Boroxine-/boronate-esters

A second peptide coupling employing 2-pyrazine carboxylic acid (24) and TBTU affords 25. Finally the boronic ester moiety is removed under acidic conditions using isobutyl boronic acid. This regenerates pinanediol boronic ester 16, which can be used in another batch run of the process. Crystallization from ethyl acetate gives bortezomib in its anhydride (boroxine) form 26. The overall yield for the route is 35% with a typical purity of > 99% w/w. 

Fig. 9 (a) Reversible synthesis of a bisiminoboronate-guanosine derivative from guanosine and a bisiminoboronic acid, (b) Dynamic polymeric networks based on reversible boroxine formation, (c) Formation of a macrocycle in a [4 + 4 + 2] condensation via simultaneous reversible formation of imine bonds and boronic esters... 

The solvothermal conditions used to synthesize boroxine- and boronate-ester COFs (temperature, solvent, solvent-to-head-space ratio) have a strong effect on the product morphology [14,20] and it seems that particular solvents give rise to ordered crystalline materials whereas others do not. hi principle, this may be understood in terms of templating effects but could equally arise from differences in monomer solubility, for example, which in turn affects the rate of network formation. 

The matter of purity is of real importance for SPC and should therefore be addressed. As boron-based functional groups, free boronic acid or one of the many cyclic boronic esters are used. The latter esters may hydrolyze to the acids during polymerization, which then enter the normal cross-coupling or follow an independent mecha-nism.f " Boronic acids always contain some water. Otherwise they are partially or completely condensed to cyclic boroxines. This water content has to be precisely... 

Achiral (a-bromo-sec-alkyl)boronic esters are readily accessible via radical bromi-nation of sec-alkylboronic esters [36]. Bromination of boronic anhydrides (trialkyl-boroxines) is much more facile and is a recommended alternative [37]. Dropwise addition of bromine to triisopropylboroxine under fluorescent room lights produced bromo derivative 44 essentially quantitatively (Scheme 8.9). Addition of ethylene glycol produced the boronic ester 45, which was easily distilled [37]. 

The general formula for boric acid esters is B(OR)2. The lower molecular weight esters such as methyl, ethyl, and phenyl are most commonly referred to as methyl borate [121 -43-7] ethyl borate [130-46-9J, and phenyl borate [1095-03-0] respectively. Some of the most common boric acid esters used in industrial appHcations are Hsted in Table 1. The nomenclature in the boric acid ester series can be confusing. The lUPAC committee on boron chemistry has suggested using trialkoxy- and triaryloxyboranes (5) for compounds usually referred to as boric acid esters, trialkyl (or aryl) borates, trialkyl (or aryl) orthoborates, alkyl (or aryl) borates, alkyl (or aryl) orthoborates, and in the older Hterature as boron alkoxides and aryloxides. CycHc boric acid esters, which are trimeric derivatives of metaboric acid (HBO2), are known as boroxines (1). 

Hydrocarbon Oxidation. The oxidation of hydrocarbons (qv) and hydrocarbon derivatives can be significantly altered by boron compounds. Several large-scale commercial processes, such as the oxidation of cyclohexane to a cyclohexanol—cyclohexanone mixture in nylon manufacture, are based on boron compounds (see Cylcohexanoland cyclohexanone Eibers, polyamide). A number of patents have been issued on the use of borate esters and boroxines in hydrocarbon oxidation reactions, but commercial processes apparently use boric acid as the preferred boron source. The Hterature in this field has been covered through 1967 (47). Since that time the Hterature consists of foreign patents, but no significant appHcations have been reported for borate esters. 

Trialkyl derivatives of boron, and in fact many other molecules such as boroxines with carbon-boron bonds, react readily with oxygen. The initial products are peroxy derivatives with BOOR bonds, which tend to react further to form borate esters. The ease of the initial reaction is shown by the fact that reported examples of vinyl polymerization induced by trialkyl borons require oxygen and are actually radical processes induced by the boron oxygen reaction or intermediate peroxides (7). 

Ebelman and Bouquet prepared the first examples of boric acid esters in 1846 from boron trichloride and alcohols. Literature reviews of this subject are available. B The general class of boric acid esters includes the more common orthoboric acid based trialkoxy- and triaryloxyboranes, B(0R)3 (1), and also the cyclic boroxins, (ROBO)3, which are based on metaboric acid (2). The boranes can be simple trialkoxyboranes, cyclic diol derivatives, or more complex trigonal and tetrahedral derivatives of polyhydric alcohols. Nomenclature is confusing in boric acid ester chemistry. Many trialkoxy- and triaryloxyboranes such as methyl, ethyl, and phenyl are commonly referred to simply as methyl, ethyl, and phenyl borates. The lUPAC boron nomenclature committee has recommended the use of trialkoxy- and triaryloxyboranes for these compounds, but they are referred to in the literature as boric acid esters, trialkoxy and triaryloxy borates, trialkyl and triaryl borates or orthoborates, and boron alkoxides and aryloxides. The lUPAC nomenclature will be used in this review except for relatively common compounds such as methyl borate. Boroxins are also referred to as metaborates and more commonly as boroxines. Boroxin is preferred by the lUPAC nomenclature committee and will be used in this review. 

Boronic. . . (s. a. Mercuride-boronation) 20, 488 Boronic acid anhydrides s. Boroxines ---- esters 16, 156... 

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