Boronate ester functionalization boranes

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). 

A direct synthesis of substituted aryl boronic esters has been recently reported by Masuda [12,13]. Thus, coupling of pinacol borane 30 with aryl halides or triflates in the presence of a catalytic amount of PdCl2(dppf) and a base like triethylamine allows the preparation of aryl boronates having a variety of functional groups such as carbonyl, cyano, nitro, and acylammo of type 31 in high yields. The product distribution 31 versus 32 (reduced product) is strongly dependent on the choice of the base employed. In the presence of triethylamine as a base, selective formation of boronates 31 was observed with negligible amount of the reduced aromatic compound 32 (Scheme 3.8). 

The facile reduction of the -COOH group by BHj THF or BH3 SMej has been employed for chemoselective reductions of the carboxyl group in the presence of ester or lactone functionalities using a stoichiometric quantity of the borane. The carbonyl group in triacylboranes resembles the reactivity of an aldehyde or a ketone more than of an ester (ester resonance) due to electron delocalization from the acyl oxygen into the p orbital of boron. 

A special case of molecular boron suboxides is found in the hydroxylated polyhedral boranes see Boron Hydride. For example, the icosahedral anion c7o50-[Bi2Hi2] and related compounds can be completely hydroxylated to form discrete molecular boron suboxides, including closo [Bi2 (OH)i2] . These can be regarded as members of a class of formally hydrated suboxides related to compounds such as tetrahydroxydiborane, B2(HO)4 (BO H2O). The terminal hydroxyl groups of these compounds can be functionalized as esters or ethers to produce a variety of large molecule derivatives. 

Functionalized alkenyl diamino- and dialkoxyboranes have been produced regio-and stereoselectively through addition of carbon- or heteroatomgenerated from bromotrichloromethane, thiols, phosphines and tributyltin hydride) to ethynylbis(diisopropylamino)boranes. The synthetic utility of these reactions was illustrated by the preparation of stereodefmed (Z)- or ( )-alkenylboronic esters via palladium-catalyzed crossStille reaction and a Suzuki coupling under basic conditions can be further conducted (Scheme 9.14) [33]. 

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