Imidovanadium(V) complexes as reaction partners for kinetically stabilizedphosphaalkynes: Synthesis of 1,2,4-azaphosphavanada(V)-cyclobutenes, 1,3,5-triphosphabenzenes, and 1H-1,2,4-azadiphospholes

Cycloaddition reactions of the kinetically stabilized phosphaalkynes 1 with the imidovanadium(v) trihalides 9 furnish the 1,2,4-azaphosphavanada(v)cyc lobutenes 10. The stability of these novel metallacyclic compounds depends solely on the substitutents of the imido unit. Thus, the imidovanadium(v) s pecies 9 with tertiary alkyl groups on the N atom form stable addition prod ucts with 1 while in the cases of compounds 9 with a lower degree of substi tution at N (primary and secondary alkyl groups) the primarily formed adduc ts 10 undergo irreversible decomposition to afford the 1H-1,2,4-azadiphosph ores 13. Reactions of an excess of the phosphaaalkyne 1 with the vanadium c omplexes 9 furnish the corresponding triphosphabenzenes 8 in good yields (3 6-68%). A catalytic reaction course has been demonstrated for the all-tert- butyl system 1a/9a in which the metallacyclic species 10a serves as the cat alytically active species. Poisoning of the catalyst leads to a second reac tion pathway, which results in formation of the azatetraphosphaquadricyclan es 16. By means of the stepwise use of different phosphaalkynes 1a,b this m ethodology provides the first access to the differently substituted triphos phabenzenes through cyclotrimerization.