Gold and rhodium transmetalation: Mechanistic insights and dual-metal reactivity

Article abstract of DOI:10.1021/om2001316

A transmetalation reaction between rhodium(III) and sp²- and sp³-hybridized organogold(I) compounds proceeds rapidly at ambient temperature (15 min to 6 h). Mechanistic experiments demonstrate that ligand dissociation on rhodium(III)

Full text of DOI:10.1021/om2001316

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pubs.acs.org/Organometallics
Gold and Rhodium Transmetalation: Mechanistic Insights and
Dual-Metal Reactivity
Yili Shi and Suzanne A. Blum*
Department of Chemistry, University of California, Irvine, California 92697-2025, United States
S Supporting Information
b
ABSTRACT: A transmetalation reaction between rhodium-
(III) and sp2- and sp3-hybridized organogold(I) compounds
proceeds rapidly at ambient temperature (15 min to 6 h).
Mechanistic experiments demonstrate that ligand dissociation
on rhodium(III) precedes transmetalation; synthetic applica-
tions of the gold/rhodium transmetalation are highlighted by
linking rhodium-mediated CÀH activation, conjugate addition,
and reductive elimination to transmetalation from organogold compounds.
Transmetalation reactions are relatively poorly understood,
which causes difficulty in accurately predicting reactivity;1 for
example, the oxidation states and coordination environments
around the two metals can result in unanticipated transmetala-
tion reactions2À4 that elude the simplicity of the typical electro-
negativity-based hard/soft method of predictive analysis.5 Herein
we report the first transmetalation reaction between organogold
and rhodium complexes.6,7 This reaction exhibits marked oxida-
tion state and coordination environment effects. The predictive
power derived from these fundamental observations enables
access to the chemistry of rhodium from organogold complexes:
rhodium-mediated CÀH activation/transmetalation and rho-
dium-catalyzed conjugate addition showcase the unique reactiv-
ity available to this dual-metal system. The demonstrated
compatibility and reactivity of this dual-metal system increases
the functionalization possibilities for the numerous reported
organogold intermediates8 by providing complementary reactiv-
ity to the recent functionalization of organogold intermediates by
palladium.6
(78%; eq 4). Rhodium complexes 3a,b displayed restricted bond
rotation at ambient temperature, as observed by 1H NMR
spectroscopy, consistent with the reported characterization data
for the analogous Cp* rhodium compounds described by Jones.12
Treatment of Cp*RhCl2(PPh3) (1) with the electron-rich and
electron-poor arylgold complexes 2a,b at ambient temperature
resulted in transmetalation products 3a,b (66% and 76%, 2 and 6
h, respectively; eq 1). The arylrhodium product 3a was air stable
and could be isolated by column chromatography.9 Notably, the
transmetalation reaction shown in eq 1 could be carried out
under “open flask” conditions,10 in air and in wet solvent, without
decreased yield (69%), demonstrating the operational facility of
both organogold(I) and rhodium(III) complexes and their
combined reaction chemistry.11 Transmetalation between 1
and the vinylgold species 2d was rapid (<15 min) and yielded
the previously unknown vinylrhodium complex 3d, as observed
by 1H NMR spectroscopy (eq 3). Upon precipitation of the gold
byproducts from the crude reaction mixture complex 3d decom-
posed; however, transmetalation between the (trimethylphos-
phine)rhodium complex 4 and organogold species 2d generated
3e, a stable trimethylphosphine analogue of 3d, which was robust
toward isolation by precipitation, permitting full characterization
Received: February 10, 2011
Published: March 16, 2011
r
2011 American Chemical Society
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Figure 2. Rh/Au bimetallic π complex 7, showing π interactions similar
to those proposed in transition state 5c. Thermal ellipsoids are shown at
the 50% probability level, and hydrogen atoms and outer-sphere triflate
counterion are omitted for clarity.
Figure 1. (a) Three possible transition states for the Rh/Au transme-
talation reaction that differ by the coordination environment around
rhodium. (b) Proposed protodeauration transition state, 5d, showing π
system involvement analogous to 5c. (c) Possible transition state for the
Rh/Au transmetalation reaction that involves a σ complex.
inverse first-order kinetic dependence on free phosphine
ligand;21 coordination of the tin reagent in place of a dissociated
phosphine ligand on palladium has been proposed to account for
this observation.18,22
In order to probe the coordinative nature of the transition
state, 10 equiv of PPh3 was added to 1 prior to addition of 2a
(eq 1). Under these conditions of excess phosphine, no trans-
metalation product was detected after 24 h. This inhibition
suggests that PPh3 dissociation from coordinatively saturated 1
precedes transmetalation (transition states 5b,c instead of 5a).
The difference in transmetalation reaction rates between
triphenylphosphine complex 1 and trimethylphosphine complex
4 provided additional evidence for ligand dissociation preceding
transmetalation. Elevated temperatures (12 min at 80 °C) were
required to promote rapid transmetalation with 4 (eq 4); at
ambient temperature transmetalation with 4 proceeded signifi-
cantly more slowly than with 1 (4, 2 days at 23 °C; 1, 15 min at
23 °C). The rate of PMe3 ligand dissociation has previously been
shown to be slower than that of PPh3 from late-metal
complexes;13 a slower initial ligand dissociation is consistent
with the observed slower overall reaction rate with trimethylpho-
sphine complex 4.
We next considered the plausibility of the bimetallic π-arene
interaction proposed in transition state 5c. We were unable to
prepare a complex demonstrating the arene interaction using
rhodium(III)/gold; however, switching to the softer rhodium(I)
metal center allowed for isolation of a stable bimetallic π-arene
adduct.23,24 Specifically, treatment of the organogold compound
2a with Rh(cod)OTf (6) gave rapid and quantitative formation
of complex 7 wherein rhodium was η6 coordinated to the
arylgold π system (eq 6).23 Structural confirmation of the π-
bonding interactions in the bimetallic complex was provided by
X-ray crystallographic characterization of 7 (Figure 2). Interest-
ingly, the reaction of 2a and 6 did not proceed further to give
transmetalation products, possibly because the transmetalation
products were thermodynamically disfavored relative to the
stabilizing soft/soft23 π interaction present in the rhodium(I)/
gold bimetallic complex 7.25 While the comparison between
rhodium(I) and rhodium(III) bonding is inexact, formation of
bimetallic arene complex 7 is consistent with the interaction
proposed in transition state 5c. Nevertheless, transition state 5b
cannot currently be ruled out.
With standard transmetalation conditions for sp2-hybridized
carbon in hand, we next explored the hybridization dependence of
this reaction. Transfer of the sp3-hybridized methyl group from
Ph3PAuMe (2c) occurred on a time scale (65%, 2 h; eq 2) similar
to that for sp2-hybridized carbon. This similarity in reaction rate
between methyl and aryl groups was unexpected because of the
well-established kinetic preference for transfer of sp2- over sp3-
hybridized groups in traditional cross-coupling reactions.13,14 In
contrast to the clean transfer reactions of sp2- and sp3-hybridized
carbon from gold, the sp-hybridized alkynylgold compound2ewas
consumed upon treatment with rhodium complex 1, as observed
by 1H NMR spectroscopy, but the organorhodium compound 3f
was not detected, possibly due to the tendency of alkynylrhodium
complexes to form heterobridged dinuclear complexes that could
result in product decomposition (eq 5).15
We next investigated the mechanism of transmetalation
between organogold(I) and rhodium complexes with the goal
of understanding the functional relationship between the co-
ordination environment at rhodium and the propensity toward
transmetalation. Several distinct mechanisms have been estab-
lished for transmetalation steps in various transition-metal
catalyzed reactions, and mechanistic insight into a transmetala-
tion step can translate into improved reaction efficiency.16 For
the rhodium/gold transmetalation we considered three mechan-
istic scenarios, shown as three transition states which differ by
their coordination environments: 5aÀc (Figure 1).17 In 5a,
transmetalation proceeds through a four-membered transition
state without dissociation of a ligand.13 In 5b, transmetalation
requires prior phosphine dissociation in order to provide an open
coordination site at Rh.18 In 5c, transmetalation requires prior
phosphine dissociation and the resulting open coordination site
forms an η2 complex with the π system of the transmetalating
arylgold complex.19 The involvement of the arylgold π system
has been previously implicated in the protodeauration of arylgold
complexes (5d),20 raising the possibility that transmetalation and
protodeauration proceed through analogous transition states
that differ by Rh(III) or proton acting as the electrophile.20
Interactions with a transmetalation partner’s π system have been
proposed for other metals (e.g., furylstannane and palladium19),
producing further interest in this hypothesis. It has been reported
that transmetalation between palladium and tin reagents exhibits
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Scheme 1. Rh/Au CÀC Bond Forming Transmetalation and
Reductive Elimination
Figure 3. 1H NMR spectra starting from complex 9 with portionwise
addition of PPh3. The distinct RhÀH region is shown, as increasing the
amount of PPh3 causes the equilibrium to shift toward 12.
suggested the broader application of mechanistic knowledge
gained from the study of 1. Further supporting the requirement
for an open coordination site, addition of excess PPh3 (4 equiv)
to 9 completely inhibited transmetalation from 2a. When a lesser
amount of PPh3 (2 equiv) was premixed with complex 9, the
transmetalation reaction with organogold 2a was slowed (4 h for
2 equiv of PPh3, <15 min for no added phosphine) but not
stopped. Titration of PPh3 to 9 produced a progressive downfield
shift of the RhÀH 1H NMR resonance until addition of 8 equiv
of PPh3 produced minimal additional change in chemical shift
(Figure 3). This spectroscopic data established that the equilib-
rium favored coordinatively saturated 12 under the same condi-
tions of excess phosphine that fully inhibited transmetalation to 9
(eq 9, Scheme 1). Qualitative line shape analysis verified that a
rapid equilibrium existed between complexes 9 and 12 at
ambient temperature. From the equilibrium data, Keq for the
ligand association reaction could be calculated in the starting
Interestingly, excess PPh3 (10 equiv) also inhibited the trans-
metalation of sp3-hybridized methylgold (eq 2). These data
supported the operation of a preceding ligand dissociation step
evenin theabsence of a π systemthat could coordinate to rhodium
and facilitate transmetalation. Inthiscase, liganddissociation could
be required to allow formation of a AuÀMe/Rh σ complex as an
intermediate prior to transmetalation or to allow formation of σ
complex character in the transition state, 5e (i.e., the σ analogue of
5c); the σ complex depicted in 5e also could involve metalÀmetal
bonding.26 The proposed AuÀMe/Ru σ interaction is supported
by the similar PtÀMe/Cu σ complex transmetalation intermedi-
ate characterized by Chen.1
This mechanistic insight permitted the successful linkage of the
CÀH activation chemistry of rhodium with organogold chemistry,
by determining the oxidation state and coordination environment at
rhodium that would generate the most successful transmetalation
partners. Suggs previously reported that the CÀH activation of
8-quinolinecarbaldehyde with Wilkinson’s catalyst produced the
rhodium(III) complex 8.27 On the basis of the coordination
environment studies described earlier in this paper, complex 8
was not anticipated to transmetalate with organogold 2a because it
lacked an open coordination site; Suggs demonstrated that the
triphenylphosphine ligands in 8 do not dissociate even at elevated
temperature.27 Indeed, no transmetalation reaction with 8 occurred
(eq 7, Scheme 1). Abstraction of chloride from 8 by AgSbF6
generated coordinatively unsaturated 9, which initiated transmetala-
tion reactivity to yield 10 and 11, the products of transmetalation
followed by bifurcation between CÀC and CÀH reductive elim-
ination pathways at rhodium (eq 8, Scheme 1). The ratio of 10 to 11
could be modified to favor CÀC bond formation by addition of a
small excess of PPh3 (10:11 = 60:40; 2 equiv of PPh3).28
presence of 1.0 equiv of 9 and 1.0 equiv of PPh3: Keq = 60 MÀ1
.
Information gained about the optimal oxidation state and
coordination environment at rhodium for transmetalation re-
vealed conditions to employ the catalytic conjugate addition
reactivity of rhodium(III)29 as a method to functionalize orga-
nogold compounds. The complex [Cp*RhCl2]2 (5 mol %)
catalyzed the conjugate addition of phenylgold to methyl pro-
piolate to give 14 (89% 1H NMR yield). The following mechan-
ism is proposed for this rhodium-catalyzed carboauration: (i)
transmetalation between 2e and 15 to give organorhodium
complex 16, (ii) migratory insertion of the alkyne,30 and (iii)
transmetalation between 17 and Ph3PAuCl to regenerate the
rhodium catalyst and form the functionalized organogold pro-
duct 14 (Scheme 2). In this catalytic reaction, dissociation of the
dimeric rhodium precatalyst provides the necessary open co-
ordination site required for the complex to participate in
transmetalation with organogold complex 2e.
In conclusion, a new transmetalation reaction between rho-
dium and organogold(I) complexes was identified. Understand-
ing of the optimal oxidation state, coordination environment,
and hybridization of the reacting partners provided fundamental
insight useful for reaction design, as highlighted by the develop-
ment of a rhodium-catalyzed conjugate addition reaction and
rhodium-mediated CÀH activation/CÀC bond forming reac-
tion. This study suggests a future extension of the rhodium/gold
dual reactivity that employs catalytic organogold intermediates
generated via organic substrate rearrangement instead of stoi-
chiometric organogold reagents.31 Given the unique transforma-
tions available to rhodium, the fundamental understanding of
this Au/Rh transmetalation reaction is anticipated to allow the
development of rhodium/gold dual catalytic reactions as
The transmetalation requirement for an open coordination
site in both rhodium(III) complex 1 and structurally distinct 9
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Scheme 2. Rh-Catalyzed Conjugate Addition Reaction Em-
ploying Au/Rh Transmetalation
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’ ASSOCIATED CONTENT
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Supporting Information. Text and figures giving experi-
b
mental procedures and compound characterization data and a
CIF file giving crystallographic data for 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: blums@uci.edu.
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’ ACKNOWLEDGMENT
We thank the NSF for funding (CAREER Award CHE-
0748312) and Dr. Joseph Ziller and Mr. Ryan Zarkesh for
assistance with X-ray crystallography. Y.S. is grateful for a
University of California Regents Dissertation Fellowship.
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