sp–sp3 Coupling reactions of alkynylsilver cations, RCCAg2+ (R = Me and Ph) with allyliodide

Abstract

Alkynylsilver cations, RCCAg₂⁺ (where R = Me and Ph) have been prepared in the gas phase using multistage mass spectrometry experiments in a quadrupole ion trap mass spectrometer. Two methods were used: (i) electrospray ionisation (ESI) of a mixture of AgNO₃ (in MeOH/H₂O/acetic acid) and the alkyne carboxylic acid to yield the appropriate silver acetylide cations RCCAg₂⁺, via a facile decarboxylation of the RCCCO₂Ag₂⁺ precursor; (ii) ESI of silver acetylides, RCCAg, which yields a cluster of the type, [(RCCAg)₁₂Ag₂Cl]⁺. Regardless of the method of preparation, these alkynylsilver cations, RCCAg₂⁺, undergo ion-molecule reactions with allyliodide to yield the ionic products Ag₂I⁺ and [(RCCCH₂CHCH₂)Ag]⁺. The CID spectrum of [(PhCCCH₂CHCH₂)Ag]⁺ was compared to that of an authentic sample of the silver adduct of 5-phenyl-1-penten-4-yne. Both ions fragment to yield Ag⁺ and the radical cation, PhCCCH₂CHCH₂⁺˙, confirming that C–C bond coupling has taken place in the gas phase. DFT calculations were carried out on these C–C bond coupling reactions for the system R = Me. The reaction is highly exothermic and involves the initial coordination of the allyliodide to both silver atoms, with the iodine coordinating to one atom and the alkene moiety coordinating to the other. The overall mechanism of C–C bond coupling involves oxidative addition of the allyliodide followed by reductive elimination of RCCCH₂CHCH₂, to ultimately yield two sets of reaction products: (i) Ag₂I⁺ and RCCCH₂CHCH₂; and (ii) [(RCCCH₂CHCH₂)Ag]⁺ and AgI.