Ruthenium-Catalyzed Selective Hydroboration of Nitriles and Imines

Abstract

Ruthenium-catalyzed hydroboration of nitriles and imines is attained using pinacolborane with unprecedented catalytic efficiency. Chemoselective hydroboration of nitriles over esters is also demonstrated. A simple [Ru(p-cymene)Cl] complex (1) is used as a catalyst precursor, which upon reaction with pinacolborane in situ generates the monohydrido-bridged complex [{(η-p-cymene)RuCl}(μ-H-μ-Cl)] 2. Further oxidative addition of pinacolborane to intermediate 2 leading to the formation of mononuclear ruthenium hydride species is suggested. Mass spectral analysis of the reaction mixture and independent experiments with phosphine-ligated ruthenium complexes indicated the involvement of mononuclear ruthenium intermediates in the catalytic cycle. Consecutive intramolecular 1,3-hydride transfers from the ruthenium center to coordinated nitrile and boronate imine ligands, leading to the reduction and resulting in the formation of diboronate amines, are proposed as a plausible reaction mechanism.

Catalyst‐Free and Solvent‐Free Facile Hydroboration of Imines

“…Both the C ‐ and N ‐phenyl ring of the imine moiety bearing diverse substituents resulted the desired products. Various substituents on C ‐phenyl ring afforded the secondary amines ( 3 a – e ) in excellent yields of 85–92 % and these results are significantly better than those observed for Co II ‐CP, Ru II , and (MeCp) 3 La systems which require higher temperature (60–70 °C) and longer reaction duration. Presence of substituents at the N ‐phenyl ring demands longer reaction span than their analogues C ‐phenyl substituted imines ( 3 h – j vs. 3 a – e ).…”

Ruthenium‐Catalyzed Tandem Activation of C≡N and B−H Bonds under Dihydrogen: Synthesis of BN Heterocycles

“…Compound 4 a formally resulted from reduction of the nitrile function and activation of the B−H bond, which led, for the first time, to the formation of the cyclic 1 H ‐2,1‐benzazaborole featuring a NH group. The reaction was catalyzed under a dihydrogen atmosphere, and notably, no 1,1‐bis(boryl)amine oligomers formally resulting from double B−H insertion– were detected. The scope of the reaction was extended to 2‐cyanophenyl(amino)boranes displaying different arene substitution patterns.…”

A greener catalyst for hydroboration of imines—external electric field modify the reaction mechanism

“…Catalytic hydroelementation of unsaturated compounds is an extremely attractive synthetic method in several fields of modern chemistry. [1][2][3][4][5][6][7][8] It is also the most straightforward and atomefficient approach for the incorporation of heteroatoms into various unsaturated systems, which is with academic and industrial importance as these reactions are used in the production of commodity and agrochemicals and material synthesis and also often encountered in chemical synthesis of complex molecules. The hydroboration reaction, a direct addition of the B H bond across unsaturated C N bond, is proved to be a powerful tool for the preparation of N-borylamines or N-methylated amines.…”

“…What is more, imines are readily available from the condensation of carbonyl compounds and primary amines and are, thus, the precursors of choice for the synthesis of unsymmetrically substituted secondary amines. [35][36][37] Although the hydroboration of imines had been reported more and more, [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]28,[38][39][40] it is worth noting that hydroboration of C N bonds with organoboranes or borohydrides is a workhorse reaction for the mild reduction of imines into amines. Especially, the use of deactivated hydroboranes, such as catecholborane (HBCat, Cat = catechol) and pinacolboane (HBPin, Pin = pinacol), requires catalysis due to the significantly decreased Lewis acidity of the boron center.…”