sp–sp 3 Coupling reactions of alkynylsilver cations , RC[triple bond, length as m-dash]CAg 2 + (R = Me and Ph) with allyliodide

George N. Khairallah, Craig M. Williams, Sharon Chow, Richard A. J. O'Hair

Alkynylsilver cations, RC≡CAg2(+) (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 RC≡CAg2(+), via a facile decarboxylation of the RC≡CCO2Ag2(+) precursor; (ii) ESI of silver acetylides, RC≡CAg, which yields a cluster of the type, [(RC≡CAg)12Ag2Cl](+). Regardless of the method of preparation, these alkynylsilver cations, RC≡CAg2(+), undergo ion-molecule reactions with allyliodide to yield the ionic products Ag2I(+) and [(RC≡CCH2CH=CH2)Ag](+). The CID spectrum of [(PhC≡CCH2CH=CH2)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, PhC≡CCH2CH=CH2(+)˙, 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 RC≡CCH2CH=CH2, to ultimately yield two sets of reaction products: (i) Ag2I(+) and RC≡CCH2CH=CH2; and (ii) [(RC≡CCH2CH=CH2)Ag](+) and AgI.