Efficient Co‐Catalyzed Double Hydroboration of Nitriles: Application to One‐Pot Conversion of Nitriles to Aldimines

Abstract

The commercially availablea nd bench-stable Co(acac) 2 /dpephos system is employeda saprecatalyst for selective and efficient room temperature hydroboration of organicn itrilesw ith HBPin to produceaseries of N,N-diborylamines [RN(BPin) 2 ], which reacti ns itu with aldehydes to give aldimines. Formation of aldimines from N,Ndiborylamines does not require ad ehydrating agent, is applicable to aw ide range of N,N-diborylamine and aldehyde substrates and is highly chemoselective, being unaffected by various common functional groups,s uch as alkenes, alkynes,s econdary amines, ketones,e sters, amides, carboxylic acids, pyridines, nitriles, and nitro compounds. The overall transformation represents as ynthetically valuable approacht oa ldimines from nitrilesa nd can be performed in as equential one-pot manner,t olerating ester, lactone, carboxamide and unactivateda lkene functionalities.

Catalytic Nitrile Hydroboration: A Route to N,N‐Diborylamines and Uses Thereof

“…[19,20] During the last decade, since the discovery of Mo-catalyzed dihydroboration of nitriles, [19,20] a number of research publications concerning other catalytic systems have appeared in the literature, with the vast majority of developments being accomplished in the area of transition metal catalysis. The newly developed methods mostly rely on Ru [22,29,30] and Co [21,23,31,32] catalysts, with a handful of examples using Ti, [33,34] Mn, [35] Fe [36] and Ni. [37] With regards to f-block elements, only two reports on using Er, Y, Dy, Gd [38] and Th [39] systems in dihydroboration of nitriles have been disclosed.…”

“…Moving beyond ruthenium systems, a series of abundant base metal catalysts (Fe, [36] Co, [21,23,31,32] Ni [37] and Mn [35] ) have been reported to facilitate hydroboration of nitriles. Thus, ironcatalyzed reactions were disclosed by Nakazawa et.…”

“…Recently, our group have reported the application of Cocatalyzed nitrile hydroboration in one-pot conversion of a diversity of nitriles to aldimines. [21] The hydroboration reactions were performed at room temperature using 2.2 equivalents of HBPin and 5 mol% of commercially available Co(acac) 2 ligated with dpephos (bis[(2-diphenylphosphino)phenyl] ether) to give a wide variety of aliphatic and aromatic N,N-diborylamines, featuring substituents of different electronic properties (Scheme 14). The system was found to tolerate ester, lactone, amide, halogens and unactivated alkenes functionalities, resulting in complete conversions of nitriles within a few hours.…”

“…The suggested mechanism of Co(acac) 2 /dpephos-catalyzed hydroboration of nitriles includes activation of the pre-catalyst with HBPin to form catalytically active (dpephos)Co(I) hydride species, [52] which undergoes insertion of the nitrile into the CoÀ H bond, followed by the reaction with HBPin. [21] The obtained N,N-diborylamines were conveniently coupled with aldehydes without additional purification (or catalyst separation) to afford aldimines in good to excellent isolated yields (Scheme 14). This second condensation step did not require a dehydrating agent and proved exceptionally chemoselective, being unaffected by common functional groups, such as secondary amines, ketones, carboxylic acids, esters, amides, alkenes, alkynes and nitro groups (Scheme 14).…”

“…[19,20] Recently, we have reported an improved protocol for such reactions and expanded the synthetic utility of this method for one-pot conversion of nitriles to a diversity of aldimines. [21] In 2019, N,N-diborylamines have been applied in Pd-catalyzed N-arylation reactions with aryl bromides [22] and in the synthesis of secondary carboxamides via treatment with aromatic carboxylic acids. [23] In addition, N-borylmines, derived from primary and secondary amines have been used as iminium ion generators for Mannich-type CÀ C bond formation reactions [24] and as powerful reagents for preparation of imines.…”