Rhodium-catalysed hydroboration employing new Quinazolinap ligands; an investigation into electronic effects

As part of an ongoing effort to improve the efficiency and substrate scope of our Quinazolinap ligand series in the rhodium-catalysed asymmetric hydroboration of vinyl arenes, 2-(p-trifluoromethylphenyl)-Quinazolinap and 2-(p-methoxyphenyl)-Quinazolinap have been synthesised and resolved in good yield. These, along with the previously reported 2-(2-pyridyl)-Quinazolinap and 2-(2-pyrazinyl)-Quinazolinap, form part of an electronic series of Quinazolinap ligands synthesised in order to explore electronic effects in this ligand class. The application of this series of ligands to the rhodium-catalysed asymmetric hydroboration of a range of vinylarenes is described. Good conversions and regioselectivities as well as excellent enantioselectivities up to 97% were obtained. 2-(p-Methoxyphenyl)-Quinazolinap demonstrated consistently high enantioselectivities in the hydroboration of sterically demanding vinylarenes.

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(S,R)-cis-[Dimethyl(1-(1-naphthyl)ethyl)aminato-C2,N]-[1-(2-p-trifluoromethylphenyl-quinazolin-4-yl)(2-naphthyl)diphenylphosphine]palladium(II) chlorideC55H52ClF3N2PPd[α]D20=-48.7 (c 1.18, CHCl3)Ee ⩾99% (by NMR)Source of chirality: (R)-(+)-1-(1-naphthyl)ethylamineAbsolute configuration: (S,R)

(S,R)-cis-[Dimethyl(1-(1-naphthyl)ethyl)aminato-C2,N]-[1-(2-p-methoxyphenyl-quinazolin-4-yl)(2-naphthyl)diphenylphosphine]palladium(II)chlorideC55H55ClN2OPPd[α]D20=-44.2 (c 0.65, CHCl3)Ee ⩾99% (by NMR)Source of chirality: (R)-(+)-1-(1-naphthyl)ethylamineAbsolute configuration: (S,R)

(S)-Diphenyl(1-(2-p-trifluoromethylphenyl-quinazolin-4-yl)(2-naphthyl)phosphineC37H24F3N2P[α]D20=+40.9 (c 0.70, CHCl3)Ee ⩾ 99% (by NMR of (R)-dimethyl(1-(1-naphthyl)ethyl)amine Pd complex)Source of chirality: Resolution using Pd complex of (R)-dimethyl(1-(1-naphthyl)ethyl)amine)Absolute configuration: (S)

(S)-Diphenyl(1-(2-p-methoxyphenyl-quinazolin-4-yl)(2-naphthyl)phosphineC37H27N2OP[α]D20=+38.0 (c 0.65, CHCl3)Ee ⩾99% (by NMR of (R)-dimethyl(1-(1-naphthyl)ethyl)amine Pd complex)Source of chirality: Resolution using Pd complex of (R)-dimethyl(1-(1-naphthyl)ethyl)amine)Absolute configuration: (S)