Synthesis, reactivity and X-ray crystal structure of an uncomplexed 1-phosphabarrelene: application to the synthesis of 1,1′-bis(dimethylsilylphosphinine)ferrocenes

Abstract

The 5,6-diphenyl-3-tertiobutyl-1,2-azaphosphinine 3 reacts with bis(phenylethynyldimethylsilyl)ferrocene, 2, to afford a mixture of the phosphinine 4, resulting from the cycloaddition of 3 with one alkynyl group of 2, and 1-phosphabarrelene, 5, whose formation results from an intramolecular [4 + 2] cycloaddition between the remaining alkynyl group of the ferrocenyl ligand and the 1,4-phosphabutadienic system of 4. The X-ray structure of compound 5 was obtained and shows no particular strain in the molecule. Competitive experiments have shown that, at high temperature, phosphinine 4 equilibrates with barrelene 5. Synthesis of a bidentate ligand 6, incorporating two dimethylsilyl-substituted phosphinines, was achieved by reacting the azaphosphinine 3 with half an equivalent of the ferrocene derivative 2. The mixed ligand 8, incorporating two different phosphinine subunits, was also prepared using a two-step sequence by reacting a mixture of 4 and 5 with diazaphosphinine 1. This reaction first produced an intermediate azaphosphinine-phosphinine ligand 7, which was then trapped with trimethylsilylacetylene to afford ligand 8.