Alkynylboronic acids

Like their aryl and alkenyl counterparts, alkynylboronic adds can be made by displacement of magnesium or lithium acetylides with borate esters. For example, Mat-teson and Peacock have described the preparation of dibutyl acetyleneboronate from ethynylmagnesium bromide and trimethyl borate [294]. The C-B hnkage is stable in neutral or acidic hydrolytic solvents but readily hydrolyzes in basic media such as aqueous sodium bicarbonate. Brown and co-workers eventually applied their organo-lithium route to boronic esters to the particular case of alkynylboronic esters, and in this way provided a fairly general access to this class of compounds [295]. 

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Transmetallation of alkenyl- and alkynylboronic acid esters with ArMgX reagents provides an access to mixed alkenyl-, aryl-, and alkynylborinic acids. In contrast, treatment of alkynylboronic acid esters with alkyl RMgX compounds displaces the alkynyl group and alkylboronic acid ester is formed ° ... 

In contrast to arylboronic acids, early reports document the great stability of alkyl-boronic acids under aqueous acidic solutions. For example, various simple alkyl-boronic adds were unaffected by prolonged heating in 40% aqueous HBr or HI [40]. Like arylboronic acids, however, deboronation is observed in hot basic aqueous solutions [76]. Alkenylboronic esters undergo protonolysis in refluxing AcOH [85], and alkynylboronic acids were reported to be quite unstable in basic aqueous solutions (Section 1.3.5). 

Boron Conjugate addition of alkenylboronic acids PhCH=CHB(OH)2 and alkynylboronic esters to ArCH=CHCOMe (Ar = unprotected indoles), carried... 

As described earlier, a variety of transition metal catalysts have been used to promote the addition of B-H bonds across alkynes. The development of metal-free approaches has been challenging to design due to the unique reactivity profile afforded by transition metal catalysts. To this end, a metal-free catalyst system has been devised for the addition of pinacolborane to internal alkynes (Scheme 6.48) [91]. Surprisingly, only a carboxylic acid additive was needed to effect the hydroboration reaction. In addition to common alkynes, an alkynylboronate was successfully borylated using the carboxylic acid catalyst. The functional group tolerance was remarkably broad, and a host of alkynes were successfully functionalized. Given the wide substrate scope, availability of the precursors and catalysts, and the metal-free nature of the chemistry, this is a reasonable approach to selective hydroboration reactions. 

An interesting metal-free approach has been developed starting from an alkynylboronate. A carboxylic acid-catalyzed hydroboration afforded 1,1-diboronates in moderate to excellent yields [91]... 

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