Direct Allylic C(sp3)-H Alkylation with 2-Naphthols via Cooperative Palladium and Copper Catalysis: Construction of Cyclohexadienones with Quaternary Carbon Centers

Article abstract of DOI:10.1021/acs.orglett.8b02910

Oxidative allylic C-H alkylation with 2-naphthols was accomplished with excellent chemoselectivities and broad substrate scope through Pd(PPh₃)₄/Cu(MeCN)₄PF₆ cooperative catalysis under mild base-free conditions. Special tolerance was observed with peptides, allowing late-stage modifications of peptides. The transformation provides a general protocol to obtain functionalized cyclohexadienones with quaternary carbon centers under two alternative sets of conditions and serves as a complementary catalysis system for the dearomatization of 2-naphthols.

Full text of DOI:10.1021/acs.orglett.8b02910

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Direct Allylic C(sp3)‑H Alkylation with 2‑Naphthols via Cooperative
Palladium and Copper Catalysis: Construction of Cyclohexadienones
with Quaternary Carbon Centers
Ming Jin,† Wei Ren,† Dang-Wei Qian,† and Shang-Dong Yang*,†,‡
†State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
‡Key Laboratory of Functional Molecular Engineering of Guangdong Province, South China University of Technology, Guangzhou
510640, P. R. China
S
* Supporting Information
ABSTRACT: Oxidative allylic C−H alkylation with 2-naphthols was accomplished with excellent chemoselectivities and broad
substrate scope through Pd(PPh3)4/Cu(MeCN)4PF6 cooperative catalysis under mild base-free conditions. Special tolerance
was observed with peptides, allowing late-stage modifications of peptides. The transformation provides a general protocol to
obtain functionalized cyclohexadienones with quaternary carbon centers under two alternative sets of conditions and serves as a
complementary catalysis system for the dearomatization of 2-naphthols.
the prospects of these reactions is through the discovery of a
new activation mode for phenol dearomatization.
yclohexadienones with ortho-quaternary carbon centers
1
Care pivotal skeletons with special bioactivities for drug
In recent years, transition-metal-catalyzed oxidative allylic
C−H activation has witnessed significant breakthroughs and
diverse C−H functionalizations have been well estab-
lished.10−12 Cooperative catalysis,13 in which multiple chemical
interactions participate in synergy to achieve significant
enhancement in catalytic activity and/or selectivity, has also
enabled many new and challenging processes. Metal Lewis acid
catalysis has long been established as one of the most powerful
tools in organic synthesis.13e,14 Inspired by these advances, we
hypothesized that Lewis acids might be efficient to activate
naphthols for dearomatization as bases and to ensure good C/
O-allylation selectivities by changing the properties15 of
phenolic hydroxyl groups synchronously through coordination.
With the advantage of a cooperative Lewis acid catalyst,
transition-metal-catalyzed allylic C−H activation may allow
direct allylic C−H dearomatization to be conducted under
mild base-free conditions, allowing good chemical selectivities
and functional-group tolerance. On the basis of our endeavors,
a new method for the direct allylic C−H alkylation with 2-
naphthols through palladium/copper cooperative catalysis
discovery, excellent photoelectric performance for material
science,2 and great importance3 in the total synthesis of natural
products. Therefore, the efficient construction of cyclo-
hexadienone motifs has been of significant synthetic interest
in the chemical community.4 Recently, transition-metal-
catalyzed asymmetric allylic dearomatization, commonly
known as CADA reaction,5 has proven to be the most
effective strategy for preparing the target molecules from
simple phenol compounds.6 Previous work in this area focused
largely on preoxidized allylic electrophiles7 for the dearoma-
tization of naphthols, which required extra steps for
prefunctionalization. To overcome this limitation, some
elegant transformations have been developed (Scheme 1A),
including two reports on iridium-catalyzed allylic dearomatiza-
tion reaction by directly using allylic alcohols8 and one
example of a palladium-hydride-mediated dearomative allyla-
tion process.9 Although successful, one major constraint that
remained is that an alkaline system6b is needed for effective
dearomatization processes and good C/O-allylation selectiv-
ities; this is introduced either through the use of stoichiometric
amounts of basic additives or by using preoxidized substrates
in situ, which results in poor tolerance with competitive
electrophiles in more complex substrates. One way to improve
Received: September 11, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02910
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Strategies for Transition-Metal-Catalyzed Allylic
Dearomatization of 2-Naphthols
Table 1. Optimization of Reaction Conditions
under base-free conditions with high C-allylation selectivity
was developed (Scheme 1B). The novel chemistry enables
effective preparation of diverse cyclohexadienones with ortho-
quaternary carbon centers and shows special tolerance with
peptides.
Initially, naphthol 1a and allylbenzene 2a were chosen as
model substrates for the allylic dearomatization reaction. To
our delight, a palladium-BQ combination in acetonitrile was
found to catalyze the reaction, albeit in low yield of
dearomative product 3aa (Table 1, entry 1) after preliminary
screening. Subsequent control experiments indicated the
significance of palladium catalyst (entry 2) and BQ (entry 3)
for the reaction. With these initial results in hand, an extensive
exploration of additives, catalysts and ligands, oxidants, and
temperature was conducted (entries 4−28). The examination
of Lewis acids revealed that Ag2CO3, Sc(OTf)3, and CeCl3
were ineffective in the reaction, and the yield was greatly
improved to 42% with Cu2(OH)2CO3 (entries 4−7). Cu-
(CH3CN)4BF4 showed a comparable effect (entry 8) to
Cu2(OH)2CO3, and the best copper salt screened was
Cu(CH3CN)4PF6 (49% yield; entry 9). Elevated or reduced
amounts of copper salts all provided diminished yields (entries
7−9). The catalyst was then evaluated. The combinations of
PPh3 and palladium salts such as Pd(OAc)2, Pd(acac)2, and
Pd(dba)2 displayed reduced reactivity (entries 10−12).
Palladium chlorides failed to catalyze the reaction (entries
13−14). An extensive screening of phosphine ligands gave no
improved results (see Supporting Information (SI) for details).
Therefore, readily available Pd(PPh3)4 was chosen for further
screening. Subsequently, the effects of a series of benzoqui-
none-type oxidants were investigated (entries 15−18) and the
best result was achieved with 2,6-DMBQ (69% yield; entry
16). Notably, with the assistance of Cu(CH3CN)4PF6, the
reaction proceeded smoothly at 40 °C with a dramatically
improvement on yield (87%; entry 22). Elevated temperatures
provided lower yields16 (entries 19−21), and a reduction in
temperature to room temperature significantly decreased the
yield with the appearance of byproduct 3a′a′ (entry 23).
Furthermore, a diminished yield was observed when the
catalyst loading was reduced to 5 mol % (entry 24). Under air,
no reaction was detected (entry 25), indicating serious adverse
a
b
Yield of isolated product 3aa. Additive (20 mol %) was used.
c
d
e
Additive (100 mol %) was used. Conditions A. Yield of isolated
f
g
byproduct 3a′a′. Palladium (5 mol %) was used. Under air.
h
Conditions B: Palladium (2.5 mol %), complex of copper and ligand
L-26 (5.0 mol %) was used (see the SI for details). nr = no reaction.
effects of air on the active catalyst. As demonstrated by the
control experiments (entries 26−27), palladium catalyst and
the copper salt were both essential to the reaction. On the
basis of these results, a cooperative role of palladium catalyst
and copper additive allowed us to move forward to
investigating chirality and reaction optimization. Multiple
chiral ligands to control enantioselective C−H dearomatization
with palladium or copper catalysts were investigated, and it was
found that ligands had a vital effect on the transformation; only
racemic products were obtained with the screened ligands (see
the SI for details). Further optimization revealed that addition
of an achiral ligand L-26 with copper catalyst dramatically
enhanced the catalytic activity and 92% yield of dearomatiza-
tion product was isolated with only 2.5 mol % Pd(PPh3)4
(Table 1, entry 28). In all, the optimization studies provided
two sets of conditions for the palladium/copper-catalyzed
allylic C−H dearomatization reaction: the general conditions
(conditions A) and an alternative set of conditions with lower
loading of catalysts (conditions B).
With the optimal conditions in hand, we explored the scope
of Pd-catalyzed allylic C−H dearomatization of 2-naphthols
under conditions A. The scope of the reaction with respect to
2-naphthols was first investigated (Scheme 2, conditions A).
1,3-Dialkyl substituted naphthols were good substrates for
allylic alkylation and all gave excellent yields (81−91%, 3aa,
B
DOI: 10.1021/acs.orglett.8b02910
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,
b
a,b
Scheme 2. Scope of Substituted Naphthols
Scheme 3. Scope of Allylbenzenes
a
Conditions A: Pd(PPh3)4 (0.02 mmol), Cu(CH3CN)4PF6 (0.10
mmol), 1a−n (0.20 mmol), 2,6-DMBQ (0.20 mmol), 2a (0.30
mmol), MeCN (1.0 mL), under argon for 48 h. Conditions B:
Pd(PPh3)4 (0.005 mmol), Cu(CH3CN)4PF6 (0.01 mmol), ligand L-
26 (0.01 mmol), 1a−n (0.20 mmol), 2,6-DMBQ (0.20 mmol), 2a
a
Conditions A: Pd(PPh3)4 (0.02 mmol), Cu(CH3CN)4PF6 (0.10
mmol), 1a (0.20 mmol), 2,6-DMBQ (0.20 mmol), 2a−i (0.30 mmol)
or 2j−n (0.24 mmol), MeCN (1.0 mL), under argon for 48 h.
Conditions B: Pd(PPh3)4 (0.005 mmol), Cu(CH3CN)4PF6 (0.010
mmol), ligand L-26 (0.010 mmol), 1a−n (0.20 mmol), 2,6-DMBQ
(0.20 mmol), 2a−i (0.30 mmol) or 2j−n (0.24 mmol), MeCN (1.0
b
(0.30 mmol), MeCN (1.0 mL), under argon for 64 h (see SI). Yield
of isolated product. O-Allylation byproduct was detected. nr = no
c
b
reaction. nd = not detected.
mL), under argon for 64 h (see SI). Yield of isolated product.
c
d
e
Byproduct was detected. Complex system was observed. The dr
value was determined by HPLC analysis.
3ca−da). However, a lack of substituents on the 3-position
greatly diminished the yield (51%; 3ba). In view of the
observations noted above, substitutes on the 3-position were
examined further. Fluorine and chlorine groups were well
tolerated in the reaction, but the more active iodine atom
completely restrained the transformation (3ea−ga). Naphthols
with substituted phenyl groups on the 3-position also reacted
smoothly with good yields (3ha−ia). Furthermore, excellent
yields (77−80%, 3ja−la) were also observed with methyl and
phenyl groups on the other aromatic ring. Notably, alkynyl
groups were not compatible with the oxidative procedure and
only naphthofuran compound 3ma was obtained. Phenanthrol
1n, with weaker aromaticity, gave almost equivalent yield
(97%; 3na).
The scope of the reaction with allylbenzenes was then
explored (Scheme 3; conditions A). Various allylbenzenes
showed good reactivities to naphthol 1a, regardless of whether
they had electron-donating or -withdrawing substituents (70−
86%, 3aa−aj). 1,4-Diene also worked in this reaction, albeit in
low yield (3ak). In comparison, only a complex system was
observed with the reaction of phenyl olefin 2l. Estrone-derived
allylbenene 2m was a good substrate for this reaction. Notably,
substrates 2n−p, with peptide fragments, were well tolerated
with the chemistry, and excellent yields were achieved,
indicating good tolerance of the Pd/Cu catalytic system and
the potential utilities in the late-stage modification of peptides.
In comparison with conditions A, comparable or better
yields were achieved under conditions B for most products
(Schemes 2 and 3) and good tolerance with peptides remained
(Scheme 3, 3an−ap), indicating the robustness of this catalysis
system. However, for naphthol 1c with a large n-Bu group and
electron deficient allylbenzene 2j, the transformations failed
when a steric bulky ligand L-26 was introduced to the reaction.
Under conditions B, the catalysis was sensitive to C−Cl bonds
and substrates such as 1f and 2h were not tolerated any
further.
To further demonstrate the potential utilities, trans-
formations based on the methodology and products were
made (Scheme 4). Easily accessible 1,3,5-triallylbenzene (2q)
was applied to the chemistry, and the high molecular weight
compound 3nq was obtained with a slight modification of
conditions A (Scheme 4a). The dearomative product 3pa was
also transformed into a rare and stable 5-hydroxy-Δ1-pyrroline
5pa (Scheme 4b) in good yield through simple trans-
formations; such species are of great synthetic and
pharmaceutical importance.17
Having established the generality of the methodology,
mechanism experiments were conducted to gain insight into
the reaction pathway. Under the standard conditions, the
byproduct 3a′a′ could not be transformed into product 3aa,
showing that allyl ether 3a′a′ is not a reaction intermediate
C
DOI: 10.1021/acs.orglett.8b02910
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Organic Letters
Letter
With the results illustrated above in hand, a plausible
catalytic pathway was proposed (Scheme 6). Initially, an
Scheme 4. Synthetic Applications
Scheme 6. Proposed Catalytic Pathway
(Scheme 5a). Furthermore, to verify the allypalladium
intermediate, a π-allylpalladium(II) complex [Pd(η3-cin)-
Scheme 5. Preliminary Mechanism Investigation
electrophilic palladium(II) species is generated in situ through
oxidation of palladium(0) precatalyst and subsequent
precoordination of palladium(II) to allylbenzene and proton
abstraction21 give rise to a highly active π-allylpalladium(II)
electrophile. Synchronously, coordination of copper(I) to
oxygen modulates the nucleophilic reactivity of phenolic
hydroxyl group in 2-naphthol and activates the naphthol to
be a pro-3° carbon nucleophile. Next, an electrophonic allylic
substitution with naphthol at the carbon center (path a)
proceeds. Finally, palladium(0) is oxidized to palladium(II) by
2,6-DMBQ under acidic conditions18 and the dearomatization
product is liberated simultaneously. The byproduct is the result
of allylic substitution on the nucleophilic oxygen center (path
b), which is an unfavorable process under the current
conditions.
In summary, we have disclosed a palladium/copper
cooperative catalysis for the direct oxidative allylic C−H
dearomatization of 2-naphthols under mild conditions. The
introduction of a cooperative copper catalyst allows highly
effective and selective C-allylation of naphthols under base-free
conditions. The special tolerance of the conditions toward
peptides demonstrates the high potential for synthetic
applications.
(PPh3)2PF6] was synthesized and used in the reaction. The
complex indeed catalyzed the reaction even in the absence of
copper(I)-salt, in spite of resulting in a mixture of
dearomatization product 3aa (57% yield) and allylic ether-
ification byproduct 3a′a′ (17% yield) (Scheme 5b), indicating
the involvement of a highly electrophilic allypalladium species.
Furthermore, the effect of copper salt was investigated
(Scheme 5c). The product was obtained in 87% yield under
the standard conditions A, whereas no product was detected
without Cu(CH3CN)4PF6 or with other additives such as
AgPF6, KPF6, or NaPF6, showing the indispensable role of the
copper cation for the allylic dearomatization process. The
effects of copper additive were further evaluated. During the
reaction, Cu(MeCN)4PF6 may increase the activity of
palladium catalyst through anion exchange and act as a
Lewis acid to activate naphthols for dearomatization. On the
other hand, copper(I) can also be oxidized to CuII and be
involved in the oxidation18 of palladium(0) to palladium(II)
with 2,6-DMBQ. To illustrate these processes, ESR experi-
ments (see SI for details) were conducted to detect a possible
CuII signal. The ESR spectra of the reaction system showed no
signal corresponding to CuII,19 thereby discounting a
cooperative oxidation role of copper salt. On the basis of
previous reports, a more likely role of copper(I) salt as a Lewis
acid20 for naphthol dearomatization is proposed.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02910.
Experimental details and characterization data for all
new compounds (PDF)
Accession Codes
CCDC 1839679 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.
D
DOI: 10.1021/acs.orglett.8b02910
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yangshd@lzu.edu.cn.
ORCID
́
R.; Pilarski, L. T.; Pershagen, E.; Szabo, K. J. J. Am. Chem. Soc. 2012,
134, 8778−8781. (h) Braun, M. G.; Doyle, A. G. J. Am. Chem. Soc.
2013, 135, 12990−12993. (i) Jiang, H.-F.; Yang, W.-F.; Chen, H.-J.;
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(j) McAtee, J. R.; Yap, G. P. A.; Watson, D. A. J. Am. Chem. Soc.
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M.; Thaisrivongs, D. A. Angew. Chem. 2012, 124, 5034−5037.
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Shang-Dong Yang: 0000-0002-4486-800X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the NSFC (nos. 21472076 and 21532001)
and the Open Fund of the Key Laboratory of Functional
Molecular Engineering of Guangdong Province (no.
2016B01017) financial support.
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