Transmetalation of alkyl ligands from Cp*(PMe3)IrR 1R2 to (cod)PtR3X

Article abstract of DOI:10.1021/om400289s

Transmetalation of alkyl and carboxylate ligands between Cp*(PMe ₃)Ir complexes and d⁸ Pt and Pd complexes is described. Studies on the scope and kinetics of this reaction support a mechanism in which carboxylate dissociation from the Pt or Pd center precedes alkyl transfer from Ir. Subsequent reaction of the resulting Pt or Pd alkyl complex enables functionalization of the hydrocarbyl ligands and suggests new opportunities for catalytic, nondirected C-C and C-H bond functionalization reactions.

Full text of DOI:10.1021/om400289s

Communication
pubs.acs.org/Organometallics
Transmetalation of Alkyl Ligands from Cp*(PMe3)IrR1R2 to (cod)PtR3X
Landon J. Durak and Jared C. Lewis*
Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: Transmetalation of alkyl and carboxylate ligands
between Cp*(PMe3)Ir complexes and d8 Pt and Pd complexes
is described. Studies on the scope and kinetics of this reaction
support a mechanism in which carboxylate dissociation from
the Pt or Pd center precedes alkyl transfer from Ir. Subsequent
reaction of the resulting Pt or Pd alkyl complex enables
functionalization of the hydrocarbyl ligands and suggests new opportunities for catalytic, nondirected C−C and C−H bond
functionalization reactions.
any transition metal complexes, [M1], activate C−H
bonds1 and C−C bonds2 in organic substrates to
This system is notable both for its relevance to the dual
catalytic cycle shown in Scheme 1 and for the fundamental
importance of transmetalation to a wide range of organo-
metallic processes,4 including cross-coupling reactions5 be-
tween main group organometallic reagents and organic halides.
Even in these ubiquitous reactions, precise mechanisms for
transmetalation are often unknown and remain the subject of
active investigation.6 The mechanistic picture is even more
complex for the transmetalation of ligands between two
transition metals. Several intra-7 and intermolecular8 ligand
transfer reactions in bimetallic systems have been described,
and transmetalation has been invoked in mechanisms for Pd/
Cu-catalyzed arylation of azoles,9 Pd-catalyzed arylation of
pyridine N-oxides,10 and Ru/Pd-catalyzed arylation of quinolyl-
8-aldehydes.11 The authors of the last two examples invoked
mechanisms analogous to that shown in Scheme 1; however,
both of these required a directing group for C−H activation,
and only the latter involved two different metals.
M
produce adducts containing a metal−carbon bond, [M1]−R.
However, functionalizing the resulting organic ligand, R, in a
manner that regenerates [M1] to enable catalytic C−H and C−
C bond functionalization remains challenging in the absence of
catalyst directing groups on the substrate.2,3 The same stability
that enables isolation of these [M1]−R complexes leads to high
reaction barriers for their subsequent turnover. We envision
that a second metal complex, [M2], might provide a unique
pathway for ligand functionalization in these [M1]−R
complexes and potentially enable catalytic C−H and C−C
bond functionalization via transmetalation (Scheme 1). We
Scheme 1. Dual Catalytic C−H Functionalization Involving
Transmetalation of an Organic Fragment (red sphere)
By focusing on [M1] complexes relevant to nondirected C−
C and C−H bond activation1,2 and [M2] complexes relevant to
C−C bond formation,5 we hope similar studies might not only
provide fundamental information on transmetalation processes
but ultimately enable dual catalytic functionalization of C−C
and C−H bonds (Scheme 1). This would mark a significant
departure from existing systems since such M1 complexes1,2
possess steric and electronic properties very different from
organometallic reagents typically used for transmetalation (e.g.,
organoboron or Grignard reagents)4,5 or directed C−H
activation.2,3
now report that Pt(II) and Pd(II) complexes can promote
transmetalation of ligands from Ir(III) hydrocarbyl complexes
and even functionalization of alkyl ligands on Ir(III) dialkyl
complexes (Scheme 2).
We initially investigated Cp*(PMe3)Ir (Cp* = pentam-
ethylcyclopentadiene) dihydrocarbyl, dihydrido, and hydrido-
hydrocarbyl complexes (1) as models for this process since the
Cp*(PMe3)Ir fragment is known to promote C−H1a and C−
Scheme 2. General Scheme for Ir/Pt Transmetalation
C
12 bond activation of hydrocarbon substrates (Scheme 2). We
identified Cp*(PMe3)IrBn2, 1a, as a convenient probe for alkyl
group (benzyl, Bn) transmetalation due to the unique 1H NMR
Received: April 5, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om400289s | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
resonances of the Bn ligands, the lack of β-hydrogen atoms, and
the ease of isolating potential benzyl-containing organic
products.
this fact and the observed increases in conversion with
increasingly labile Pt−X ligands (Table 1, entries 1−3), we
hypothesized that transmetalation could occur via ionization of
2c followed by irreversible ligand transfer from 1a to
intermediate 5 and subsequent association of TFA with Ir
intermediate 6 (Scheme 3). The rate law for this process,
assuming a steady-state concentration of 5, is shown in eq 1.
On the basis of extensive precedent for transmetalation to d8
square planar Pt complexes4 and the stability of Pt(II)-dialkyl
complexes,13 we explored the reactions of a series of
(cod)PtMeX complexes (cod = 1,5-cyclooctadiene), 2, with 1
at 75 °C in d6-benzene (Table 1). Initial experiments indicated
k1k2[2c][1a]
k−1[TFA] + k2[1a]
rate = kobs[2c] =
Table 1. Scope of Ir/Pt Transmetalation
(1)
a
a
entry
1
R1
Bn
R2
Bn
2
X
% 3
% 4
Scheme 3. Putative Mechanism for Reaction of 1 and 2
1
2
a
a
a
b
c
d
e
f
a
b
c
c
c
c
c
c
Cl
20
35
90
100
0
11
30
98
90
0
Bn
Bn
Bn
Me
p-Tol
Ph
OAc
TFA
TFA
TFA
TFA
TFA
TFA
b
3
Bn
4
5
Me
p-Tol
Bn
c
6
57
13
7
72
4
d
7
e
p-Tol
H
H
8
H
a
Reactions conducted in sealed NMR tubes in a pre-equilibrated oil
bath using 14.0 mM 1 and 2. Yields calculated relative to 1,3,5-
b
trimethoxybenzene internal standard. Optimal yields were obtained
The validity of our mechanistic hypothesis was explored by
kinetic analysis of the reaction between 1a and 2c under
pseudo-first-order conditions (>15 equiv of 1a). Clean first-
order consumption of 2c was observed over reaction times
spanning at least four half-lives, which confirmed the order in
2c expected from eq 1. The values for kobs measured for
reactions conducted in benzene, THF, and methylene chloride
increased with the increasing polarity of these solvents (Table
2), consistent with the proposed ionization process. Fur-
c
after 8 h reaction time. No evidence of Ph transfer was observed.
d
Resonances consistent with both H and p-Tol transfer were observed.
e
Reaction monitored at room temperature for 2 h prior to heating for
an additional 18 h.
that the chloride (X = Cl, 2a) and acetate (X = OAc, 2b)
complexes both underwent transmetalation and produced the
two expected transmetalation products (e.g., entry 1, 3aa and
4aa) in similar yields. The analogous trifluoroacetate complex
(X = TFA, 2c) led to markedly increased rates of trans-
metalation and provided 3ac and 4ac in 90% and 98% yields,
respectively. Exploration of more weakly coordinating ligands
(e.g., triflate) led to complex reaction mixtures.
Table 2. Effects of Solvent Polarity on Transmetalation
a
entry
solvent
benzene
ET(30)14
kobs (10−6 s−1
)
1
2
3
34
37
41
6.4
37
tetrahydrofuran
dichloromethane
Several additional Cp*(PMe3)Ir complexes were explored to
probe the effects of hydride and different hydrocarbyl ligands
on transmetalation. Methyl complex 1b underwent facile
reaction with 2c, indicating that the unique reactivity often
associated with benzylic intermediates was not essential. Diaryl
complexes were not reactive; however, mixed aryl alkyl
complexes underwent alkyl transmetalation, albeit with lower
yield than the dialkyl complexes. Interestingly, in the case of
hydridoaryl complex 1e, the overall conversion was low, but a
small degree of aryl transfer was observed. This is notable,
because such a hydridoaryl complex can be directly obtained via
arene C−H bond activation.1a An Ir-containing product,
Cp*(PMe3)Ir(p-Tol)TFA, consistent with hydride transfer,
was also observed, but formation of the expected Pt(II) hydride
complex could not be confirmed. Finally, rapid reaction
occurred between dihydrido complex 1f and 2c to provide an
uncharacterized mixture of products, including Cp*(PMe3)Ir-
(TFA)H, consistent with hydride transmetalation. These
findings, particularly the differences in the rates of hydride
transfers observed for 1e and 1f and of aryl transfers observed
for 1e and 1c, suggest that the nature of both R1 and R2 impacts
the transmetalation reaction.
60
a
Reactions conducted at 43 °C using 6.5 mM 2c. See Supporting
Information for plots of ln[2c] vs time.
thermore, a plot of kobs versus [1a]0 showed clear saturation
behavior (Figure 1). This result both confirms the complex rate
dependence on 1a expected from eq 1 and provides another
line of evidence supporting initial TFA dissociation, since this
equilibrium translates mathematically to a saturation curve.
To gain deeper insight into the nature of this transformation,
a detailed analysis of the reaction of 1a with 2c was performed.
No reaction was observed between the isolated transmetalation
products of these compounds at 75 °C for 24 h, which
indicated that the transmetalation process is irreversible. Given
Figure 1. kobs for reactions between 1a and 2c at various starting
concentrations of 1a. Reactions were conducted at 43 °C using 7.5
mM 2c. See Supporting Information for plots of [2c] and ln[2c] vs
time.
B
dx.doi.org/10.1021/om400289s | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Dissociation of TFA was also probed by examining the
effects of added (n-Bu)4NTFA on the rate of transmetalation
between 1a and 2c. Rearranging eq 1 affords an expression with
a linear relationship between 1/kobs and [TFA]/[1a] (eq 2),
which enables kinetic analysis of the reaction at a constant
concentration of 1a under pseudo-first-order conditions.15
Consistent with the mechanism outlined in Scheme 2, the
observed rate constant decreased as the concentration of TFA
was increased, and a plot of 1/kobs versus [TFA]/[1a] was
linear (Figure 2). Thus, three lines of evidence support the
resulting in a [Pd(benzyl)2] species that undergoes reductive
elimination.18 However, no intermediates were detected during
this reaction, and the Ir product could not be characterized
under the reaction conditions, so alternate pathways may be
operable.19
In conclusion, we have demonstrated that hydrocarbyl
ligands on model Ir(III) complexes relevant to nondirected
C−H and C−C activation of hydrocarbon substrates can be
transferred to Pt(II) complexes in high yield. Kinetic studies
support a mechanism in which carboxylate dissociation from
Pt(II) precedes irreversible alkyl ligand transfer from Ir(III).
The reported reaction proceeds smoothly on a variety of alkyl-
substituted Ir(III) complexes, but aryl-substituted complexes
were less reactive. In addition to undergoing an apparently
analogous transmetalation, a Pd(II) complex was able to
catalyze functionalization of the alkyl fragment. The reported
reaction validates a key step toward a dual catalytic system for
nondirected C−H and C−C bond functionalization, which is a
subject of ongoing studies in our group.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data for new
compounds are provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 2. Plot of 1/kobs vs [TFA]/[1a] at various starting
concentrations of (n-Bu)4N·TFA. Reactions were conducted at 42
°C using 7.1 mM 2c in C6D6.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jaredlewis@uchicago.edu.
■
initial dissociation step shown in Scheme 3. This mechanism
highlights the importance of generating an electrophilic Pt
cation intermediate to promote transmetalation and should
serve as an important guide for further experiments into the
scope and utility of this reaction.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by The University of Chicago Louis
Block Fund for Basic Research and Advanced Study.
■
⎛
⎞
⎟
⎛
⎜
⎞
⎟
k−1
1
kobs
1
k1
[TFA]
=
+
⎜
⎝
k k ⎝ [1a] ⎠
⎠
(2)
1
2
REFERENCES
■
During preliminary studies on transmetalation involving
Pd(II) analogues of 2, organic products consistent with
reductive elimination from Pd following transmetalation were
also observed, indicating that functionalization of organic
ligands formerly bound to iridium as outlined in Scheme 1 is
possible. To facilitate this process, 1a was heated with a
catalytic amount of Pd(OAc)2, benzoic acid, and Ag2O, which
promotes the exchange of acetate for benzoate ligands, thereby
increasing the electrophilicity of the Pd(II) species,16 and
oxidizes Pd(0) to Pd(II).17 To our delight, the organic product
1,2-diphenylethane was formed in 59% yield (Scheme 4).
Control experiments established that no conversion was
observed in the absence of Pd with all other reaction
components present, and all reaction components were
required to achieve high conversion. Bibenzyl formation is
consistent with two alkyl ligand transfers from Ir to Pd,
(1) (a) For representative examples, see: Bengali, A.; Arndtsen, B.;
Burger, P.; Schultz, R.; Weiller, B.; Kyle, K.; Moore, C.; Bergman, R.
G. Pure Appl. Chem. 1995, 67, 281−288. (b) Labinger, J. A.; Bercaw, J.
E. Top. Organomet. Chem. 2011, 35, 29−60. (c) Pamplin, C. B.;
Legzdins, P. Acc. Chem. Res. 2003, 36, 223−233. For a survey of
additional examples, see: (d) Shilov, A. E.; Shulpin, G. B. Chem. Rev.
1997, 97, 2879−2932.
(2) Jun, C.-H.; Park, J.-W. Top. Organomet. Chem. 2007, 24, 117−
143.
(3) (a) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147−
1169. (b) Arockiam, P. B.; Bruneau, C.; Dixneuf, P. H. Chem. Rev.
2012, 112, 5879−5918.
(4) (a) Partyka, D. V. Chem. Rev. 2011, 111, 1529−1595. (b) Cross,
R. J.; Gemmill, J. J. Chem. Soc., Dalton Trans. 1984, 2, 199−204.
(c) Cross, R. J.; Davidson, M. J. Chem. Soc., Dalton Trans. 1986, 9,
1987−1992.
(5) de Meijere, A.; Diederich, F., Eds. Metal-Catalyzed Cross-Coupling
Reactions; Wiley-VCH, 2004.
(6) (a) Zhao, P.; Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc.
2007, 129, 1876−1877. (b) Jin, L.; Xin, J.; Huang, Z.; He, J.; Lei, A. J.
Am. Chem. Soc. 2010, 132, 9607−9609. (c) Liu, Q.; Lan, Y.; Liu, J.; Li,
G.; Wu, Y.-D.; Lei, A. J. Am. Chem. Soc. 2009, 131, 10201−10210.
(d) Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 2116−
2119.
Scheme 4. Catalytic Functionalization of Hydrocarbyl
Ligands on 1a via Putative Pd(II) Dialkyl Intermediate
(7) Ritleng, V.; Chetcuti, M. J. Chem. Rev. 2007, 107, 797−858.
(8) (a) Hirner, J. J.; Shi, Y.; Blum, S. A. Acc. Chem. Res. 2011, 44,
603−613. (b) Osakada, K.; Yamamoto, T. Coord. Chem. Rev. 2000,
198, 379−399. (c) Komine, N.; Ishiwata, T.; Kasahara, J.; Matsumoto,
C
dx.doi.org/10.1021/om400289s | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
E.; Hirano, M.; Komiya, S. Can. J. Chem. 2009, 87, 176−182.
(d) Berenguer, J. R.; Bernechea, M.; Lalinde, E. Organometallics 2007,
26, 1161−1172. (e) Cao, D. H.; Stang, P. J.; Arif, A. M.
Organometallics 1995, 14, 2733−2740.
(9) (a) Fan, S.; Chen, F.; Zhang, X. Angew. Chem., Int. Ed. 2011, 50,
5918−5923. (b) Huang, J.; Chan, J.; Chen, Y.; Borths, C. J.; Baucom,
K. D.; Larsen, R. D.; Faul, M. M. J. Am. Chem. Soc. 2010, 132, 3674−
3675. It should be noted that copper has long been used to promote
direct arylation of relatively acid C−H bonds; for example: (c) Pivsa-
Art, S.; Satoh, T.; Kawamura, Y.; Miura, M.; Nomura, M. Bull. Chem.
Soc. Jpn. 1998, 71, 467−473.
(10) Tan, Y.; Barrios-Landeros, F.; Hartwig, J. F. J. Am. Chem. Soc.
2012, 134, 3683−3686.
(11) Ko, S.; Kang, B.; Chang, S. Angew. Chem., Int. Ed. 2005, 44,
455−457.
(12) Anstey, M. R.; Yung, C. M.; Du, J.; Bergman, R. G. J. Am. Chem.
Soc. 2007, 129, 776−777.
(13) Low, J. J.; Goddard, W. A., III. J. Am. Chem. Soc. 1984, 106,
6928−6937.
(14) Reichardt, C. Chem. Rev. 1994, 94, 2319−2358.
(15) Sweeney, Z. K.; Polse, J. L.; Andersen, R. A.; Bergman, R. G. J.
Am. Chem. Soc. 1998, 120, 7825−7834.
(16) Lebrasseur, N.; Larrosa, I. J. Am. Chem. Soc. 2008, 130, 2926−
2927.
(17) Liang, Z.; Yao, B.; Zhang, Y. Org. Lett. 2010, 12, 3185−3187.
(18) (a) Konze, W. V.; Scott, B. L.; Kubas, G. J. J. Am. Chem. Soc.
2002, 124, 12550−12556. (b) Ackerman, L. J.; Sadighi, J. P.; Kurtz, D.
M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2003, 22, 3884−3890.
(19) For example, one reviewer pointed out that oxidant-induced
reductive elimination could explain this reactivity. It should be noted,
however, that reductive elimination of R1−R2 from Cp*(PMe3)IrR1R2
complexes is not favorable and typically requires very strong oxidants,
such as I2. See, for example: Tjaden, E. B.; Stryker, J. M. J. Am. Chem.
Soc. 1990, 112, 6420−6422. Why Pd(II) would be required for such
an oxidative pathway is also not clear.
D
dx.doi.org/10.1021/om400289s | Organometallics XXXX, XXX, XXX−XXX