A new strategy for gold and palladium dual-catalytic reactivity and turnover, called catalyzed catalysis, enhanced the synthetic usefulness of vinylgold intermediates by providing dual-catalytic carbon-carbon cross-coupling as an alternative to protodemetalation. This protocol enabled the synthesis of substituted butenolides and isocoumarins from allyl esters. Kinetic and spectroscopic experiments support a mechanism in which the Lewis acidic gold complex catalyzes both an initial rearrangement step and a subsequent Lewis basic palladium oxidative-addition step.

Using two transition metals to simultaneously catalyze a reaction can offer distinct opportunities for reactivity and selectivity when compared to using single-metal catalyst systems. Creating dual transition metal catalytic systems is complicated, however, by challenges in predicting compatible reactivities and designing turnover pathways for both metals. In this Account, we describe our development of dual-metal catalysis reactions involving gold and a second transition metal. The unique rearrangement intermediates accessible through gold-only catalysis, which exploits the soft Lewis acidity of Au(I), make gold an attractive partner for dual-metal catalysis reactions. Because of the complexity of achieving simultaneous turnover of two catalysts and predicting compatibilities, our approach has been to first gain a fundamental understanding of the reactivity of the two metals with each other, both in stoichiometric and monocatalyzed reactions. To this end, we have investigated the combined reactivity of organogold compounds with palladium, nickel, and rhodium. We narrate the intricacies of turning over two catalysts simultaneously and thereby illuminate the valuable role of fundamental studies in identifying the optimal conditions to promote desirable two-metal reactivity and compatibility. Transmetalation, redox reactivity, and new mechanisms for dual-metal catalytic turnover were probed from this standpoint. We have applied the knowledge gained through these studies to the development of reactions that are dual-catalyzed by gold and palladium, as well as nickel- and rhodium-catalyzed reactions of organogold compounds. More broadly, these new reactions expand the reactivity available to catalytic organogold intermediates via trapping and functionalization reactions with other transition metals. Our investigations reveal strategies useful for designing dual-metal reactions with gold. First, the versatility of gold as a transmetalation partner suggests that many potential methods may exist to intercept catalytic organogold intermediates with a second transition metal. Second, ligands on both metals should be selected carefully in order to prevent catalyst deactivation. Finally, reactions must be designed such that any oxidative steps involving the second metal outcompete undesired reactions with redox-active organogold compounds. We believe that the application of these principles will allow for the design of a diverse set of dual-catalyzed functionalizations befitting the wide variety of gold-catalyzed transformations already established.

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A new palladium-catalyzed syn carboauration of alkynes proceeds in 2 h at ambient temperature with complete regioselectivity. The resulting α-ester vinyl−gold intermediates are resistant to rapid protodemetalation, permitting their participation in new one-pot palladium-and-gold cross-coupling reactions and electrophilic trapping reactions.

Two new Rh(I)-catalyzed methods for the synthesis of chiral alpha-branched amines via addition of arylboronic acids to N-tert-butanesulfinyl and N-diphenylphosphinoyl imines have been developed. The syntheses are more functional group tolerant than alternative methods utilizing Grignard or organolithium reagents, and the imine activating groups used are easily removed. These methods are both high-yielding (70-97% yield) and very selective (>93:7 dr and 88-94% ee). Significantly, the N-tert-butanesulfinyl imine method works for aliphatic imines with enolizable protons. In addition, a one-pot procedure for the synthesis of N-tert-butanesulfinyl protected alpha-branched amines from aldehydes has been developed.