Catalytic Dehydrogenative Borylation of Terminal Alkynes by a SiNN Pincer Complex of Iridium

Compounds
with carbon–boron bonds are versatile intermediates
for building more complex molecules via the elaboration of the carbon–boron
bonds into other carbon–element bonds. The synthesis of carbon–boron
bonds by catalytic dehydrogenative borylation of carbon–hydrogen
bonds with dialkoxyboranes (RO)2BH is particularly attractive.
It has been demonstrated for a variety of carbon–hydrogen bond
types but not for the C­(sp)–H bonds of terminal alkynes, for
which hydroboration of the triple bond is a competing process. We
report a new iridium catalyst that is strictly chemoselective for
C–H borylation of terminal alkynes. The key to the success
of this catalyst appears to be the new ancillary SiNN pincer ligand
that combines amido, quinoline, and silyl donors and gives rise to
structurally unusual Ir complexes. A variety of terminal alkynes (RCCH)
can be converted to their alkynylboronates (RCCBpin,
where pin = pinacolate) in high yield and purity within minutes at
ambient temperature.