Cobalt-Catalyzed C-H Activation and [3 + 2] Annulation with Allenes: Diastereoselective Synthesis of Indane Derivatives

Article abstract of DOI:10.1021/acs.orglett.1c01521

An unprecedented bidentate directing-group-assisted cobalt-catalyzed oxidative C-H activation of aryl hydrazones followed by asyn-diastereoselective [3 + 2] annulation reaction has been achieved, employing allenes as the annulation partners. The selective 2,3-migratory insertion of allenes with arylcobalt(III) species and the subsequent intramolecular diastereoselective nucleophilic addition ofη¹-allylcobalt onto the imine resulted in [3 + 2] annulation over the alternative [4 + 2] annulation. Furthermore, the oxidative annulation obviates the need for stoichiometric metal oxidants and proceeds under aerobic conditions.

Full text of DOI:10.1021/acs.orglett.1c01521

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Letter
Cobalt-Catalyzed C−H Activation and [3 + 2] Annulation with
Allenes: Diastereoselective Synthesis of Indane Derivatives
Arnab Dey and Chandra M. R. Volla*
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ABSTRACT: An unprecedented bidentate directing-group-assis-
ted cobalt-catalyzed oxidative C−H activation of aryl hydrazones
followed by a syn-diastereoselective [3 + 2] annulation reaction has
been achieved, employing allenes as the annulation partners. The
selective 2,3-migratory insertion of allenes with arylcobalt(III)
species and the subsequent intramolecular diastereoselective
nucleophilic addition of η1-allylcobalt onto the imine resulted in
[3 + 2] annulation over the alternative [4 + 2] annulation.
Furthermore, the oxidative annulation obviates the need for
stoichiometric metal oxidants and proceeds under aerobic
conditions.
ransition-metal-catalyzed oxidative functionalization and
annulation reactions of inert C−H bonds are emerging as
ing naturally abundant molecular oxygen or air as a terminal
oxidant in cobalt catalysis.12
T
one of the most appealing and powerful tools for the atom- and
step-economic synthesis of heterocycles, drugs, and natural
products.1 This strategy has been dominated by rare and
expensive noble transition-metal catalysts.2 Capitalizing on this
concept with more abundant and inexpensive first-row
transition-metal catalysts has recently gained more interest.3
In this regard, earth-abundant cobalt(II) salts have attracted
considerable attention for accomplishing oxidative C−H
annulation reactions with different π-organic components
such as alkenes, alkynes, and allenes.4 On a different note,
the allylation of imines with allyl metal reagents is an efficient
and straightforward method for accessing valuable homoallyl
derivatives.5 Although the advantages of cobalt salts have been
recognized in various organometallic reactions6 like hydro-
genation, hydrofunctionalization, cross-coupling, and cyclo-
addition, their use in nucleophilic allylation reactions is
surprisingly limited.7 As a result, the development of
nucleophilic allylation reactions with η1-allylcobalt species is
highly desired.
With our continuous interest in bidentate directing-group-
assisted C−H activation,13 we recently reported a Co(II)-
catalyzed [4 + 2] annulation reaction with 1,3-diynes.14 The
reaction ensues via a key Co-alkenyl intermediate Int-B under
redox-neutral conditions with the N−N bond acting as an
internal oxidant (Scheme 1b). To expand the synthetic
repertoire of allenes, we envisioned a 2-hydrazineylpyridine-
assisted annulation reaction of allenes with Co(II) salts
(Scheme 1c). The outcome of this anticipated strategy relies
on two independent competitive pathways: (1) Reductive
elimination followed by N−N bond cleavage (analogous to
1,3-diynes) can produce [4 + 2] annulated isoquinoline
derivatives, or (2) a nucleophilic attack onto the imine can
afford [3 + 2] annulated indane derivatives. On the basis of this
hypothesis, the reaction between hydrazone derivatives and
allenes was studied in the presence of Co(II) salts, and to our
delight, a preferential nucleophilic attack on the imine was
observed, furnishing indane derivatives having two new
stereogenic centers with high syn diastereoselectivity for the
aryl and azo groups.15 The observed high stereocontrol in the
[3 + 2] annulation is a manifestation of the highly selective 2,3-
migratory insertion of allene with the organocobalt(III)
species, leading to Co-σ-allyl intermediate Int-C. Furthermore,
Allenes8 are versatile organic motifs with three reactive
carbon centers, and they exhibit complex patterns of reactivity
for migratory insertion based on electronic and steric
factors.9,10 Unravelling these issues, various oxidative C−H
annulation strategies with allenes, proceeding via Co-π-allyl
intermediate Int-A, have been documented (Scheme 1a).11
Although elegant, these oxidative transformations require
stoichiometric amounts of transition-metal oxidants based on
Mn(III) or Ag(I) for the regeneration of a cobalt catalyst,
which inevitably limits the overall efficiency of the trans-
formation. Therefore, extensive efforts have been devoted to
exploring green and efficient oxidative transformations employ-
Received: May 5, 2021
Published: June 16, 2021
https://doi.org/10.1021/acs.orglett.1c01521
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5018−5023
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a
Scheme 1. Overview of the Work
Table 1. Optimization of Reaction Conditions
b
entry
variation from standard conditions
yield (%)
c
1
2
3
4
none
83 (78)
50
10
20 mol % Mn(OAc)2
without Mn(OAc)2
NaOAc instead of Mn(OAc)2
NaOPiv instead of Mn(OAc)2
Cu(OAc)2 instead of Mn(OAc)2
PivOH instead of Mn(OAc)2
with CoCl2·6H2O
with Co(acac)2
with Co(NO3)2·6H2O
with CoSO4·7H2O
25
40
d
c
5
d
6
d
7
8
d
20
35
18
19
e
9
e
10
11
12
13
14
15
e
e
HFIP, TFE, t-BuOH, MeOH, DCE, CH3CN, acetone
at room temperature
at 100 °C
<25%
72
without Co(OAc)2·4H2O
a
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), Co(OAc)2·
4H2O (20 mol %), Mn(OAc)2 (40 mol %), EtOH (2 mL) at 70 °C
for 6 h. Yield is calculated based on H NMR of the crude reaction
mixture using 1,3,5-trimethoxybenzene as an internal standard. Yield
refers to isolated yield after column chromatography. 20 mol %
b
1
c
d
e
additive was used. Using 20 mol % NaOPiv.
dropped to 72% at 100 °C (entries 13 and 14). As expected,
no product was detected in the absence of cobalt salt (entry
15).
the divergent reactivity observed between Int-B (Co-alkenyl)
and Int-C (Co-σ-allyl) is the main highlight, and it can be
ascertained to the higher nucleophilicity of the Co-σ-allyl
species. It is worth mentioning that the cobalt catalyst was
regenerated using air as the green oxidant.
With the optimized reaction conditions in hand, we started
to explore the substrate scope of our methodology by treating a
variety of substituted hydrazone derivatives 1 with phenyl
allene 2a (Table 2). We were pleased to see the formation of
the indane derivatives 3a−3o in moderate to good yields with
diversely substituted hydrazone derivatives 1, irrespective of
the electronic and steric nature of the substituents. Both
electron-donating and electron-withdrawing substituents were
compatible under the optimized reactions. Interestingly, meta-
chloro-substituted hydrazone 1n and 2-fluorenyl hydrazone 1o
afforded the corresponding products by selective activation of
the less sterically hindered C−H bond (3n and 3o in 75 and
67% yield, respectively).
To further investigate the generality and scope of the
methodology, several allenes 2 were tested with hydrazone 1a.
As evident from Table 3, various substituents on the arylallene
were compatible to deliver the indane derivatives 3p−3ad in
53−95% yield. The structure and stereochemistry of indanes
were further confirmed by the X-ray crystallography analysis of
3w. The 2-naphthyl-substituted allene 2m produced the
corresponding product 3ab in an excellent yield of 95%.
Captivatingly, disubstituted allenes 2n and 2o reacted
smoothly under the standard conditions to provide the indanes
3ac and 3ad in 53 and 59% yield, respectively, with the
selective formation of the E-isomer, as confirmed by NOE
analysis.15 The preferential formation of the E-isomer is
consistent with the proposed 2,3-migratory insertion of allene
with the arylcobalt(III) species, leading to the Co-σ-allyl
complex Int C. We subsequently evaluated the amenability of
We commenced our studies with hydrazone 1a and
phenylallene 2a as the model substrates. After rigorous
optimization of various reaction parameters, we arrived at
the following optimized conditions: 20 mol % Co(OAc)2·
4H2O and 40 mol % Mn(OAc)2 at 70 °C in EtOH under an
air atmosphere to observe 83% of the desired [3 + 2]
annulation product 3a (isolated yield of 78%) (Table 1, entry
1).15 The 1H NMR of the crude reaction mixture indicated the
formation of a single diasteromer, and the syn configuration of
the phenyl moiety and the azo group in the product 3a were
confirmed by a nuclear Overhauser effect (NOE) study and
single-crystal X-ray diffraction analysis of 3a. Lowering the
loading of Mn(OAc)2 resulted in lower yields, and 3a was
observed in only 10% yield in the absence of Mn-
(OAc)2(entries 2 and 3). Whereas, additives like NaOAc or
NaOPiv instead of Mn(OAc)2 produced 3a in inferior yields
(entries 4 and 5), the use of Cu(OAc)2 or PivOH was found to
be deleterious (entries 6 and 7). Other Co(II) salts did not
show much improvement, as 3a was observed in 18−35% yield
(entries 8−11). The screening of various polar protic and
nonprotic solvents indicated the crucial role of EtOH as a
solvent for the success of the reaction (entry 12). The
temperature is found to be optimum at 70 °C, as no reaction
was observed at room temperature, and the yield of 3a
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Table 2. Substrate Scope with Different Hydrazone
Table 3. Substrate Scope with Different Allenes
Derivatives
this approach for the late-stage modification of biologically
relevant hydrazone derivatives. When an apocynin-derived
hydrazone derivative was subjected to this protocol, the target
product 3ae was isolated in 66% yield. Similarly, other
bioactive molecules like aspirin-, L-menthol, indole-, and
morpholine-substituted hydrazones also smoothly underwent
the annulation to afford the corresponding functionalized
indane derivatives 3af−3ai in 50−58% yield.
A gram-scale reaction was conducted (Scheme 2) by
reacting 1.0 g of 1a and 1.6 g of 2a, and we were pleased to
observe the formation of 1.16 g of indane 3a (75% yield). To
further expand the synthetic utility, we performed selective
reduction reactions on 3a. Reduction using Fe powder (20 mol
%) and NH4Cl (1 equiv) specifically reduced the azo group to
hydrazine functionality and provided 4 in 98% isolated yield.
Under Pd-catalyzed hydrogenation with 1 atm of H2, the exo-
methylene moiety was selectively reduced to generate 5 in
quantitative yield as a single diastereomer (Scheme 2b).
To get more insights into the reaction mechanism, various
control, competitive, and deuterium labeling experiments were
performed, as shown in Scheme 3. Under the optimized
conditions, the control reaction between N-phenyl hydrazone
6 and 2a did not provide any indane product, indicating the
importance of bidentate pyridine coordination with the cobalt
catalyst (Scheme 3a). Another control experiment was
conducted by using N-methyl hydrazone 7 instead of 1, and
no product was detected in this case either, suggesting that an
initial N−Co bond formation is crucial for C−H bond
activation (Scheme 3a). The exchange experiments on 1a and
[D]5-1a either with allene or without the allene implied no H/
D exchange or D/H exchange (see the Supporting
Scheme 2. Scale-up Reaction and Product Transformations
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Scheme 3. Preliminary Mechanistic Investigation
Scheme 4. Proposed Reaction Mechanism
allenes to access indane derivatives using simple and
commercially available cobalt(II) salts. Various allenes reacted
smoothly with diversely substituted hydrazones irrespective of
the steric and electronic nature of the substituents. The syn
configuration of the product was confirmed by a NOE study
and X-ray diffraction analysis. The use of simple cobalt(II)
salts and air as the greener oxidant makes the transformation
more valuable and appealing. Furthermore, the protocol was
compatible with the functionalization of biologically relevant
hydrazones.
Information). When a 1:1 mixture of 4-Me 1f- and 4-F 1j-
substituted hydrazone derivatives was reacted under our
standard conditions with 2a, a 1.1:1 mixture of 3f and 3j
was obtained, suggesting that electrophilic cobaltation is
unlikely (Scheme 3b). Parallel kinetic studies with 1a and
[D]5-1a afforded a kinetic isotope effect value (kH/kD) of 1.23
(Scheme 3c) and a competitive isotopic effect between 1a and
[D]5-1a produced a product distribution value of (PH/PD) 1.56
(Scheme 3d). These combined studies suggest that C−H
cobaltation is irreversible and might not be the rate-limiting
step.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01521.
Additional experimental procedures and X-ray crystallo-
graphic analysis (PDF)
On the basis of the preliminary mechanistic studies and
literature precedence,12b,16 a plausible reaction mechanism has
been proposed, as shown in Scheme 4. The catalytic cycle is
initiated by the interaction of hydrazone derivative 1 with the
Co(II) catalyst to generate intermediate A. Intermediate A,
after aerial oxidation followed by a Mn(OAc)2-assisted (acting
as a base) concerted metalation deprotonation (CMD)
process, leads to the C−H metalated complex C. The
subsequent coordination of the allene and selective 2,3-
migratory insertion of the less-substituted double bond lead to
the allyl cobalt intermediate E (σ-allyl complex).15 The
intramolecular nucleophilic addition of the allylcobalt species
onto the imine carbon proceeds via a six-membered cyclic
transition state TS followed by elimination, furnishing the
Co(I) intermediate F. Decomplexation gives the product 3 and
Co(I), which, after aerial oxidation, regenerates the active
Co(II) catalyst.
Accession Codes
CCDC 2023190−2023192 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Chandra M. R. Volla − Department of Chemistry, Indian
Institute of Technology Bombay, Mumbai 400076, India;
orcid.org/0000-0002-8497-1538; Email: chandra.volla@
chem.iitb.ac.in
Author
In conclusion, we have demonstrated a bidentate directing-
group-assisted oxidative C−H activation and a syn-diaster-
eoselective [3 + 2] annulation strategy of hydrazones with
Arnab Dey − Department of Chemistry, Indian Institute of
Technology Bombay, Mumbai 400076, India; orcid.org/
0000-0001-6176-8775
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A. S. K. New and Selective Transition Metal Catalyzed Reactions of
Allenes. Angew. Chem., Int. Ed. 2000, 39, 3590−3593.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01521
(9) For reviews on allenes in C−H activation, see: (a) Santhoshku-
mar, R.; Cheng, C.-H. Fickle Reactivity of Allenes in Transition-
Metal-Catalyzed C-H Functionalizations. Asian J. Org. Chem. 2018, 7,
1151−1163. (b) Shukla, R. K.; Nair, A. M.; Volla, C. M. R. Allenes:
Versatile Building Blocks in Cobalt-Catalyzed C-H Activation. Synlett
2021, DOI: 10.1055/a-1471-7307.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(10) For selected references, see: (a) Zeng, R.; Fu, C.; Ma, S. Highly
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Glorius, F. Mild Rhodium(III)-Catalyzed CCH Activation and
Intermolecular Annulation with Allenes. Angew. Chem., Int. Ed.
We are grateful to SERB, India for supporting this activity
(CRG/2019/005059). We also thank Mr. Darshan Mhatre,
IIT Bombay for the single-crystal X-ray diffraction analysis.
2012, 51, 7318−7322. (c) Nakanowatari, S.; Muller, T.; Oliveira, J. C.
̈
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