Page 6 - Organogold Reactivity with Palladium, Nickel, and Rhodium: Transmetalation, Cross-Coupling, and Dual Catalysis

Organogold Dual Metal Catalysis Hirner et al.

FIGURE 1. (a) Hybridization dependence on protodeauration rate. (b) Proposed transition state showing the donation of the AuC bond into the
organogold π-system.
by scavenging the labile phosphine ligand from palladium. gold-only catalysis, 79,27 we examined the kinetic basicities
Spectroscopic identification indicated that this bisphosphine of a variety of organogold compounds (29). Quantitative
gold complex formed rapidly upon addition of stoichio- data on the relative kinetic basicities of organogold com-
metric triphenylphosphine to PPh 3 AuOTf. pounds were obtained relative to phenyl(triphenylphos-
In addition to their ability to install a heterocyclic scaffold phino)gold(I) (30) through a series of competition experi-
and to functionalize organogold intermediates, these reac- ments (eq 4). 28 A full table of relative protodeauration rates
tions are interesting to us for the simultaneous metal is available in the original publication, but we find the
substratemetal interactions that give rise to this unique broader conclusion of these results to merit highlighting in
reactivity. The term “dual-metal catalysis” encompases this narrative. The basicities of organogold compounds did
many different mechanisms. As shown in the gold/palla- not follow the hybridization trend that we expected on the
dium mechanism detailed in Scheme 4, we consider that basis of the pK a values for the corresponding carbon acids
2
3
the gold catalyst potentiates the subsequent palladium- (i.e., basicity increases sp < sp <sp ). Organogold com-
3
catalyzed oxidative addition step: both metals communicate pounds instead exhibited a different basicity trend: sp <sp
2
through substrate 17 via an electronic “push/pull” that <sp (Figure 1a). In analogy to the role of hyperconjugation
activates the substrate toward oxidative addition. 26 We find in the protodemetalation of group 14 organometallics, 29 we
it helpful conceptually to distinguish the potentiated activa- reasoned that our hybridization dependence could be ex-
tion as a “catalyzed catalysis” mechanism to differentiate it plained by a rate enhancement stemming from a hypercon-
from more common dual-metal catalysis mechanisms jugative donation from the AuC σ-bond into the π-system
3
wherein the two metals act in sequence. From a broad during the transition state (Figure 1b). Because sp -hybri-
perspective, this dual-catalytic cross-coupling method pres- dized organogold compounds do not have a π-system, they
ents an alternative means to functionalize the numerous are not subject to this donation and therefore undergo
organogold intermediates proposed in many catalytic protodeauration at a slower rate. This understanding of
cycles. 79 We therefore view these two rearrangements the involvement of the organogold π-system in steps that
more as conceptual advances in reaction design rather than functionalize the CAu σ-bond has extended into other
a new route to a specific class of compounds, illustrative of our studies within our group: subsequent experimental data
paradigm that gold and a second transition metal catalyst can suggested a transition state analogy between protodeaura-
provide reactivity not accessible with only a single catalyst. tion and transmetalation (see section VIII).
Just as our investigation of the Au/Pd transmetalation step
allowed us to develop this dual-catalyzed rearrangement, we
envision that similar advances in the knowledge of funda-
mental metalmetal compatibility will allow for the future
design of additional dual-catalyzed reactions.

VI. Relative Kinetic Basicities of Organogold
Compounds
In order to use a second metal catalyst to intercept organo-
gold intermediates, a deeper knowledge of the stability and VII. Ni-Catalyzed Cross-Coupling of Organo-
reactivity of organogold compounds was needed. Seeking gold Reagents
to quantitatively understand the susceptibility of catalytic To this point in our research program, our investigation
organogold intermediates toward protodeauration, which of dual-metal reactivity had been focused on Au and Pd.
is commonly employed as a catalyst-regenerating step in These successes taught us valuablelessons in reaction design,

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