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.