Boranes boron acid esters

An alkyl- or alkenylboron compound, as a suitable organoboron component (a borane, boronic acid or ester) can be prepared through hydroboration of an... 

Boranes and, to a lesser extent, boronic acids can undergo slow hydrolysis (protode-boration) in the presence of protic solvents. This unwanted reaction can become predominant if a cross-coupling reaction only proceeds slowly (e.g. with electron-rich, sterically demanding, or unreactive halides Scheme 8.20 see also Scheme 8.14) or if the boron derivative is particularly sensitive, for example 2-formylphenylboronic acid. In such instances the reaction should be performed under anhydrous conditions in an aprotic solvent with a boronic acid ester [151] or a stannane. 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Boronic acids RB(OH)2 were first made over a century ago by the unlikely route of slow partial oxidation of the spontaneously flammable trialkyl boranes followed by hydrolysis of the ester so formed (E. Frankland, 1862) ... 

Alkenylboronic acids and esters have been prepared by thermal or catalyzed hydroboration of 1-alkynes with catecholborane (HBcat), pinacolborane (HBpin), or dihaloboranes 41-43, followed by hydrolysis to boronic acids or alcoholysis to boronic esters. A convenient alternative to improve chemo- and regioselectivity is the hydroboration of alkynes with dialkylboranes. For selective removal of dummy groups, the oxidation of two cyclohexyl groups was conduced by treatment of l-alkenyl(dicyclohexyl)borane intermediates with Me3N-0 (Equation (7)).116 The... 

Tris(organoamino)boranes have been utilized to prepare, in reasonable yields,4,5 mono- and dihalo(organoamino)boranes which are often difficult to obtain by direct amination of the boron trihalides. Carboxylic acids, 1,3-diketones, ketones, and /3-ketoesters have been converted into carboxamides, enamino-ketones, enamines, and j -enamino-amides, respectively, by reaction with an appropriate tris(organoamino)borane under very mild conditions.6 Sulfenamides (R2NSC6H5) have also been prepared in high yield from selected tris(organoamino)boranes and sulfenic esters under relatively mild conditions.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. 

The first two and last two steps for the catalytic cycle of Suzuki cross-coupling are much the same as those for the Stille reaction (Scheme 12.18) the transmeta-lation step, however, is unique. Transmetalation involves transfer of R to Pd from a borane, borate ester, or boronic acid. Both recent experimental investigations and analysis using DFT calculations indicate that transmetalation is not simply a concerted process as suggested by transition state structure 38. 

In addition, arylthiophene 76 was obtained by a one-pot Suzuki coupling of p-methoxyiodobenzene and 3-bromothiophene via an in situ boronate formation using one equivalent of bis-pinacolato borane [66]. This method avoided the isolation of boronic acids and is advantageous when base-sensitive groups such as aldehyde, nitriles, and esters are present. However, the cross-coupling yields are low when both aryl halides are electron-poor because of competitive homocoupling during the reaction. 

Yang et al. used catechol to immobihze borane and boronic acids [58], since support-bound boronic catachol ester hnkage is stable enough to perform amidation reactions. Custom-derivatized boronic acids could be obtained, which could be hberated from the support with THF/H2O/ACOH 90 5 5 (v/v/v). Hebei et al. reported Suzuki-mediated release of biaryls from polyglycerol esters of various boronic acids and aryl bromides [59]. 

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). 

Starting from a vinyl-substimted resin, hydroboration with 9-BBN yields a homobenzylb-orane (Scheme 9). This intermediate can be coupled with various functionalized aryl iodides as well as vinyl and alkyl iodides giving rise to resins with amide, ester, or protected hydroxy functionaUties.t Similarly, bromostyrene could be coupled with functionalized boranes for the attachment of preformed handles, for example, for the construction of the silicon traceless linker.t i The carbometaUation of certain alkenes such as tropanes and the subsequent treatment with aryl boronic acid gives rise to two new C—C bonds (Scheme 10). 

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