Domino Palladium-Catalyzed Double Norbornene Insertion/Annulation Reaction: Expeditious Synthesis of Overcrowded Tetrasubstituted Olefins

Article abstract of DOI:10.1021/acs.orglett.9b01543

A Pd-catalyzed domino process involving a double norbornene insertion/annulation reaction was developed for the expeditious synthesis of overcrowded olefins. In this one-pot reaction, four new C-C bond formations were achieved by three consecutive Heck ca

Full text of DOI:10.1021/acs.orglett.9b01543

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Domino Palladium-Catalyzed Double Norbornene Insertion/
Annulation Reaction: Expeditious Synthesis of Overcrowded
Tetrasubstituted Olefins
Kanagaraj Naveen,†,‡ Paramasivan Thirumalai Perumal,*,‡ and Deug-Hee Cho*,†
†Advanced Industrial Chemistry Research Centre, Korea Research Institute of Chemical Technology, 45, Jongga-ro, Jung-gu, Ulsan
44412, Republic of Korea
‡Organic and Bio-organic Chemistry, Central Leather Research Institute (CSIR), Adyar, Chennai 600 020, India
S
* Supporting Information
ABSTRACT: A Pd-catalyzed domino process involving a double
norbornene insertion/annulation reaction was developed for the
expeditious synthesis of overcrowded olefins. In this one-pot reaction,
four new C−C bond formations were achieved by three consecutive Heck
carbopalladations and C−H activation across the CC triple bond of 2-
alkynyl bromobenzenes with two norbornene rings via a reactive vinylic-
Pd(II) species. This protocol provides a step-economical synthetic system
to access the structurally identical molecular machine motifs of
overcrowded olefins with high yields.
method offers a straightforward way to achieve a wide range of
vercrowded tetrasubstituted olefin compounds have
Oattracted much interest in the past decade due to their norbornene-fused tetrasubstituted olefins in good yields.
Though the norbornene insertion reactions with a CC
iterative application in the field of artificial molecular machines
triple bond or a C−C single bond are scarce,10 most of the
disclosed methods deal with the bond formation of a CC
double bond and a norbornene ring.11 For example, Kosugi et
al. developed the stereoselective double norbornene insertion
reactions with 1-alkenyltin trichlorides (Scheme 1a).11e In this
contrast, we set out to explore double norbornene insertion
reactions with alkyne to construct the synthetically robust bis-
fluorene scaffold. This unexplored double norbornene
(AMMs) designing.1 Recently, various sterically overcrowded
olefin derivatives were synthesized and employed in the light-
driven molecular switches and molecular motors,2 and further,
these kinds of motifs were used as a fluorescent material in the
organic light-emitting diodes (OLEDs) and fluorescent
bioprobes designing.3 Typically, McMurry coupling or
Barton-Kellogg reactions are applied for the synthesis of
symmetrical and unsymmetrical tetrasubstituted olefins.4 In
this context, Lautens et al. developed an attractive domino
approach based on the Catellani reaction to access sterically
crowded olefins via a Pd-catalyzed multiple C−H activation.5
Consecutively, Tietze et al. and Perumal et al. realized the
synthesis of multisubstituted olefins through Pd-catalyzed
domino reactions.6 Although these methods have made a
significant contribution to the synthesis of sterically hindered
tetrasubstituted olefins, the development of a simple and step-
economical domino approach for their efficient preparation is
still highly desirable.
Scheme 1. Strategies for Pd-Catalyzed Double Norbornene
Insertion Reactions
The insertion of norbornene and its derivatives into the
organic molecules is creating a great impact in the field of
medicinal and pharmaceutical chemistry.7 These types of
insertion reactions are established with particular functional
groups in the literature.8 In our continuous research on Pd-
catalyzed reactions, we recently disclosed a first Pd-catalyzed
single norbornene insertion reaction across alkyne derivatives
for the efficient synthesis of tetrasubstituted olefins.9 This
Received: May 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01543
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
insertion reaction provides a synthesis of structurally similar
molecular machine motifs of overcrowded olefins. Therefore,
we desire to develop this unexplored reaction in a step
economical manner using easily accessible starting materials
such as 2-alkynylaryl bromobenzene and norbornene in one-
pot (Scheme 1b). To the best of our knowledge, this is the first
reductive difunctionalization reaction of alkyne with norbor-
nene to synthesize the multisubstituted olefin compounds.
To investigate this domino Pd-catalyzed double norbornene
insertion/annulation reaction, 1-bromo-2-(p-tolylethynyl)-
benzene 1a and norbornene 2 were used as trial substrates
under different reaction conditions (see Supporting Informa-
tion for more details). An extensive survey of various reaction
parameters revealed that 10 mol % of Pd(OAc)2, 20 mol % of
PPh3, and 3.0 equiv of K2CO3 in DMF at 100 °C under N2
atmosphere were considered as optimal reaction conditions to
this title reaction. Single-crystal X-ray diffraction analysis
confirmed the structure of tetrasubstituted olefin 3a (Scheme
at the para-position of the alkynylaryl moiety. All of these
substitutions underwent smooth conversion to afford the
desired product of multisubstituted olefins (3b,c and 3e−i) in
74−95% yields. Remarkably, the biphenyl substrate 1d
exhibited good reactivity to produce the desired product 3d
in 82% yield. Interestingly, when the bromobenzene ring
bearing various substituents at the meta-position of the R1 (H,
−Me, −CO2Me, −CN, and −Cl) were treated with
norbornene, it provided the corresponding products (3k−o)
in 80−95% yields. Subsequently, different substitutions on
both rings were also explored in our title reaction conditions
with norbornene and were found to provide the resultant olefin
products (3p−s) in 72−96% yields. Under the optimized
conditions, 2,4-dichloroalkynylaryl bromobenzene 1j was
successfully transformed to the expected product 3j in 70%
yield. Furthermore, the substrate 1t has two possible C−H
activation sites. The reaction proceeded preferentially at the
least sterically crowded site to generate the major product
3t(a) and the minor product 3t(b) from the crowded site.
Next, a gram-scale reaction of 1a (1.0 g) was performed to test
the practicality of this protocol, which produced an isolated
yield of 90% (1.26 g) of 3a (Scheme 2). Functional groups like
ether, amine, ester, keto, nitro, nitrile, and chloro were highly
compatible in our reaction conditions, providing opportunities
for further elaboration of these multisubstituted olefins. In this
protocol, 2-alkynylaryl bromobenzene, bearing electron-with-
drawing groups, provided excellent yields compared with
electron-rich 2-alkynylaryl bromobenzene. Further, the exact
structure of the major isomer 3k was confirmed by single-
crystal X-ray analysis.12
2).12 Further characterization by H and 13C NMR spectros-
1
copy of the diastereomeric mixture 3a/3′a was performed and
identified 3a as a major diastereomer.
Scheme 2. Scope of 2-Alkynylaryl Bromobenzenes with
a
Norbornene
To further explore the synthetic versatility of the double
norbornene insertion/annulation reactions, the scope of 2-
alkynylheteroaryl bromobenzenes (1u−z) was subjected to
optimized reaction conditions (Scheme 3). The Pd-catalyzed
domino reaction of 2-alkynylthienyl 1u with norbornene
afforded our desired product 3u in 72% yield. A slightly
lower yield of product 3v was obtained for the 3-alkynylthienyl
substrate. Gratifyingly, excellent yields of norbornene-incorpo-
rated overcrowded olefin products (3w,x) were isolated when
electron-withdrawing groups (−CO2Me and −CN) were
Scheme 3. Scope of 2-Alkynylheteroaryl Bromobenzenes
a
with Norbornene
a
Reactions were carried out with 1a−t (0.3 mmol), norbornene 2
(3.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), and K2CO3
(3.0 equiv) in DMF (4 mL) at 100 °C under N2 atmosphere. Isolated
yield by flash column chromatography, and ratios of the diastereomers
b
were determined by 1H NMR. 20 mol % of Pd(OAc)2 and 40 mol %
c
of PPh3 for 5 h. 3.68 mmol scale of 1a. Major isomer only presented
in this table.
Under optimized reaction conditions, the scope of the Pd-
catalyzed double norbornene insertion/annulation reaction of
various 2-alkynylaryl bromobenzenes (1a−t) was examined
with norbornene 2 (Scheme 2). The experimental results
indicated that the electronic effects of the starting materials
played a significant role in reaction efficiency, regardless of the
diastereomeric ratio of the products. Initially, we investigated
the reaction of the different R2 substitution, which possesses
electron-donating (−OMe and −NEt2) and electron-with-
drawing (−CO2Me, −COMe, −NO2, −CN, and −Cl) groups
a
Reactions were carried out with 1u−z (0.3 mmol), norbornene 2
(3.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), and K2CO3
(3.0 equiv) in DMF (4 mL) at 100 °C under N2 atmosphere. Isolated
yield by flash column chromatography, and ratios of the diastereomers
were determined by 1H NMR. Major isomer is only presented in this
table.
B
DOI: 10.1021/acs.orglett.9b01543
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Letter
introduced at the bromobenzene ring (R) of 3-alkynylthienyl
substrates (1w,x). The double norbornene insertion reaction
was not successful when we employed the 2-alkynylpyridyl and
3-alkynylindole bromobenzene (1y,z) substrates in our general
reaction. In addition, the standard reaction conditions were
favorable for norbornadiene 4 to undergo the desired domino
reaction with 1a to provide the norbornadiene-fused over-
crowded olefin 5a/5′a in 30% yield (Scheme 4).
Scheme 6. Competitive Experiments
Scheme 4. Pd-Catalyzed Domino Reaction of 1a with
Norbornadiene
was obtained between 3f/3a, which suggests that the electronic
effects do not have a strong influence. Further, these results
confirm an incompatibility for an electrophilic aromatic
substitution mechanism in the C−H activation step. A
competition experiment between substrate 1e with norbornene
2 and diphenylacetylene 9 was conducted (Scheme 6c). This
study demonstrates the developed catalytic system was only
applicable for selective norbornene insertion reactions across
2-alkynyl bromobenzenes, and norbornene was the key
substrate in the success of this transformation.
After establishing the scope of 2-alkynylaryl/heteroaryl
bromobenzene derivatives, we examined the influence of the
π-system containing 2-alkynylaryl bromobenzenes (6a,b) with
norbornene (Scheme 5). Under optimal reaction conditions,
Scheme 5. Reaction of π-System Containing 2-Alkynylaryl
Bromobenzenes
Based on previous reports11 and competition experimental
studies, a plausible reaction mechanism for a double
norbornene insertion/C−H activation annulation reaction
was proposed (Scheme 7). First, the oxidative addition of 1k
Scheme 7. A Plausible Mechanism for Pd-Catalyzed Double
Norbornene Insertion/Annulation Reaction
the 2-phenoxy 6a substrate afforded a mixture of products with
the desired double norbornene moiety 7a/7′a in 38% yield and
another single norbornene insertion product 7b13 in 48% yield.
The substrate 6b did not undergo the desired reaction to
provide product 8a, instead it yielded a single norbornene
inserted product 8b with 40% yield by C−Hb activation.9 This
is due to the stronger coordination of the aryl ring π-system
(phenyl compare to phenoxy) with active vinylic-Pd(II)
intermediate and thereby limits the reactivity of a second
norbornene insertion reaction and did not produce a double
norbornene insertion product. This result strongly suggests the
regio- and stereoselective formation of a key vinylic-Pd(II)
intermediate C via syn-carbopalladation of cis-exo-arylnorborn-
yl Pd(II) bromide complex B (Scheme 7).
Next, the mechanistic insight was obtained through
intramolecular kinetic isotope effect (KIE) studies using the
substrate 1a-D1 and norbornene 2. Under standard conditions,
the corresponding products 3a/3a-D1 were obtained in 95%
yield with an intramolecular KIE of 3.3 (Scheme 6a). This high
KIE clearly indicates that the C−H activation step was
involved in the C−H bond cleavage and is incompatible for an
electrophilic aromatic substitution-type mechanism.14 To
further deduce the mechanistic pathway, an intermolecular
competition experiment between the electron-rich 1a and
electron-deficient 1f of 2-alkynylaryl bromobenzenes with
norbornene 2 was performed (Scheme 6b). A ratio of 1.1:1.0
to Pd(0) would afford the active organo Pd(II) intermediate A,
which undergoes intermolecular carbopalladation with norbor-
nene 2 on the exo face exclusively to generate the cis-exo-
arylnorbornyl Pd(II) bromide complex B. This intermediate
undergoes intramolecular syn-carbopalladation on the tethered
alkyne to generate an active key vinylic-Pd(II) species C with
regio- and stereoselective manner. Subsequently, C would
C
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Letter
undergo intermolecular carbopalladation via either the steri-
cally favored endo face or sterically nonfavored exo face of
another norbornene molecule, leading to intermediates D or F,
respectively. Formation of D is favored over that of F due to
less steric hindrance in the former. Further, these intermediates
undergo C−H activation with the adjacent aryl ring to form
the corresponding palladacycles E and G. Finally, reductive
coupling of the intermediates E and G gives the final products
3k and 3′k with regeneration of the active Pd(0) catalyst in the
presence of a base.
Notably, all of the synthesized overcrowded olefins displayed
interesting photoluminescence (PL) properties.15 For instance,
the PL spectrum of 3k/3′k in a thin-film state showed
absorption and emission maxima at 357 and 466 nm,
respectively (Figure 1a). This result indicates the aggrega-
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01543.
■
S
Experimental procedures, optimization details, character-
ization data, copies of NMR spectra of all compounds,
and crystallographic data for 3a, 3k and 7b (PDF)
Accession Codes
CCDC 1020303, 1483642, and 1872729 contain the
supplementary crystallographic 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 Authors
■
*E-mail: dhcho@krict.re.kr.
*E-mail: ptperumal@gmail.com.
ORCID
Figure 1. (a) Normalized absorption (black line) and PL (blue line)
spectra of 3k/3′k in a thin-film state. (b) PL spectra of 3k/3′k in
THF and THF/H2O mixtures. Concentration: 10 μM and excitation
wavelength: 310 nm. (c) Plot of PL peak intensity of 3k/3′k vs water
fractions in THF/H2O mixtures. Photos of 3k/3′k in THF/H2O
mixtures (fw = 99, 90, 80, and 70%) taken under UV luminescence.
Paramasivan Thirumalai Perumal: 0000-0003-2057-6368
Deug-Hee Cho: 0000-0002-0682-7020
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
tion-induced emission (AIE) phenomenon of 3k/3′k
attributed to the restriction of intramolecular motions.3 To
investigate the AIE behavior of 3k/3′k, its PL spectra were
recorded in THF/H2O mixtures (Figure 1b and 1c). In THF/
H2O mixtures with fw = 0−80%, 3k/3′k emitted a faint
fluorescence. The fluorescence intensity of 3k/3′k was
We thank the Korea Electric Power Corporation Open R & D
program (KEPCO-RI7XH03) for financial support. The
author K.N. thanks the Council of Scientific and Industrial
Research (CSIR), New Delhi, India for the research fellowship.
REFERENCES
■
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In conclusion, we have demonstrated a highly efficient Pd-
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E
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