Highly Chemo-, Regio- and E/Z-Selective Intermolecular Heck-Type Dearomative [2 + 2 + 1] Spiroannulation of Alkyl Bromoarenes with Internal Alkynes

Article abstract of DOI:10.1021/acs.orglett.9b00099

Described herein is a palladium-catalyzed dearomative annulation of alkyl bromoarenes with internal alkynes. Challenges in this spiroannulation include the chemoselectivity among [2 + 2 + 1], [2 + 2 + 2], and [3 + 2] annulations and the E/Z-selectivity associated with the generated exocyclic double bond. In the presence of Pd(OAc)₂ and a phosphine ligand, a variety of highly functionalized spirocyclopentadienes with an exocyclic carbon-carbon double bond are provided in good to excellent yields with high chemo-, regio-, and E/Z-selectivity via a Heck-type pathway.

Full text of DOI:10.1021/acs.orglett.9b00099

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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Highly Chemo‑, Regio- and E/Z-Selective Intermolecular Heck-Type
Dearomative [2 + 2 + 1] Spiroannulation of Alkyl Bromoarenes with
Internal Alkynes
Xingrong Liao, Deping Wang, Yueyuan Huang, Yudong Yang,* and Jingsong You*
Key Laboratory of Green Chemistry and Technology of the Ministry of Education, College of Chemistry, Sichuan University, 29
Wangjiang Road, Chengdu 610064, P.R. China
S
* Supporting Information
ABSTRACT: Described herein is a palladium-catalyzed
dearomative annulation of alkyl bromoarenes with internal
alkynes. Challenges in this spiroannulation include the
chemoselectivity among [2 + 2 + 1], [2 + 2 + 2], and [3 +
2] annulations and the E/Z-selectivity associated with the
generated exocyclic double bond. In the presence of
Pd(OAc)2 and a phosphine ligand, a variety of highly
functionalized spirocyclopentadienes with an exocyclic carbon−carbon double bond are provided in good to excellent yields
with high chemo-, regio-, and E/Z-selectivity via a Heck-type pathway.
he development of highly efficient methods for the rapid
assembly of complex molecules with a high level of
such as indoles, pyrroles, and pyridines, as substrates owing to
their favorable keto−enol tautomerism and/or lower reso-
nance stabilization energy (22 kcal/mol for pyrrole and 27
kcal/mol for pyridine).5,9−14 Furthermore, as compared with
the extensively investigated enantioselective dearomatization
reactions,11,12,13c the transition-metal-catalyzed dearomative
spiroannulations involving an E/Z selectivity is seldom
disclosed. Thus, the development of novel dearomative
spirocyclizations that enable researchers to overcome the
above substrate restriction is still in high demand. Herein, we
disclose a palladium-catalyzed intermolecular dearomative
annulation of readily available alkyl bromoarenes with internal
alkynes for the one-step synthesis of spiro[4,5]-
cyclopentadienes with an exocyclic double bond (Scheme 1).
The introduction of an alkyl group onto the aromatic ring
allows the reaction to undergo a [2 + 2 + 1] annulation, in
which a Heck-type pathway is involved, rather than a common
[2 + 2 + 2] or [3 + 2] annulation.
Our investigation began with the reaction of 1-bromo-2-
methylnaphthalene (1a) and diphenylacetylene (2a) as the
model substrates (Table 1). To our delight, the desired
dearomatizative product 3a was obtained in 70% yield in the
presence of Pd(OAc)2 (5 mol %), PPh3 (12 mol %), and
Cs2CO3 (1.0 equiv) in 1,4-dioxane (Table 1, entry 1). Other
common phosphine ligands, including PCy3, P(p-tol)3, P(p-
anisyl)3, JohnPhos ((2-biphenyl)di-tert-butylphosphine), and
Cy-JohnPhos ((2-biphenyl)dicyclohexylphosphine), were also
effective for this transformation (Table 1, entries 2−6). P(p-
anisyl)3 proved to be the most suitable ligand, giving 3a in 94%
yield (Table 1, entry 4). However, the trial with X-Phos led to
a rather poor yield (Table 1, entry 7). The reaction could also
T
chemo-, regio-, and stereoselectivity from easily available
precursors is a long-standing objective in organic synthetic
chemistry.1,2 In particular, the achievement of chemo- and
regioselectivity switch by alternation of the reaction pathway to
afford distinctly different architectures would be highly
attractive.3 Transition-metal-catalyzed annulation reactions of
haloarenes with diarylacetylenes are considered as one of the
most straightforward and efficient approaches for the
construction of highly functionalized polycyclic hydrocarbons,4
which widely exist in organic optical and electronic materials.
Typically, the in situ generated arylmetallic species would
attack the internal alkyne(s) to form an alkenylmetallic
intermediate, which then approach the peri-sites of the
aromatic ring to give a formal [2 + 2 + 2] or [3 + 2]
annulated product.5 In contrast, the attack of the alkenylme-
tallic species at the ipso-position to deliver a spirocyclic
skeleton is rather difficult owing to the challenge in breaking
the aromaticity (resonance stabilization energy for phenyl ring,
ca. 30 kcal/mol).6 Moreover, the highly congested structures
of the target products (at least five carbon cycles around the
pentadiene motif) further hamper the [2 + 2 + 1] annulation of
haloarenes with diarylacetylene.
Spirocycles are prevalent core structures in natural products,
bioactive molecules, organic functional materials, and chiral
ligands.7,8 Among the known synthetic methods, transition-
metal-catalyzed dearomatization of aromatic compounds,
which is pioneered by Hamada,9 Buchwald,10 You,11 Feringa,12
and Luan,13 is regarded as one of the most efficient and reliable
approaches to construct three-dimensional spirocycles from
planar arenes. Despite significant progresses, the reported
methods typically focus on using electronically activated
aromatics, such as phenols and naphthols, and heteroarenes,
Received: January 8, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00099
Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters
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a
Scheme 1. Transition-Metal-Catalyzed Dearomative
Spiroannulation of Aromatics with Internal Alkynes
Scheme 2. Scope of Internal Alkynes
a
Table 1. Optimization of the Reaction Conditions
a
Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), Pd(OAc)2 (5
mol %), P(p-anisyl)3 (12 mol %), and Cs2CO3 (1.0 equiv) in 1,4-
b
b
dioxane (1 mL) at 130 °C for 12 h under an N2 atmosphere. Using
entry
ligand
PPh3
PCy3
base
solvent
yield (%)
c
1-iodo-2-methylnaphthalene instead of 1a. Three millimole scale at
1
2
3
4
5
6
7
8
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
Na2CO3
K3PO4
KOtBu
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
toluene
DMF
70
65
76
94 (88)
72
d
e
18 h. Using Cy-JohnPhos instead of P(p-anisyl)3. Without ligand.
f
1,6-Diyne (0.3 mmol) as the substrate and toluene as the solvent.
P(p-tol)3
c
P(p-anisyl)3
JohnPhos
Cy-JohnPhos
X-Phos
with 1a to afford the corresponding dearomatizative products
in moderate to good yields. Diaryl alkynes containing electron-
donating, -neutral, and -withdrawing groups at different
positions smoothly underwent the expected transformation
under the standard conditions (Scheme 2, 3b−h). Good
functional group tolerance was observed, and even chloro
could survive in the present palladium catalytic conditions. In
general, the spiroannulation reaction using unsymmetrical
alkynes may lead to three regioisomers. Interestingly, the
attempts with unsymmetrical phenyl alkyl alkynes exclusively
delivered the head-to-tail regioisomers in moderate yields
(Scheme 2, 3i and 3j). The resulting unsymmetrical cyclo-
pentadiene moieties were identified by the signals for alkyl
groups in 1H and 13C NMR spectra. Surprisingly, when dialkyl
alkynes were subjected to the standard conditions, [3 + 2]
annulation rather than [2 + 2 + 1] annulation took place
predominately, leading to a large amount of methyl-1,2-
dialkylacenaphthalenes. However, the desired spiroannulation
reactions could proceed smoothly in the absence of a
phosphine ligand, giving 3k and 3l in moderate yields. In
addition, 1,6-diyne (2m) also worked under the palladium
catalytic conditions, furnishing a tetracyclic architecture in 45%
yield. The attempts using terminal alkynes such as phenyl-
acetylene and 1-hexyne gave rise to the alkynylation products.
It would be highly challenging to achieve the E/Z-selectivity
in the [2 + 2 + 1] annulation of alkynes and bromoarenes
bearing other alkyl groups rather than the methyl group owing
to the geometric isomerism of the generated exocyclic double
bond. Indeed, when 1-bromo-2-ethylnaphthalene was sub-
jected to the standard conditions, a mixture of E/Z-isomers
(E/Z = 3/1) were obtained in an 85% total yield (see SI). To
improve the stereoselectivity (E/Z-selectivity), the phosphine
ligands were further optimized. Cy-JohnPhos was found to be
92
<10
42
62
43
<10
<10
80
34
30
−
9
P(p-anisyl)3
P(p-anisyl)3
P(p-anisyl)3
P(p-anisyl)3
P(p-anisyl)3
P(p-anisyl)3
P(p-anisyl)3
P(p-anisyl)3
P(p-anisyl)3
10
11
12
13
14
15
16
17
THF
DCE
dioxane
n.d.
d
78
a
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), Pd(OAc)2 (5
mol %), ligand (12 mol %), and base (1.0 equiv) in solvent (1 mL) at
b
130 °C for 12 h under an N2 atmosphere. NMR yield using
c
dibromethane as an internal standard. Isolated yield in parentheses.
d
Under an air atmosphere. JohnPhos = (2-biphenyl)di-tert-butyl-
phosphine; Cy-JohnPhos = (2-biphenyl)dicyclohexylphosphine.
take place even in the absence of a phosphine ligand (Table 1,
entry 8). The attempts with several common bases instead of
Cs2CO3 all led to inferior results (Table 1, entries 9−12).
Subsequently, the solvents were screened. Toluene exhibited a
slightly inferior efficiency for this reaction (Table 1, entry 13).
Polar solvent DMF and ether solvent THF afford 3a in only
34% and 30% yields, respectively (Table 1, entries 14 and 15).
No desired reaction was detected when DCE was used (Table
1, entry 16). Finally, an attempt under air was conducted,
giving the target product 3a in 78% yield (Table 1, entry 17).
With the optimized condition in hand, we next explored the
substrate scope of alkynes (Scheme 2). A diverse set of
symmetrical and unsymmetrical internal alkynes could react
B
DOI: 10.1021/acs.orglett.9b00099
Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters
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the most effective, providing 4a in 79% yield with a high E/Z
value (E/Z = 10/1). Other alkyl bromoarenes also successfully
delivered the desired products in moderate to excellent yields
with a high E/Z value (E/Z > 20/1) (Scheme 3, 4b−h). The
Scheme 4. Pd-Catalyzed Dearomative [3 + 2]
Spiroannulation Reaction
a
Scheme 3. Scope of Alkyl Bromoarenes
properties indicated that the electron-donating substituents
were more favorable to this transformation (Scheme 5a).
Scheme 5. Competition Experiments and Kinetic Isotope
Effect Experiments
a
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), Pd(OAc)2 (5 mol
%), P(p-anisyl)3 (12 mol %), and Cs2CO3 (1.0 equiv) in 1,4-dioxane
Surprisingly, when the reaction with an equimolar mixture of
diphenyl acetylene and methyl phenyl acethylene was
conducted, only one reigoisomer 3i was observed, which
resulted from the annulation of 1a with 2i (Scheme 5b).
Finally, a kinetic isotopic effect (KIE) value of 1.22 was
observed in the competition reaction between 1a and 1a-d3
with 2a (Scheme 5c). This result suggested that β−H
elimination might not be in the rate-determining step.
On the basis of the above results and previous
reports,5a,c,13d,15 a tentative mechanism is proposed in Scheme
6. Initially, migratory insertion of internal alkyne 2 into the in-
situ-formed arylpalladium intermediate I delivers an alkenyl-
b
(1 mL) at 130 °C for 12 h under an N2 atmosphere. Using Cy-
c
JohnPhos instead of P(p-anisyl)3. Using Pd(dppf)Cl2 instead of
Pd(OAc)2 and P(p-anisyl)3. Pd(dppf)Cl2
(diphenylphosphino)ferrocene]dichloropalladium(II).
= [1,1′-bis-
configuration of 4d was confirmed unambiguously by X-ray
crystallographic analysis. When the chain length of the ortho-
substituted alkyl groups are longer than that of the propyl
group, the employment of Cy-JohnPhos instead of P(p-anisyl)3
was not necessary (Scheme 3, 4e−h). In addition to alkyl
bromonaphthalenes, alkyl-substituted bromophenanthrenes
and bromobenzenes could also participate in this reaction.
However, the reaction with 3-butyl-4-bromophenanthrene
gave only the spirocyclic product 4h in 38% yield, probably
owing to large steric hindrance. It is worth noting that the
bromoarenes with alkyl substituents at the para site were also
effective substrates (Scheme 3, 4k). When bromobenzenes
bear aliphatic chains at both ortho and para positions, the β−H
elimination selectively took place at the para substituent
(Scheme 3, 4l and 4m).
Scheme 6. Plausible Mechanistic Pathway
Additionally, the biaryl-based substrates, which bear the
bromo and alkyl groups at different aromatic rings, could also
work under the standard conditions, as exemplified by the
reaction of 1-(2-bromophenyl)-4-methylnaphthalene (1n)
with diphenylacetylene (Scheme 4).
Next, two intermolecular competition experiments were
conducted to clarify the reactivity of alkynes. The competition
reaction between two diaryl alkynes with different electronic
C
DOI: 10.1021/acs.orglett.9b00099
Org. Lett. XXXX, XXX, XXX−XXX

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2018, 5, 3410. (e) Zhu, G.; Li, Y.; Bao, G.; Sun, W.; Huang, L.; Hong,
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dearomative [2 + 2 + 1] annulation reaction of simple alkyl
bromoarenes with internal alkynes to afford a variety of highly
functionalized spirocyclopentadienes in good to excellent
yields with high chemo-, regio-, and E/Z-selectivity. The
alkyl group on the aromatic ring of bromoarenes proved to be
critical for the 5-exo-trig cyclization via a Heck-type pathway.
Further extension of the substrate scope and investigation on
the applications of the spirocyclopentadienes are ongoing.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00099.
■
S
Detailed experimental procedures, characterization data,
1
and H and 13C NMR spectra of final products (PDF)
Accession Codes
CCDC 1877013 contains 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(10) Rousseaux, S.; García-Fortanet, J.; Del Aguila Sanchez, M. A.;
Buchwald, S. L. J. Am. Chem. Soc. 2011, 133, 9282.
AUTHOR INFORMATION
Corresponding Authors
(11) (a) Wu, Q.-F.; He, H.; Liu, W.-B.; You, S.-L. J. Am. Chem. Soc.
2010, 132, 11418. (b) Wu, Q.-F.; Liu, W.-B.; Zhuo, C.-X.; Rong, Z.-
Q.; Ye, K.-Y.; You, S.-L. Angew. Chem., Int. Ed. 2011, 50, 4455.
(c) Zhuo, C.-X.; Liu, W.-B.; Wu, Q.-F.; You, S.-L. Chem. Sci. 2012, 3,
205. (d) Zheng, J.; Wang, S.-B.; Zheng, C.; You, S.-L. J. Am. Chem.
Soc. 2015, 137, 4880. (e) Cheng, Q.; Wang, Y.; You, S.-L. Angew.
Chem., Int. Ed. 2016, 55, 3496. (f) Wu, W.-T.; Xu, R.-Q.; Zhang, L.;
You, S.-L. Chem. Sci. 2016, 7, 3427. (g) Xia, Z.-L.; Zheng, C.; Wang,
S.-G.; You, S.-L. Angew. Chem., Int. Ed. 2018, 57, 2653. (h) Yang, Z.-
P.; Jiang, R.; Wu, Q.-F.; Huang, L.; Zheng, C.; You, S.-L. Angew.
Chem., Int. Ed. 2018, 57, 16190.
■
*E-mail: yangyudong@scu.edu.cn.
*E-mail: jsyou@scu.edu.cn.
ORCID
Yudong Yang: 0000-0002-7142-2249
Jingsong You: 0000-0002-0493-2388
Notes
The authors declare no competing financial interest.
(12) Rudolph, A.; Bos, P. H.; Meetsma, A.; Minnaard, A. J.; Feringa,
B. L. Angew. Chem., Int. Ed. 2011, 50, 5834.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
(NSFC) for financial support (21502123 and 21432005).
■
(13) (a) Nan, J.; Zuo, Z.; Luo, L.; Bai, L.; Zheng, H.; Yuan, Y.; Liu,
J.; Luan, X.; Wang, Y. J. Am. Chem. Soc. 2013, 135, 17306. (b) Gu, S.;
Luo, L.; Liu, J.; Bai, L.; Zheng, H.; Wang, Y.; Luan, X. Org. Lett. 2014,
16, 6132. (c) Yang, L.; Zheng, H.; Luo, L.; Nan, J.; Liu, J.; Wang, Y.;
Luan, X. J. Am. Chem. Soc. 2015, 137, 4876. (d) Bai, L.; Yuan, Y.; Liu,
J.; Wu, J.; Han, L.; Wang, H.; Wang, Y.; Luan, X. Angew. Chem., Int.
Ed. 2016, 55, 6946. (e) Zuo, Z.; Wang, H.; Fan, L.; Liu, J.; Wang, Y.;
Luan, X. Angew. Chem., Int. Ed. 2017, 56, 2767. (f) Fan, L.; Liu, J.; Bai,
L.; Wang, Y.; Luan, X. Angew. Chem., Int. Ed. 2017, 56, 14257. (g) Bai,
L.; Liu, J.; Hu, W.; Li, K.; Wang, Y.; Luan, X. Angew. Chem., Int. Ed.
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DOI: 10.1021/acs.orglett.9b00099
Org. Lett. XXXX, XXX, XXX−XXX