Prying open a Reactive Site for Allylic Arylation by Phosphine-Ligated Geminally Diaurated Aryl Complexes

Gas-phase ion–molecule reaction
experiments, theoretical
kinetic modeling, and computational chemistry were used to examine
the role of a second gold center in promoting allylic arylation. Geminally
diaurated complexes [(L)nAu2Ph]+ are demonstrated to participate in C–C bond
formation reactions with allyl halides, CH2CHCH2X (X = Cl, Br, and I), given a favorable phosphine ligand
architecture. Thus, while [(Ph3P)2Au2Ph]+, 1, is unreactive toward the allyl halides,
[(dppm)­Au2Ph]+, 2 (where dppm =
bis­(diphenylphosphino)­methane, (Ph2P)2CH2), reacts via C–C bond coupling to produce [(dppm)­Au2X]+. The reaction kinetics (efficiencies) follows
the expected leaving group ability, X = I (58%) > Br (2%) >
Cl (0.02%).
DFT calculations were carried out to examine the potential mechanism
for the C–C bond coupling reactions of 2 with
each of the three allyl halides. The most favorable mechanism for
C–C bond coupling of 2 requires the active participation
of both gold centers in a redox couple mechanism wherein the allylic
halide oxidatively adds across the gold centers to form a AuIAuIII complex with a weak Au···Au interaction,
followed by intramolecular reductive elimination of allyl benzene
from the AuIII center.