Page 7 - Organogold Reactivity with Palladium, Nickel, and Rhodium: Transmetalation, Cross-Coupling, and Dual Catalysis

Organogold Dual Metal Catalysis Hirner et al.

TABLE 4. Representative Products of the Ni-Catalyzed Cross-Coupling of Organogold Reagents

SCHEME 5. Proposed Mechanism for Ni-Catalyzed Cross-Coupling of Organogold Compounds

allowing us to expand our aim to other combinations of two organogold oxidant and nickel precatalyst 36 may be op-
transition metals. We selected Au and Ni as our next target. erative in generating Ni(III) precatalyst 40, although the
Offering reactivity both parallel to 30 and unique from that of organobromide could serve as an alternative oxidant. 36
Pd, 31 Ni offered the potential to build on our previous experi- Additional insight into the redox processes available to
ence with Pd while also providing access to eventual different organogold complexes and nickel is provided by examining
classes of bond disconnections. Unlike Pd, however, Ni is more the substrate scope illustrated in Table 4: electron-poor
readily susceptible to single-electron redox processes, and the organobromides were required for high cross-coupling pro-
stability of organogold(I) compounds in the presence of a duct yields. When electron-rich organobromides were em-
single-electron reducing metal was uncertain. We therefore ployed, gold mirror was observed and cross-coupling
explored the compatibility of the two metals 18 through a product yields were low. In this case, the slower oxidative
Ni-catalyzed cross-coupling of organogold reagents. 32 In the addition reactions of electron-rich organobromides 37 may
presence of a Ni catalyst, we foundorganogoldreagents permit kinetically competitive reduction reactions of orga-
34ah to undergo cross-coupling reactions with aryl and vinyl nogold compounds by nickel intermediates to predominate.
bromides 33ah in high yield under mild conditions (Table 4). These results illuminate the valuable role of fundamental
In analogy to other Ni-catalyzed cross-coupling reac- studies in identifying the optimal conditions that promote
tions 33 and on the basis of EPR studies in our group, this desirable two-metal reacitivity and compatibility.
reaction likely proceeds through a Ni(I)/Ni(III) catalytic cycle This new cross-coupling reaction is an early demonstra-
(Scheme 5). Notably, single-electron reduction of the orga- tion of the compatibility of homogeneous nickel and orga-
nogold reagent was not detected under standard reaction nogold complexes with one another. 18 This reactivity
conditions 34 after the Ni(I) catalyst-generating steps despite knowledge formed the basis for an investigation currently
the potential thermodynamic favorability of this redox underway in our group to combine the unique reactivity of
reaction. 35 Nevertheless, a reaction redox between an both Au and Ni catalytically. Using the lessons learned from

Vol. 44, No. 8 ’ 2011 ’ 603–613 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 609