From alkynyl borates

Alkynylboranes as useful synthetic intermediates are stronger Lewis acids and are easily hydrolyzed compared with alkylboranes. The Pd-catalyzed reaction of stable methoxy(alkynyl)borate complexes 71 derived from alkynyllithium gives arylacetylenes and stereodefined enynes 72 efficiently [Eq. (27)] [40]. 

Heterocycles. Alkenyl and aryl boronates deliver their organic residues to the opposition of the nitrogen atom of cyclic carbinolamines to replace the hydroxyl group. Synthesis of 2,3,5-trisubstituted furans, from a catalyzed reaction of alkynyl borates and enones, is achieved in one step. 

Another reaction between alkynes and CO using alkali metals yields a,3-unsaturated acids. Starting from alkynyl lithium, the lithium trial kylalkynyl borates can be prepared, which react with CO yielding the lithium salt of an unsaturated acid. After further treatment with acetjc acid the stereospecific ., p-unsaturated acids are obtained in yields of about 70-80 % [97] (Figure 34). For instance, trihexylbo-rane and hexynyl lithium led to 2-butylnonenoic acid in yields of 83 %. 

Alkynyl(methoxy)borates prepared in situ from an alkynyllithium or sodium and 9-methoxy-9-BBN coupled with 1-alkenyl and aryl halides (Equation (210)).899-902 Addition of triisopropylborate to lithium acetylide yielded an air stable and isolable ate complex that couples with aryl and alkenyl halides (Equation (211)).903 904 Air and moisture stable alkynyltrifluoroborates were probably the most convenient reagents that allow handling in air and coupling reactions in basic aqueous media (Equation (212)).46... 

However, the application of these classical procedures for 1-alkenylboronic acid or ester synthesis may suffer from the formation of small amounts of the opposite stereoisomers, or from bis-alkenylation leading to the boronic acid derivatives. Also, formation of trialkylboranes may occur. A recent useful variant utilizes organolithium reagents and triisopropyl borate, followed by acidification with HCl to give directly alkyl-, aryl-, 1-alkynyl-, and 1-alkenylboronic esters in high yields, often over 90% (Scheme 2-6) [27]. Triisopropyl borate was shown to be the best of the available alkyl borates to avoid multiple alkylation of the borates. 

Suzuki couplings. Pd-catalyzed reactions of B-allyl borate complex derived from 6-methoxy-9-borabicyclo[3.3.1]nonane with aryl triflates give allylarenes. This technique broadens the scope of the Suzuki coupling to allow transfer of Me, TMSCHj, and alkynyl groups which has eluded conventional manipulations. 

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