Synthesis of 2-Ferrapyridine Complexes and Their Use as Precursors for Substituted Pyridinones and Pyrroles

The 2-ferra-3-azetine complexes Fe₂(μ-CHCH=NR)(CO)₆, which form from the reaction of Fe₂(μ- CH₂)(CO)₈ with phosphinimines, have been found to react photochemically with a variety of alkynes (R¹C=CR²) by inserting the alkyne into the four-membered metallacycle to give the 2-ferrapyridine complexes Fe₂(μ-C(R¹)C(R²)CHCH=NR)(CO)₆. Two of the 2-ferrapyridine complexes have been crystallographically characterized and shown to consist of six-membered metallacycles having adjacent iron and nitrogen atoms with this metallacycle π-coordinated to the second iron atom by three carbon atoms and an Fe-Fe bond. Terminal alkynes (R¹ = Me, Ph, Bu^t, SiMe₃) undergo the insertion regioselectively to give only the 2-ferrapyridine isomer with the substituted carbon adjacent to the iron atom. Unsymmetrical internal alkynes insert to give generally a mixture of isomers which is influenced by the steric and electronic nature of the alkyne substituents. The 2-ferrapyridine complex Fe₂(μ-C(Ph)CHCHCH=NBu^t)(CO)₆ was shown to undergo a ring contraction when heated at 160 °C under CO (500 psi) to give the new 2-ferra- 3-azetine complex Fe₂{μ-C[CH=C(Ph)(H)]CH=NR}(CO)₆, which has also been crystallographically characterized and has a vinyl substituent attached to the 3-carbon of the 2-ferra-3-azetine ring. When heated, the 2-ferrapyridine complexes released 2-pyridinones and pyrroles with the ratio of these products dependent upon the ring substituents and the conditions employed. For example, the 2-ferrapyridine complexes prepared from terminal alkynes gave 2-pyridinones as the major products. The reaction of 2-ferrapyridine complexes prepared from internal alkynes gave mixtures of 2-pyridinones and pyrroles. The presence of electron-donating substituents on the 2-ferrapyridine ring favored the formation of 2-pyridinones, as did the presence of halide ion and an atmosphere of CO in the thermolysis reactions.