Co(III)-Catalyzed [4+1] Annulation of Amides with Allenes via C?H Activation

Article abstract of DOI:10.1002/adsc.201801335

A Co(III)-catalyzed [4+1] annulation of amides with allenes to synthesize isoindolone and 1,5-dihydro-pyrrol-2-one derivatives is reported. A wide range of aromatic and vinylic amides react with allenes to give the corresponding annulation products in good to excellent yields. The mechanistic studies strongly support that the catalytic reaction proceeds through an amide-directed C?H activation, followed by carbocobaltation of allene, β-hydride elimination, and an intramolecular 1,2-hydroamination. (Figure presented.).

Full text of DOI:10.1002/adsc.201801335

Title: Co(III)-Catalyzed [4 + 1] Annulation of Amides with Allenes via C–
H Activation
Authors: Boobalan Ramadoss, Rajagopal Santhoshkumar, and Chien-
Hong Cheng
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801335
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10.1002/adsc.201801335
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Co(III)-Catalyzed [4 + 1] Annulation of Amides with Allenes via
C–H Activation
Ramadoss Boobalan,a Rajagopal Santhoshkumar,a and Chien-Hong Chenga,*
a
Department of Chemistry, National Tsing Hua University, Hsinchu, 30013 (Taiwan); Fax: (+886) 3572-4698; E-mail:
chcheng@mx.nthu.edu.tw
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
hydroarylation and annulation from the three reactive
carbons of allene.[10e-g,12] With this background and our
continued progress in C–H functionalization,[12d-e,13]
we are highly interested in developing an economical
method for annulation of amides with allenes using
CoCp* catalyst system. Herein, we report a Co(III)-
catalyzed [4 + 1] annulation of amides with allenes to
give isoindolinones and 1,5-dihydropyrrol-2-one,
where allene acts as a one-carbon source.[12d,14]
Isoindolinone and pyrrolone motifs have been found in
many natural products and bioactive compounds.[15,16]
Abstract. A Co(III)-catalyzed [4 + 1] annulation of
amides with allenes to synthesize isoindolone and 1,5-
dihydro-pyrrol-2-one derivatives is reported. A wide
range of aromatic and vinylic amides react with allenes to
give the corresponding annulation products in good to
excellent yields. The mechanistic studies strongly support
that the catalytic reaction proceeds through an amide-
directed C–H activation, followed by carbocobaltation of
allene, β-hydride elimination, and an intramolecular 1,2-
hydroamination.
Keywords: Cobalt; Allenes; Annulation; Amides; C–H
Activation
The metal-catalyzed annulation reactions are one of
the efficient approaches in organic synthesis because
it provides straightforward method for the synthesis of
cyclic products from easily accessible substrates in a
rapid manner.[1] In particular, the reactions through C–
H activation[2,3,4] offer an atom- and step-economical
process[5] to construct complex organic molecules,[6]
which has a number of applications in the synthesis of
natural products, bioactive molecules, and materials.[7]
Amide is a versatile directing group in C–H
functionalization
to
access
N-heterocyclic
compounds.[8] Similar to alkynes and alkenes,
allenes[9] also underwent diverse reactions including
annulation,[10a-g] allylation,[10h-j] allenylation,[10k,l] and
dienylation[10m,n] with amides (Scheme1a).[2d] However,
due to the presence of orthogonal cumulative carbon-
carbon double bonds, allenes showed distinct
reactivity from alkynes and alkenes.[11] In 2016, Lu et
al reported a Rh(III)-catalyzed oxidative annulation of
amides with allenes to afford γ‑ lactams.[10a] The
reaction is limited to allenes bearing a hydroxyl group.
The coordination of hydroxyl group to the rhodium
center might be the key for the regioselectivity of the
allenes (Scheme 1b).
Recently, earth-abundant first-row transition metal
catalysts[3,4] have emerged as an economical
alternative to precious metals. As a first-row metal,
cobalt showed its unique reactivity in C–H
functionalization.[4] Specifically, the reaction with
allenes showed different reactivity including
Scheme 1. C–H functionalization of amides with allenes.
The reaction of benzamide 1a and allene 2a was
examined in the presence of Cp*Co(CO)I2 under
oxidative reaction conditions (Table 1, entries 1‒18).
From these studies, we found the optimization
conditions consisting of 10 mol% Cp*Co(CO)I2, 20
mol% AgSbF6, 20 mol% Mn(OAc)3·2H2O, and 1.0
equiv Ag2O in 1,2-dichloroethane (DCE) at 110 °C for
15 h (Table 1, entry 9), which gave the highest yield
(87%) of annulation product 3aa. The additive AgSbF6
is essential for the success of the reaction (Table 1,
entry 16); the possible role of this salt is to quickly
remove the iodide from Cp*Co(CO)I2 to provide
vacant sites for catalysis. In addition, a catalytic
amount of acetate source plays a vital role in
1
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10.1002/adsc.201801335
Advanced Synthesis & Catalysis
enhancing the reaction yield (Table 1, entry 17). These
results show that a high concentration of acetate ion
would suppress the coordination of substrates to the
cobalt (Table 1, entries 1, 3, 5, and 6), however, a
catalytic amount of acetate ion is necessary to abstract
the proton in the C−H activation step. Thus, the barely
soluble Ag2O or Ag2CO3 acts as better oxidants, which
might neutralize the AcOH produced during the
reaction to maintain a low concentration of OAc− in
the catalytic solution. It is noteworthy that the reaction
in the presence of RhCp* catalyst gave 3aa in 61%
yield (see supporting information), which was
unsuccessful in the Lu’s reaction conditions.[10a]
smoothly under the optimized reaction conditions to
give products 3ca-ga in good to excellent yields. o-
Substituted substrates 1h-j reacted well with allene 2a,
to provide the corresponding products 3ha-ja in good
yields. In the reactions of 3-methyl and 3,4-dimethoxy
substituted benzamides 1k-l, the C–H activation
highly favored the less-hindered ortho position of
amides to give products 3ka and 3la, respectively. It is
worth
mentioning
that
electron-rich
2,3-
dimethoxybenzamide 1m and thiophene containing
amide 1n went well with the reaction conditions,
affording products 3ma-na in 80 and 63% yields,
respectively. Different N-substituted benzamides 1o-p
also involved in the reaction, however, N-methyl
benzamide was the best one. The present reaction was
not limited to aromatic amides; it was also viable with
vinylamide 1q to provide annulation product 3qa,
albeit in lower yield. Furthermore, the structures of 3ia,
3ka, and 3la were unambiguously confirmed by X-ray
crystallographic analysis.[17]
Table 1. Optimization studies[a,b]
Yield
(%)
Entry
Additive 1
Additive 2
Oxidant
1[c]
2[c]
3[c]
4[c]
5
--
--
AgSbF6 Cu(OAc)2·H2O
0
AgSbF6 Mn(OAc)3·2H2O trace
Mn(OAc)2
--
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
CuO
Mn(OAc)2
Ag2O
AgOAc
Ag2CO3
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
O2
31
11
70
37
65
77
87
65
24
63
16
15
0
Mn(OAc)2
--
6[c]
7
Mn(OAc)2
Cu(OAc)2
8
9
Mn(OAc)3·2H2O AgSbF6
10
11
12
Fe(OAc)2
NaOAc
AcOH
AgSbF6
AgSbF6
AgSbF6
13 Mn(OAc)3·2H2O AgOTf
14 Mn(OAc)3·2H2O AgPF6
15 Mn(OAc)3·2H2O NaSbF6
16 Mn(OAc)3·2H2O
--
0
Scheme 2. Co(III)-Catalyzed Coupling of Amides 1 with
Allene 2a. Standard reaction conditions: 1 (0.30 mmol), 2a
(0.45 mmol), Cp*Co(CO)I2 (0.030 mmol), AgSbF6 (0.060
mmol), Mn(OAc)3·2H2O (0.060 mmol), Ag2O (0.30 mmol),
and DCE (4.0 mL) at 110 °C for 15 h under N2; Isolated
yields.
17 --
18[d] Mn(OAc)3·2H2O AgSbF6
AgSbF6
20
30
[a] Unless otherwise mentioned, all reactions were carried
out using 1a (0.18 mmol), 2a (0.27 mmol), Cp*Co(CO)I2
(0.018 mmol, 10 mol%), additive 1 (0.036 mmol), additive
2 (0.036 mmol), oxidant (0.18 mmol) and DCE (2.5 mL) at
110 °C for 15 h under N2. [b] Yields were determined by the
Next, we examined the scope of allenes for the
catalytic reaction. A variety of allenes 2b-l with
benzamide 1a were tested (Scheme 3). The reaction of
benzamide 1a with ortho-, meta-, and para-substituted
benzylallenes 2b-g proceeded efficiently under the
optimized reaction conditions, to afford the desired
products 3ab-ag in 68-87% yields. Likewise, 1-
naphthylmethyl allene 2h also furnished product 3ah
in good yield. The use of long chain aryl and alkyl
allenes 2i-j with 1a provided the annulation products
3ai-aj in 61% and 45% yields, respectively. 1,3-
Disubstituted cyclic allene 2k was transformed into the
[c]
1H NMR integration method.
Oxidant (0.36 mmol).
dReaction at 80 °C under O2 balloon.
With the optimized reaction conditions, we
investigated the scope of different amides 1 with
benzyl allene 2a (Scheme 2). Thus, the reaction of
unsubstituted amide 1b with 2a afforded the
annulation product 3ba in 81% yield. Likewise,
electron-donating, halogen, and electron-withdrawing
group substituted benzamides 1c-g proceeded
2
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10.1002/adsc.201801335
Advanced Synthesis & Catalysis
desired spiro compound 3ak, albeit in 39% yield.
Unsymmetrical 1,3-disubstituted allene 2l also
underwent annulation reaction with 1a to form
regioisomeric products, 3al+3al', in a 2:1 ratio. Finally,
electron-poor benzamides 1g was tested with allenes
2b and 2g to give products 3gb and 3gg. These results
show that the catalytic reaction highly favors electron-
poor benzamides and allenes. Moreover, a concerted
metallation-deprotonation mechanism is operative in
C–H activation step.[18] However, the present reaction
failed to afford the desired annulation products with
1,1-disubstituted and simple aliphatic allenes
containing ester, silyl, and hydroxyl group. It is
noteworthy that hydroxymethylallene does not
undergo annulation with benzamide 1a, in contrast to
Scheme 4. Co(III)-catalyzed [4 + 2] annulation of amide 1a
with phenylallene 2m.
In order to gain insight into the mechanism of this
cobalt-catalyzed oxidative annulation of amides with
allenes, a series of isotope-labeling experiments were
conducted (Scheme 5). Thus, heating penta-deuterated
benzamide [D5]-1b without 2a under standard reaction
conditions gave 1b with 50% D/H exchange at the
ortho position of amide (Scheme 5a), whereas the
reaction of [D5]-1b with 2a gave the annulation
product [D5]-3ba in 80% yield with a trace of D/H
exchange. These results indicate that the C–H
activation step becomes irreversible in the presence of
allene likely due to the faster allene insertion than
protonation of C-cobalt bond. Next, the competitive
and parallel experiments of [D5]-1b with 2a provided
an intermolecular kinetic isotopic effect (KIE) of 3.5
and 1.9, respectively (Scheme 5b). Similarly, the
reaction of [D1]-1b with 2a gave an intramolecular
KIE of 3.2 (Scheme 5c). These results clearly evidence
that the C–H bond cleavage is the rate-limiting step.
Moreover, [D7]-2a was subjected to the annulation
reaction with 1b for 1 h, giving [D7]-3ba in 30% yield,
the
RhCp*-catalyzed
oxidative
annulation
reaction.[10a]
where
a
benzylic deuterium was completely
transferred to the methyl group in [D7]-3ba (Scheme
5d). This result is in agreement with the proposed
mechanistic steps (III-V) in Scheme 6.
Scheme 3. Co(III)-catalyzed coupling of amide 1a with
allenes 2. The reaction conditions are similar to that in
Scheme 2 footnote. [a] Reaction with 2 (0.60 mmol).
In addition, phenylallene 2m was also subjected to the
annulation conditions (Scheme 4). The reaction gave [4 + 2]
annulation product 4am in 30% yield,[10e] along with other
unidentified products. Although a number of conditions had
been tested, we failed to improve the yield of product 4am.
3
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10.1002/adsc.201801335
Advanced Synthesis & Catalysis
Scheme 6. Plausible catalytic cycle.
In conclusion, we have demonstrated a first-row
transition metal Co(III)-catalyzed oxidative annulation
of amides with allenes to form isoindolinones and 1,5-
dihydro-pyrrol-2-one. The reaction tolerated different
amides and allenes. The mechanistic studies strongly
indicate that the reaction proceeds through C–H
activation, diene formation, and an intramolecular 1,2-
hydroamination. Further studies on the reactivity of
allenes with different arenes in the presence of first-
row metal catalysts are underway.
Scheme 5. Mechanistic studies.
Based on our preliminary mechanistic studies and
literature precedent,[10a,m,n,12e,14c,19]
a
plausible
mechanism for cobalt-catalyzed [4 + 1] annulation of
benzamides with allenes is depicted in Scheme 6. The
reaction starts with the formation of active cobalt
Experimental Section
General procedure for the synthesis of 3: To a sealed tube,
N-methyl benzamides 1 (0.30 mmol), allenes 2 (0.45 mmol),
CoCp*(CO)I2 (0.030 mmol), Ag2O (0.30 mmol),
Mn(OAc)3·2H2O (0.060 mmol) and AgSbF6 (0.060 mmol)
were added inside a glove box and the tube was sealed with
a rubber septum. DCE (4.0 mL) was added to the sealed tube
via syringe and the reaction mixture was allowed to stir at
110 °C for 15 h. Then, the reaction mixture was cooled and
diluted with CH2Cl2 (10 mL). The reaction mixture was
filtered through a celite pad and the celite pad washed with
CH2Cl2 (3 × 10 mL). The filtrate was concentrated under
reduced pressure and the residue was purified by silica gel
column chromatography using n-hexane/ethyl acetate (50%
EA) as eluent to afford the desired pure product 3.
Benzamides 1a-q, [D1]-1b, [D5]-1b[20a,b] and allenes[12e,20c]
were synthesized according to literature procedures.
species
I
from CoCp*(CO)I2, AgSbF6, and
Mn(OAc)3·2H2O. Coordination of amide 1b with I and
ortho C–H cobaltation give five-membered
cobaltacycle II. Then, insertion of allene 2a, followed
by β-hydride elimination delivers intermediate IV. The
Co–H in IV inserts into the diene moiety to form V,
which then undergoes reductive elimination to give
product 3ba. The cobalt(I) species generated from the
catalytic cycle is reoxidized by Ag2O to give active
cobalt(III) catalyst I.
Acknowledgements
We thank the Ministry of Science and Technology of the Republic
of China (MOST 105-2633-M-007-003) for support of this
research.
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Advanced Synthesis & Catalysis
UPDATE
Co(III)-Catalyzed [4 + 1] Annulation of Amides
with Allenes via C–H Activation
Adv. Synth. Catal. Year, Volume, Page – Page
Ramadoss Boobalan, Rajagopal Santhoshkumar,
and Chien-Hong Cheng*
7
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