The application of novel boron complexes in asymmetric transfer hydrogenation of aromatic ketones

Asymmetric transfer hydrogenation using iso-PrOH as a hydrogen source offers an attractive route for reducing simple unsymmetrical functionalized ketones to chiral alcohols. The combined use of organometallic and coordination chemistry has produced a number of new and powerful synthetic methods for important classes of compounds in general and for optically active substances in particular. For this aim, the (S,Z)-1-((1-hydroxy butane-2-yl imino)methyl)naphthalene-2-ol chiral ligand was chosen to obtain boron complexes. Boronic derivative compounds such as phenylboronic acid, 6-methoxynaphthalen-2-ylboronic acid, 4-methyl-3-nitrophenylboronic acid and 1,4-phenylenediboronic acid were applied to obtain complexation with chiral based ligands. The structures of these ligands and their complexes have been elucidated by a combination of multinuclear NMR spectroscopy, LC–MS/MS, TGA/DTA, UV–Vis., elemental analysis, XRD, SEM, and FTIR. These boron complexes have also been tested as catalysts in the enantioselective transfer hydrogenation of acetophenone derivatives to afford the corresponding product, (S)-1-phenylethanol with high conversions (up to 99%) and modest enantioselectivities (up to 70% ee). The substituents on the backbone of the ligands had a significant effect on both the activity and % ee.

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(S,Z)-1-((1-Hydroxy butan-2-ylimino)methyl)naphthalen-2-olC15H17NO2[α]D20 = −11.9 (c 0.5, DMSO)Source of chirality: (S)-2-aminobutan-1-olAbsolute configuration: (S)

(S,Z)-5-Ethyl-2-phenyl-4,5-dihydronaphtho[2,3-h][1,3,6,2]dioxazaboronineC21H20BNO2[α]D20 = +1.5 (c 0.5, DMSO)Source of chirality: (S)-2-aminobutan-1-olAbsolute configuration: (S)

(S,Z)-3-Ethyl-6-(6-methoxynaphthalen-2-yl)-3,4-dihydronaphtho[1,2-h][1,3,6,2]dioxazaboronineC26H24BNO3[α]D20 = −3.5 (c 0.5, DMSO)Source of chirality: (S)-2-aminobutan-1-olAbsolute configuration: (S)

1,4-Bis((S,E)-5-ethyl-4,5-dihydronaphtho[2,3-h][1,3,6,2]dioxazaboronin-2-yl)benzeneC36H34B2N2O4[α]D20 = −10.6 (c 1, DMSO),Source of chirality: (S)-2-aminobutan-1-olAbsolute configuration: (S,S)

(S,Z)-5-Ethyl-2-(4-methyl-3-nitrophenyl)-4,5-dihydronaphtho[2,3-h][1,3,6,2]dioxazaboronineC22H21BN2O4[α]D20 = −6.7 (c 0.5, DMSO)Source of chirality: (S)-1-amino-2-propanolAbsolute configuration: (S)