Reactivity of the α-Ketoacyl Ligand in Cp'(L)(NO)Mn-C{O{C{O{Tol with Electrophiles, Nucleophiles, and Alkynes

The reactivity of the α-ketoacyl complexes Cp'(L)(NO)Mn~C{O}C{O}Tol (Cp' = C₅H₄Me; 1, L = CO; 2, L = PPh₃) with a variety of electrophiles, nucleophiles, and alkynes has been explored. Complex 2 undergoes protonation at the α-carbonyl oxygen at low temperature to produce a hydroxy-carbene complex. This species rearranges upon warmup by breaking the carbon-carbon bond of the α-ketoacyl ligand to give 4-methylbenzaldehyde and [Cp'Mn(PPh₃)(NO)(CO)]⁺ (6). Oxidation of complex 2 with either [Cp₂Fe]⁺ or Ag⁺ also results in the formation of complex 6 along with a mixture of organic products in which TolC(O)C{O}Tol dominates. These products indicate that oxidation induces cleavage of the α-ketoacyl carbon-carbon bond to form 6 and TolC{O}^• radicals. Complex 1 reacts with NEt₂H under photochemical conditions to give the α-ketoamide TolC(O)C{O}NEt₂. Upon photolysis, complex 1 reacts with alkynes Rceee≡Cr' to form metallacycles of the type Cp(CO)(NO)Mn=CR—CR/=C(C{O}Tol)O that result from coupling of the alkyne and the α-ketoacyl ligand. The metallacycles can be demetalated by treatment with HC1 to give vinyl α-diones that are the net result of cis addition of the α-ketoacyl ligand and a proton across the alkyne triple bond.