Insertion of imines and carbon monoxide into manganese-alkyl bonds: Synthesis and structure of a manganese-α-amino acid derivative

Two new routes have been developed to generate manganese-chelated amides from imine and carbon monoxide building blocks. The reaction of (CO)₅MnR (R = CH₃, Ph) with (p-tolyl)(R¹)=NR² (R¹ = H, ^tBu; R² = alkyl, H) results in the generation of the cyclometalated imine complexes (CO)₄Mn[η²-4-CH₃-2- (C(R¹)=NR²)-C₆H₃] in 50-84% isolated yield. However, when the reaction of (p-tolyl)(H)C=NCH₃ with (CO)₅MnCH₃ is performed in the presence of AlCl₃, the product of sequential carbon monoxide and imine insertion is generated in 35% yield: (CO₄)Mn[η²-C(H)(p-tolyl)N (CH₃)COCH₃]. The addition of PPh₃ to the latter complex results in replacement of a carbonyl ligand and formation of the isolable fac-(CO)₃(PPh₃)- Mn[η²-C(H)(p-tolyl)N(CH₃)COCH₃]. In an alternative route to metal-bound amides, the oxidative addition of the N-acyl iminium salt (p-tolyl)(H)C=N(CH₃)COPh⁺Cl^- to (CO)₅Mn^-Na⁺ has been found to lead to the generation of the product of subsequent CO insertion, (CO₄)Mn- [η²-COC(H)(p-tolyl)N(CH₃)COPh]. The latter represents the first example of a acyl-bound transition-metal-chelated α-amino acid complex and has been characterized by X-ray crystallography. Preliminary reactivity studies demonstrate that (CO)₄Mn[η²-COC(H)(p-tolyl)N(CH₃)COPh] reacts with PPh₃ to generate the unchelated cis-(CO)₄(PPh₃)Mn[COC-(H)(p-tolyl)N(CH₃)COPh], while reaction with NaBH₄ leads to the liberation of the amide (p-tolyl)CH₂N(CH₃)COPh, rather than the amino acid residue.