Chemoselective Hydroboration of Propargylic Alcohols and Amines Using a Manganese(II) Catalyst

Abstract

The first manganese-catalyzed hydroboration of propargylic alcohols and amines as well as internal alkynes is reported. High regio-and stereoselectivity is achieved by applying 2 mol % of a manganese pre-catalyst based on the readily accessible bis(imino)pyridine ligand and MnCl2 as metal source. Propargylic alcohols and amines, as well as symmetric internal alkynes, were efficiently converted into the corresponding functionalized alkenes, which can serve as important and valuable intermediates for further synthetic applications such as cross-coupling reactions.

The Rise of Manganese-Catalyzed Reduction Reactions

“…132 The Adhikari/Maji and Garcia groups both, separately, using Mn(I) catalysts 63/64 and 62 (Scheme 11), reduced nitriles using different hydrogen sources: the ammoniaborane complex and butan-2-ol, respectively. 133 Both groups could isolate the primary amine in high yields; however, the group of Maji could also isolate the secondary amine by modifying the polarity of the solvent and optimizing with a modified catalyst (63). In addition, the nitrile could be coupled with a primary amine to create more diverse secondary amines (Scheme 11C).…”

“…In 2020, the group of Rueping used a bis(imino)pyridine (BIP) ligand -also used by Thomas (catalyst 7, Scheme 2) -to synthesize a (BIP)MnCl 2 catalyst for the single reduction of propargylic alcohols and amines as well as internal alkynes. 63 This afforded highly valuable, functionalized alkenes, with a possible application as substrates for cross-coupling reactions. Even more recently, the group of Kirchner first used their Mn(I) catalyst 21 (Scheme 2) with HBPin to also reduce terminal alkenes into the respective linear organoborates at low catalyst loadings (0.25 mol%).…”

Hydroboration of terminal alkenes and trans-1,2-diboration of terminal alkynes catalyzed by a Mn(I) alkyl complex

“…As manganese is concerned, several examples for Mn(I)-catalyzed hydroborations of polarized C-X multiple bonds such as carbonyls, [12] nitriles, [13] and CO2 [14] were reported. Interestingly, the hydroboration of alkenes [15] and alkynes [16] is as yet restricted to manganese complexes in the oxidation state +II containing potentially non-innocent ligands such as terpyridine or benzylic imines. An overview of manganese-based hydroboration catalysts for alkenes and alkynes is depicted in Scheme 1.…”

Hydroboration of Terminal Alkenes and trans‐1,2‐Diboration of Terminal Alkynes Catalyzed by a Manganese(I) Alkyl Complex

“…As manganese is concerned, several examples for Mn I ‐catalyzed hydroborations of polarized C‐X multiple bonds such as carbonyls, [12] nitriles, [13] and CO 2 [14] were reported. Interestingly, the hydroboration of alkenes [15] and alkynes [16] is as yet restricted to manganese complexes in the oxidation state +II containing potentially non‐innocent ligands such as terpyridine or benzylic imines. An overview of manganese‐based hydroboration catalysts for alkenes and alkynes is depicted in Scheme 1 .…”