Enantiomerically Pure Axially Chiral Aminocarbene Complexes of Chromium

Aminocarbene complexes having a methyl group in the ortho-position to the aminocarbene moiety on the aromatic ring, (S)-pentacarbonyl[(N,N-dimethylamino)(3-methoxycarbonyl-2-methylphenyl)carbene]chromium(0) [(S)-6] and (R)-pentacarbonyl[(N,N-dimethylamino)(5-methoxycarbonyl-2-methylphenyl)carbene]chromium(0) [(R)-7], were prepared in enantiomerically pure form by the crystallization of diastereoisomeric esters with (S)-1-(1-naphthyl)ethanol. In the case of (S)-6 the racemization barrier ΔG⧧rac = 121 ± 0.5 kJ·mol−1 was established. The substitution of o-methyl group with an isopropyl group virtually did not change the racemization barrier (ΔG⧧rac = 120.5 ± 0.5 kJ·mol−1), while the introduction of an o-phenyl group led to substantial lowering of ΔG⧧rac. Attempts to transfer chirality in thermal reactions of the complexes (S)-6 and (R)-7 with alkynes, palladium-catalyzed insertion into a C−H bond, and photochemical formation of β-lactams were unsuccessful.