Switching of Reaction Pathway from C?C Rollover to C?N Ring-Extension Annulation

Article abstract of DOI:10.1002/chem.201703687

This work discloses that a simple change in the anion of a copper(II) reagent along with the reaction solvent can dramatically alter the course of a Cp*Rh^III-catalyzed C?H activation–annulation reaction leading to completely switchable chemoselective products. The nature of the anion in terms of its coordinating ability and basicity, and also the polarity of the solvent have been found to be the crucial factors in the observed divergence.

Full text of DOI:10.1002/chem.201703687

A Journal of
Accepted Article
Title: Switching of Reaction Pathway from C-C Rollover to C-N Ring-
Extension Annulation
Authors: Joyanta Choudhury, Ranjeesh Thenarukandiyil, and
Champak Dutta
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Switching of Reaction Pathway from C–C Rollover to C–N Ring-
Extension Annulation
Ranjeesh Thenarukandiyil,[a] Champak Dutta,[a] and Joyanta Choudhury*[a]
Abstract: This work discloses that a simple change in anion of a
copper(II) reagent along with the reaction solvent can dramatically
alter the course of a Cp*Rh(III)-catalyzed C–H activation-annulation
reaction leading to completely switchable chemoselective products.
The nature of the anion in terms of its coordinating ability and
basicity, and also the polarity of the solvent have been found to be
the crucial factor in the observed divergence.
Introduction
Advancement of fundamental knowledge and basic concepts in
chemistry has had
a tremendous impact on modern-day
catalysis. It has benefited to widen the spectrum of catalyst
development from simple and highly efficient to all sorts of
complex and smart catalysts, thereby addressing the issues of
sustainability and diversity. It is an (unrealistic) dream of any
catalysis researcher to develop one single catalyst performing a
large number of different synthetic reactions with an equal great
degree of efficiency. Similarly, to achieve different chemo-/regio-
/enantio-divergent products from a common substrate with high
selectivity control by using same catalyst is an ambitious and
formidable task. To tackle the intricacies present in transition-
metal-based this type of diversity-oriented catalysis, a) a good
understanding of bond-breaking/bond-making mechanistic sce-
narios, and b) ability to control the property and reactivity of
organometallic intermediate(s) are required. In chemodivergent
processes, as expected, strategies involving tuning the nature of
ligand and/or metal, design of substrate, or use of different
solvents and reagents have been applied to perturb the
chemoselectivity-controlling step(s), and thus alter the reaction
outcome (Figure 1a).[1] At this juncture, we paid attention to
Cp*Rh(III)-catalyzed C–H activation-functionalization reactions
which are prevalent and versatile in modern synthetic chemistry,
and considered as step-economic, straightforward, and highly
desirable transformations.[2] Most of these methodologies often
use stoichiometric amount of a copper(II) compound (CuX2) as
oxidant. Interestingly, CuX2 is known to participate in anion-
exchange processes with the catalyst [Cp*RhCl2]2, generating
‘Cp*RhX2’ type species under the reaction conditions. We
hypothesized that the coordinating ability and basicity of X can
affect the nature (electrophilicity and Lewis acidity of the metal
center) as well as reactivity of the subsequent organo-Rh
intermediate(s), and C–H activation step of the reaction.
Therefore, a judicious change in X can cause a mechanistic
dichotomy and provide the opportunity to explore a novel way to
Figure 1. Controlling chemodivergent processes: a) different tools; b) anion as
a tool; c) hypothesis of this work based on the nature of the anion.
accomplish switchable chemoselective C–H functionalization
protocol under tuned reaction conditions.
The proof-of-concept of this hypothesis has been described
herein by choosing C–H activation-functionalization of pyridine-
appended imidazole substrates to the fused imidazopyridine
derivatives. Imidazolium motif containing molecules are known
to be of significant interest in diverse research areas, but there is
a lack of functionalization protocols of these molecules. On the
other hand, the fused imidazopyridines represent an important
class of compounds known as pharmaceutically active and
potential drugs (Figure 1b, and Supporting Information).[3]
Additionally, such types of N-doped extended π-conjugated
[a]
R. Thenarukandiyil, C. Dutta, Dr. J. Choudhury
Organometallics & Smart Materials Laboratory
Department of Chemistry
Indian Institute of Science Education and Research Bhopal
Bhopal 462 066, India
E-mail: joyanta@iiserb.ac.in
Supporting information for this article is given via a link at the end of
the document.
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polycyclic aromatic hydrocarbons are considered as potential
electronic materials as well.[4] After the initial pyridine-directed
imidazole C(2) –H activation by [Cp*RhCl2]2, the resulting five-
membered cyclometalated intermediate (Int, Figure 1b) might be
prone to chloride dissociation-exchange with available anion (X).
When X = OAc, its coordinating and fairly basic character[5] may
trigger the intermediate to undergo a ‘rollover’ C–H activation[6]
of the coordinated pyridine backbone via internal base-assisted
metalation (BAM)/ concerted metalation-deprotonation (CMD)
pathway. Subsequent insertion of an alkyne into Rh–Cpyridine or
Rh–Cimidazole followed by reductive elimination may produce C–C
annulated product (Figure 1b,c). On the contrary, if X = BF4 or
CF3SO3, due to their feebly basic character,[5] a base-assisted
similar rollover C–H activation at the Rh center may not be
facilitated any more. Rather, their non-/less-coordinating
nature[7] may enhance the electrophilicity of Rh through the
formation of cationic complex. This, in turn, would increase the
electrophilic character of coordinated alkyne to trigger insertion
processes prior to a C–N reductive elimination step providing the
C–N annulated product (Figure 1b,c). Because of the cationic
nature of the intermediate, the C–N reductive elimination would
also be favored. Notably, this type of anion-dependent divergent
outcomes in transition-metal catalysis are severely limited to
only a few gold-catalyzed reactions.[8] Comparatively, in a
broader sense, anion-guided chemistry was previously explored
in the field of crystals and materials, host-guest chemistry, self-
assembled systems such as cages, macrocycles, metal-organic
frameworks etc.[9]
Scheme 1. Examples of C–C annulation. 1 (0.1 mmol), 2 (0.2 mmol),
Cu(OAc)2.H2O (0.12 mmol), [Cp*RhCl2]2 (5 mol%), toluene (1 mL). [a]<10% of
the other regioisomer was observed.
Next, when the same substrates 1a and 2b were treated by
employing 2.0 equiv. of Cu(BF4)2 instead of Cu(OAc)2 in a more
polar solvent MeOH (ε 32.7) at 100 °C in the presence of 5
mol% catalyst, it did not result any pyridyl C–H activation
product, but it furnished a new product in <10% yield (Scheme
2). 1H NMR analysis of this new product indicated that it
consisted of two sets of protons from the alkyne moiety and all
the aromatic protons from the pyridyl benzimidazole backbone
except the imidazole-NCHN proton. The same product was
formed in 92% yield when an elevated temperature of 140 °C
was used under the above conditions (Scheme 2). The product
was later confirmed as double alkyne-inserted imidazole C–N
annulated compound 4b by 13C{1H} NMR and ESI-HRMS
analysis (Supporting Information for details). Later the loading of
the catalyst (3 mol%) and Cu(BF4)2 (1.5 equiv.) was optimized
(Scheme 2). During the screening of the reaction conditions, it
was confirmed that an expected mono alkyne-inserted
Results and Discussion
Keeping the above-mentioned hypothesis in mind, the
prospective divergent catalytic reactions were explored under
two distinct reaction conditions (e.g., polarity of the solvents
used) to deal with the plausible neutral or cationic rhodacyclic
intermediates. Thus, the rollover C–C annulation catalysis was
achieved via the reaction of various N-pyridylbenzimidazoles (1)
with different internal alkynes (2) using 1.2 equiv. of Cu(OAc)2
and catalytic [Cp*RhCl2]2 in non-polar solvent toluene (ε 2.38),
as shown in Scheme 1. N-pyridylbenzimidazole 1a was
successfully transformed to the C–C coupled products not only
with diarylalkynes but also with dialkylalkynes in good yields (3a-
3c, 63-80%). It is to note that only one specific example of the
reaction of 1a with diphenylacetylene producing 3a was reported
earlier[6a-b]. However, in this work, this protocol has not only been
generalized for the N-pyridylimidazole class of substrates, but a
mechanistic rationale and viewpoint has also been provided.
Introduction of substituents at the ortho or para position of the
pyridyl moiety did not alter the efficiency of the reaction (3d-3f,
84-60%). Similar to the pyridine, quinoline substitution also
effectively furnished the rollover C–C annulated product in good
yield (3g, 82%). Importantly the reactions with unsymmetrical
alkyne provided single regioisomers as major products with
better yields (3h-3i, 80-87%). The imidazole version of 1a also
gave the product but in lesser yield (3j, 40%).
Scheme 2. Reaction conditions for C-N annulation.
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pyridine-N annulated compound 4b’ was not formed even when
1 equiv. of alkyne was used, and only 4b was obtained as the
sole product (Scheme 2). It is important to mention that all non-
coordinating anion containing oxidizing agents such as Cu(BF4)2,
Cu(OTf)2, and AgBF4 successfully produced C–Nimidazole
annulated product (see Supporting Information).
Next, we examined the reactions with different N-pyridyl
benzimidazoles and various internal alkynes (Scheme 3).
Diarylalkynes as well as dialkylalkynes furnished the doubly
inserted C–N coupled cationic products in high yields (4a-4c, 75-
84%). Ortho and para substitution on the pyridine ring did not
affect the reaction much and provided excellent yields (4d-4h,
85-91%). The quinoline substituted benzimidazole also led to the
similar product formation in higher yield (4i, 82%). The
transformation of 2-(1H-imidazol-1-yl)pyridine to the C–N
coupled product in good yield (4j, 70%) under this condition
widened the substrate scope. The fact that the formation of
exclusively C–C product in ~43% yield with Cu(OAc)2 and C–N
product in ~77% yield with Cu(BF4)2 under identical reaction
conditions in a common and moderately polar solvent 1,2-
dichloroethane (ε 10.36) emphasized the major role of anion and
a less-pronounced role of solvent in guiding the selectivity in the
present chemistry (see Supporting Information). Notably, no C–
Nimidazole coupled product formed with N-phenylimidazole as
substrate, which emphasized that probably the initial ortho
pyridine coordination was essential for the catalysis to proceed.
Similarly, N-pyridylindole did not provide any C–Npyridine coupled
product under Cu(BF4)2 conditions, suggesting that the
monoannulation reaction from a putative pyridine-coordinated
intermediate was not facile under the employed reaction
conditions (see Supporting Information).
Scheme 4. Mechanistic investigation.
Mechanistic investigation was performed for this novel divergent
chemistry. five-membered cyclometalated rho-dium(III)
A
intermediate 5 was isolated in good yield (64%) from the
reaction of the substrate 1a (0.3 mmol) and [Cp*RhCl2]2 (0.15
mmol) in the presence of NaOAc (2.4 mmol) as base in
ClCH2CH2Cl (Scheme 4a). Full characterization of the complex 5
1
was accomplished by H and 13C{1H} NMR spectroscopic, and
mass spectrometric (ESI-MS) methods. The structure of 5 was
confirmed by single crystal X-ray diffraction studies.[10] It is
important to mention that there was no indication of the
presence of N–H proton in the 1H NMR spectrum, which
eliminated the possibility of the formation of a protic NHC
complex[11] under this condition. This was further confirmed by
comparing the <N-C-N bond angle value of 110.8(3)° in 5 with a
known Cp*Rh-complex (109.4(2)° for imidazolyl-Rh versus
104.4(2)° for NHC-Rh).[12]. The intermediacy of 5 for the two
different modes of reactivity was confirmed via performing two
separate control experiments by treating 5 with alkyne 2b in
presence of Cu(OAc)2 and Cu(BF4)2 which provided high yields
of 3b (91%) and 4b (85%) respectively (Scheme 4b).
Significantly, the seven-membered double alkyne-inserted
metallacyclic intermediate 6 for this new C–N annulation
reaction could also be isolated from a reaction with complex 5
and dimethylacetylene dicarboxylate (DMAD) as alkyne in the
absence of Cu(BF4)2 (Scheme 4c). The presence of the Cl
ligand was probably found to be essential to stabilize the
complex 6. Full characterization of 6 was accomplished by
Scheme 3. Examples of C–N annulation. 1 (0.1 mmol), 2 (0.25 mmol),
Cu(BF4)2.6H2O (0.15 mmol), [Cp*RhCl2]2 (3 mol%), MeOH (1 mL).
spectroscopic and single-crystal X-ray diffraction analysis[10]
.
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10.1002/chem.201703687
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Scheme 5. Proposed catalytic cycles.
However, complex 6 was proved extremely reluctant to further
reductive elimination reaction, indicating that DMAD is
dramatically different from normal alkynes in electronic nature.
Plausible catalytic cycles for both the transformations were
proposed based on our hypothesis, the mechanistic information
gained through the control experiments, and previous literature
reports[2] (Scheme 5). After the generation of a cyclometalated
intermediate 5 from 1 with the rhodium center, the next step is
the chloride dissociation-exchange with the X of CuX2 and the
intermediate formation is highly dependent on the nature of X. In
the presence of Cu(OAc)2 (X = OAc), the reaction follows
pathway-A. Being a reasonably basic and coordinating ligand,
OAc can first coordinate to form IA, and then direct the
intermediate to undergo a ‘rollover’ C–H activation[6] leading to
the formation of a new cyclometalated intermediate IIA. Then
alkyne insertion to IIA generates IIIA which upon reductive
elimination produces the C–C annulated product 3. Re-
oxidationof the [Cp*RhI] species triggers the catalytic cycle to
continue. On the other hand, in presence of Cu(BF4)2, (X= BF4),
the reaction proceeds through pathway-B. The less basic and
weakly/non-coordinating nature of BF4 does not favor the
rollover C–H activation at the Rh center. Rather it prefers to form
a cationic complex from which a facile formation of alkyne
coordinated intermediate IB is expected. From here, the
insertion of alkyne into Rh–C can provide the seven-membered
rhodacycle intermediate IIB. However, we never observed a
possible C–Npyridine reductive eliminated product from IIB, even
with one equivalent of alkyne. So it is logical to think that either
this intermediate does not form or it is transformed immediately
into more stable five-membered metallacycle intermediate IIIB.
Indeed, the latter is the case as DFT calculation suggested that
IIIB is more stable than IIB by ~ 18.6 kcal (see Supporting
Information for details). The second alkyne insertion to IIIB can
the C–Nimidazole coupled product 4 and Cp*RhI species for further
regeneration and continuation of the catalytic cycle. It is to note
that, even if the BF4 anion is not directly coordinated to the
catalytic metal center, it tunes the electrophilicity and Lewis
acidity of the metal center to control reactivity throughout the
catalytic cycle.
Conclusions
In summary, a controlled mechanistic dichotomy led to switch
the pathway of a Cp*Rh(III)-catalyzed C–H functionalization
reaction. With the basic and coordinating OAc anion, a rollover
C–H activation pathway was favored resulting C-C annulation
products in non-polar solvent. In contrast, feebly basic and
less/non-coordinating BF4 anion triggered double alkyne
insertion instead of C–H activation leading to C–N annulated
cationic products in polar solvent. The key to the success of this
switchable chemodivergent protocol is the control over the
nature and reactivity of the organometallic rhodacyclic
intermediates based on anion and solvent polarity. Not only the
above novel aspect of reaction control, but the presented
chemistry of easily switchable selectivity in Rh(III)-catalyzed C–
H activation reaction allows also to access neutral and ionic π-
conjugated organic materials.
Experimental Section
General procedure for catalytic C–C rollover annulation
In an oven-dried sealed tube, 1 (0.1 mmol), [RhCp*Cl2]2 (5 mol%), 2 (0.2
mmol) and Cu(OAc)2.H2O (0.12 mmol) were taken. To this mixture, 1 mL
of toluene was added and the reaction mixture was stirred at 140 °C for 4
lead to the formation of
a seven-membered rhodacycle
intermediate IVB. The reductive elimination from IVB provides
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h. After that, the mixture was cooled to room temperature and the solvent
was removed by evaporation in vacuo. Then CH2Cl2 (10 mL) was added
to this residue followed by washing with aqueous K2CO3. The organic
layer was collected. The extraction was repeated with CH2Cl2 (10 mL x 2)
and the organic layers were combined and evaporated to dryness. This
crude mixture was adsorbed on to silica and purified by silica gel column
chromatography by using 2% EtOAc in Et2O solvent system.
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annulation
pKa(DCE of HO f = − . HBF4 = − . and cOH = . . . Kütt .
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mmol) and Cu(BF4)2.6H2O (0.15 mmol) were taken. To this, 1 mL of
MeOH was added and the reaction mixture was stirred at 140 °C for 12 h.
After that, the mixture was cooled to room temperature and the solvent
was removed by evaporation in vacuo. Then CH2Cl2 (7 mL) was added to
this mixture followed by washing with ethylenediamine in water (300 µL
ethylenediamine in 5 mL water). The organic layer was collected and the
extraction was repeated with CH2Cl2 (7 mL x 2) and the organic layers
were combined and evaporated to dryness. This crude mixture was
adsorbed on to silica and purified by silica gel column chromatography by
using 5% MeOH in CHCl3 solvent system.
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Acknowledgements
This work was financially supported by DST-SERB (grant no.
EMR/2016/003002) and IISER Bhopal. R.T. and C.D.
acknowledge doctoral fellowships from UGC and IISER Bhopal
respectively. We thank Suraj K. Gupta for help in DFT study.
[9]
Keywords: chemoselectivity • rhodium • C–H activation •
annulation • anion.
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Entry for the Table of Contents
FULL PAPER
A simple change in anion of a
copper(II) reagent dramatically
altered the course of a
Ranjeesh Thenarukandiyil,
Champak Dutta, and Joyanta
Choudhury*
Cp*Rh(III)-catalyzed C–H
activation-annulation reaction
leading to completely
switchable chemoselective
products in suitably chosen
solvents. Different coordinating
ability and basicity of the
Page No. – Page No.
Switching of Reaction
Pathway from C–C Rollover to
C–N Ring- Extension
Annulation
anions and the polarity of the
solvents played crucial role in
the observed divergence.
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