Cobalt-Catalyzed sp2-C?H Activation: Intermolecular Heterocyclization with Allenes at Room Temperature

Article abstract of DOI:10.1002/anie.201604956

The reactivity of allenes in transition-metal-catalyzed C?H activation chemistry is governed by the formation of either alkenyl–metal (M–alkenyl) or metal–π-allyl intermediates. Although either protonation or a β-hydride elimination is feasible with a M–alkenyl intermediate, cyclization has remained unexplored to date. Furthermore, due to the increased steric hindrance, the regioselectivity for the intramolecular cyclization of the metal–π-allyl intermediate was hampered towards the more substituted side. To address these issues, a unified approach to synthesize a diverse array of biologically and pharmaceutically relevant heterocyclic moieties by cobalt-catalyzed directed C?H functionalization was envisioned. Upon successful implementation, the present strategy led to the regioselective formation of dihydroisoquinolin-1(2H)-ones, isoquinolin-1(2H)-ones, dihydropyridones, and pyridones.

Full text of DOI:10.1002/anie.201604956

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201604956
German Edition: DOI: 10.1002/ange.201604956
À
C H Activation
2
À
Cobalt-Catalyzed sp -C H Activation: Intermolecular
Heterocyclization with Allenes at Room Temperature
Neetipalli Thrimurtulu,* Arnab Dey, Debabrata Maiti,* and Chandra M. R. Volla*
Abstract: The reactivity of allenes in transition-metal-cata-
À
lyzed C H activation chemistry is governed by the formation
of either alkenyl–metal (M–alkenyl) or metal–p-allyl inter-
mediates. Although either protonation or a b-hydride elimi-
nation is feasible with a M–alkenyl intermediate, cyclization
has remained unexplored to date. Furthermore, due to the
increased steric hindrance, the regioselectivity for the intra-
molecular cyclization of the metal–p-allyl intermediate was
hampered towards the more substituted side. To address these
issues, a unified approach to synthesize a diverse array of
biologically and pharmaceutically relevant heterocyclic moi-
À
eties by cobalt-catalyzed directed C H functionalization was
envisioned. Upon successful implementation, the present
strategy led to the regioselective formation of dihydroisoqui-
nolin-1(2H)-ones, isoquinolin-1(2H)-ones, dihydropyridones,
and pyridones.
A
llenes have two orthogonal carbon–carbon double bonds
and have been explored in a wide variety of metal-catalyzed
[1]
À
transformations, including C H functionalization reactions.
The pattern of insertion of an organometallic intermediate R–
M with allene is governed by the nature of the metal M and
also depends on the steric and electronic nature of the
substituents R1 and R2. Carbometallation of R–M can take
place either on C1 or C2 of the allene, which will lead to either
an M–alkenyl or M–p-allyl intermediate, respectively
(Scheme 1). Protonation or a b-hydride elimination has
previously been explored with an M–alkenyl intermediate
by employing rhodium[2] and iridium[3] catalysts.
Scheme 1. Overview of the work.
oxidative annulation of allenes with benzamides, which takes
place preferentially at the less hindered side (Scheme 1,
path B).[6] In 2010, the groups of Cramer[7] and Takai[8]
independently described an imine directed cyclization with
allenes. However, regioselective cyclization reactions at the
more substituted end of the allene remain unexplored for aryl
and alkenylamides. Herein, we report a unified approach for
synthesizing versatile heterocycles by controlling the elec-
tronic nature of the allene and cobalt catalyst (first-row
In a pioneering study, Ma demonstrated the RhIII-
catalyzed hydroarylation reaction of allenes proceeding
through the protonation of a Rh–alkenyl intermediate
(Scheme 1, path A).[4] However, the cyclization pathway
leading to biologically and pharmaceutically relevant com-
pounds through an M–alkenyl intermediate has never been
realized. Additionally, the M–p-allyl intermediate can
undergo Tsuji–Trost type substitutions in the presence of
external nucleophiles to afford the functionalized products. In
particular, benzamides usually undergo cyclization at the less
substituted end of the allene to avoid steric hindrance (M–p-
allyl, M = Rh and Pd).[5] Glorius reported the RhIII-catalyzed
À
transition metal) through a directed C H functionalization
(Scheme 1).
À
Recently, cobalt catalyzed C H activation has gained
much interest, and excellent reports in this area have come
from the groups of Nakamura,[9] Kanai,[10] Yoshikai,[11]
Ackermann,[12] Daugulis,[13] and others.[14] Based on the
prior success of allene interaction with organometallic
intermediates,[5,15] benzamide was employed as the model
2
[*] Dr. N. Thrimurtulu, A. Dey, Prof. D. Maiti, Prof. C. M. R. Volla
Department of Chemistry, Indian Institute of Technology Bombay
Powai, Mumbai 400076 (India)
À
substrate for cobalt-catalyzed sp -C H activation. Anchoring
cobalt in close proximity to benzamide and stabilizing the
metallacycle for the allene insertion was planned using the
bidentate chelator 8-aminoquinoline.[16] The allene was
expected to interact with the cobaltacycle intermediate
E-mail: uluiict@gmail.com
dmaiti@chem.iitb.ac.in
chandra.volla@chem.iitb.ac.in
À
generated by facile C H activation of benzamide.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201604956.
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Table 1: Synthesis of dihydroisoquinolin-1(2H)-ones and dihydropyrido-
A combination of benzamide (1a) and phenylallene was
reacted with readily available Co(acac)2 at room temperature.
After optimization (see Supporting Information), we were
delighted to observe the formation of dihydroisoquinolin-
1(2H)-one (2a) as the major isomer (5:1 regioisomers, as
observed by 1H NMR), the structure of which was confirmed
by X-ray crystallography. Detailed optimization revealed that
the use of Mn(OAc)3·2H2O as the oxidant and NaOPiv·H2O
as the base in trifluoroethanol was crucial for obtaining good
yields of 2a (for crystallographic data, see Ref. [18]). The
reaction showed a high regioselectivity for the heterocycliza-
tion at the more substituted side, which proceeded through
nes.[a]
a Co–p-allyl intermediate. This is in sharp contrast with the
III
Rh -catalyzed C H activation reactions,[6] in which reductive
À
elimination took place predominantly at the less sterically
hindered side. We attribute this difference in selectivity to the
smaller ionic radius of cobalt compared to rhodium. Sub-
sequently, a broad range of electron-poor and electron-rich
aromatic substrates were investigated (2b–p, Table 1). Sev-
eral valuable functional groups such as methoxy (2d), fluoro
(2e), chloro (2 f), and bromo (2g) groups were tolerated at
different positions of 1, providing ample opportunity for
further derivatization of the products. Meta-substituted
benzamides resulted in complicated mixtures of regioisomers
with phenylallene.
The reaction was also applicable to 2-biphenyl and 1-
naphthyl derivatives, which provided the corresponding
highly functionalized products 2j and 2k in synthetically
useful yields. The structure of 2k was confirmed by X-ray
crystallography. Interestingly, the same regioselectivity was
observed for sterically demanding 1,1-dimethylallene (2m
and n) and 1,1-diphenyl substituted allene (2o), which
furnished the corresponding dihydroisoquinolin-1(2H)-ones
in good yields. Notably, 1,3-diphenylallene provided the
product 2p in excellent yield.
Gratifyingly, the reaction was not restricted to aromatic
amides. Olefinic carboxamides under the same conditions
provided the expected dihydropyridones. As shown in
Table 1, the heterocyclization of cinnamic acid amide with
phenylallene afforded the corresponding dihydropyridone
2
À
(2q) through alkenyl sp C H activation. Evidently, irrespec-
tive of the steric demand in arylallene and 1,1-dimethylallene,
both allenes provided the corresponding dihydropyridones
(2q–v) in good yields (Table 1).
[a] Reaction conditions: 1 (0.2 mmol), allene (0.8 mmol), Co(acac)2
(20 mol%), Mn(OAc)3·2H2O (0.2 mmol), NaOPiv·H2O (0.4 mmol), TFE
(2 mL), 48–60 h, rt, isolated yield for combined two isomers (5:1
regioisomers determined by crude 1H NMR). [b] brsm (based on
recovered starting material) yield, [c] 2:1 (E/Z) ratio, [d] determined by
crude1H NMR analysis, [e] 2:1 regioisomers.
Variation of the electronic properties of the allene has the
potential to change their reactivity patterns. Interestingly,
changing the allene partner from aryl allene to an electron
deficient allene such as allenyl phosphonate led to the
formation of isoquinolinones (3a–j) or pyridone 3k as the
sole products with aryl or alkenyl amides, respectively
(Table 2; for crystallographic data, see Ref. [18]). As
expected, 1-phenyl-substituted allenyl phosphonate also pro-
vided the same regioisomeric product 3j with excellent yield
(Table 2). In addition to aryl amides, heterocyclic derivatives
like thiophene (3g) and quinoline (3h) were also compatible.
After evaluating the scope with allenylphosphonates, other
allenes were tested under the optimal reaction conditions. In
the case of cyclohexylallene, the desired isoquinolinone (3l)
and pyridones (3m–p) were obtained as the major regioiso-
meric products. Alkenyl amides gave the corresponding
pyridone products with methoxyallene (3q–s) in moderate
yields. These differences in the yield and regioselectivity of
the products may be due to the back donation of lone pair
electrons from the oxygen atom of the allene (Table 2).
To study the scalability of the present method, the
reaction of 1b with allenylphosphonate was performed in
the gram scale and 3b was isolated in 67% yield (1.3 g,
Scheme 2). To demonstrate the synthetic utility of the
2
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Table 2: Synthesis of isoquinolin-1(2H)-ones and pyridones.[a]
Scheme 3. Application: Horner–Wittig reaction.
[a] Reaction conditions: 1 (0.2 mmol), allene (0.24 mmol), Co(acac)2
(20 mol%), Mn(OAc)3·2H2O (0.2 mmol), NaOPiv·H2O (0.4 mmol), TFE
(2 mL), 24–60 h, rt, isolated yield. [b] brsm yield, [c] 2:1 regioisomers,
1
[d] determined by crude H NMR analysis, [e] 4:1 regioisomers, [f] 10:1
regioisomers.
Scheme 4. Preliminary mechanistic investigation.
1e was subjected to the standard conditions, a 1:1.3 mixture of
products was obtained, which suggests that electrophilic
cobaltation is unlikely (Scheme 4A). An intermolecular
competition between 1a and [D]5-1a gave a product distri-
bution value of 1.1 {[PH/PD]} (Scheme 4B). This value
Scheme 2. Gram scale synthesis of dihydroisoquinolin-1(2H)-ones.
À
indicates that C H bond cleavage might not be involved in
isoquinolin-1(2H)-one products, a Horner–Wittig[17] reaction
was performed with 3d. The reaction of the stabilized
phosphonate carbanion with 9-anthraldehyde resulted in the
formation of olefin 4 with excellent E-selectivity. The
structure of 4 was confirmed by X-ray crystallography
(Scheme 3; for crystallographic data, see Ref. [18]).
The mechanism of the reaction was studied in detail using
intermolecular competition and deuterium labeling experi-
ments. The kinetic investigation of the reaction with benza-
mide 1a suggested a first-order rate dependence (see
Supporting Information) with respect to 1a. When a 1:1
mixture of o-methoxybenzamide 1d and o-fluorobenzamide
the rate-limiting step. Furthermore, no D/H exchange was
observed when [D]5-1a was treated with phenylallene under
standard conditions. Similarly, no deuterium incorporation in
the product 2c was observed when 1c was treated with
isotopically labelled CD3OD as a cosolvent. These results
À
suggest that the C H cobaltation step is irreversible (Sche-
me 4C).
Considering our preliminary mechanistic studies and
literature precedent, we propose the mechanism shown in
Scheme 6A. In accordance with the known coordination
properties of CoIII with arenes, we hypothesize that the
catalytic reaction starts with a CoIII species. To support the
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formation of an active CoIII complex, we reacted benzamide
1a with Co(acac)2 in the presence of K2S2O8 at room
temperature, upon which we observed the formation of
CoIII intermediate A (Scheme 5; for crystallographic data, see
Ref. [18]), which was confirmed by X-ray crystallography. We
were delighted to observe the formation of the key inter-
mediate B along with A, when 1c was used instead of 1a
under the same conditions. The formation of intermediate B
can be proposed based on high-resolution mass spectrometry
(HRMS) and 1H NMR studies (see Supporting information).
Scheme 5. Isolation of intermediate A.
Coordinative insertion of the double bond of an allene
À
into a C Co bond of intermediate C affords the seven
membered cobaltacycle intermediate D or E, depending on
the electronic nature of the allene. Carbocobaltation of
allenes is known to proceed through the addition of aryl to the
central carbon of the allene to produce a Co–p-allyl complex.
For R = Ph, insertion with the more electron-rich double
bond is favored and can account for the selective formation of
the stable Co–s-allyl complex D (Scheme 6B). Reductive
elimination of intermediate D gives cyclized product 2 and
regenerates the CoIII species after oxidation of CoI with
Mn(OAc)3·2H2O.
Scheme 6. Proposed reaction mechanism.
In contrast, when either electron-poor allenes or sterically
hindered allenes were employed, carbocobaltation of inter-
mediate C takes place through a different pathway (Co–
alkenyl) leading to E (Scheme 6C). Reductive elimination of
E leads to F, which undergoes a 1,3-hydrogen shift, which
provides the corresponding isoquinolin-1(2H)-ones or pyr-
idones 3.
In conclusion, we have reported a heterocyclization
reaction of (hetero) arylamides and alkenylamides with
allenes under operationally simple conditions at room
temperature. Variation of the electronic properties of the
allenes changed the reactivity pattern for the regioselective
Acknowledgements
This activity is supported by CSIR (funding to D.M., 02
(0242)/15/EMR-II), DST (funding to C.M.R.V., EMR/2015/
002047). Financial support was received from DSTunder Fast
Track Scheme (N.T.) and IIT-B (A.D.). The authors thank
Soham Maity (D.M. lab) for initial experiments, starting
materials, and insightful discussions.
À
Keywords: cobalt catalysts · C H activation · heterocyclization ·
metal intermediates
À
cyclization with aryl/alkenyl C H activation using an inex-
pensive and commercially available cobalt catalyst. A novel
and efficient synthesis of cobalt-catalyzed intermolecular
heterocyclization for the synthesis of dihydroisoquinolin-
1(2H)-ones, isoquinolin-1(2H)-ones, dihydropyridones, and
pyridones through either M–alkenyl or M–p-allyl intermedi-
ates was developed. This reaction features high regioselec-
tivity and impressive substrate scope for both the coupling
partners.
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Received: May 20, 2016
Revised: August 2, 2016
Published online: && &&, &&&&
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À
C H Activation
N. Thrimurtulu,* A. Dey, D. Maiti,*
C. M. R. Volla*
&&&& — &&&&
2
À
Cobalt-Catalyzed sp -C H Activation:
Intermolecular Heterocyclization with
Allenes at Room Temperature
À
Two paths to ring formation: Heterocyc-
lization reactions of aryl and alkenyla-
mides with allenes at room temperature
are reported. The reactions, which involve
C H activation using a Co catalyst,
feature a high regioselectivity and
impressive substrate scope for both cou-
pling partners.
6
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