Alkynylboron reagents

In addition to their use as precursors for alkynylboron, tin or zinc compounds, alkynylmagnesium reagents show a moderate reactivity toward the coupling with haloarenes and haloalkenes. They are often commercially available, or easy to prepare. Their main drawback is their low chemoselectivity and high nucleophilicity, which implies incompatibilities with functional groups such as nitro and carbonyl. 

Alkynylboronates are also suitable substrates when combined with the reagent obtained from PhIO and Tf20 at 0°C, i.e. PhI(OTf)OI(OTf)Ph [136]. Another approach is the direct reaction of terminal alkynes with 3-hydroxy-4-nitrofurazan and PhIO (Scheme 46) [137]. 

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 ° ... 

An alternative, less general procedure for the preparation of alkynyl(phenyl)iodonium triflates in moderate yields employs the reaction of alkynylsilanes [493], alkynylstannanes [494], or alkynylboronates [297] with Zefirov s reagent, 2PhI0-Tf20. This method is also applicable to the synthesis of the parent ethynyl(phenyl)iodonium triflate [495]. 

The double arylation reaction (Scheme 10.98, path B) also proceeded in this case to form the desired product 294, albeit in low yield. It has been mentioned that the developed carbozirconation also has a potential application to the regio- and stereoselective synthesis of oHgosubstituted alkenes [79]. A consistent extension of this work disclosing the appUcation of the Negishi reagent [81] in carbometallations of alkynylboronates has been pubHshed recently (Scheme 10.99) [82]. 

Initial treatment of an alkynyl Grignard reagent with trimethylborate followed by the addition of MIDA generated the MIDA alkynylboronates [106]... 

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