Capturing Elusive Cobaltacycle Intermediates: A Real-Time Snapshot of the Cp*CoIII-Catalyzed Oxidative Alkyne Annulation

Article abstract of DOI:10.1002/anie.201704744

Despite Cp*Co^III catalysts having emerged as a very attractive alternative to noble transition metals for the construction of heterocyclic scaffolds through C?H activation, the structure of the reactive species remains uncertain. Herein, we rep

Full text of DOI:10.1002/anie.201704744

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201704744
German Edition: DOI: 10.1002/ange.201704744
À
C H Activation
Capturing Elusive Cobaltacycle Intermediates: A Real-Time Snapshot
of the Cp*CoIII-Catalyzed Oxidative Alkyne Annulation
Jesffls SanjosØ-Orduna, Daniel Gallego, Al›ria Garcia-Roca, Eddy Martin, Jordi Benet-
Buchholz, and Mónica H. PØrez-Temprano*
Abstract: Despite Cp*CoIII catalysts having emerged as a very
attractive alternative to noble transition metals for the con-
À
struction of heterocyclic scaffolds through C H activation, the
structure of the reactive species remains uncertain. Herein, we
report the identification and unambiguous characterization of
two long-sought cyclometalated Cp*CoIII complexes that have
À
been proposed as key intermediates in C H functionalization
reactions. The addition of MeCN as a stabilizing ligand plays
a crucial role, allowing the access to otherwise highly reactive
species. Mechanistic investigations demonstrate the intermedi-
acy of these species in oxidative annulations with alkynes,
including the direct observation, under catalytic conditions, of
a previously elusive post-migratory insertion seven-membered
cobaltacycle.
À
D
irected C H functionalization has become one of the most
powerful synthetic tools for the construction of organic
scaffolds.[1] For decades, this field has been dominated by
the use of noble metals, such as Pd or Rh.[1] Earth-abundant
first-row transition-metal catalysts,[2] in particular Cp*CoIII
III
À
Scheme 1. Cp*Co -catalyzed C H functionalization reactions.
nes.[4b,l,m,10] These coupling reactions provide a facile route for
the construction of privileged heterocyclic frameworks of
particular interest in medicinal chemistry and materials
science.[4a,b,11] Challenged by the lack of mechanistic informa-
tion, we wondered whether it would be possible to design
a new approach to access some of the putative cobaltacycle
complexes,[3] have recently shown their potential to construct
[4]
À
À
C C and C X bonds, not only representing a low-cost
alternative to Cp*RhIII catalysts[1a,l,5] but also offering
a unique catalytic reactivity.[3e] Despite the significant prog-
III
À
ress in this field, Cp*Co -catalyzed C H functionalization is
still at its infancy, in part, due to the limited fundamental
organometallic understanding of these systems.
intermediates aiming at a more comprehensive picture of
III
À
Cp*Co -catalyzed C H oxidative alkyne annulation. Herein,
In contrast to Rh-based systems,[6] the nature of the
putative cobalt reactive species within the catalytic cycle of
we reveal previously inaccessible mechanistic intricacies of
these transformations, including: 1) The intermediacy of
cyclometalated CoIII species (A and B) in the reaction
mechanism (Scheme 1); 2) the higher catalytic activity of A-
type complexes compared to the widely used [Cp*CoI2(CO)]
catalyst; and 3) the first experimental evidence of the
formation of a key seven-membered cobaltacycle intermedi-
ate, B, under catalytic conditions.
We initiated our study by targeting the synthesis of
cationic 2-phenylpyridine-derived cyclometalated Cp*CoIII
complexes. Such species, that is, A in Scheme 1, have been
proposed as key transient intermediates in Cp*CoIII-catalyzed
À
directed C H functionalization reactions still remains elusive.
À
This is likely due to the proposed reversibility of the C H
cleavage en route to A,[4b–n,7] along with the high reactivity of
the targeted metallacyclic intermediates, which hampers their
isolation and characterization (Scheme 1).[4j,8,9] Unravelling
À
how Cp*Co-based catalysts promote C H functionalization
would be particularly relevant, not only for organometallic
chemistry but also to set the basis for designing fundamental
new reactivity within Co catalysis.
In particular, Cp*CoIII systems have demonstrated their
great capability to catalyze oxidative annulations with alky-
[4a,f,h,j,7a,b,12]
À
C H functionalization reactions,
including annula-
tion processes.[10d,f] Due to the proposed reversible C H
cobaltation using Cp*CoIII systems, we envisioned that an
alternative strategy, involving a ligand-assisted Csp2-I oxida-
tive addition to CoI followed by halide abstraction, would
À
[*] J. Sanjosꢀ-Orduna, Dr. D. Gallego, A. Garcia-Roca, Dr. E. Martin,
Dr. J. Benet-Buchholz, Dr. M. H. Pꢀrez-Temprano
Institute of Chemical Research of Catalonia (ICIQ)
Avgda. Paꢁsos Catalans 16, 43007 Tarragona (Spain)
E-mail: mperez@iciq.es
À
facilitate the access to a direct analogue of a C H-activated
CoIII metallacycle.[13,14] As shown in Scheme 2, the reaction of
2-(2-iodophenyl)pyridine (1-I) with 2-CO in THF, at 608C for
24 h, resulted in the formation of the cobaltacycle 3-I in 81%
isolated yield.[15,16] Interestingly, when a more labile ligand,
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201704744.
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comparable yields when using [Cp*CoI2(CO)] as the catalyst
(entry 2). Importantly, we do not observe a decrease in yield
when the reaction is performed with only 1 mol% of 3-MeCN
(entry 3), the lowest catalyst loading reported to date for this
type of reactions. This is in stark contrast with the significant
decrease in yield obtained with [Cp*CoI2(CO)] (entry 4).
Notably, 3-MeCN also showed a superior reactivity compared
to [Cp*CoI2(CO)] at even lower temperatures (358C;
entries 5,6).[20,21]
Encouraged by the mild conditions shown in entries 5 and
6, we questioned whether we could detect key reactive cobalt
intermediates under catalytic conditions. Although no tran-
1
sient species was detected by H NMR spectroscopy when
[Cp*CoI2(CO)] was used as the catalyst precursor, we were
delighted to observe the in situ formation of a long-lived
intermediate (6) using 3-MeCN as pre-catalyst (Figure 1),[22]
suggesting that MeCN plays a crucial role in facilitating the
access/detection of otherwise elusive cobalt intermediates.
Scheme 2. Oxidative addition of 1-I to [Cp*CoIL2] and formation of the
cationic species 3-MeCN. ORTEP structures for 3-I and 3-MeCN are
shown with thermal ellipsoids set at 50% of probability (H atoms and
counter anion BF4À have been omitted for clarity).
such as vinyltrimethylsilane (VTMS), is bound to the CoI
metal center (2-VTMS), the oxidative addition proceeded
smoothly at room temperature (89% isolated yield) in less
than 30 min. Initial attempts at forming the desired cationic
cobaltacycle led to rapid decomposition upon treatment of 3-I
with AgBF4 in CH2Cl2. We solved this problem by performing
the reaction of 3-I with AgBF4 in MeCN, which affords the
desired cationic cobaltacycle stabilized by acetonitrile coor-
dination (3-MeCN) (Scheme 2). In addition to ESI-MS,[17] the
structure of 3-MeCN was unequivocally confirmed by NMR
spectroscopy and single-crystal X-ray diffraction.[18]
Figure 1. Detection of long-lived intermediate (6) under catalytic
conditions in CD2Cl2 at 308C.
With a reliable route in hand to one of the most widely
III
À
invoked intermediates in Cp*Co -catalyzed C H function-
alization reactions, we next investigated not only whether 3-
À
MeCN could be catalytically competent in oxidative C H
To elucidate the molecular structure of this transient
species and gain further insights into the mechanism of these
processes, we performed a series of stoichiometric studies
between 3-MeCN and 4 (Figure 2). Gratifyingly, when we
monitored the progress of the alkyne annulation reaction by
1H NMR spectroscopy at 308C in CD2Cl2, we observed the
fast conversion of 3-MeCN to the same cobalt complex (6)
observed in the catalytic experiments. Consumption of 6 was
alkyne annulation reactions but also its efficiency compared
to [Cp*CoI2(CO)] (Table 1).[4c] To our delight, using diphe-
nylacetylene (4) as a coupling partner,[19] 3-MeCN efficiently
catalyzes the formation of the annulation product 5 (89%
isolated yield) in less than 2 h (entry 1). 5 was obtained in
À
accompanied by formation of the C N-coupled product, 5
Table 1: Comparison between the catalytic activity of 3-MeCN and
[Cp*CoI2(CO)] in oxidative alkyne annulation.[a]
(Figure 2a). Moreover, the reductive elimination step was
considerably slowed down by the addition of an excess of
MeCN (20 equiv) allowing the clean observation of inter-
mediate 6 (Figure 2b). Despite being highly reactive, 6 could
be unambiguously characterized by X-ray crystallography
(Figure 2).[23] X-ray analysis reveals a seven-membered cat-
ionic cobaltacycle structure generated by the migratory
entry
Catalyst
mol%
T [8C]
t [h]
Yield [%][b]
insertion of the alkyne 4 into the CoIII Csp2 bond. Whereas
À
1
2
3
4
5
6
3-MeCN
[Cp*CoI2(CO)]
3-MeCN
10
10
1
1
10
10
100
100
100
100
35
2
2
2
2
24
24
89
84
90
66
64
39
numerous reports have postulated the intermediacy of
analogous cobaltacycles in annulation reactions, until now
none of them provided structural evidences of their forma-
tion. The experimental kinetic data shown in Figure 2 also
provide other very interesting mechanistic features of the
annulation reaction such that the migratory insertion of 4
seems to be faster than the final C N reductive elimination
step. Furthermore, the strong inhibitory effect of MeCN on
the reaction progress implies that 3-MeCN and 6 are off-cycle
[Cp*CoI2(CO)]
3-MeCN[c]
[Cp*CoI2(CO)][c]
35
[a] Reaction conditions: 1 (0.1 mmol), 4 (0.3 mmol), [Cp*CoIII]
(10 mol%, 0.01 mmol), AgBF4 (0.01 mmol), Cu(OAc)2 (0.1 mmol), DCE
(1.0 mL) under Ar. [b] Reactions were run in duplicate and the yields are
for the isolated product. [c] Reaction carried out in DCM.
À
2
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Figure 2. Kinetic profile of the stoichiometric reactions of 3-MeCN
Scheme 4. Proposed mechanism.
with 3 equiv of 4 in CD2Cl2 at 308C: a) in absence of MeCN, b) in the
presence of 20 equiv of MeCN. ORTEP structure for 6 is shown with
thermal ellipsoids set at 50% of probability (H atoms and counter
reductive elimination from 8 affords the desired coupled
product 5 and a [Cp*CoI] species. The resultant CoI species is
re-oxidized to the active [Cp*CoIII] by copper or silver salts,
À
anion BF4 have been omitted for clarity).
À
species that coexist in equilibrium with highly reactive
that ultimately regenerates 7 upon reversible C H activa-
cationic cobaltacycles possessing
a
vacant coordination
tion.[24]
site.[17,23] This is consistent with 6 being the resting state in
In conclusion, we have described a mechanistic study that
À
catalysis. The direct observation and full characterization of
unravels the role of cobaltacycle intermediates in C H
À
a cobalt resting state during Cp*Co-catalyzed C H function-
alization is unprecedented.
oxidative annulation reactions. Our work unambiguously
demonstrates the intermediacy of cyclometalated Cp*CoIII
species, due to the unique ability of MeCN to stabilize highly
reactive cobalt intermediates. We anticipate that this work
will set the basis for future developments in the area of cobalt
catalysis. Work on these goals is currently ongoing in our
laboratory.
Efforts to isolate and characterize the remaining [Cp*CoI]
species after the formation of the annulated product were
unsuccessful. However, the addition of 1-I to the reaction
mixture resulted in the clean formation of 3-I, suggesting
indirectly the presence of this [Cp*CoI] species after the
reductive elimination step (Scheme 3). Taken together, these
results provide strong evidence for a catalytic cycle consisting
of a CoI/CoIII couple.
Acknowledgements
We thank the CERCA Programme/Generalitat de Catalunya
and the Spanish Ministry of Economy, Industry and Com-
petitiveness (MINECO: CTQ2016-79942-P, AIE/FEDER,
EU, and Severo Ochoa Excellence Accreditation 2014–2018,
SEV-2013-0319) for the financial support. J.S.-O. thanks
Severo Ochoa Excellence Accreditation for a pre-doctoral
contract. We thank to the Research Support Area of ICIQ.
The authors also thank Prof. Ruben Martin and Dr. Alex
Shafir for useful discussions.
I
À
Scheme 3. Indirect detection of Cp*Co species after the C N reductive
elimination step.
On the basis of the above experimental data, a plausible
catalytic cycle using 3-MeCN as pre-catalyst is depicted in
Scheme 4. First, the cationic cobaltacycle 7 is generated from
3-MeCN. Then, alkyne coordination followed by a fast
Conflict of interest
The authors declare no conflict of interest.
migratory insertion into the CoIII C bond leads to the
À
À
formation of 8, which is stabilized by MeCN in an equilibrium
forming 6, a seven-membered cyclometalated compound that
Keywords: C H activation · cobalt catalysis · heterocycles ·
reaction mechanisms · structure elucidation
À
is the resting state of the catalytic cycle. Subsequent C N
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a cobaltacycle synthesized by Kanai,
[5] For selected reviews on Cp*RhIII-catalyzed C H functionaliza-
À
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(33% reported yield, see Ref. [4b]). See Supporting Information
for full characterization.
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4
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[16] Chang and co-workers showed the catalytic activity of the 3-I
results, we hypothesized that the presence of excess of MeCN
analogue synthetized by Kanai, Matsunaga, and co-workers in
inhibited the reaction. See Supporting Information for more
details.
À
the cobalt-catalyzed C H amidation with dioxazolones, see
Ref. [4i].
[22] See Supporting Information for more details.
[23] CCDC 1540813 (6) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre. 6 was also
fully characterized by NMR spectroscopy and ESI-MS. The
cationic species (8) without the acetonitrile coordinated was also
detected by ESI-MS. See Supporting Information for more
details.
[24] As expected, we observed H/D scrambling at the ortho position
of 1 under catalytic conditions at 308C in CD2Cl2, in the absence
of 4 and AgBF4 and in the presence of D2O using 3-MeCN as
catalyst.
[17] By using ESI-MS, we observed complex 3-MeCN along with the
cationic species without the acetonitrile coordinated (7).
[18] CCDC 1540811 (3-I) and 1540812 (3-MeCN) contain the sup-
plementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge
Crystallographic Data Centre.
[19] We selected analogous reaction conditions to those reported
previously by Wang and co-workers (Ref. [9f]): 1 (1 equiv), 4
(3 equiv), [Cp*CoIIII2(CO)] (10 mol%), AgBF4 (1 equiv), Cu-
(OAc)2 (1 equiv), DCE, 1008C, 24 h.
[20] While 5 could be obtained in 78% at 358C with 3-I, the
combination of 3-I (10 mol%) with MeCN (10 mol%) afforded
a comparable yield relative to 3-MeCN. In sharp contrast,
a significant erosion in yield is observed when using 3-I at 1008C.
At present, we believe these results showcase the inherent
proclivity of cobaltacycles to decomposition. See Supporting
Information for more details.
Manuscript received: May 8, 2017
Revised manuscript received: June 1, 2017
Accepted manuscript online: June 6, 2017
Version of record online: && &&, &&&&
[21] The yield of 5 decreased dramatically when [Cp*Co(MeCN)3]-
(BF4)2 was used as precatalyst. Based on our stoichiometric
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
À
C H Activation
Cyclic Co complex caught: The first direct
observation and full characterization of
elusive cobaltacycle species in Cp*CoIII-
J. Sanjosꢁ-Orduna, D. Gallego,
A. Garcia-Roca, E. Martin,
J. Benet-Buchholz,
À
catalyzed C H oxidative alkyne annula-
tion is reported. The exceptional ability of
MeCN to stabilize otherwise highly reac-
tive intermediates has paved the way to
uncover previously inaccessible mecha-
nistic features of these transformations.
M. H. Pꢁrez-Temprano*
&&&& — &&&&
Capturing Elusive Cobaltacycle
Intermediates: A Real-Time Snapshot of
the Cp*CoIII-Catalyzed Oxidative Alkyne
Annulation
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!