Hydroboration And Hydroarylation Of Heteroatom-Substituted Acetylenes

The hydroaddition reaction of alkynes is one of the important methods to build alkenes. Among these, the control of regio- and stereoselectivity still remains a significant challenge. By using thiol or silyl substituents as the directing groups, some regio- and stereoselective hydroaddition reactions of alkynes, including hydroboration of thioalkynes and hydroarylation of silylacetylenes, have been developed here, which provide new efficient methods for the access of trisubsituted alkenes via the iterative cross-coupling strategy. The main content is divided into two parts:First, a Cu-catalyzed divergent hydroboration of thioacetylenes has been achieved, providing (Z)-l-thio- or (Z)-2-thio-l-alkenyl boronates in moderate to high yields with excellent regio- and stereoselectivity, by using pinacolborane or bis(pinacolato)diboron as the hydroborating reagents, respectively. DFT calculations indicate that the sulfur atom plays a key role in determining the regioselectivity through polarizing the C-C triple bonds and participating in the HOMO orbitals. Moreover, the SR group can also serve as a good leaving group, resulting in the concise synthesis of six regio- and stereoisomers of trisubstituted alkenes via the iterative cross-coupling of C-B and C-S bonds. Clearly, it will be valuable for assembling stereochemically diverse trisubstituted olefins in organic synthesis.Second, an efficient Pd-catalyzed addition of boronic acids to silylacetylenes is described, providing β,β-disubstituted (E)- or (Z)-alkenylsilanes in satisfactory yields with excellent regio- and stereoselectivity under mild reaction conditions. It represents the first highly regio- and stereoselective addition of boronic acids to aryl and alkenyl silylacetylenes. Moreover, the sequential Pd-catalyzed boron addition/ N-halosuccinimidemediated halodesilylation reaction results in a stereodivergent approach to β,β-disubstituted alkenyl halides, which can serve as the versatile synthetic intermediates for the stereodivergent assembly of (E)- and (Z)- trisubstituted alkenes via transition-metal-catalyzed cross-coupling reactions.