[RhIIIcp*]-catalyzed dehydrogenative aryl-aryl bond formation

Article abstract of DOI:10.1002/anie.201107842

Directed, undirected! Rhodium(III)-catalyzed double CH bond activation (one directed, one undirected) provides an efficient route to biaryls (see scheme; DG=directing group). Significant kinetic isotope effects for both reaction partners and H/D scrambling between them are interesting experimental findings. While the mechanism is still unclear, a rhodium(V) species is invoked in the catalytic cycle. Copyright

Full text of DOI:10.1002/anie.201107842

Angewandte
Chemie
DOI: 10.1002/anie.201107842
À
Double C H Activation
III
À
[Rh Cp*]-Catalyzed Dehydrogenative Aryl Aryl Bond Formation**
Joanna Wencel-Delord, Corinna Nimphius, Frederic W. Patureau, and Frank Glorius*
The omnipresence of biaryl scaffolds in natural products,
medicinal agents, and organic materials places their con-
struction amongst the key transformations of organic chemis-
tion reactions, cationic RhIII catalysts have recently been
shown to be particularly suitable for the activation of this
latent bond,[3] mostly resulting in oxidative Heck reactions,[4]
alkynylations,[5] and nucleophilic addition type transforma-
tions.[6] Many new synthetic disconnections for the synthesis
of many highly valuable compounds (such as derivatives of
indenoles[5j] and indoles,[5g] fulvenes,[5j] pyrroles,[5i,g] isoqui-
nolines,[5e] acenes,[5d] and anthrylazoles[5a]) could be estab-
lished. However, to the best of our knowledge, no RhIII-
À
try. Indeed, cross-coupling reactions leading to this C C bond
formation have seen tremendous development over the past
decades. However, in the context of green and sustainable
À
chemistry, construction of these biaryl moieties by using C H
À
bonds as latent functional groups through twofold direct C H
bond functionalization has emerged as an attractive alter-
native.[1] Indeed, catalyzed cross-dehydrogenative couplings
(CDC) obviate the need for prefunctionalized coupling
partners (minimizing reaction steps and waste) and are only
accompanied by the formal formation of hydrogen as the sole
byproduct. However, multiple obstacles such as unfavorable
À
catalyzed dehydrogenative Ar Ar bond formation proceed-
[7]
À
ing through C H bond activation has been developed.
Inspired by our very recent observation that [RhCp*(SbF6)2]
(Cp* = pentamethylcyclopentadienyl) is capable of perform-
ing undirected C H activation of bromoarenes,[8] we focused
À
À
thermodynamics, the generally low reactivity of C H bonds,
our effort on the use of this reactivity for biaryl formation.
À
À
and selectivity issues (functionalization of one C H bond in
Herein, we report the first example of Rh-catalyzed Ar Ar
À
the presence of others, competition between heterocoupling
and homocoupling) make this synthetic pathway particularly
challenging. Over the last few years important efforts have
been made to overcome these difficulties, and several Pd-
catalyzed dehydrogenative biaryl formations have been
reported.[2] Very recently, a seminal contribution in this field
was made by Yu et al.[2k] By using Pd(OAc)2 as a catalyst in
combination with a “F+” oxidant, they succeeded in a para-
cross-coupling by means of double C H activation
(Scheme 1).
We commenced our study using tertiary benzamide 1a
and bromobenzene as coupling partners, [(RhCp*Cl2)2]
combined with AgSbF6 as the catalyst, Cu(OAc)2, and
PivOH. We noticed with delight that the desired product 3a
was formed and could be isolated in 23% yield as a 2.6:1
mixture of meta/para regioisomers (Scheme 2). Interestingly,
no coupling at the ortho position of bromobenzene was
observed. The efficiency of this reaction was, however,
compromised by a significant amount of homocoupling of
1a.[9] Intriguingly, the employment of a catalytic amount of
CsOPiv (20 mol%) together with 1.1 equiv of PivOH pre-
À
selective CDC, consisting of C H bond activation of benz-
amides followed by an electrophilic aromatic palladation of
simple arenes.
However, alternative methods and mechanisms are
required, since an electrophilic aromatic metalation step
limits the substrate scope (electron-rich substrates preferred)
and selectivity pattern (ortho/para-directing groups). Besides
À
the development of Pd-catalyzed C H bond-functionaliza-
[*] Dr. J. Wencel-Delord, C. Nimphius, Prof. Dr. F. Glorius
Universitꢀt Mꢁnster, Organisch-Chemisches Institut
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.oc/glorius/
index.html
Prof. Dr. F. W. Patureau
Fachbereich Chemie – Organische Chemie, Technische Universitꢀt
Kaiserslautern, Erwin-Schrçdinger-Strasse, Geb. 52, 67663 Kaisers-
lautern (Germany)
[**] We thank Nadine Kuhl for discussions, Dr. Klaus Bergander for
special NMR experiments, and Dr. Heinrich Luftmann for help with
MS analyses. This work was supported by the European Research
Council under the auspices of the European Community’s Seventh
Framework Program (FP7 2007-2013)/ERC Grant agreement no.
25936 and the Alexander von Humboldt Foundation (F.P.). The
research of F.G. has been supported by the Alfried Krupp Prize for
Young University Teachers of the Alfried Krupp von Bohlen und
Halbach Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107842.
Scheme 1.
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Scheme 2. Initial study of the CDC of benzamides and PhBr.
vented the undesired homocoupling and thus improved the
yield of 3a to 55%. Replacement of PivOH by a stoichio-
metric amount of either CsOPiv or CsOAc shut down the
reactivity. Further improvement was achieved by a slight
modification of the directing group. When benzamide 1b
bearing a NiPr2 group was used, the cross-coupling product 3b
could be isolated as a meta/para mixture (2.8:1) in 81% yield.
Control reactions showed that omission of either the Rh
precatalyst or Cu(OAc)2 led to total inactivity of this catalytic
system. Similarly, the use of a catalytic amount of Cu(OAc)2
(25 mol%) or increased concentration (0.4m and 1.0m)
resulted in a severe decrease of the efficiency of the coupling
(3b isolated in 22, 69 and 49% yield, respectively).
Under these optimized conditions, the scope of arenes
bearing no chelating group site was examined (Scheme 3, 3b–
h). Both chloro- and iodobenzene provided the cross-coupled
products 3c and 3d in good yields (76 and 66%, respectively),
albeit an increased reaction temperature (1608C) was
required to reach full conversion. Intriguingly, utilization of
fluorobenzene as the coupling partner shut down the reac-
tivity and coupling of toluene with 1b afforded 3 f in
drastically decreased yield (16%).[10] When 1-bromonaph-
thalene and 1,2-bromotoluene were engaged in the reaction,
the expected products 3g and 3h were formed efficiently as
mixtures of three and two regioisomers, respectively. Fur-
thermore, an array of synthetically useful benzamides were
reacted with bromobenzene (Scheme 3, 3i–o). A number of
functional groups at either the meta or para position of the
aryl ring were compatible with our catalytic system.[11]
Benzamides bearing electron-donating groups such as Me
or OMe in meta position were arylated smoothly to give the
desired products 3i and 3j, isolated in 82 and 72% yield,
respectively. Introduction of the OMe group in para position
of the benzamide had a beneficial effect on the efficiency of
the reaction giving corresponding 3k with an improved yield
(89%). In contrast, benzamides containing electron-with-
drawing substituents reacted more sluggishly and an increase
of the reaction temperature to 1608C was required for
completion of these CDCs. Importantly, brominated benza-
mide could also be efficiently used as coupling partner to give
the dibrominated biphenyl product 3n in 69% of yield. When
Scheme 3. CDC of benzamides with monosubstituted arenes. General
reaction conditions: 1 (1.0 mmol, 1.0 equiv), PhBr (5 mL),
[(RhCp*Cl2)2] (2.5 mol%), AgSbF6 (10 mol%), Cu(OAc)2 (2.2 mmol,
2.2 equiv), PivOH (1.1 mmol, 1.1 equiv), CsOPiv (0.2 mmol, 20
mol%), 140–1608C, 21 h. [a] At 1608C. [b] At 1408C.
1608C). Subsequently, numerous different 1,3-disubstituted
arenes turned out to be efficient coupling partners. Intrigu-
ingly, while 1-bromo-3-methylbenzene gave good to moder-
ate yields, slight improvement of the efficiency of these CDCs
was observed when electron-deficient bromoarenes were
used. Thus coupling of a meta-substituted electron-rich
benzamide with electron-poor 1-bromo-3-trifluoromethyl-
benzene led to the formation of biphenyl 4i in 84% yield.
1-Bromo-3-methoxybenzene showed drastically lower reac-
tivity (4e isolated in 23% yield). In addition, regioselective
cross-coupling was also successful using 1,2-dibromobenzene.
To additionally establish the power of this methodology,
a ketone directing group was also evaluated (Scheme 5). The
initial attempt to couple 2,2-dimethyl-1-phenylpropan-1-one
(5) and bromobenzene under standard reaction conditions
gave the desired product 6a, but the reaction was significantly
more sluggish. Higher reaction temperatures led to full
conversion and 6a could be isolated in 72% yield as a 2.1:1
mixture of meta/para regioisomers. As expected, when 1,2-
À
naphthalene 2-carboxamide was used, C H activation oc-
curred at only one position leading to 3o in good yield.
Encouraged by these results showing that steric hindrance
disfavors cross-coupling in the ortho position of bromoben-
zene, we turned our attention to 3-substituted bromoben-
zenes (Scheme 4). Gratifyingly, coupling of the standard
benzamide 1b with 1-bromo-3-methylbenzene gave the cross-
coupling product 4a as a single regioisomer (67% yield at
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Scheme 6. Study of the kinetic isotope effect.
coupling between a 1:1 mixture of 1b and [D5]-1b and either
bromobenzene (experiment B) or [D5]bromobenzene (exper-
iment D).[13]
À
These results clearly suggest that true C H bond activa-
tion occurs on both coupling partners.[14] The important KIEs
observed for experiments A and C, combined with data from
À
previous reports on CDCs indicate that the C H bond
cleavage of the simple arene is of substantial importance for
the total rate of this cross-coupling. Additionally to these
KIEs studies, a noticeable H/D scrambling on both reaction
partners was observed. Likewise, H/D exchange was observed
when 3b was submitted to the reaction conditions in the
presence of pentadeuterated bromobenzene (solvent). These
Scheme 4. Highly regioselective CDC of benzamides using 1,3-halo-
arenes. For general reaction conditions, see Scheme 2. [a] On
a 3 mmol scale. [b] At 1408C.
À
observations clearly indicate the reversible nature of both C
H activation events.
On the basis of the data above and precedent literature,
a plausible catalytic cycle is proposed in Scheme 7. It is
postulated that, in the presence of AgSbF6, a Rh precatalyst
generates a highly electrophilic Rh species A, which coor-
dinates to the carbonyl group of the benzamide. A chelate-
À
assisted reversible C H activation occurs at the ortho position
and could proceed by means of a base-assisted concerted
metalation/deprotonation (CMD) pathway.[15] The five-mem-
bered rhodacycle B has a free coordination site and could
therefore coordinate bromobenzene in a h2 manner. At this
stage of the catalytic cycle, two distinct pathways are
plausible. In the first case, pathway a, the h2-coordinated
Scheme 5. CDC of ketone 5.
dibromobenzene was employed as a coupling partner, the
corresponding biphenyl ketone 6b was isolated in moderate
yield as a single regioisomer. A slight decrease of the yield
(47%) was observed in the case of 1-bromo-3-methylbenzene.
For obtaining mechanistic insight, and determining the
values of the kinetic isotope effects (KIEs) for both benz-
amide and bromobenzene, a series of cross-coupling reactions
involving deuterated coupling partners were performed
(Scheme 6). An intermolecular competition experiment
between bromobenzene and [D5]bromobenzene, with sub-
strate 1b as the reaction partner, showed a significant KIE of
3.5 (experiment A). Likewise, when the deuterated benz-
amide [D5]-1b was submitted to the same reaction conditions,
a similar KIE of 3.4 was measured (experiment C).[12]
Furthermore, KIEs of 1.8–2.0 on the benzamide were
determined by comparison of the initial reaction rate of the
À
bromobenzene would undergo C H activation (probably by
s-bond metathesis or CMD mechanism) affording the neutral
rhodium complex D. After sequential reductive elimination
of the cross-coupling product and reoxidation of the RhI
intermediate E, the regenerated RhIII catalyst A could re-
enter the catalytic cycle. In contrast, pathway b starts with
À
a C H activation of bromobenzene affording the rhodium(V)
hydride complex F. This high oxidation state of rhodium
would enhance the reductive elimination of the biphenyl
adduct,[16] and a subsequent hydride abstraction/oxidation
would provide the regenerated catalyst A. The experimental
observations of important H/D scrambling on both coupling
partners and the biaryl product could be explained either by
À
the existence of Rh H intermediates (which support path-
way b) or by the protonolysis of intermediate D. The
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Scheme 8. Potential synthetic applications of halogen-substituted
biaryls. For reaction conditions, see the Supporting Information.
opens opportunities for the development of new RhIII-
À
catalyzed C H bond activations, leading to more efficient
and milder transformations .[19]
Scheme 7. Postulated catalytic cycle. (Not all ligands on Rh are
depicted.)
Received: November 7, 2011
Published online: January 27, 2012
existence of a rhodium(V) hydride species bearing a Cp*
ligand has already been evoked;[17,18] however, no direct proof
of its existence in this case could be obtained so far.
Consequently, it is difficult to draw a conclusion concerning
the mechanism of this transformation. Moreover, the role of
Cu(OAc)2 and the particular effectiveness of bromoarenes as
coupling partners (which could potentially act as an oxi-
dant[7,8] and/or a catalyst modifier[8]) remain unclear.
Finally, in order to highlight the synthetic value of this new
strategy, we investigated possible transformations of the
generated biaryl products (Scheme 8). Gratifyingly, product
4h underwent smooth bromine/lithium exchange, and sub-
sequent quenching of the reaction with water led to the
formation of the one meta-Br substituted biaryl product 7. An
additional advantage of this transformation is the fact that
many different electrophiles can be introduced. We were
pleased to see that under unoptimized conditions the Suzuki
coupling of 4b with 4-methoxyphenylboronic acid gave the
desired product 8 in 51% yield. This new triphenyl moiety
still bears a chloro substituent, allowing for further function-
alization.
In conclusion, a novel RhCp*-catalyzed aryl–aryl bond
formation has been disclosed, a selective dehydrogenative
cross-coupling between arenes bearing a directing group and
halogen-substituted (I, Br, Cl) benzene derivatives. The scope
of this transformation is broad with regard to both coupling
partners and the employment of 1,3-disubstituted bromo-
arenes leads to the regioselective formation of valuable meta-
substituted biphenyl products with moderate to good yields.
Finally, mechanistic studies indicate that this catalytic system,
in contrast to other Pd-catalyzed CDCs, proceeds by “real”
À
Keywords: aryl–aryl bond formation · biaryls · C H activation ·
cross-dehydrogenative coupling · rhodium catalysis
.
À
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À
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À
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[9] High-resolution ESI mass spectrometric analysis of the crude
reaction mixture shows also the presence of dehalogenated
biaryl product. During the whole project, the formation of trace
amounts of these dehalogenated products was observed.
[10] Application of benzene as a coupling partner led to the
formation of only trace amounts of the desired coupling product
(reaction run at 1408C).
[11] Introduction of a methyl substituent at the ortho position of the
benzamide led to inhibition of the reaction and the desired
product was not obtained.
À
[12] A comparable KIE for the undirected Pd-catalyzed C H
activation of simple arenes was previously observed by Dong
et al.[2i]
[13] a) Recently, Bergman and Ellman have reported a KIE of 2 for
[6b]
the RhIII-catalyzed C H bond activation of acetanilides.
À
[5] For selected examples of RhIII-catalyzed alkynylation reactions
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b) Likewise, for the alkynylation of an acetanilide a KIE of 4.2
À
was measured for the C H activation step; see: D. R. Stuart, P.
Alsabeh, M. Kuhn, K. Fagnou, J. Am. Chem. Soc. 2010, 132,
À
18326 – 18339. Chang et al. measured a KIE of 2.3 for the C H
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À
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À
[16] In the case of Pd-catalyzed C H activation reactions, high-
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[7] In 1993 Barrett et al. reported the homocoupling of phenols in
the presence of a RhIII catalyst; however, this reaction probably
À
À
does not occur by means of C H bond activation: A. G. M.
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2234.
[19] For a recent review on C H activations under mild conditions,
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