Enaminones as Synthons for a Directed C?H Functionalization: RhIII-Catalyzed Synthesis of Naphthalenes

Article abstract of DOI:10.1002/anie.201603943

The use of enaminones as effective synthons for a directed C?H functionalization is reported. Proof-of-concept protocols have been developed for the Rh^III-catalyzed synthesis of naphthalenes, based on the coupling of enaminones with either alkynes or α-diazo-β-ketoesters. Two inherently reactive functionalities (hydroxy and aldehyde groups) are integrated into the newly formed cyclic framework and a broad range of substituents are tolerated, rendering target products readily available for further elaboration.

Full text of DOI:10.1002/anie.201603943

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International Edition: DOI: 10.1002/anie.201603943
German Edition: DOI: 10.1002/ange.201603943
Carbocyclization
À
Enaminones as Synthons for a Directed C H Functionalization:
RhIII-Catalyzed Synthesis of Naphthalenes
Shuguang Zhou, Jinhu Wang, Lili Wang, Chao Song, Kehao Chen, and Jin Zhu*
Abstract: The use of enaminones as effective synthons for
carbocycles have been obtained thus far.[7] To enable the
À
a directed C H functionalization is reported. Proof-of-concept
construction of an alternative scaffold, the six-membered
naphthalene ring structure, a weaker directing group, an
alkene moiety, is positioned one carbon atom away from the
phenyl ring to eliminate the five-membered-ring synthetic
pathway (Scheme 1b).[8] Using a weakly coordinating alkene
group, however, translates to, inevitably, several major draw-
backs associated with that PdII-based catalytic system:
1) required use of a strong acid for the generation of
a highly electrophilic PdII species, 2) no synthetically useful
functionalities integrated into the target framework during
the ring-forming process, 3) poor regioselectivity associated
with the alkyne coupling partner, and 4) incompatibility with
synthetically useful handles such as bromo and iodo sub-
stituents.
protocols have been developed for the RhIII-catalyzed synthesis
of naphthalenes, based on the coupling of enaminones with
either alkynes or a-diazo-b-ketoesters. Two inherently reactive
functionalities (hydroxy and aldehyde groups) are integrated
into the newly formed cyclic framework and a broad range of
substituents are tolerated, rendering target products readily
available for further elaboration.
N
aphthalenes are an important class of organic compounds
with applications in a wide range of academia and industrial
sectors.[1] A plethora of conventional synthetic methods have
been developed for accessing this type of highly useful
polyaromatic structures, which include: Diels–Alder reac-
tions, transition-metal-mediated cyclizations, ring
rearrangements or expansions, and Lewis acid
catalyzed cyclizations.[2] Despite progress, essentially
all of the methods reported thus far rely on
a laborious and tedious prearrangement of a complex
set of functional groups.[3] The Lewis acid based
intermolecular coupling protocols allow partial
circumvention of the issue but come at the expense
of limited substrate scope.[4]
Transition-metal-catalyzed, directed intermolec-
À
ular C H functionalization offers a powerful tool for
the synthesis of cyclic scaffolds, which complements
conventional methods.[5] Typical end products of
these reactions are heterocycles, where heteroatom-
containing directing groups, in part or as a whole, are
incorporated into target frameworks.[6] The synthesis
of carbocyclic compounds by this route is technically
more challenging as it requires a dominant cycliza-
tion reactivity at a typically less reactive site. There-
fore, a less strong directing group is resorted to for
avoiding competing heteroatom cyclization path-
ways. The ketone group has been the primary focus
Scheme 1. Schematic representation of different carbocyclization strategies.
of attention along this line of research because of its
high electrophilic reactivity (Scheme 1a). However,
in spite of tremendous progress, only five-membered
For both of the above carbocyclization strategies, direct-
ing and cyclization site are located on the same functional
group (ketone or alkene, respectively). We envisioned that
a distinct reaction pathway should be possible when separat-
ing those two sites. A ketone group is typically a preferred site
for coordination compared with an alkene group but the
reactivity of an alkene group can override that of a ketone
group under certain circumstances (e.g., organometallic
migratory insertion, polarity matching). Herein, we report
on the use of enaminones, which contain both ketone and
[*] S. Zhou, J. Wang, L. Wang, C. Song, K. Chen, Prof. Dr. J. Zhu
Department of Polymer Science and Engineering
School of Chemistry and Chemical Engineering
State Key Laboratory of Coordination Chemistry
Nanjing National Laboratory of Microstructures
Nanjing University, Nanjing 210093 (China)
E-mail: jinz@nju.edu.cn
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603943.
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À
Table 1: Synthetic scope for the reaction between enaminones and
alkene groups, as effective synthons for the directed C H
functionalization (Scheme 1c). Proof-of-concept protocols
for the synthesis of naphthalene derivatives have been
developed based on the RhIII-catalyzed coupling of enami-
nones with either alkynes or a-diazo-b-ketoesters. Enami-
nones, or enamines of 1,3-diketone or 3-ketoester compounds,
represent a class of highly versatile synthetic intermediates
because of their push–pull electronic structure.[9] This intrigu-
alkynes.[a,b]
= À = À
ing conjugated O C C C N system can exhibit reactivity as
ambident nucleophile (enamine) and electrophile (enone).[10]
Its broad synthetic utility is highlighted by the ability to
construct a diversified set of cyclic scaffolds but nevertheless
À
has never been demonstrated in the C H functionalization
context.[11] We used readily available aromatic enaminones
(prepared from acetophenones and N,N-dimethylformamide
dimethylacetal) for target transformations and importantly,
the naphthalene derivatives obtained contain two oxygen-
containing functional groups, aldehyde and hydroxy, that are
amenable to further synthetic elaboration.
À
First, we examined the C H functionalization/cyclization
reaction between (E)-3-dimethylamino-1-phenylprop-2-en-1-
one (1a) and diphenylacetylene (2a) using a variety of
additives and solvents at 608C with [{Cp*RhCl2}2](2 mol%)/
AgSbF6 (8 mol%) as the catalyst precursor (Table S1). With
2 equiv of either Cu(OAc)2 or AgOAc, the target product 1-
hydroxy-3,4-diphenyl-2-naphthaldehyde (3a) was formed in
> 10% yield. The combination of AgOAc and dichloroethane
(DCE) gave the best result (25% yield). Further screening of
additives revealed that the addition of HOAc/H2O (2.0 equiv
each) promoted the yield to a promising 56%. A further
improvement in the yield (79%) was possible by increasing
the amount of HOAc (6.0 equiv) and H2O (8.0 equiv). The
yield could be boosted to 83% with a slight adjustment of the
reaction temperature to 808C.
Under optimized conditions, the scope of the enaminones
was examined by using 2a as the coupling partner (Table 1).
Enaminones bearing an electron-rich substituent (1b, methyl;
1c, methoxy) on the phenyl ring afforded the desired products
in good yields (3b, 76%; 3c, 83%). Substrates containing
electron-deficient substituents (1d, fluoro; 1e, chloro; 1 f,
bromo; 1g, iodo) were slightly less reactive, and the
corresponding products 3d–3g were isolated in 66, 73, 69,
and 51% yields, respectively. The compatibility with bromo
and iodo substituents is an appealing feature differentiating
the Rh catalysis system from the Pd system because oxidative
addition of the polar carbon–bromo/carbon–iodo bond is
a typical reactivity pattern observed for Pd. Impor-
tantly, our catalytic reaction worked even for sub-
strates containing a highly electron-withdrawing
group (1h, nitro; 1i, trifluoromethyl; 1j, methoxy-
carbonyl), and the corresponding products 3h–3j
were obtained in moderate yields (57, 59, and 63%,
respectively). For meta-substituted substrates (1k,
methyl; 1l, fluoro; 1m, chloro), the preferred cou-
pling site for the phenyl ring is the carbon atom para
to the substituent (3k–3m), likely reflecting the
operation of a steric effect. Among the substituents
examined, the methyl group (1k) is the most bulky,
[a] Conditions: enaminone (0.5 mmol), alkyne (1.5 equiv), DCE
(2.0 mL). [b] Yield of isolated product.
(3kA). The less sterically hindered fluoro (1l) and chloro
(1m) groups cannot completely prevent the formation of
ortho-coupled regioisomers (3lB, 3mB), which accounted for
a small percentage of the final products. The reaction can also
be applied to multi-substituted (1n, dimethoxy) and polycyc-
lic (1o, naphthyl ring) substrates, and the corresponding
products (3n, 3o) were acquired in 59 and 37% yields,
respectively. With the enaminone substrate scope examined,
we next investigated the alkyne scope by using asymmetri-
cally substituted alkynes. Most significantly, regioselectivity is
excellent for such a type of substrate (phenyl/alkyl alkyne: 2p,
methyl; 2q, ethyl; 2r, propyl). Compound 3p could be easily
recrystallized and its single-crystal X-ray diffraction study
provided unambiguous evidence for the formation of a naph-
thalene framework with pendant aldehyde and hydroxy
groups.[12]
The reaction mechanism was investigated initially through
a deuterium labeling study with 1a. A key evidence for the
thus offering the generation of a single product Scheme 2. Deuterium labeling study with 1a.
2
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ketone group as directing group came from the
identification of three hydrogen/deuterium scram-
bling sites: the two ortho carbon atoms on the
phenyl ring, and the alkene carbon atom next to the
dimethylamino group (Scheme 2). Indeed, a p-coor-
dination of the alkene group to RhIII would allow C H
À
activation only at the two ortho carbon sites; whereas
a coordination from the ketone oxygen atom would
À
enable simultaneous C H activation at all three sites,
through the formation of two types of five-membered
rhodacycles.
A large kinetic isotope effect (KIE) was observed
(kH/kD = 5.3) in an experiment between 1a/[D5]-1a
À
and 2a, suggesting that C H activation is involved in
the turnover-limiting step. A competition experiment,
reaction of electron-rich (1c) and electron-poor (1d)
enaminones with 2a, indicated a slightly higher
reactivity for the electron-rich substrate, providing
evidence for an electrophilic aromatic substitution
(EAS) pathway. No reaction occurred in a control
experiment performed with 1-phenylprop-2-en-1-one
and 2a, suggesting that the reaction outcome can be
substantially influenced by even a remote substituent
(most likely through electronic effects).
Scheme 3. Proposed mechanism for the reaction involving enaminones and
alkynes.
Based on these results, a plausible catalytic cycle is
proposed in Scheme 3: reaction of [{Cp*RhCl2}2] and AgSbF6
to produce a cationic [Cp*Rh]2+ species, coordination of
608C, under which 5a could be obtained in 81% yield.
Single-crystal X-ray analysis confirmed the structure assign-
ment.[12]
Based on the optimized reaction conditions, the enami-
none substrate scope was then investigated by using 4a as the
coupling partner (Table 2). Both electron-rich and electron-
poor substituents on the phenyl ring were tolerated. In
OAcÀ and the enaminone 1a, turnover-limiting C H activa-
À
tion, migratory insertion of the alkyne 2a, further migratory
insertion of the alkene, b-hydride elimination to release 3a-N
and [Cp*RhH]+, reoxidation of [Cp*RhH]+ to afford
[Cp*Rh]2+ and H+, nucleophilic attack of 3a-N by H2O to
generate the final product 3a. The proposal of a late-stage
nucleophilic attack by H2O, most likely after cyclization, is
Table 2: Synthetic scope for the reaction between enaminones and
a-diazo-b-ketoesters.[a,b]
À
based on the fact that 1) the C N bond remains intact after
À
C H activation (Scheme 2), 2) as a control, substitution (with
a methyl group) at the alkene carbon atom next to the ketone
group renders the enaminone prone to hydrolysis by H2O, and
3) in the absence of H2O, a product can still be formed
(speculated to be 3a-N) but is difficult to separate by silica gel
chromatography.
With the satisfactory results obtained for alkyne sub-
strates, we envisioned that a-diazo-b-ketoesters might equally
well serve as competent coupling partners, considering the
nucleophilic reactivity of alkene carbon atoms next to
a ketone group and expected retention of the electrophilic
ketone group from a-diazo-b-ketoesters after a typical RhIII
catalytic cycle. Indeed, a screening of reaction conditions
under [{Cp*RhCl2}2](2 mol%)/AgSbF6 (8 mol%) catalysis in
DCE, by using 1a and ethyl 2-diazo-3-oxobutanoate (4a) as
the coupling partners, confirmed a reaction with participation
of an additive (Table S7). As expected, no oxidant was
necessary. Among the additives, 2,2-dimethylbutyric acid
(HODmb, 2 equiv)/H2O(8 equiv) gave the best yield (46%)
for 5a. A change from DCE to other solvents negatively
impacted the yield. An increase in the amount of catalytic
species and additive proved to be helpful. Optimized reaction
conditions
include
[{Cp*RhCl2}2](4 mol%)/AgSbF6
[a] Conditions: enaminone (0.5 mmol), a-diazo-b-ketoester (1.5 equiv),
(16 mol%), HODmb (2.5 equiv), H2O (10 equiv), DCE,
DCE (2.0 mL). [b] Yield of isolated product.
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general, the electronic effect mirrors that from the
alkyne coupling partner. Electron-rich substituents
(1b, 1c) gave better yields than electron-poor
substituents (1d–1j). And again, the reaction
could still proceed for substrates bearing very
electron-deficient substituents (1h–1j), and the
respective products were obtained in moderate
yields. Compared to the alkyne case, the regiose-
lectivity for meta-substituted substrates is better for
the fluoro substituent (1l) but worse for the chloro
substituent (1m), likely reflecting a mixed play of
steric and electronic effects. A disubstituted sub-
strate (1n) reacted equally well. An examination of
the a-diazo-b-ketoester scope revealed compatibil-
ity with the cyclopropyl group (4o) as well as with
a variety of substituents (4p, methyl; 4q, fluoro; 4r,
bromo) on the phenyl ring (para to the ketone
group).
Mechanistic experiments revealed a large KIE
(kH/kD = 6.1 for the reaction between 1a/[D5]-1a
and 4a) and a small electronic effect (preferred
reaction with 4a for electron-rich 1c over electron-
deficient 1d), again suggesting an EAS pathway for
Scheme 4. Proposed mechanism for the reaction involving enaminones and a-
diazo-b-ketoesters. The hydrolysis process is similar to that described in
Scheme 3.
À
the turnover-limiting C H activation step.
Based on the above results and literature
precedents,[13] a plausible reaction mechanism is proposed in
Scheme 4: generation of a cationic [Cp*Rh]2+ species for the
III
À
C H activation, formation of a Rh –carbene species, 1,1-
migratory insertion and proto-demetalation to produce
intermediate 5a-AC and release of RhIII for a new catalytic
cycle, two consecutive nucleophilic attacks for the ring
closure, and formation of the aldehyde.
The oxygen-containing functional groups of the formed
naphthalene derivatives are amenable to further transforma-
tions. Briefly described herein are several conversions using
3a as model reactant (Scheme 5). As expected, the aldehyde
group reacted with an amino group (from aniline), affording
an imine product (6a). And the hydroxy group provided
a handle for the generation of ethers (7a and 8a) through
reaction with a bromo-containing molecule.[14] Significantly,
participation of both aldehyde and hydroxy groups enabled
the synthesis of another ring structure on the original
naphthalene framework (9a).[15]
À
In summary, we have proposed an enaminone-directed C
H functionalization method and demonstrated its utility in the
RhIII-catalyzed synthesis of naphthalenes, through coupling
with either alkynes or a-diazo-b-ketoesters. The reaction
Scheme 5. Transformations of 3a.
Acknowledgements
À
takes advantage of the C H activation directing capability of
ketone groups and the unique reactivity pattern associated
with enaminones. Two inherently reactive functionalities
(aldehyde and hydroxy) have been built into the naphthalene
framework and a broad range of substitution patterns is
tolerated, rendering target products readily available for
further synthetic manipulation.
J.Z. gratefully acknowledges support from the National
Natural Science Foundation of China (21425415, 21274058)
and the National Basic Research Program of China
(2015CB856303).
À
Keywords: carbocyclization · C H activation · enaminones ·
naphthalenes · rhodium
4
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Pharm. Bull. 2013, 61, 1239 – 1247; b) M. Potowski, C. Golz, C.
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Received: April 22, 2016
Revised: May 14, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Carbocyclization
An easy way to carbocycles: Alkynes or a-
diazo-b-ketoesters are the partners for
enaminones in the RhIII-catalyzed syn-
thesis of the naphthalene framework, as
shown in the picture. The installed alde-
hyde and hydroxy functional groups can
be harnessed for further synthetic
manipulation.
S. Zhou, J. Wang, L. Wang, C. Song,
K. Chen, J. Zhu*
&&&& — &&&&
Enaminones as Synthons for a Directed
III
À
C H Functionalization: Rh -Catalyzed
Synthesis of Naphthalenes
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!