A Comparison of Two Zinc Hydride Catalysts for Terminal Alkyne C–H Borylation/Hydroboration and the Formation of 1,1,1-Triborylalkanes by Tandem Catalysis Using Zn–H and B–H Compounds

The synthesis of 1,1,1-triborylalkanes
from terminal alkynes and
pinacolborane (HBPin) is reported. This transformation proceeds via
initial Zn-catalyzed alkyne C–H borylation, which can be achieved
using a NacNacZnH complex. Combinations of a NacNacZn-alkynyl formed
via C–H zincation of a terminal alkyne and HBPin exist in equilibrium
with the alkynyl-BPin and NacNacZnH. The consumption of NacNacZnH
by irreversible reaction with a terminal alkyne evolving H2 is essential for driving alkyne C–H borylation to completion.
The alkynyl-BPin compounds undergo hydroboration catalyzed by Zn–H
complexes at raised temperatures with a {7DIPP}­ZnH­(NTf2) complex (7DIPP = 1,3-bis­(2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-1H-1,3-diazepin-3-ium-2-ide) a more active catalyst for hydroboration
than a NacNacZnH complex. Calculations indicate the {7DIPP}­Zn–H
congener has a more pronounced biphilic character than that of NacNacZnH
(greater electrophilicity at Zn while maintaining a basic hydride).
Of the two hydroboration steps, the hydroboration of alkynylBPin is
catalyzed by Zn–H complexes, while the hydroboration of 1,1-diborylalkenes
is catalyzed more effectively by B–H-containing species, including
boranes formed in situ from HBPin. These observations led to a one-pot
protocol being developed for converting terminal alkynes into 1,1,1-triborylalkanes
that utilizes {7DIPP}­ZnPh­(NTf2) as a precatalyst for the
formation of 1,1-diborylated alkenes with subsequent addition of BH3-THF as catalyst for the final step.