(η⁴-1,5-Cyclooctadiene)(η⁶-phosphinine)iron(0): Novel Room-Temperature Catalyst for Pyridine Formation^,¹

Abstract

Two routes are described for the synthesis of (η4-1,5-cyclooctadiene)(η6-phosphinine)iron(0) complexes. A three-component reaction of iron vapor with COD and 2-(trimethylsilyl)-4,5-dimethylphosphinine at low temperature yields 20% (COD)(2-(trimethylsilyl)-4,5-dimethylphosphinine)Fe(0) (3), whereas ligand exchange of one COD ligand of the in situ prepared metal vapor product (COD)2Fe by 2-(trimethylsilyl)-4,5-dimethylphosphinine or (η1-2-chloro-4,5-dimethylphosphinine)Cr(CO)5 gives corresponding (COD)(phosphinine)Fe(0) complexes in more than 80% isolated yield. As the phosphinine derivatives are prochiral, complexation leads to the racemate of two enantiomers. Both enantiomers are found in the unit cell of single crystalline 3 and are related by an inversion center. Complex 3 is a novel room-temperature catalyst for the [2 + 2 + 2] cyclic addition reaction of one molecule of butyronitrile with two molecules of methyl propargyl ether giving up to 160 mol of pyridine derivatives/mol 3. A chemically robust species, 3 is an air-stable crystalline material, but exposure to oxygen in solution causes slow decomposition.