Unprecedented (η5-Formylcyclohexadienyl)Mn(CO)3 Complexes: Synthesis, Structural and Theoretical Characterizations, and Resolution of the Planar Chirality

The synthesis of the (η5-formylcyclohexadienyl)Mn(CO)3 complexes 5−8 was successfully achieved after lithiation and then electrophilic quench by DMF of variously substituted (η5-cyclohexadienyl)Mn(CO)3 complexes. Their full experimental characterization is reported, together with a theoretical study that allowed the consequences of the regiochemistry on the electronic properties of the complexes to be understood. Rearomatization of complex 6 led to the formation of the cationic [(η6-2-methoxybenzaldehyde)Mn(CO)3]+ complex 11, first example of a benzaldehyde derivative coordinated to the Mn(CO)3 + entity. The electrophilic reactivity of the formyl group of complex 5 was tested with hydrides and a Grignard reagent as the nucleophiles. The corresponding alcohols were isolated in high yields and the excellent diastereoselectivity could be explained by steric factors clearly identified thanks to the crystal structure determination of the starting aldehyde 7b. By using the enantiopure (R,R)-N,N′-dimethylcyclohexane-1,2-diamine, the corresponding aminals were obtained and their resolution afforded, after acidic treatment, the enantioenriched (η5-formylcyclohexadienyl)Mn(CO)3 with very high enantiomeric excess. The X-ray analysis of two of these aminals 14 and 15 allowed the assignment of the absolute configurations of the planar chiral η5 moieties.