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

AbstractRhodium(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.