1,4-Conjugate Addition of Aryl boronic Acids on Cyclohexenone as Catalyzed by Rhodium(I) Complexes of C2-Symmetric Bioxazoline Fused N-heterocyclic Carbenes

Abstract

Rhodium(I) complexes (1-3) of chiral C2-symmetric tricyclic bioxazoline fused imidazole derived N-heterocyclic carbene ligands were efficient precatalysts for the 1,4-conjugate addition of aryl boronic acids to cyclohexenone yielding 3-aryl cyclohexanones in moderate to excellent yields but without any chirality transfer. Quite significantly, the intermediates of the proposed catalytic cycle namely, a rhodium hydroxo species (A), a phenyl bound rhodium species (B) and a 3-phenyl cyclohexenolate bound rhodium species (D) have been detected by mass spectrometry. Lastly, the rhodium(I) precatalysts of the type, {(3R,7R)-3,7-di-R-2,3,7,8-tetrahydrodioxazolo[3,2-c:3′,2′-e]imidazol-5-ylidene}Rh(COD)Cl, [R=s-butyl (1), i-butyl (2), i-propyl (3), COD=1, 5-cyclooctadiene] were prepared by the direct metalation of tricyclic oxazoline fused imidazole derived N-heterocyclic carbene precursors namely, (3R,7R)-3,7-di-R-2,3,7,8-tetrahydrodioxazolo[3,2-c:3′,2′-e]imidazol-4-ium trifluoromethanesulfonate, [R=s-butyl, i-butyl, i-propyl] with {(COD)RhCl}2 in presence of t-BuOK at room temperature.