Palladium End-Capped Polyynes via Oxidative Addition of 1-Haloalkynes to Pd(PPh3)4

Abstract

Reported here is the use of 1-haloacetylenes and 1-halopolyynes as synthons for the preparation of new palladium(II) end-capped polyynes. The 1-haloalkynes were obtained in a series of transformations from para-substituted bromoarenes that included Sonogashira coupling followed by halogenation and chain elongation via Cadiot–Chodkiewicz protocol. The key step for the synthesis of metal complexes was oxidative addition of 1-haloalkynes to Pd(PPh3)4, which allowed obtaining a series of metal compounds 1–5-CnX with carbon chains up to hexatriyne in 75–100% yield. All the compounds were characterized by NMR and HRMS or elemental analysis. The 13C spectra of the 1-haloalkynes showed interesting, although expected, shifts of the carbon chain atoms close to the halogen termini. X-ray crystal structures were obtained for three polyynes—two butadiynes (2-C4[Pd]Br and 3-C4[Pd]Br) and one hexatriyne (1-C6[Pd]Br)—and the latter is the first reported X-ray crystal structure of palladium end-capped hexatriyne.