Novel C2-symmetric bisoxazolines with a chiral trans-(2R,3R)-diphenylcyclopropane backbone: preparation and application in several enantioselective catalytic reactions

Various chiral bisoxazoline ligands with a chiral trans-(2R,3R)-diphenylcyclopropane backbone have been efficiently synthesized (five examples). These chiral ligands were tested and compared in palladium(0)-catalysed enantioselective allylic alkylations (up to 97% ee), copper(I)-catalysed enantioselective cyclopropanations (up to 89% ee) and aziridinations (up to 90% ee). We observed that the presence of a stereogenic centre on the oxazoline moiety is mandatory in order to obtain acceptable enantioselectivities.

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(2R,3R)-1,1-Bis-[oxazolin-2′-yl]-2,3-diphenylcyclopropaneC21H20N2O2[α]D20=+99 (c 1.0, CHCl3)Source of chirality: Sharpless asymmetric dihydroxylationAbsolute configuration: (2R,3R)

(2R,3R)-1,1-Bis-[(4′S)-4′-tert-butyloxazolin-2′-yl]-2,3-diphenylcyclopropaneC29H36N2O2[α]D20=+19 (c 1.0, CHCl3)Source of chirality: Sharpless asymmetric dihydroxylation, (S)-tert-leucinolAbsolute configuration: (2R,3R,4′S)

(2R,3R)-1,1-Bis-[(4′R)-4′-tert-butyloxazolin-2′-yl]-2,3-diphenylcyclopropaneC29H36N2O2[α]D20=+160.2 (c 1.0, CHCl3)Source of chirality: Sharpless asymmetric dihydroxylation, (R)-tert-leucinolAbsolute configuration: (2R,3R,4′R)

(2R,3R)-1,1-Bis-[(4′S)-4′-phenyloxazolin-2′-yl]-2,3-diphenylcyclopropaneC33H28N2O2[α]D20=+18.2 (c 1.0, CHCl3)Source of chirality: Sharpless asymmetric dihydroxylation, (S)-phenylglycinolAbsolute configuration: (2R,3R,4′S)

(2R,3R)-1,1-Bis-[(4′R)-4′-phenyloxazolin-2′-yl]-2,3-diphenylcyclopropaneC33H28N2O2[α]D20=+193.6 (c 1.0, CHCl3)Source of chirality: Sharpless asymmetric dihydroxylation, (R)-phenylglycinolAbsolute configuration: (2R,3R,4′R)