Selective Hydroboration of Alkynes Enabled by a Silylene Iron(0) Dinitrogen Complex

Silylenes, isoelectronic with carbenes, are a kind of key intermediates in organosilicon chemistry. They possess a lone pair and an empty orbital on the silicon center, and thus could be used as donors and acceptors. Consequently, they could form complexes with various metals to support new structures and chemistry similar to both carbenes and phosphines. Iron complexes played important roles in the development of catalysts because of the inexpensive, nontoxic and sustainable characteristics. Catalytic hydroboration of alkynes presents the most atom-economic and straightforward protocol for the synthesis of vinylboranes which are indispensable intermediates for C-C coupling reactions. For the catalytic hydroboration of alkynes with iron catalysts, Enthaler's group developed the first iron catalytic system for hydroboration of alkynes by using Fe-2(CO)(9) (A, Chart 1) as the catalyst. Almost at the same time, Thomas's group reported the bis(imino)pyridine derived iron complexes (B) in combination with an activator for catalytic hydroboration of alkynes and alkenes. In 2017, Nishibayashi and co-workers employed an iron(II) hydride complex (C) supported by a PNP pincer ligand for catalytic E-selective hydroboration of alkynes. In 2020, Findlater et aL reported the regioselective hydroboration of alkynes and alkenes with iron complexes supported by bis(2,6-diisopropylaniline)acenaphthene ligands. However, these catalysts still suffered from limited substrate scope or harsh conditions. The development of highly selective catalysts for a wide substrate scope is still desirable. On the basis of our design on silylene ligands for iron chemistry, we are interested in the silylene-iron complexes for catalytic hydroboration reactions. In this paper, hydroborylation of terminal allcynes catalyzed by a neutral silylene-imine iron(0) dinitrogen complex D was studied. The reaction is highly regio- and stereoselective and almost exclusively gave E-hydroboration products. The optimized reaction conditions are as following: To a dried Schlenk tube were added complex D (0.006 g, 0.01 mmol), toluene (1.0 mL), alkyne (0.20 mmol), and catechol bonnie (0.02 g, 0.20 mmol). After the mixture was stirred at 80 degrees C for 24 h, it was cooled down to room temperature. The solvents were removed under vacuum and the residue was purified by flash chromatography on silica gel to afford the desired products.