1,6-Addition Arylation of para-Quinone Methides: An Approach to Unsymmetrical Triarylmethanes

Article abstract of DOI:10.1002/ejoc.201600385

A 1,6-addition arylation reaction of para-quinone methides with α-isocyanoacetamides and electron-rich aromatic compounds under metal-free conditions has been developed. BF₃·Et₂O plays two roles in the reaction: catalyzing the cyclization of α-isocyanoacetamides to give oxazoles, and activating the para-quinone methides to achieve the 1,6-addition arylation process. The reaction shows good functional group tolerance, scalability, and regioselectivity. It is a consice protocol for the synthesis of diverse unsymmetrical triarylmethanes. Further transformation of the resulting triarylmethanes provides an efficient route to some functionalized molecules.

Full text of DOI:10.1002/ejoc.201600385

DOI: 10.1002/ejoc.201600385
Full Paper
Unsymmetrical Triarylmethanes
1,6-Addition Arylation of para-Quinone Methides: An Approach
to Unsymmetrical Triarylmethanes
Shang Gao,[a][‡] Xiuyan Xu,[a][‡] Zhenbo Yuan,[a] Haipin Zhou,[a] Hequan Yao,*[a] and
Aijun Lin*[a]
Abstract: A 1,6-addition arylation reaction of para-quinone arylation process. The reaction shows good functional group
methides with α-isocyanoacetamides and electron-rich aro-
matic compounds under metal-free conditions has been devel-
oped. BF3·Et2O plays two roles in the reaction: catalyzing the
cyclization of α-isocyanoacetamides to give oxazoles, and acti-
vating the para-quinone methides to achieve the 1,6-addition
tolerance, scalability, and regioselectivity. It is a consice protocol
for the synthesis of diverse unsymmetrical triarylmethanes. Fur-
ther transformation of the resulting triarylmethanes provides
an efficient route to some functionalized molecules.
Introduction
The unsymmetrical triarylmethane motif (a triarylmethane with
three different aromatic groups) is a privileged structural motif
that is widely found in functional materials, pharmaceutical
agents, and biologically active molecules.[1] The development
of promising strategies for the synthesis of unsymmetrical tri-
arylmethanes has attracted considerable attention in recent
decades. Lewis-acid- or Brønsted-acid-catalyzed Friedel–Crafts
reactions between nucleophilic electron-rich arenes and un-
symmetrical diarylmethanols or related derivatives were the tra-
ditional method for the construction of unsymmetrical triaryl-
methanes (Scheme 1, a).[2] Recently, transition-metal-catalyzed
cross-coupling reactions[3] and C–H functionalization[4] have
emerged as alternative methods to synthesize these skeletons
(Scheme 1, b). Despite this progress, these methods still have
shortcomings, including a limited substrate scope, a need for
prefunctionalization of the substrates, the use of expensive
transition metals, and the formation of undesired regioisomers.
This has hampered their wider use to some extent. Thus, the
development of a practical method that will improve the reac-
tion efficiency, simplify the operation, and expand the structural
scope, using readily available starting materials and an environ-
mentally benign catalyst is still desirable.
Scheme 1. Approaches to unsymmetrical triarylmethanes.
Recently, para-quinone methides (p-QMs), an important and
readily available class of synthetic intermediates, have been
used to construct diarylmethines through 1,6-conjugate addi-
tion.[5,8c] In 2015, Anand's group reported a Pd-catalyzed annu-
lation of ortho-alkynylanilines followed by 1,6-addition of p-
QMs for the synthesis of unsymmetrical triarylmethanes bearing
indole groups.[6] Sun's group achieved the synthesis of unsym-
metrical triarylmethanes containing pyrrole groups through the
chiral-phosphoric-acid-catalyzed 1,6-conjugate addition of in-
situ-generated p-QMs with 2-methylpyrroles.[7] To construct un-
symmetrical triarylmethanes containing more heteroaromatic
groups, and also as part of our ongoing interest in p-QMs,[8] in
this paper we report our results on the BF3·Et2O-catalyzed 1,6-
addition arylation of p-QMs with α-isocyanoacetamides.[9] This
results in the direct synthesis of diverse unsymmetrical triaryl-
methanes with oxazole groups and phenol groups in good to
[a] State Key Laboratory of Natural Medicines (SKLNM) and Department of
Medicinal Chemistry, School of Pharmacy, China Pharmaceutical
University,
Nanjing, 210009, P. R. China
E-mail: hyao@cpu.edu.cn
ajlin@cpu.edu.cn
[‡] S. Gao and X. Xu contributed equally.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600385.
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excellent yields under mild conditions (Scheme 1, c). Unsym- could be finally isolated in 88 % yield (Table 1, entries 19 and
metrical triarylmethanes containing both these two groups are 20).
important structures that are found in natural products and
Having established optimized reaction conditions, we ex-
pharmaceuticals, but the synthesis of this kind of unsymmetri-
plored the substrate scope and generality of this 1,6-addition
cal triarylmethane has never been reported. To further exploit
arylation reaction, and the results are summarized in Scheme 2.
our strategy, we also used dialkyl anilines, methyl indolizine-1-
p-QM derivatives bearing electron-donating groups (-Me, -MeO,
-Me2N), halogen atoms (F, Cl, Br), or electron-withdrawing
groups (-CF3, -CN, -NO2) at the para position of the benzene
ring gave 3ba–3ja in 76 to 93 % yields. p-QM derivatives with
carboxylate, furans, thiophenes, 2-naphthol, and 2-hydroxy-1,4-
naphthalenedione as electron-rich aromatic compounds to syn-
thesize other functional unsymmetrical triarylmethanes.
an -OMe, -Cl, or -Br group at the ortho or meta position gave
3ka–3qa in 77–92 % yields, proving that this reaction is insensi-
tive to the ortho, meta, and para substituents on the phenyl
Results and Discussion
rings. The fact that halogen atoms (Cl, Br) are tolerated in differ-
ent positions means that further transformation of the unsym-
metrical triarylmethane products should be possible. p-QM de-
rivatives with phenyl, alkenyl, thiophenyl, and 1-naphthyl
groups were also tested, and the corresponding arylated prod-
ucts (i.e., 3ra–3ua) were obtained in 72–95 % yields. Changing
the R1 group of the p-QM derivatives to an isopropyl group
resulted in the formation of 3va in 50 % yield. Next, we ex-
plored the scope of the reaction with respect to the α-isocyano-
acetamide component. Substrates with electron-donating or
electron-withdrawing groups at the para-position of the benzyl
group were well tolerated, giving 3ab–3ae in 76–90 % yields.
When R2 was varied to naphthalen-2-ylmethyl and methyl
groups, 3af and 3ag were formed in 83 and 81 % yields. More-
over, 3ah, with the C-4 position of the oxazole ring unsubsti-
tuted, was obtained in 86 % yield. When R3 was changed from
a morpholino to a thiomorpholino or another cyclic amino
group, 3ai–3al were formed in 46–88 % yield. When α-iso-
cyanoacetamide 2a was replaced with α-isocyanoacetate, the
corresponding product (i.e., 3am) was not obtained. To probe
the efficiency of our reaction, a gram-scale experiment (1a:
4 mmol) was carried out. When the catalyst loading was de-
creased to 15 mol-%, 3aa was still obtained in 85 % yield
(1.84 g).
Initially, our investigations focused on the combination of para-
quinone methide 1a and α-isocyanoacetamide 2a to optimize
the reaction conditions (Table 1). The synthesis of unsymmetri-
cal triarylmethane 3aa containing a oxazole, a benzene, and a
phenol group could be achieved in 60 % yield when BF3·Et2O
was used as the catalyst (Table 1, entry 9). Other Brønsted or
Lewis acids such as AcOH, (PhO)2POOH, Sc(OTf)3, AlCl3,
Cu(OTf)2, Ni(OTf)2, Mg(OTf)2, or Zn(OTf)2 gave 3aa in lower
yields (Table 1, entries 1–8). A solvent screening revealed that
acetonitrile was better than other solvents, giving 3aa in 75 %
yield (Table 1, entries 10–16). Raising or lowering the reaction
temperature did not give a better result (Table 1, entries 17 and
18). By adjusting the equivalent ratio of 1a/2a to 1:2, 3aa
Table 1. Screening of reaction conditions.[a]
Entry Catalyst
Solvent
T
Ratio 1a/2a
Yield [%][b]
[°C]
To explore the detailed mechanism of our reactions, control
experiments were carried out as shown in Scheme 3. When
the reaction was run without 1a, α-isocyanoacetamide 2a was
transformed into 4,5-disubstituted oxazole 4a in 88 % yield in
5 min under standard conditions.[9f] When 2a was treated with-
out BF3·Et2O, only a trace amount of 4a could be detected
[Scheme 3, Equation (1)]. These results indicated that in our
system, the cyclization of 2a to form 4a occurs before the 1,6-
addition of p-QMs; this is different from previously reported
reactions involving α-isocyanoacetamides, which proceed
through nucleophilic addition followed by cyclization. Combin-
ing 4a with 1a under standard conditions, the arylation path-
way proceeded smoothly to give 3aa in 84 % yield, even when
the reaction time was shortened to 5 h. Compound 3aa was
only obtained in 22 % yield in the absence of BF3·Et2O
[Scheme 3, Equation (2)]. Based on these data, we speculate
that BF3·Et2O plays two roles in our reaction: it catalyzes the
cyclization of α-isocyanoacetamide 2a to form oxazole 4a, and
it activates para-quinone methide 1a to achieve the 1,6-addi-
tion arylation process via intermediates I and II. A detailed cata-
lytic cycle is proposed in Scheme 3.
1
2
3
4
5
6
7
8
AcOH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
r.t.
80
50
50
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1.5
1:2
n.r.
32
53
51
35
20
8
36
60
36
<5
14
57
21
18
75
25
48
(PhO)2POOH
Sc(OTf)3
AlCl3
Cu(OTf)2
Ni(OTf)2
Mg(OTf)2
Zn(OTf)2
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
9
10
11
12
13
14
15
16
17
18
19
20
EtOH
toluene
(CH2Cl)2
dioxane
DMF
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
78
93 (88)[c]
[a] Reaction Conditions: 1a (0.2 mmol), 2a, and catalyst (0.04 mmol) in solvent
(1.0 mL) at the stated temperature for 20 h. [b] The yields were determined
by 1H NMR spectroscopy, with dibromomethane as an internal standard.
[c] Isolated yields.
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Scheme 2. Substrate scope. Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), BF3·Et2O (0.04 mmol) in acetonitrile (1.0 mL) at 50 °C for 20 h. Isolated yields are
given. [a] 4 mmol scale.
Based on the above reaction mechanism, we supposed that in 95, 80, and 84 % yields. Cyclic amines, such as pyrrolidine,
nucleophilic electron-rich aromatic compounds could also be morpholine, N-methylindoline, and N-methyltetrahydroquinol-
used to construct unsymmetrical triarylmethanes using our cat- ine, gave 6ad–6ag in 78–92 % yields. When N,N-dimethyl-
alytic system. Since nitrogen-containing unsymmetrical triaryl- naphthalen-1-amine (5h) was examined, 6ah was obtained in
methanes are important for dyes[10] and medicinal com- 97 % yield. A substrate bearing a secondary amine gave 6ai in
pounds,[11] various nitrogen-containing aromatic compounds 41 % yield. When aminodiphenylmethane was tested, double
were then tested, and the results are summarized in Scheme 4. 1,6-addition arylated product 6aj was assembled quickly in
Dialkyl anilines 5a–5c gave arylated products 6aa, 6ab, and 6ac 88 % yield in 5 min; this compound can serve as the core struc-
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Scheme 3. Control experiments and proposed catalytic cycle.
ture of dendritic architectures. After checking the arylamines, ated, and gave target arylated products 6va in 52 % yield, and
we then turned our attention to other important heteroaro- 6ya in 83 % yield. 1-Naphthoquinone 4-methide (p-NMQ) also
matic compounds. Generally, the reaction proceeded smoothly reacted smoothly to give 6xa in 61 % yield under our reaction
to give the unsymmetrical triarylmethanes in moderate to good conditions. To our delight, when the catalyst loading was low-
yields at room temperature within short reaction times. Indole ered to 15 mol-%, compound 6aa was still formed in 92 % yield
and 3-methylindole gave C-3- and C-2-arylated products 6ak on a 5 mmol scale (1.91 g).
and 6al in 92 and 98 % yields, while 2,3-dimethylindole gave
With the aim of gaining insight into the utility of the
C-5-arylated product 6am in 66 % yield. Methyl indolizine-1- reaction, the arylated product was further transformed. Tetra-
carboxylate reacted selectively at the C-3 position to give 6an substituted furan 7 was obtained conveniently from 3aa
in 98 % yield. Other heteroaromatic rings such as furan, thio- and an alkyne in 43 % yield through a Diels–Alder reaction
phene, and benzofuran could also be converted into the de- (Scheme 5). Furthermore, an AlCl3-promoted de-tert-butylation
sired products (i.e., 6ao–6aq) in good yields. 2-Naphthol, of 6aa gave compound 8 in 85 % yield at room temperature.
4-hydroxycoumarin, N-methyl-4-hydroxyquinol-2-one, and 2- In addition, 6aa and 6aj could be oxidized with DDQ (2,3-
hydroxy-1,4-naphthalenedione were also used in the reaction, dichloro-5,6-dicyano-1,4-benzoquinone) to give 9 in 99 %
and gave 6ar–6au in good yields from 83 to 92 %. When the yield, and 10 in 53 % yield; these compounds are also core
phenyl group of the p-QM component was changed to a structures of many biologically active molecules,[12] materials
naphthyl or methyl group, the expected products (i.e., 6ua and for dye-sensitised solar cells (DSC),[13] and chemodosimeters
6wa) were formed in 88 and 71 % yields, respectively. p-QM for the selective and chromogenic sensing of cyanide
derivatives with isopropyl and methyl groups were also toler- anions.[14]
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Scheme 4. Substrate scope. Reaction conditions: 1 (0.2 mmol), 5 (0.24 mmol) and BF3·Et2O (0.04 mmol) in CH2Cl2 (1.0 mL) at 40 °C for 10 h under Ar. Isolated
yields are given. [a] 5 mmol scale with 15 mol-% BF3·Et2O as catalyst at 40 °C for 24 h under Ar. [b] 50 °C. [c] 24 h. [d] 120 h. [e] r.t., 5 min. [f] 1a (0.4 mmol),
5 (0.2 mmol) and BF3·Et2O (0.04 mmol) in CH2Cl2 (1.0 mL). [g] r.t., 24 h. [h] 40 °C, 24 h.
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Scheme 5. Transformation of triarylmethanes.
mixture was purified by flash column chromatography on neutral
alumina to give pure product 6.
Conclusions
In conclusion, we have developed a BF3·Et2O-catalyzed 1,6-ad-
dition arylation of para-quinone methides (p-QMs) with α-isocy-
anoacetamides. The reaction allows the direct synthesis of un-
symmetrical triarylmethanes containing both oxazoles and
phenols in good to excellent yields under mild conditions for
the first time. Moreover, our method was extended to other
electron-rich aromatic compounds. This metal-free arylation
strategy showed good functional-group tolerance and scalabil-
ity up to gram scale. Transformation of the unsymmetrical tri-
arylmethane products further increased the range of accessible
compounds, and revealed potential applications of the prod-
ucts.
Acknowledgments
Generous financial support from the National Natural Science
Foundation of China (NSFC) (grant numbers NSFC21572272 and
NSFC21502232), the Natural Science Foundation of Jiangsu
Province (grant number BK20140655), and the Innovative Re-
search Team in University (grant number IRT_15R63) is grate-
fully acknowledged.
Keywords: Synthetic methods · Polycycles · Carbocycles ·
Triarylmethanes
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Experimental Section
1,6-Addition Arylation of para-Quinone Methides (p-QMs) with
α-Isocyanoacetamides: p-QM 1 (0.2 mmol, 1.0 equiv.) and substi-
tuted α-isocyanoacetamide 2 (0.4 mmol, 2.0 equiv.) were dissolved
in CH3CN (1.0 mL), and then BF3·Et2O (5.1 μL) was added. The reac-
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reaction was complete (detected by TLC), the solvent was removed
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column chromatography on silica gel to give pure product 3.
1,6-Addition Arylation of para-Quinone Methides (p-QMs) with
Electron-Rich Aromatic Compounds: p-QM
1 (0.2 mmol,
1.0 equiv.) and substituted arylamine or heteroaromatic compound
5 (0.24 mmol, 1.2 equiv.) were dissolved in CH2Cl2 (1.0 mL), and
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Received: March 29, 2016
Published Online: June 8, 2016
Eur. J. Org. Chem. 2016, 3006–3012
www.eurjoc.org
3012
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim