Reactivity of cyclopalladated compounds. Part 18. Compared reactivity of the Pd–C bonds of two closely related six-membered palladocyclic rings with substituted alkynes. X-Ray and molecular structures of [Pd{C(Ph)C(R)C(Ph)C(R)(o-C6H4NCMeNHPh)}Cl](R = CO2Et) and [Pd{C(R)[C(CO2Me)C(R)C(R)C(R)C(R)][o-C6H4NCMe(OH)]}Cl](R = CO2Me)

Abstract

The reactions between alkynes and the chloride-bridged cyclopalladated dimers derived from orthopalladation of acetanilide, [{P[graphic omitted])}₂(µ-Cl)₂](1), and amidine, [{P[graphic omitted]Ph)}₂(µ-Cl)₂](2), afforded organometallic products in which one, two, or three alkynes had been inserted into the Pd–C bonds of (1) and (2), the ratio of alkyne : palladium in the final products being independent of the stoicheiometry of the reaction. Compound (1) gave with hexafluorobut-2-yne the mono-inserted product [{P[graphic omitted])](µ-Cl)}₂]. With diphenylacetylene it gave an unstable product (7) in which two alkynes had been added per palladium atom forming with the acetanilide unit a spirocyclic ligand η³-bonded to the palladium centre. Compound (7) decomposed on heating to afford quantitatively the pentasubstituted naphthalene derivative C₁₀H₃Ph₄(NHCMeO). Compound (2) reacted with diphenylacetylene and ethyl 3-phenylpropionate to give the di-inserted products [P[graphic omitted]CMeNHPh)}Cl](11, R = Ph; 12, R = CO₂Et). The structure of (12) was ascertained by an X-ray structure determination, which confirmed the regioselectivity of the insertion reaction and that (12) is an eight-membered palladocyclic ring through the co-ordination to the metal of the nitrogen atom immediately adjacent to the phenyl ring. Triple insertion of alkynes was observed when reacting (1) and (2) with dimethyl acetylenedicarboxylate affording [P[graphic omitted]CMeYH)]}Cl](9, Y = O; 13, Y = NPh). The structure of (9) has been confirmed by an X-ray diffraction study. A related triple insertion occured when treating (1) with but-2-yne affording a thermally unstable intermediate; its depalladation led to trans-o-C₆H₄(NHCMeO)CHCH(C₅Me₅).