Metal free catalytic hydroboration of multiple bonds in methanol using N-heterocyclic carbenes under open atmosphere

Abstract

An easy to operate method of catalytic hydroboration of unsaturated compounds has been developed with wide substrate scope. Reactions of various aldimines, ketimines, α,β-unsaturated carbonyl compounds, and alkynes were successfully executed with bis(pinacolato)diboron and N-heterocyclic carbenes in methanol without requiring a transition metal or inert atmosphere.

Alkenyl Boronates: Synthesis and Applications

“…In 2013, Sun achieved this reaction using N-heterocyclic carbenes as the catalyst under air (Scheme 2a). [11] As terically hindered benzimidazole derivative gave excellent results in b-hydroboration,e ven for non-protected propargylic alcohols. Jin established the catalytic activity of carboxylic acids for this reactionb yh eating the mixturei n octane at 100 8C( Scheme 2b).…”

Practical Solvent‐Free Microwave‐Assisted Hydroboration of Alkynes

“…A plethora of transition metals has been used to catalyzed and control the regio‐ and stereoselectivity of this process , . More recently, transition metal‐free protocols have been reported to obtain the trans isomers of alkenyl boronates, including the use of several boranes, N ‐heterocyclic carbenes, 4‐(dimethylamino)benzoic acid, different bases (NaOH, LiO t Bu, NaO t Bu/PhI(OAc) 2 ) or main‐group metals as catalysts. Furthermore, non‐catalyzed reactions were previously studied by Brown and Knochel .…”

Highly Enantiospecific Borylation for Chiral α‐Amino Tertiary Boronic Esters

“…Indeed, there have been only a limited number of reports on asymmetric approaches to more sterically congested α‐amino tertiary boronic esters, which involves a particular challenge for the stereoselective construction of N‐substituted quaternary carbon stereogenic centers (Scheme ). For example, significantly lower enantioselectivities were observed for the synthesis of sterically hindered α‐amino tertiary boronic esters compared to secondary boronic esters by borylation of imines and Cu‐catalyzed hydroamination of alkenyl dan‐boronates . Recently, Ellman reported a copper‐catalyzed diastereoselective borylation of chiral N ‐tert‐butanesufinyl ketimines with the involvement of a chiral auxiliary to synthesize α‐amino tertiary boronic esters [Scheme , Eq.…”