Stereo- and regioselective boryllithiation and -sodiation of alkynes: reductive approach using flow microreactors

Abstract The stereo- and regioselective syn-boryllithiation and -sodiation of alkynes have been achieved by reductive borylmetallation using flow microreactors. This method involves the fast mixing of a solution of an alkyne and stoichiometric alkoxypinacolborane with an alkali metal arenide solution, which efficiently generates highly reactive β-borylalkenyllithium and -sodium species by suppressing undesired diboration that inevitably occurs in a batch reactor. Unlike the conventional three-component borofunctionalization, the intermediates can be generated in the absence of electrophiles, which has enabled diverse electrophiles to participate in various syn-borofunctionalizations such as borylsilylation, borylhalogenation, borylcarbonylation, borylsulfenylation and borylarylation. Trapping with aldehydes and ketones provides a series of oxaboroles of biological interest. Furthermore, unsymmetrical diarylacetylenes undergo highly regioselective borylmetallation, which is applicable to the stereo- and regiocontrolled syntheses of multisubstituted oxaboroles and differently tetrasubstituted alkenes.