Iridium(i)-catalyzed hydration/esterification of 2-alkynylphenols and carboxylic acids

Article abstract of DOI:10.1039/c9cc09984k

An iridium(i)-catalyzed hydration/esterification of 2-alkynylphenols and carboxylic acids is described. Various 2-alkynylphenols and carboxylic acids could be used in this process to furnish aromatic ortho-acyloxyketones via a regio- and stereo-selective addition reaction followed by intramolecular rearrangement. This protocol features mild reaction conditions and broad substrate scope. Further transformations were conducted to demonstrate the synthetic application.

Full text of DOI:10.1039/c9cc09984k

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DOI: 10.1039/C9CC09984K
COMMUNICATION
Iridium(I)-Catalyzed Hydration/Esterification of 2-Alkynylphenols
and Carboxylic Acids
Linwei Zeng, a Renjie Chen, a Chen Zhang,a Hujun Xie*,b and Sunliang Cui*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
(A) Addition reactions of alkynes and carboxylic acids
An
iridium(I)-catalyzed
hydration/esterification
of
2-
R1
R2
alkynylphenols and carboxylic acids is described. Various 2-
alkynylphenols and carboxylic acids could engage in this process to
furnish aromatic ortho-acyloxyketones via a regio- and stereo-
selective addition reaction followed by intramolecular
rearrangement. This protocol features mild reaction conditions and
broad substrate scope. Further transformations were conducted to
demonstrate the synthetic application.
O
O
Cat.
R1
R2
+
R3 OH
or catalyst-free
R3
O
O
(B) This work
OH
1 mol %
[Ir(cod)Cl]2
R2
O
O
O
+
R1
R2 OH
rt
R1
Ir(I)
R1
The addition reaction between carboxylic acids and alkynes has
become a powerful tool in the synthesis of enol esters which are
highly versatile building blocks in organic synthesis.1 A variety of
transition metals, including Ru, Rh, Co, Ag, Au, Ir, Pd and Re,
have shown capable in this type reactions with high efficiency
and selectivity (Scheme 1A).2 For representative examples, Shvo
and Bianchini reported a Ru-catalyzed and a Rh-catalyzed
synthesis of enol esters from alkynes and carboxylic acids
respectively.3 Li reported a Co-catalyzed regio- and stereo-
selective addition of carboxylic acids to alkynes.4 Satoh
H
O
silica gel
R2
O
regio- and stereoselective addition
mild reaction conditions
broad scope
HO
pure (E)-vinyl esters
Scheme 1 Addition reactions between alkynes and carboxylic acids
the hydroacyloxylation of alkynes and further transformation
cascade toward multiple bond formations in
manipulation.
a simple
established
a Ag-promoted enol esters formation from
2-Alkynylphenols were versatile synthons toward structurally
divergent benzofuran derivatives and other valuable
compounds.9 For example, Cho reported a TMSOTf-promoted
addition of aldehydes and 2-alkynylphenols for the synthesis of
chroman-4-ones.10 This reaction relied on the electron-
donating property of alkyne to attack the activated aldehyde.
Very recently, our group reported an iridium(I)-catalyzed
regioselective Huisgen cycloaddition between 2-alkynylphenols
and organic azides, in which the phenolic hydroxyl could serve
as a directing group.11 Encouraged by above work, we
hypothesized that an addition reaction might be realized
between 2-alkynylphenols and carboxylic acids, and the
obtained enol esters might rearrangent to ortho-acyloxyketone
derivatives, which are precursor of Baker-Venkataraman
rearrangement.12 In continuation of our research in alkynes,13
herein we would like to report an iridium(I)-catalyzed
hydration/esterification between 2-alkynylphenols and
carboxylic acids to achieve ortho-acyloxyketone derivatives
(Scheme 1B).
carboxylic acids and internal alkynes.5 Zhang and Nolan
developed Au-catalyzed addition between alkynes and
carboxylic acids for the synthesis of enol esters.6 Ishii and
Merola independently described an Ir-catalyzed addition
reaction between carboxylic acids and alkynes, which is
applicable to terminal alkynes and suffered from slightly low
yields and poor selectivity.7 Additionally, electron-rich alkynes,
such as ynamides and ynol ethers, have been widely studied in
the addition reactions with carboxylic acids.8 However, all these
reactions end with hydroacyloxylation of alkynes upon the
formation of enol esters. It would be interesting to develop
a. Institute of Drug Discovery and Design, College of Pharmaceutical Sciences,
Zhejiang University, Hangzhou 310058, China. E-mail: slcui@zju.edu.cn
b. Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou
310018, China. E-mail: hujunxie@gmail.com
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Journal Name
O
R3
We commenced our study by investigating this reaction
between 2-(phenylethynyl)phenol 1a and phenylacetic acid 2a.
We initially treated 1a and 2a with 2 mol % [Ir(cod)Cl]2 in CH2Cl2
at room temperature, and a new product 3a was isolated in 87%
yield after silica gel column chromatography (Table 1, entry 1).
NMR spectroscopy and mass spectrometry showed that 3a was
2-(2-phenylacetyl)phenyl 2-phenylacetate, indicating the
hydration/esterification occurs in this transformation. A variety
of catalysts, including [Cp*IrCl2]2, [Rh(cod)Cl]2, [Cp*RhCl2]2,
[Ru(p-cymene)Cl2]2, Cp*Ru(cod)Cl, AuClPPh3, IPrAuCl/AgNTf2,
Cu(OTf)2, Zn(OTf)2 and Pd(OAc)2, were then screened and found
completely shut down the reactivity (entries 2-11). The next
survey of solvents revealed that 1,2-dichloroethane (DCE) was
effective to give a slightly lower yield (entry 12, 80 %). The use
of MeOH, dioxane and toluene significantly decreased the yield
(entries 13-15, 31%-68%), while THF, DMF and CH3CN
suppressed the reaction completely (entries 16-18). Decreasing
the amount of [Ir(cod)Cl]2 to 1 mol % could give a comparable
88% yield (entry 19). Notably, a 5 mmol scale reaction was
conducted and 3a could be isolated in 85% yield. Moreover, the
reaction was supressed in the absence of [Ir(cod)Cl]2 (entry 20).
OH
View Article Online
O
R3 OH
2
O
(1) [Ir(cod)Cl]2
(2) silica gel
(1 mol %), DCHOCl , rt
2
I:
2
10.1039/C9CC09984K
R2
+
R1
R1
O
R2
1a
3
 carboxylic acids
O
O
O
O
O
O
O
O
O
O
Br
S
O
Ph
, 49%
O
Ph
O
Ph
O
Ph
O
Ph
3b
3c
3d
3e
3f
, 91%
, 81%
, 45%
, 81%
O
O
O
HN Bz
R
O
O
O
O
O
O
NTs
O
Ph
O
O
Ph
O
Ph
O
Ph
O
Ph
O
Ph
3j
, R = OH, 67%
3k
3l
, 73%
3g
3h
3i
, 86%
, 56%
, 87%
, R = OAc, 95%
O
O
O
O
3o
3p
3q
3r
3s
3t
3u
3v
, R = H, 89%
O
O
N
O
O
, R = 2-Br, 96%
, R = 3-N(Me)2, 92%
, R = 4-NO2, 71%
, R = 4-Cl, 74%
, R = 4-CF3, 72%
O
N
O
Boc
R
O
Ph
O
O
Ph
Ph
O
Ph
, R = 4-OMe, 81%
, R = 3,4,5-tri-OMe, 81%
3w
, 98%
3n
, 86%
3m
, 79%
O
O
O
O
O
N
O
O
Ph
O
S
O
O
O
O
Ph
O
Ph
O
Ph
O
3b'
Ph
O
Ph
3x
3y
3z, 97%
3a'
, 88%
, 78%
, 96%
, 94%
Table 1 Reaction optimization.a
 2-alkynylphenols
O
O
O
O
O
O
O
O
O
O
Ph
Ph
Ph
N
O
Ph
S
Ph
(2) silica gel column chromatography
OH
Ph
O
O
O
O
(1) catalyst, solvent, rt, 12h
+
Ph
Ph
R
O
OH
O
O
O
O
O
R
Ph
3c'
3d'
3e'
1a
2a
3a
, R = 4-Me, 91%
, R = 4-OMe, 93%
, R = 4-F, 88%
, R = 4-Cl, 88%
, R = 2-Cl, 79%
3j'
, R = n-Bu, 31%
, R = cyclopropyl, 79%
3l'
, 89%
3h'
, 68%
3i'
, 71%
yield (%)b
3k'
entry
1
2
3
4
5
6
7
8
cat. (2 mol %)
[Ir(cod)Cl]2
[Cp*IrCl2]2
[Rh(cod)Cl]2
[Cp*RhCl2]2
[Ru(p-Cymene)Cl2]2
Cp*Ru(cod)Cl
AuClPPh3
solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DCE
3f'
87
3g'
O
O
Ph
0
0
0
0
O
O
O
Me
O
Ph
O
H
O
O
Ph
Cl
Ph
O
Ph
O
H
H
O
3o', 75%
Ph
O
Ph
3n', 55%
O
O
OMe
3p', 83%
0
3m'
, 81%
Scheme 2. Substrate scope. Reaction conditions: 1a (0.2 mmol), 2 (0.22 mmol), and
[Ir(cod)Cl]2 (1 mol %) were added to CH2Cl2 (2 mL) and stirred at rt for 12 h, then purified
by silica gel column chromatography; yield refers to isolated product.
trace
IPrAuCl/AgNTf2
Cu(OTf)2
0
0
0
9
10
11
12
13
14
15
16
17
18
19c
20
Zn(OTf)2
Pd(OAc)2
acyloxyketones in moderate to excellent yields (3b-3f, 45%-
91%). The sterically hindered 1-adamantanecarboxylic acid was
also suitable in this reaction to deliver 3g in 56% yield. The
functionalized carboxylic acids with the scaffold of
tetrahydropyran and piperidine could participate in this process
smoothly (3h and 3i). Interestingly, chiral carboxylic acids were
also appropriate. For example, when R-Mandelic acid and its
acetyl ester were subjected to this reaction, the R-mandelic
esters could be obtained in 67% and 95 yields respectively (3j
and 3k). Amino acids, like protected L-alanine, glycine and L-
proline, were well tolerated in the process (3l-3n). With respect
to aryl carboxylic acids, various substituted benzoic acids were
found compatible in this protocol. Numerous functional groups,
including bromo, dimethylamino, nitro, chloro, trifluoromethyl
and methoxy, regardless the substituted position, were
tolerable, delivering the corresponding products in satisfactory
yields (3o-3v). The structure of product 3r was undoubtedly
confirmed by X-ray analysis.14 Naphthyl carboxylic acids, furan-
2-carboxylic acid, thiophene-2-carboxylic acid and 1H-indole-1-
carboxylic acid were also found suitable in this process to
trace
80
31
43
68
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
[Ir(cod)Cl]2
none
MeOH
dioxane
toluene
THF
trace
0
DMF
MeCN
CH2Cl2
CH2Cl2
0
88 (85)d
0
a Reaction conditions: 1a (0.2 mmol), 2a (0.22 mmol), and catalyst (2 mol %) were
added to solvent (2 mL) and stirred at room temperature for 12 h, then purified by
silica gel column chromatography. Yield refers to isolated product. 1 mol %
catalyst was used. d 5 mmol scale reaction.
b
c
With the optimized reaction conditions in hand, we next test
the substrate scope of carboxylic acids (Scheme 2). Numerous
aliphatic acids, bearing the moiety of methyl, cyclopropyl,
alkenyl, thiophene-3-methyl and 3-bromopropyl, could well
engage in this process to deliver the corresponding ortho-
2 | J. Name., 2012, 00, 1-3
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COMMUNICATION
furnish the products in good to excellent yields (3w-3a’). The efficiently (4j-4l). With respect to 2-alkynylpheVnieowlsA,rtivclaerOionluines
unsaturated cinnamic acid was also applicable to form the functional groups were tolerated toDOdIe: 1li0v.1e0r39e/Cn9oClC0e9s9t8e4rKs
product 3b’ in an excellent 94% yield. Meanwhile, the scope of successfully (4m-4o). Thus, (E)-enol esters were proposed as
alkynes was explored using phenylacetic acid (2a) and an array key intermediates of the followed acyl migration in this Ir-
of 2-alkynylphenols. Diverse aryl substituent groups at the end catalyzed reaction.
of the alkyne, such as para-methylphenyl, para-methoxyphenyl,
To demonstrate the synthetic utility of this reaction, four
para-fluorophenyl, para-chlorophenyl, ortho-chlorophenyl, obtained ortho-acyloxyketones were treated with NaH and
thiophene-2-yl and 2-naphthyl, could be compatible in this subsequently refluxed in acetic acid. To our delight, flavones
process to furnish the ortho-acyloxyketones in excellent yields (5a-5d) were constructed in good yields, among which The
(3c’-3i’). For the alkyl substituent groups, the butyl, cyclopropyl structure of flavone 5c was confirmed by X-ray analysis (Scheme
and aminoethyl groups, were found well tolerated (3j’-3l’). In 4).16 Therefore, this method offers a facile and rapid access to
respect of the substitution in the phenol ring, different flavones from easily available carboxylic acids and 2-
functional groups, including methyl, chloro and alkynyl, were alkynylphenols.
O
O
explored and found well engage in this process as well (3m’ and
3n’). The 1-(phenylethynyl)naphthalen-2-ol was also tested
and found suitable to deliver the product in 75% yield (3o’).
Notably, this process could be applied in the structural
modification of natural products. For example, when the
natural dehydrocholic acid (2e’) was used, the dehydrocholic
acid derived ortho-acyloxyketone 3p’ was isolated in 83% yield.
Therefore, this method provides a robust approach toward
ortho-acyloxyketones from easily accessible starting materials
with high efficiency.
R1
R2
(1) NaH, DMSO, rt
O
R1
(2) HCl, AcOH, reflux
O
O
R2
5
X-ray (5c)
3
O
O
O
O
Ph
Ph
Ph
Ph
O
S
O
O
O
O
O
5a
5b
5c
5d
, 90%
, 69%
, 86%
,91%
Scheme 4. Synthetic application
At this stage, we tried to obtain the enol esters from
carboxylic acids and 2-alkynylphenols to clarify the reaction
process. Unexpectedly, changing the purified method of
products from silica gel column chromatography to direct
concentration or recrystallization could exclusively obtain (E)-
enol esters. As shown in Scheme 3, 2-(phenylethynyl)phenol 1a
and a series of benzoic acids or heterocyclic carboxylic acids
could deliver corresponding (E)-enol esters in excellent yields
and stereoselectivity (4a-4i) upon simple purification. The
structure of 4f was confirmed by X-ray analysis to demonstrate
the E-form.15 Aliphatic acids, including tetrahydro-2H-pyran-4-
carboxylic acid, 1-adamantanecarboxylic acid and natural 18α-
glycyrrhetinic acid, could give corresponding (E)-enol esters
To gain more insights into the reaction, control reactions
were conducted. As shown in Scheme 5A, a variety of alkynes
were prepared to test this process. When the phenolic hydroxyl
group of 2-(phenylethynyl)phenol 1a was changed to OMe (1o)
or NH2 (1p), the reactivity was completely shut down. When the
N-methyl-2-(phenylethynyl)aniline (1q) was used, only an
intramolecular addition was occurred to produce a indole
product. When the hydroxyl group was shifted from ortho-
substitution to meta- (1r) or para- substitution (1s), we did not
find any reaction occurring. Replacing 1a with a homologated 2-
(phenylethynyl)benzyl alcohol (1t) or removing the phenolic
hydroxyl group (1u) also suppressed the reactivity. Moreover,
using a structurally similar homopropargyl alcohol derivative
(1v) suppressed the reactivity completely. These demonstrated
that the ortho-substituted phenolic hydroxyl group is essential
in this reaction. Meanwhile, a stepwise experiment was
performed (Scheme 5B). Treating enol ester 4c with silica gel
column chromatography could deliver 3q in 92% yield, showing
the silica gel promotes the acyl migration. This process
presumably due to the hydroxyl groups on the surface of silica
gel activates the carboxyl group of enol esters 4 and initiates the
subsequent rearrangement.17
H
R1
O
OH
O
R2 OH
2
(1) [Ir(cod)Cl]2 (1 mol %), CH2Cl2, rt
(2) without silica gel
R2
O
R1
+
HO
4
1a
H
O
Ph
H
O
Ph
H
O
Ph
H
O
Ph
O
O
Me
O
O
N
Cl
HO
HO
HO
HO
, quant.
4a
4b
, 91%
, 93%
H
4d
, 89%
H
4c
Ph
H
Ph
Ph
O
O
NO2
O
O
O
O
HO
MeO
HO
, 92%
O2N
HO
91%
Ph
(A)
4f
X-ray,
4e
4f
R
4g
, 89%
H
Ph
H
Ph
H
H
O
Ph
Ph
Ph
[Ir(cod)Cl]2 (1 mol %)
O
O
O
or
1o
1p
1q
1r
1s
3
4
or
, R = 2-OMe
, R = 2-NH2
, R = 2-NHMe
, R = 3-OH
+
2a
OH
O
O
O
O
1v
CH2Cl2, rt
O
S
O
HO
HO
HO
, 85%
Cl
HO
, 83%
4h
4i
4j
, 87%
, 90%
4k
, R = 4-OH
R
HO
1t
, R = 2-CH2OH
O
1u
, R = H
H
H
H
O
H
Ph
O
O
O
H
O
O
(B)
H
O
Ph
O
silica gel column
chromatography
O
O
N
HO
HO
4m
HO
N
4l
4n
R = Me, , 81%
, 68%
, 92%
4o
R = F, , 90%
HO
O
Ph
3q
Scheme 3. Synthesis of (E)-enol esters. Reaction conditions: 1a (0.2 mmol), 2 (0.2 mmol),
and [Ir(cod)Cl]2 (1 mol %) were added to CH2Cl2 (2 mL) and stirred at rt for 12 h, then
solvent was removed to obtain the products or purified by recrystallization.
4c
, 92%
Scheme 5. Control reactions
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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COMMUNICATION
In summary,
Journal Name
10 J. Y. Park, P. R. Ullapu, H. Choo, J. K. Lee, S. J. Min, A. N. Pae,
Y. Kim, D.-J. Baek and Y. S. Cho, Eur. J. Org. ChemV.,ie2w0A0rt8ic,le5O4n6lin1e.
11 R. Chen, L. Zeng, Z. Lai, S. Cui, Adv. SDynOtI:h1.0C.1a0t3a9l/.C290C1C90,9938641K,
989.
12 (a) W. Baker, J. Chem. Soc. 1933, 1381; (b) H. S. Mahal and K.
Venkataraman, J. Chem. Soc. 1934, 1767; (c) G. A. Kraus, B. S.
Fulton, S. H. Woo, J. Org. Chem. 1984, 49, 3212; (d) N. Thasana
and S. Ruchirawat, Tetrahedron Lett., 2002, 43, 4515.
13 (a) R. Chen, Y. Liu, S. Cui, Chem. Commun. 2018, 54, 11753; (b)
B. Huang, L. Zeng, Y. Shen, S. Cui, Angew. Chem., Int. Edit.
2017, 56, 4565; (c) Y. Shen, B. Huang, L. Zeng, S. Cui, Org. Lett.
2017, 19, 4616; (d) L. Zeng, Z. Lai, S. Cui, J. Org. Chem. 2018,
83, 14834;
a
robust
Iridium(I)-catalyzed
hydration/esterification of 2-alkynylphenols and carboxylic
acids was developed. Diverse ortho-acyloxyketones could be
rapidly assembled through a regio- and stereoselective addition
reaction and sequential acyl migration. The phenolic hydroxylic
group plays an important role in this reaction. This process
features mild reaction conditions and broad substrate scope.
Furthermore, the products could further convert to flavones
efficiently, demonstrating the synthetic application.
We are grateful for the financial support by National Natural
Science Foundation of China (21971222), and Key R & D
Programs of Zhejiang Province (2019C03082).
14 CCDC 1894582.
15 CCDC 1960048.
16 CCDC 1883891.
17 For examples of carbonyl activation by silica gel, see: (a) T.
Taniguchi and D. P. Curran, Org. Lett., 2012, 14, 4540; (b) J.
Aleksic, M. Stojanovic and M. Baranac-Stojanovic, Chem.-
Asian J., 2018, 13, 1811.
Conflicts of interest
There are no conflicts to declare.
18 For oxidative addition of carboxylic acid O-H bonds to
iridium(I) complexes, see: (a) I. S. Kim and M. J. Krische, Org.
Lett., 2008, 10, 513; (b) S. A. Smith, D. M. Blake and M. Kubota,
Inorg. Chem., 1972, 11, 660.
19 F. T. Ladipo, M. Kooti and J. S. Merola, Inorg. Chem. 1993,
32,1681.
20 For selective examples of hydrometalation of alkynes, see: (a)
F. T. Ladipo and J. S. Merola, Inorg. Chem. 1993, 32,5201; (b)
A. Lumbroso, N. Abermil and B. Breit, Chem. Sci., 2012, 3, 789;
(c) A. Lumbroso, P. Koschker, N. R. Vautravers and B. Breit, J.
Am. Chem. Soc., 2011, 133, 2386.
21 For hydroxyl as directing group in Ir-catalysis, see: (a) Y. Ebe,
T. Nishimura, J. Am. Chem. Soc. 2014, 136, 9284; (b) F. Zhu, Y.
Li, Z. Wang, X. Wu, Angew. Chem., Int. Edit. 2016, 55, 14151.
Notes and references
1
2
3
For select applications of enol esters, see: (a) S. C. Bourque, F.
Maltais, W.-J. Xiao, O. Tardif, H. Alper, P. Arya and L. E.
Manzer, J. Am. Chem. Soc. 1999, 121, 3035; (b) F. Simal, A.
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Org. Lett., 2010, 12, 5498; (e) D. L. Smith, W. R. F. Goundry
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