Imine Reduction with Me2S-BH3

Abstract

Although there exists a variety of different catalysts for hydroboration of organic substrates such as aldehydes, ketones, imines, nitriles etc., recent evidence suggests that tetra-coordinate borohydride species, formed by activation, redistribution, or decomposition of boron reagents, are the true hydride donors. We then proposed that Me2S-BH3 could also act as a hydride donor for the reduction of various imines, as similar compounds have been observed to reduce carbonyl substrates. This boron reagent was shown to be an effective and chemoselective hydroboration reagent for a wide variety of imines.

Mechanistic Insight into Hydroboration of Imines from Combined Computational and Experimental Studies

“…The rate‐limiting step involves a free energy barrier of 23.6 kcal/mol and the whole pathway is exothermic by 29.2 kcal/mol. The intermediate IM10 was detected by 1 H and 11 B NMR spectroscopy when equimolar amounts of BH 3 ⋅ SMe 2 and N ‐benzylideneaniline were mixed at room temperature for 12 h (see Figure S17–19 for detail) [16] . Thus, our theoretical and experimental work shows that BH 3 ⋅ SMe 2 can serve as an efficient catalyst for hydroboration of imine 1 b .…”

Mild and Efficient Reductive N,N‐Dimethylation of Amines by Using 1,3‐Dimethylimidazole‐2‐ylidene Borane (diMe‐Imd‐BH3)

“…A suitable mechanism for the reaction indicated in Scheme 1,, 2 was proposed based on results and previously published related literature [24].…”

Applications of catalysis in hydroboration of imines, nitriles, and carbodiimides

“…They proposed that Me 2 S-BH 3 acts as a hydride donor in the reduction of various imines with good to moderate isolated yield of 44-88% (14 examples). 121 Noticeably, a catalyst free and solvent free imine hydroboration was report in 2019. This methodology is capable and compatible using 1.1-1.3 eq.…”

“…They proposed that Me 2 S-BH 3 acts as a hydride donor in the reduction of various imines with good to moderate isolated yield of 44–88% (14 examples). 121…”