Metal-Free Dehydrogenative Diels-Alder Reactions of Prenyl Derivatives with Dienophiles via a Thermal Reversible Process

Article abstract of DOI:10.1021/acs.orglett.8b02469

An efficient dehydrogenative Diels-Alder reaction of prenyl derivatives with dienophiles has been developed. The reaction exhibits broad substrate scope and provides efficient access to cyclohexene derivatives with good to excellent yields. A reasonable mechanism involving a metal-free thermal reversible process is proposed.

Full text of DOI:10.1021/acs.orglett.8b02469

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Metal-Free Dehydrogenative Diels−Alder Reactions of Prenyl
Derivatives with Dienophiles via a Thermal Reversible Process
Wen-Lei Xu, Heng Zhang, Yu-Long Hu, Hui Yang, Jie Chen,* and Ling Zhou*
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry &
Materials Science, Northwest University, Xi’an 710127, P.R. China
S
* Supporting Information
ABSTRACT: An efficient dehydrogenative Diels−Alder
reaction of prenyl derivatives with dienophiles has been
developed. The reaction exhibits broad substrate scope and
provides efficient access to cyclohexene derivatives with good
to excellent yields. A reasonable mechanism involving a metal-
free thermal reversible process is proposed.
ross-dehydrogenative coupling reactions represent a
synthetically efficient strategy for functional building
with diverse dienophiles was met with less success in our
previous report.4d DDQ is widely used in many reactions as an
excellent oxidizing reagent.5 Due to the electron-withdrawing
effect of the cyano groups, it is also a better dienophile.6
Consequently, metal-free DDQ-mediated DHDA reactions face
challenges in terms of chemoselectivity,4d,f and DDQ adducts
are usually observed. Therefore, converting the DDQ adducts
into valuable scaffolds in such DHDA reactions is highly
desirable.
C
blocks in organic chemistry because new C−C bonds are
generated via the direct functionalization of inert C(sp3)−H
bonds.1,2 The Diels−Alder reaction, which has irreplaceable
advantages in constructing cyclohexene derivatives, has
attracted considerable attention, as well.3 Particularly, synthetic
chemists have made great efforts in studying dehydrogenative
Diels−Alder (DHDA) reactions in recent years.4 In 2011,
White’s group first reported a Pd-catalyzed DHDA reaction of
terminal olefins (Scheme 1, eq 1).4b Very recently, metal-free
The Diels−Alder reaction is reversible at high temperatures,7
which are usually employed to produce reactive olefins or
metastable molecular entities and to protect a double bond
against undesired reactions.8 We envisaged that the DDQ
adducts in DHDA reactions could react with other dienophiles
to give other cycloadducts via a thermal reversible process, in
which DDQ adducts would release DDQ and dienes and DDQ
could continue to oxidize substrates to dienes; meanwhile, the
resulting dienes can react with other dienophiles to form
cyclohexene derivatives. To the best of our knowledge, the
DDQ-mediated metal-free cross-DHDA reaction between
DDQ adducts and diverse dienophiles has not been reported.
Herein, we report an efficient DDQ-mediated DHDA reaction
of prenyl derivatives with tetracyanoethylene and other
dienophiles via a metal-free thermal reversible process for the
first time, leading to a range of cyclohexene derivatives with
good to excellent yields (Scheme 1, eq 4).
Scheme 1. Dehydrogenative Diels−Alder Reactions
We began our studies by evaluating the reaction of prenyl
benzene (1a) with tetracyanoethylene (2a) in the presence of
DDQ in a ratio of 1:2:1.2 in DCM at ambient temperature for 48
h (Table 1, entry 1). The desired cross-DHDA reaction product
3a was obtained in 24% yield, together with the DDQ adduct
side product 4 in 39% yield. This result indicates that a
competition between DDQ and tetracyanoethylene did exist. It
seems that DDQ is more reactive than tetracyanoethylene in this
Diels−Alder reaction system, which is consistent with our
DHDA reactions using DDQ as oxidant were developed by
Zhang’s (Scheme 1, eq 2)4e and Antonchick’s groups.4g
Additionally, in 2015, our group also reported a metal-free
DHDA reaction of prenyl derivatives, in which DDQ plays a
dual role as both an oxidant and a dienophile (Scheme 1, eq 3).4d
Unfortunately, our attempt to realize the cross-DHDA reaction
Received: August 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02469
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Scope of Prenyl Derivatives
temp
time
(h)
yield of
yield of
b
b
entry
oxidant
DDQ
solvent
(°C)
3a (%)
4 (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
DCM
DCM
DCM
toluene
toluene
toluene
toluene
toluene
DCE
25
0
−30
80
100
110
115
120
90
115
115
115
115
48
48
48
48
48
48
48
48
48
48
72
72
72
24
5
0
39
43
49
23
18
trace
0
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
DDQ
p-chloranil
49
61
88
94
94
55
89
47
nr
0
15
trace
na
na
na
PhCl
c
toluene
toluene
toluene
d
BQ
e
oxone
trace
a
Reactions were carried out with 1a (0.2 mmol), 2a (0.4 mmol),
b
oxidant (0.24 mmol) in solvent (2.0 mL) under N2. Isolated yield.
c
d
p-Chloranil = 2,3,5,6-tetrachlorocyclohexa-2,5-diene-1,4-dione. BQ
e
= p-benzoquinone. Oxone = potassium peroxymonosulfate.
previously reported results.4d As we proposed above, changing
the reaction temperature would influence the ratio of the
products. Indeed, the reaction temperature is crucial for this
reaction; lower temperatures resulted in lower yields of 3a, and
higher temperatures resulted in higher yields (Table 1, entries
1−7). The yield of 3a was increased to 94% without side product
when the reaction was performed at 115 °C (Table 1, entry 7).
Various solvents were screened, and toluene gave the best
results. Investigation of other oxidants were also attempted, and
p-chloranil can also give the desired product, albeit with a lower
yield (Table 1, entry 11).
With the optimized conditions in hand, we examined this
synthetic method in a range of substituted prenyl derivatives. As
shown in Scheme 2, this protocol was amenable to a wide range
of 1 substrates, and the reactions all proceeded smoothly to yield
the corresponding cyclohexene derivatives in excellent yields.
Substrates with electron-withdrawing groups (F, Cl, Br, CF3) or
electron-donating groups (Me, OMe, NHAc) at the ortho-,
meta-, or para-position of the phenyl ring were well tolerated,
and the corresponding products were obtained with excellent
yields (3b−p, 84−97%). These results indicate that the electron
effect has little impact on this new DHDA reaction. Notably, the
DDQ adduct side product was not detected in these reactions.
Naphthalenyl-substituted 1q returned the desired product in
93% yield. Additionally, furyl-substituted 1r was also a suitable
substrate for this reaction, and the corresponding product 3r was
prepared in 73% yield. Prenyl ether 1s, which previously failed in
such a DHDA reaction, returned the desired product 3s in 75%
yield at a higher temperature. Interestingly, 2-methyl-2-butene
1t was also tolerated, and the corresponding product 3t was
obtained in 51% yield. Next, the substrate scope with regard to
the disubstituted substrate was examined. Substrates with the
same or different substituents were all amenable to the reaction
(1u−1ac), and the derived cyclohexene derivatives with an all-
carbon quaternary stereocenter were obtained with high yields
a
Reactions were carried out with 1 (0.2 mmol), 2a (0.4 mmol), DDQ
(0.24 mmol) in toluene (2.0 mL) at 115 °C under N2 for 48 h. The
b
yields shown are for isolated products. Gram scale (1.17 g 1a).
c
Performed at 140 °C.
(3u−3ac). The structure of 3u was further confirmed by X-ray
crystallography.9
To extend the scope and application of this DHDA reaction,
we conducted this reaction with other dienophiles (Scheme 3).
Treatment of 1a with quinones (2b, 2c) in the presence of DDQ
and LiCl for 72 h led to the desired products 5ab and 5ac in 51
and 71% yields, respectively. It is worth noting that LiCl may
play an important role in activating the LUMO of quinones.10,11
Additionally, DHDA reaction of 1a with but-2-ynedioic acid
dimethyl ester (2d) gave 1,2-bis(methoxycarbonyl)-1,4-cyclo-
hexadiene 5ad in 88% yield. When maleic anhydride 2e was used
as the dienophile, tetrahydroisoindole-1,3-dione 5ae was
generated in 61% yield with endo-selectivity, which is widely
used as a precursor for the synthesis of (poly)amides, dyes,
synthetic resins, etc.12 Moreover, methyl acrylate 2f, a weak
dienophile, is also tolerated in this DHDA reaction to yield the
corresponding product 5af in 54% yield, albeit with poor
B
DOI: 10.1021/acs.orglett.8b02469
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a
Scheme 3. Scope of Dienophiles
Scheme 4. Demonstration of Synthetic Utility of Products
with different reaction times (0.5, 12, 24, 36, and 48 h). As
shown in Scheme 5a, the yields of 3a and 4 changed with the
a
Scheme 5. Control Experiments
a
Reactions were conducted with 1a (0.4 mmol), 2b−2f (0.2 mmol),
DDQ (0.3 mmol) in toluene at 115 °C under N2 for 72 h. The yields
1
shown are for isolated products. The dr value was determined by H
b
NMR; the major isomer is shown. 0.2 mmol LiCl was used.
stereoselectivity. Oxindoles that contain a quaternary stereo-
genic center at the C3 position are fascinating targets in organic
synthesis, due to their wide-ranging utility as synthetic
intermediates for drugs, clinical pharmaceuticals, and alka-
loids.13 2,3-Olefinic oxindoles (2g−j) with N-tosyl, benzyl, or
methyl groups could react with 1a, and spirocyclic oxindoles
(5ag−aj) were obtained from this new DHDA reaction with
good to excellent yields (71−97%). Furthermore, trans-
dibenzoylethene also returned the desired product 5ak in 95%
yield.
To further validate the synthetic utility of this newly
developed protocol, several transformations of these products
were carried out. Further dehydrogenation of 5ac by Pd/C gave
the aromatized product 9,10-anthracenedione 6ac in 87%
yield,14 which is a key intermediate in pharmaceutical and
material chemistry.15 Similarly, 5ad can be further converted to
phthalate 6ad in excellent yields (91%), which acted as
plasticizers or dispersants to improve the elaboration of
plasticization efficiency.16 Notably, this two-step reaction can
be performed in one pot, and a measurable increased yield was
observed (Scheme 4), and 1,4-benzoquinone 2c returned the
desired product 7 in 60% yield by this strategy. Thus, an effective
method for the synthesis of polycyclic aromatic quinones by a
one-pot reaction from simple prenyl derivatives and quinones
was established based on the newly developed DHDA reaction.
To shed light on the DHDA reaction mechanism, some
control experiments were carried out. We conducted five parallel
model reactions (1a with 2) under the optimized conditions
a
Panel (a) shows the correlation between the yields of 3a/4 with the
reaction time of the model reaction (Table 1, entry 7).
reaction time. An increase of the yield of 3a is accompanied by a
decrease of the yield of 4, suggesting that 4 was converted to 3
through a thermal reversible process during the reaction. To
further prove this process, DDQ adduct 4 was treated directly
with 2a under the optimal conditions in the absence of DDQ,
and as expected, the desired product 3a was obtained in 87%
yield (Scheme 5b). When 2a was reduced to 1 equiv, 3a was
isolated in 79% yield with 9% of 4 recovered. Moreover,
treatment of 3a with DDQ in a ratio of 1:1 at 115 °C gave no
product 4, suggesting that the thermal stability of 3a is better
than that of 4 (Scheme 5c). These results confirm our proposal
C
DOI: 10.1021/acs.orglett.8b02469
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Letter
that a thermal reversible process occurred in this DHDA
reaction.
On the basis of the experimental results mentioned above and
previous literature,4c a reasonable mechanism was described in
Scheme 6. Dehydrogenation of 1a occurs with DDQ to generate
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(NSFC-21672170), the Natural Science Basic Research Plan in
Shaanxi Province of China (2018JC-020, 2018JM2029), the
Key Science and Technology Innovation Team of Shaanxi
Province (2017KCT-37), and the “Top-rated Discipline”
Construction Scheme of Shaanxi Higher Education for financial
support.
Scheme 6. Proposed Reaction Mechanism
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02469.
Experimental procedures, characterization data for all new
compounds, and copies of 1H, 13C NMR spectra (PDF)
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CCDC 1844174 contains the supplementary crystallographic
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chmchenj@nwu.edu.cn.
*E-mail: zhoul@nwu.edu.cn.
ORCID
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Ling Zhou: 0000-0002-6805-2961
Notes
The authors declare no competing financial interest.
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