Page 8 - Organogold Reactivity with Palladium, Nickel, and Rhodium: Transmetalation, Cross-Coupling, and Dual Catalysis

Organogold Dual Metal Catalysis Hirner et al.

our Au/Pd studies as a guide, we believe that future Au/Ni an excess of PPh 3 to the otherwise effective reaction of Rh
dual-catalyzed transformations will be accessible. complex 45 and organogold compound 49a completely
inhibited transmetalation. This experiment suggested that
VIII. Au/Rh Transmetalation and Rh-Catalyzed the PPh 3 ligand must dissociate from Rh prior to transmetala-
Conjugate Addition to Alkynes tion, thereby eliminating 49a as a plausible transition state.
Most recently, we have expanded to studying the reactivity This mechanistic evidence had direct implications for the
of Au and group IX metals. In order to further increase the development of a CC bond forming reaction between
scope of reactivity available to the many reported organo- acylrhodium complex 50 and organogold complexes. We
gold intermediates beyond cross-coupling, Rh was selected found that coordinatively saturated Rh 50 was unable to
as our next goal due to its well-documented capability for react with organogold 46a (eq 6) because the required open
unique catalytic reactions. 38 As was the case with the Au/Pd coordination site was unavailable. Upon chloride abstrac-
and Au/Ni dual-metal transformations, the transmetalation tion to provide an open coordination site, the organogold
of an organic fragment between the two metals would be gold complex transmetalated with the acylrhodium com-
the pivotal step in any potential Au/Rh dual reactivity. plex, allowing for formation of a new CC bond (eq 7). An
Aryl-, vinyl-, and methylgold compounds 46ad were understanding of the optimal coordination environment of
found to undergo transmetalation with Rh(III) complex 45 to Rh for the Rh/Au transmetalation also allowed the rational
afford organorhodium(III) products (eq 5). 39 Highlighting the design of a Rh-catalyzed addition of organogold complexes
stabilityofbothorganogoldandrhodium(III) complexes,these via conjugate addition chemistry (eq 8). 39
transmetalation reactions could be conducted in vessels open
to air and in wet solvents without diminished yields.

In order to better design future Au/Rh catalytic reactivity,
we decided to first gain knowledge about the fundamental
transmetalation step; advances in the understanding of
transmetalation historically have been used to aid in the
design of efficient catalytic cycles involving other metals. 40
Several transition states for the Au/Rh transmetalation reac-
tion were considered based upon proposed transition states
for other transmetalations (Figure 2). In transition state 49a,
the reaction occurs through a four-centered transition state
with no loss of ligands. 41 Transition state 49b requires the
loss of a ligand from Rh to form an open coordination site, 42
and 49c shows the loss of a ligand from Rh to allow for
bimetallic π-complex character during transmetalation. 43
Notably, transition state 49c is analogous to that previously
reported for the metathesis between HCl and arylgold
28
compounds (i.e., protodeauration, Figure 1b, section VI ).
This model is also in accordance with the bimetallic AuAu IX. Guiding Principles for Dual-Metal Reac-
π-complex reported by Schmidbaur et al. 44 tivity with Gold
The nature of the coordination environment about Rh Our investigations indicate several broad lessons that are
was first examined in order to distinguish coordinatively useful for the planning of new dual-metal reactions with Au.
saturated transition state 49a from 49b and 49c. Addition of A primary cause of the current dearth of dual-catalyzed

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