and zirconium has been reported [35]. The synthesis of 22 is outlined in Scheme 7.12. Treatment of hex-l-yne with HBBr2 Me2S followed by conversion of the dibromoboronic ester to the corresponding alkenyl boronic acid and esterification with propane-1,3-diol provided the alkenyl boronic ester. Hydrozirconation of this compound with 3 equivalents of the Schwartz reagent, Cp2Zr(H)Cl [57], afforded the desired product 22 in 86% yield. 

Electrolytic reduction, apparatus, 52, 23 Enol acetates, acylation of, 52,1 Enol esters, preparation, 52, 39 Epichlorohydrin, with boron trifluoride diethyl therate and dimethyl ether to give trimethyloxonium tetra-fluoroborate, 51,142 ESTERIFICATION OF HINDERED ALCOHOLS f-BUTYL p-TOLUATE,... 

Finally, compound (iv) is condensed with either trimethyl(6-methyl-3-pyridyl)tin or the boronate ester by means of Pd(PPh3)4 to afford etoricoxib. The metallated pyridine (vii) is obtained by esterification of 3-hydroxy-2-methylpyridine with triflic anhydride to give the corresponding triflate, which is treated with a tin reagent to yield the target tin intermediate. The boron lithium salt (viii) is prepared by treatment of 5-bromo-2-methylpyridine with butyllithium followed by addition of triisopropyl borate. 

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