Zinc Hydride-Catalyzed Dihydroboration of Isonitriles and Nitriles: Mechanistic Studies with the Structurally Characterized Zinc Intermediates

Abstract

The conjugated bis-guanidinate (CBG)-stabilized zinc hydride, [{LZnH} 2 ; (L = {(ArNH)(ArN)−C�N− C�(NAr)(NHAr)}; Ar = 2,6-Et 2 -C 6 H 3 )] (Zn-1), is shown to be a highly active catalyst for the double reduction of isonitriles and nitriles with pinacolborane (HBpin) in this study. A wide array of isonitriles and nitriles, including aryl and alkyl groups, underwent hydroboration to afford exclusively 1,2-and 1,1-diborylated amine products, respectively. A series of stoichiometric reactions were performed to understand the reaction mechanisms. Hydroboration of isonitrile proceeds through the formation of zinc formimidoyl complex, [LZnC(H)N( t Bu)] 2 (Zn-2), while nitrile hydroboration through zinc vinylidenamido complex, [LZnNC(H)(C 6 H 5 )] 2 (Zn-3). Moreover, compounds Zn-2 and Zn-3 are isolated and thoroughly characterized. Such molecular zinc complexes are rare in the literature.

Tri-coordinated zinc alkyl complexes with N^S/Se coordination of imino-phosphanamidinate chalcogenide ligands as precursors for efficient hydroboration of nitriles and esters

“…In these cases, trityl chloride can work as a hydride scavenger to abstract the hydride species leading to the quenching of the catalytic reaction. In light of this evidence and based on the previous literature, 43,44,53,54 the most plausible mechanism of nitrile hydroboration using a zinc alkyl complex as a pre-catalyst is depicted in Scheme 2. Here, the zinc hydride complex I serves as an active catalyst which is formed in situ in the reaction from the pre-catalyst (zinc alkyl complex) and HBpin.…”

“…In the previous literature, zinc hydride species worked as active catalysts where zinc complexes were used for catalytic hydroboration. 43,44,[52][53][54] To understand the mechanism, we performed a few controlled experiments. When complex 3b was reacted with HBpin (1 equiv.)…”

“…40,41 The Nembenna group established a conjugated bis-guanidinate zinc hydride complex as a catalyst for the hydroboration of isonitriles, nitriles and esters. 42,43 Our group introduced the imidazolin-2-iminato ligand-supported zinc alkyl complex as a catalyst for hydroboration of nitriles 44 and Ma et al recently used ZnEt 2 as a pre-catalyst for hydroboration of esters. 45 Nonetheless, the use of zinc alkyl complexes in the hydroboration of nitriles and esters is very rare.…”

Magnesium(ii) complexes supported by indole- and pyrrole-containing Schiff-base ligands: syntheses, structures, and applications as precatalysts in the hydroboration of nitriles and alkynes

“…Since Nikonov and coworkers discovered that a Mo( iv ) complex could catalyse the hydroboration of nitriles in 2012, 32,33 research on the hydroboration of nitriles or alkynes has been increasing. A plethora of transition-metal complexes, such as zirconium, 34 manganese, 35 iron, 36,37 cobalt, 22,38–43 nickel, 44–46 zinc, 47–53 ruthenium, 54–56 palladium, 57 and many rare-earth metal complexes, 58–60 can catalyse the hydroboration of nitriles or alkynes. In recent years, earth-abundant, nontoxic and inexpensive main group-metal compounds have become favourable catalysts in these research fields.…”