Catalytic Electrophilic Alkylation of p-Quinones through a Redox Chain Reaction

Article abstract of DOI:10.1002/anie.201702885

Allylation and benzylation of p-quinones was achieved through an unusual redox chain reaction. Mechanistic studies suggest that the existence of trace hydroquinone initiates a redox chain reaction that consists of a Lewis acid catalyzed Friedel–Crafts alkylation and a subsequent redox equilibrium that regenerates hydroquinone. The electrophiles could be various allylic and benzylic esters. The addition of Hantzsch ester as an initiator improves the efficiency of the reaction.

Full text of DOI:10.1002/anie.201702885

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Accepted Article
Title: Catalytic Electrophilic Alkylation of p-Quinones via a Redox Chain
Reaction
Authors: Xiao-Long Xu and Zhi Li
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10.1002/anie.201702885
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Catalytic Electrophilic Alkylation of p-Quinones via a Redox
Chain Reaction
Xiao-Long Xu, and Zhi Li*
Abstract: Allylation and benzylation of p-quinones is achieved by an
unusual redox chain reaction. Mechanistic studies suggest that the
trace existence of hydroquinone initiates a redox chain reaction that
consists of a Lewis acid-catalyzed Friedel-Crafts alkylation and a
subsequent redox equilibrium that regenerates hydroquinone. The
electrophiles could be various allylic and benzylic esters. Addition of
Hantzsch ester as initiator improves the efficiency of the reaction.
The bulk of established chemical syntheses of quinone use
a functionalization–oxidation strategy. Substituents are first
installed on hydroquinones, phenols or phenyl ethers by Friedel-
Crafts reactions,[6] radical reactions[7] and transition metal-
catalyzed coupling reactions,[8] which are then oxidized to
quinones.[9] Functionalized hydroquinone precursors could also
be obtained by nucleophilic addition[10] and cycloaddition
reaction[11] of simple quinones to substituted 2-cyclohexen-1,4-
dione intermediates, which can then rearrange to hydroquinone
and be oxidized.[12] Alternatively, Lipshutz[13] and Negishi[14]
developed efficient syntheses of CoQ10 based on metal-
catalyzed cross coupling between chloromethyl-CoQ0 and
organoaluminum or organozinc reagents. Recently emerged
direct quinone functionalization methodologies include radical
C-H functionalization with aryl boronic acids[15] and transition
metal-catalyzed C-H functionalization with broader varieties of
coupling substituents.[16] However, benzylation of quinones
remains challenging. Herein we report a mild, one-step and redox-
economic[17] catalytic allylation and benzylation of p-quinones and
the study of its unique mechanism, in which allyl and benzyl esters
serve as the electrophiles and quinones are the formal
“nucleophiles” (Scheme 1).
Quinone compounds participate in many of the naturally occurring
electron transfer processes due to their unique redox properties.[1]
For examples, ubiquinone (CoQ10) presents in most eukaryotic
cells as a key component of the aerobic cellular respiration
process;[2] plastoquinone and phylloquinone (Vitamin K) present
in green leaves as electron acceptors in photosynthesis
process.[3] Many natural or synthetic quinone compounds thus
exhibit potent biological activities as drug candidates targeting
relevant biochemical processes.[4] The electronic properties of
quinone derivatives also attract much attention from researchers
of functional materials.[5]
Table 1. Effects of Lewis Acids and Solvents.[a]
entry
catalyst
mol %
solvent
T/°C
yield/%
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
–
0
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
acetone
AcOH
EtOAc
MeOH
EtOH
Et2O
toluene
PhCl
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
0
N.R.
70
N.R.
27
70
72
92
20
9
N.R.
N.R.
N.R.
N.R.
73
53
90
60
73
HOTf
Yb(OTf)3
Bi(OTf)3
Al(OTf)3
Sc(OTf)3
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Scheme 1. Retrosynthetic Analysis of 2-Allyl-1,4-Benzoquinones: Previous
Work and This Work.
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
1
0.5
25
25
62
[a] Conditions: 1b (0.25 mmol), 2b (0.25 mmol), catalyst and 1 mL solvent
were stirred at specified temperature under N2 for 2 h. Yields were
determined by 1H-NMR of the crude reaction mixture. N.R. = No reaction
observed. ‒OTf = CF3SO3‒, trifluoromethanesulfonate.
[*]
X.-L. Xu, Dr. Z. Li
School of Physical Science and Technology
ShanghaiTech University
393 Middle Huaxia Road, Pudong, Shanghai, China 201210
E-mail: lizhi@shanghaitech.edu.cn
Supporting information for this article is given via a link at the end of
the document.
Previous
research
has
shown
that
metal
trifluoromethanesulfonate (triflate) Lewis acid catalysts allow the
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10.1002/anie.201702885
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elimination of carboxylic acid from ester by cleaving the alkoxyl
C-O bond, among which the cleavage of prenyl acetate (1a) is
particularly rapid.[18] Thus, it is possible to use allylic esters as
diene surrogates in cycloaddition reactions with dienophiles such
as 1,4-benzoquinone (2a). Surprisingly, when investigating the
reaction between 1a and 2a, we always obtained an unexpected
product 3aa, appearing to be generated from direct prenylation of
quinone (Table S1). This intriguing new product urged us to
uncover its uncommon formation mechanism.
In order to amplify alkylation over cycloaddition, 2,6-
dimethylbenzoquinone and 2-cyclohexenyl acetate was chosen to
be the model substrates for conditions optimization because the
rate of cycloaddition could be impeded by the methyl groups on
benzoquinone. The reaction works with many Lewis and Brønsted
acids as catalysts in dichloromethane solvent at ambient
temperature. As previous work indicates, the reactivity follows the
trend of Lewis acidity of the catalysts. Polar solvents inhibit the
reaction probably by poisoning the catalysts (Table 1).[18a]
amount of Q to HQ; (2) HQ undergoes alkylation to HQ-R; (3) Q
and HQ-R undergoes redox reaction to HQ and Q-R; (4) steps (2)
and (3) are repeated until all Q is converted to Q-R. The initiator
might be the added HQ itself, or the diene, or the HQ derivative
that comes from the tautomerization of Q cycloaddition by-product.
The termination of this chain reaction may involve aerobic
oxidation of the last trace of HQ-R to Q-R.
This hypothesis is supported by kinetic studies. The reaction
showed zero order kinetics on either ester or quinone precursors,
indicating neither ester nor quinone directly participates in the
rate-determining step (Figure S2a,b). Instead, it showed first
order kinetics on the Lewis acid concentration, suggesting a
rapidly and quantitatively generated Lewis acid-ester complex
might be involved in the rate-determining step (Table 1 and Figure
S2c). In addition, alkyl substitution at quinone lowers its reduction
potential.[20] As a result, a redox equilibrium between HQ-R
(easier to oxidize than HQ) and remaining Q (easier to reduce
than Q-R) is quickly established, releasing HQ and Q-R.[21]
Table 2. Effect of Initiators on Product Selectivity.[a]
entry
initiator
mol %
conv. /%
3bc / 4bc
1
2
3
0
1
2
5
50
100
100
100
100
100
100
N.D.
60 / 40
74 / 26
83 / 17
93 / 7
93 / 7
0 / 0
4
5
6[b]
7
2
86
57 / 43
8
9
(EtO)3SiH
HB(pin)
2
2
96
78
55 / 45
51 / 49
10
2
100
92 / 8
[a] Conditions: 2c (0.25 mmol) was first mixed with initiator in 1 mL CH2Cl2
at 25 °C under N2 for 0.5 h., then 1b (0.25 mmol) and Hf(OTf)4 (2 mol%)
were added and stirred at 25 °C under N2 for 2 h. Conversions of 2c (Conv.)
and selectivities of 3bc / 4bc were determined by 1H-NMR of crude reaction
mixture. HB(pin) = pinacolborane. [b] No 2c added. The initiator is insoluable
in CH2Cl2. Product is a mixture of initiator and unidentifiable complex.
Conversion was not determined (N.D.).
Scheme 2. Proposed Redox Chain Mechanism of 1,4-Benzoquinone
Prenylation.
A plausible mechanism hypothesis has to explain how these
two electrophiles join together. After evaluating several
possibilities, the most reasonable mechanism is drawn as follows,
taking the reaction between 1a and 2a as an example: prenyl
acetate acts as the source of allylic carbocation electrophile;
benzoquinone (Q) acts as the source of the real nucleophile,
hydroquinone (HQ); and the C-C bond formation step is probably
a Lewis acid-catalyzed Friedel-Crafts alkylation between HQ and
prenyl carbocation generated from Lewis acid-activated prenyl
Since the rate-determining step is presumably the Friedel-
Crafts alkylation between HQ and the Lewis acid-ester complex,
addition of a small amount of HQ resulted in dramatically
improved yield of the alkylation product (Table 2), likely because
the equilibrium concentration of HQ was directly increased.
Without the addition of HQ as an initiator, the reaction between
cyclohexyl acetate (1b) and 2,3-dimethylbenzoquinone (2c)
generates a 60 : 40 mixture of alkylation product 3bc and
cycloaddition product 4bc. The reaction may first generates 4bc
which may then initiate the redox chain pathway to 3bc. In
contrast, adding merely a few mol % of 2,3-dimethylhydroquinone
in the beginning could improve the selectivity to 93 : 7. The
acetate. HQ is supplied by
a a
redox-chain reaction,[19]
mechanism that resembles radical chain reaction and may
constantly regenerate HQ in the following sequence (also shown
in Scheme 2): (1) trace amount of an initiator reduces trace
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10.1002/anie.201702885
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COMMUNICATION
Table 3. Scope of Substrates.[a]
initiator greatly accelerated the alkylation but not the cycloaddition.
However, adding too much hydroquinone did not proportionally
improve reaction yield and selectivity. In fact, the reaction
between pure hydroquinone and prenyl ester does not work very
well in the working solvent dichloromethane, likely due to the poor
solubility of hydroquinone.[22] In order to avoid using different
hydroquinone initiators for each reaction, catalytic amount of
reductants were screened as initiator to reduce the quinone in situ.
Hantzsch ester has been found to be the superior choice over
other common reductants (Table 2).
The scope of reaction covers a variety of quinones, but not
all allylic esters perform well (Table 3). Cyclohexenyl acetate is
successful in most cases (3ba-3bl), while less satisfactory results
were obtained using other open-chain allylic esters. Electron-rich
methyl- and methoxy- substituted quinones perform better than
electron-deficient quinones such as chloroquinones. The latter
ones gave more cycloaddition by-products when the ester is
capable of elimination to a diene. Some of the products also suffer
from slow decomposition during purification (3bg, 3bh, 3bk and
3bl). Furthermore, the reaction unlikely goes through a true diene
intermediate because many benzyl esters underwent facile
conversion (3ca-3fm). When multiple alkylation sites are present
at benzoquinone, selectivity follows the classic directing group
effect (3be, 3bg and 3bj). When excess ester is fed to quinone
bearing multiple reactive sites, more than one alkylation is
observed (5), and vice versa (6). When the electronic effects of
benzyl esters are differentiated by a methoxy group, only the one
that takes the para position of the methoxy is reactive (7). Finally,
ethyl ester of asthma drug Seratrodast12 was synthesized (8),
demonstrating the potential of this methodology in pharmaceutical
synthesis.
Products and isolated yields (%)
23
50
80
22
30[b]
91
71[c]
86
64
73
32
49[d]
30
76
32
93
76
31
93
75[e]
72[e]
74[e]
76
87
74
72
70[e]
93[g]
64[i]
64[f]
Scheme 3. Definitive Necessity of Initiator.
75[h]
A puzzle to be solved next is whether the redox chain
mechanism is the only pathway that gives the alkylation product.
If so, a probe reaction that excludes all potential redox chain
initiators should not proceed at all. Benzylation of quinone could
be a good probe reaction because benzyl esters cannot afford
any potential reductants such as HQs, alkenes or dienes during
C-O bond cleavage. Indeed, the reaction of 4-methoxybenzyl
acetate and benzoquinone only gave the oligomerization product
of the acetate. But as expected, addition of catalytic amount of
Hantzsch ester allowed benzylation in 74% yield, suggesting the
redox chain mechanism is the only possible pathway (Scheme 3,
Table S3). Although less effective, allyl esters (1a and 1b) and
1,3-dienes (isoprene and 1,3-cyclohexadiene) were all shown to
be initiators, explaining why reactions of 1a and 1b do not require
an external initiator (Table S3).
[a] Conditions: quinone (0.25 mmol) was first mixed with Hantzsch ester (2
mol %) in 1 mL CH2Cl2 at 25 °C under N2 for 0.5 h., then alkyl acetate (0.25
mmol) and Hf(OTf)4 (2 mol %) were added and stirred at 25 °C under N2 for
2 h unless otherwise noted. Yields are isolated yields. [b] 2.0 equiv. alkyl
acetate added over 4 h via a syringe pump. [c] 4.0 equiv. alkyl acetate added
over 24 h via a syringe pump. [d] 2 mL CH2Cl2, 4.0 equiv. alkyl acetate, 4 h.
[e] 35 °C for 5 h. [f] 4 mol % Hf(OTf)4, 3.0 equiv. alkyl acetate. [g] 4 mol %
Hf(OTf)4, 0.5 equiv. alkyl acetate. [h] 4 mol % Hf(OTf)4, 4 mol % Hantzsch
ester. [i] 5 mol % Hf(OTf)4, 4 mol % Hantzsch ester, 16 h.
Effects of leaving groups were also investigated. Reactivity
of different carboxylates follows steric bulk trend: the bulkier the
carboxylate, the slower the reaction. Alkyl halides were also good
electrophiles, as shown in classic Friedel-Crafts alkylation
reactions (Table S4).
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In summary, allylation and benzylation of quinone were
accomplished by an unexpected mechanism: redox chain
reaction. This transformation would be useful in the syntheses of
allyl and benzyl quinone derivatives with potential utility in
pharmaceutical and materials research. These efforts are
ongoing in this lab.
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Acknowledgements
Financial support for this work was generously provided by the
“1000 Talents Plan for Young Professionals” Start-up funding,
National Natural Science Foundation of China (Grant No.
21673141) and ShanghaiTech University start-up funding. We
thank Shanghai Advanced Research Institute, CAS for supporting
lab space, and the graduate program of Shanghai Institute of
Organic Chemistry, CAS for supporting X.X. We thank Dr. Huawei
Liu for NMR expertise, and Dr. Fei Zhao for HRMS expertise.
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Keywords: alkylation • quinones • redox chemistry • synthetic
methods
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hydroquinone (See supporting information).
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Xiao-Long Xu, Zhi Li*
Page No. – Page No.
Catalytic Electrophilic Alkylation of p-
Quinones via a Redox Chain Reaction
Redox-economic and catalytic quinone C-H allylation and benzylation are achieved
by coupling quinone and allyl or benzyl ester. The mechanism is a redox chain
reaction, in which the consumption of the actual reactant hydroquinone was
replenished by redox equilibrium between alkylated hydroquinone and remaining
quinone (See Scheme).
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