Gold promoted arylative cyclization of alkynoic acids with arenediazonium salts

Article abstract of DOI:10.1039/c7ob02447a

Alkynoic acids derived from salicylic acid and analogues undergo arylative cyclization with arenediazonium salts promoted by gold in the absence of external ligands. The reaction is thermally induced and proceeds even in the absence of light. A difference in regioselectivity has been found compared with that observed in the cycloisomerization process of the same type of compounds.

Full text of DOI:10.1039/c7ob02447a

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Gold promoted arylative cyclization of alkynoic
acids with arenediazonium salts†
Cite this: DOI: 10.1039/c7ob02447a
Ulises A. Carrillo-Arcos and Susana Porcel
*
Received 2nd October 2017,
Accepted 7th December 2017
Alkynoic acids derived from salicylic acid and analogues undergo arylative cyclization with arenediazo-
nium salts promoted by gold in the absence of external ligands. The reaction is thermally induced and
proceeds even in the absence of light. A difference in regioselectivity has been found compared with that
observed in the cycloisomerization process of the same type of compounds.
DOI: 10.1039/c7ob02447a
rsc.li/obc
In recent years an interest has emerged to expand the chem-
istry of gold, typically governed by the oxidation state I, by
engaging it in redox processes. This has led to the develop-
ment of a variety of strategies aiming at circumventing the
high redox potential of the Au(I)/Au(III) couple (E0 = +1.41 V).1
Initial efforts involved the addition of strong external oxidants
with the concomitant problems of toxicity, low functional
group compatibility or atom economy.2 More recently, milder
approaches have been described which relied on the use of
arenediazonium salts as electrophiles.3 In most of these strat-
egies, gold is used in combination with a photoredox catalyst
which promotes Au(I)/Au(III) oxidation stepwise, by the gene-
ration of aryl radicals.4 In a few cases, it has been noticed that
the oxidation can also be induced in the absence of the photo-
catalyst, under direct light irradiation, thermally or by using a
base.5 In particular, our group showed that arenediazonium
chlorides are able to oxidize [AuCl(L)] (L = SMe2, PPh3) com-
plexes under thermal conditions.6 With the aim of incorporat-
ing the [AuArCl2(L)] intermediates generated in this pathway in
a two-step transformation, we decided to explore the arylative
cyclization of alkynoic acids. Examples of the arylative cycliza-
tion of alkynylphenols,4q,r heteroatom tethered alkynes4s and
chiral homopropargyl sulphonamides4t have appeared recently
in the bibliography, but to the best of our knowledge this is
the first study on alkynoic acids.
Scheme 1 Gold(I) catalysed cycloisomerization of alkynoic acids.7
To undertake the present study, we chose 2-(2-butyn-1-
yloxy)benzoic acid (1a) and p-nitrobenzenediazonium chloride
as model substrates. As a first approach, we decided to study
the transformation sequentially by preforming the aryl Au(III)
intermediate. Following the methodology previously reported
by us, a mixture of p-nitrobenzenediazonium chloride pre-
pared in situ from aniline and [AuCl(SMe2)] was heated in
DMSO at 50 °C for 4 h. Thereafter, 1a was added and the
mixture was heated for 16 h at 50 °C. Under these conditions,
the decomposition of 1a along with 14% of 4,4′-dinitrobiphe-
nyl resulting from the parasite homocoupling reaction was
observed. Gratifyingly, the addition of Li2CO3 promoted aryla-
tive cyclization leading to a mixture of 7-(1b) and 8-(1c) mem-
bered ring enol lactones in 61% yield (Table 1, entry 2).
Employing Cs2CO3 as a base the same mixture of lactones was
obtained, although in a lower yield (54%, entry 3). It is impor-
tant to note that comparatively, the Au(I) catalysed cycloiso-
merization reaction over the same substrate led exclusively to
the corresponding 2H-chromene.7 This result points out that
the oxidation state of gold has a marked influence on the
regioselectivity of the cyclization. Au(III), according to its
harder Lewis character,8 promotes the attack of the more
nucleophilic moiety, the carboxylic acid, whereas, Au(I) pro-
motes the attack of the softer nucleophile, the aryl ring.
A previous report from our group showed that gold(I) cata-
lyses the cyclization of alkynoic acids derived from salicylic
acid, leading to lactones and 2H-chromenes (Scheme 1).7
Particularly, it was observed that 2H-chromenes were preferred
over lactones when a methyl or an electron-rich arene was
present at the end of the alkyne.
Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior
s/n, Ciudad Universitaria, 04510 México D.F., Mexico. E-mail: sporcel@unam.mx
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7ob02447a
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Table 1 Sequential arylative cyclization of 1a a
Table 3 One-pot arylative cyclization of 1a with p-nitrobenzenediazo-
nium chloridea
t, T
Yield% (1b/1c)b
1
2
3
4
3 h, 25 °C
2.5 h, 50 °C
2.5 h, 25 °C
3 h, 25 °C
75 (77/23)
86 (72/28)
76 (76/24)
68 (72/28)
Blue LEDs (25 W)
Darkness
a Aniline : tbuONO : HCl (1 : 1.2 : 2), [1a] = 0.05 mmol ml−1 b Isolated
.
yield.
Entry
Base
Yield% (1b/1c)b
1
2
3
—
—
Li2CO3 (2 equiv.)
Cs2CO3 (2 equiv.)
61 (54/46)
54 (64/36)
all cases, the 7-membered ring lactone 1b was favoured over
1c, and 2H-chromene was not observed.
a Aniline : tBuONO : HCl (1 : 1.2 : 2), [1a] = 0.05 mmol ml−1 b Isolated
.
yield.
At this point, and with the aim of increasing the yield of
the arylative cyclization, we decided to review the effect of the
counteranion of the diazonium salt on the transformation.
With this goal, we studied the reaction of 1a using p-nitro-
benzenediazonium chloride under optimized conditions with
p-nitrobenzenediazonium tetrafluoroborate (Table 3, entry 1).
Pleasantly, the yield increased to 75%. Moreover, employing
MeOH : MeCN (3 : 1) as a solvent and heating at 50 °C for
2.5 h, increased the yield up to 86% (entry 2). This result was
congruent with a previous observation of our group which
showed that the coupling of arenediazonium salts with silver
acetylides promoted by gold works better when the counter-
anion of the diazonium salt is a chloride.6 After that, we ana-
lysed whether the reaction could be accelerated by irradiation
with blue LEDs (entry 3). However, the yield was similar to that
obtained when the reaction was carried out under sunlight at
25 °C (entry 3 vs. entry 1). In addition, we performed the reac-
tion in the dark to test whether the sunlight is necessary for
the reaction to work. Interestingly, the yield decreased only
slightly (68%, entry 4), suggesting that in this transformation
light does not promote the reaction of gold with the diazo-
nium salt.9 One possibility is that the base induces the
decomposition of the diazonium salt to an aryl radical, pro-
moting the formation of the aryl Au(III) intermediate, as the
group of Shi has described recently.5h,10
Next, we examined whether it was possible to perform the
same transformation in a one-pot sequence. In this case, we
decided to use the diazonium salt with a tetrafluoroborate
anion, as this anion allows us to store the diazonium salt for
prolonged times,3 avoiding the precaution of preparing it
in situ. When a mixture of 1a, p-nitrobenzenediazonium tetra-
fluoroborate and [Au(SMe2)Cl] was stirred in DMSO at 25 °C,
1a was consumed after 3 h leading to a mixture of 1b/1c in
45% yield (Table 2, entry 1). Although the yield was lower, this
result encouraged us as it showed that the arylative cyclization
could be performed in one pot. Then, we studied the effect of
the addition of a phosphine by using [Au(PPh3)Cl] instead of
[Au(SMe2)Cl]. However, in the presence of PPh3 the yield
lowered to 30% (entry 2). On the other hand, switching the
solvent to MeCN lowered the yield to 35%, but by increasing
the polarity with a mixture of MeOH : MeCN (3 : 1), the yield
increased to 57% (entry 4). Regarding the regioselectivity, in
Table 2 One-pot arylative cyclization of 1a with p-nitrobenzenediazo-
nium chloridea
The scope of the reaction was analysed over a variety of alky-
noic acids with different substitution patterns (Table 4). The
introduction of π-donor substituents (Cl, OMe) at position 4 of
the aryl ring decreased the yield slightly compared with the
same substituents at position 5 (69% and 68% vs. 75% and
80%, entry 1). In addition, the substituents at position 4
favoured the 7-membered ring lactone over the 8-membered
ring lactone (2b/2c, 5b/5c = 88 : 12). On the contrary, the intro-
duction of a methyl group at position 3 lowered the regio-
selectivity (83%, 4b/4c = 47/53). Likewise, the replacement of
the oxygen atom at the ortho position of the carboxylic acid by
[Au(I)]
Solvent
t (h)
Yield% (1b/1c)b
1
2
3
4
[Au(SMe2)Cl]
[Au(PPh3)Cl]
[Au(SMe2)Cl]
[Au(SMe2)Cl]
DMSO
DMSO
MeCN
MeOH : MeCN (3 : 1)
3
7
3
3
45 (75/25)
30 (72/28)
35 (75/25)
57 (78/22)
a [1a] = 0.05 mmol ml−1 b Isolated yield.
.
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Table 4 Evaluation of the alkynoic acid scopea
Yield% (Xb/Xc)b
Scheme 2 Arylative cyclization of ortho-alkynyl benzoic acids.
Alkynoic acid
1
69%, 2b/2c = 88/12
75%, 3b/3c = 79/21
83%, 4b/4c = 47/53
68%, 5b/5c = 88/12
80%, 6b/6c = 68/32
Table 5 Evaluation of the aryldiazonium salt scope and limitationsa
2
70%, 7b/7c = 57 : 43
76%, 8b/8c = 44 : 56
X
Yield% (Xb/Xc)b
3
4
84%, 9b/9c = 47 : 53
1
2
3
4
5
6
7
8
H
4-Cl
4-Br
4-CN
4-COPh
4-OMe
2-Cl
—
89%, 17b/17c = 73/27
68%, 18b/18c = 77/23
80%, 19b/19c = 67/33
80%, 20b/20c = 71/29
70%, 21b/21c = 60/40
—
80%, 10b/10c = 67 : 33
3-Cl
—
5
75%, 11b/11c = 79/21
62%, 12b/12c = 68/32
45%, 13b/13c = 30/70
a [1a] = (0.03 mmol ml−1). b Isolated yield.
6
35%, 14b/14c = 30/70
In this case, we observed the mixture of lactones 14b/14c
(35%) arising from monoarylation, in which the 8-membered
ring lactone was the major isomer (15b/15c = 30/70).11
With the aim of examining the regioselectivity of arylative
cyclization for the formation of 5 and/or 6 membered ring lac-
tones, we studied the reaction over the ortho-alkynyl benzoic
acids 15a and 16a (Scheme 2). The reaction took place in both
cases with good yields, and in the case of 15a led exclusively to
a [Xa] = (0.03 mmol ml−1). b Isolated yield.
a sulfur atom or an NTs group decreased the regioselectivity the 5-membered ring lactone.
(entry 2, S: 70%, 7b/7c = 57 : 43; NTs: 76%, 8b/8c = 44 : 56).
Finally, to complete the study of the reaction scope, we
Alkynoic acid 9a, derived from 1-hydroxy-2-napthoic acid, inspected the performance of the reaction over different diazo-
underwent arylative cyclization to give the corresponding tri- nium salts (Table 5). It was observed that the reaction took
cyclic lactones 9b/9b (84% yield, entry 3), with the same regio- place for both electron-donor and electron-withdrawing groups
selectivity as that observed for the alkynoic acid with a methyl at the para position, with yields and regioselectivities similar
group at position 3 (47/53). The introduction of a larger substi- to those provided by p-nitrobenzenediazonium chloride
tuent at the end of the alkyne (n-butyl, 10a) did not signifi- (entries 2–7). However, the reaction failed for benzenediazo-
cantly affect the performance of the reaction and the corres- nium chloride, o-chlorobenzenediazonium and m-chloro-
ponding lactones 10b/10c were obtained in 80% yield (entry 4). benzenediazonium chlorides (entries 1, 7 and 8), which can be
On the other hand, the incorporation of p-MeOC6H5, C6H5- or attributed to a minor stability of the diazonium salts.
p-NO2C6H5 groups at the end of the alkyne led to lactones
A plausible mechanism for the one-pot arylative cyclization
11b/11c, 12b/12c and 13b/13c in 75%, 62% and 45% yields, is depicted in Scheme 3. We propose that initially the arene-
respectively (entry 5). In this group of alkynoic acids it could diazonium salt oxidizes [Au(SMe2)Cl] to form the arylgold(III)
be observed that the regioselectivity of the reaction depends complex, since we have previously observed that the cyclo-
on the electronic nature of the phenyl ring. Thus, for isomerization of alkynoic acids with [Au(L)Cl] only works in the
p-MeOC6H5 and C6H5 the 7-membered ring lactone was the presence of a silver salt as a halogen scavenger.7 Once formed,
major regioisomer (11b/11c = 79/21, 12b/12c = 68/32), while for the arylgold(III) species coordinates with the alkyne, promoting
p-NO2C6H5 the regioselectivity was reversed (13b/13c = 30/70). the nucleophilic attack of the carboxylic acid moiety. Lastly, a
In addition, we examined the reaction over alkynoic acid 15a reductive elimination takes place, furnishing the 7- and
bearing an atom of hydrogen at the end of the alkyne (entry 6). 8-membered ring enol lactones. Although in this proposed
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Conclusions
In summary, we have shown that arylgold(III) complexes gener-
ated from arenediazonium salts can promote the arylative
cyclization of alkynoic acids to furnish 7/8 and 5/6-membered
ring enol lactones.13 Unlike in the cycloisomerization of alky-
noic acids catalysed by Au(I), 2H-chromenes are not formed.
The reaction proceeds in the absence of light, but the trapping
of aryl radicals with TEMPO suggests that a radical path is
involved.
Scheme 3 Proposed mechanism.
mechanism, the [Au(L)Cl] complex is regenerated, all attempts
to carry out this transformation with a catalytic amount of
gold were unsuccessful.12
Experimental
General experimental methods
In order to prove the participation of radicals in this
process, we test the reaction in the presence of 1 equiv. of
TEMPO (Scheme 4a). Under these conditions, the yield of the
cyclization decreased to 25%, and by mass spectrometry the
product arising from the trap of the p-nitroaryl radical with
TEMPO could be detected (22, m/z = 278.35).12 By removing
the alkynoic acid from the medium (Scheme 4b), again one of
the major peaks observed is m/z = 278.35, which belongs to 22.
Lastly, we examined the reaction of the preformed complex
pNO2C6H5Au(L)Cl2 in DMSO with TEMPO (Scheme 4c). In this
case, we observed by NMR spectroscopy the decomposition of
the complex, and by mass spectrometry we could identify the
peaks belonging to TEMPO and 2,2,6,6-tetramethylpiperidine.
All these results suggest that under the reaction conditions,
the diazonium salt decomposes quickly to give an aryl radical
that can be trapped with TEMPO. However, the possibility that
the Au(III) complex can be formed via a 2 electron process,
which occurs in parallel with the formation of the aryl radicals,
can’t be ruled out.
All reactions were performed under a nitrogen atmosphere
using standard Schlenk techniques. Solvents were dried using
standard methods when necessary. Commercial reagents were
purchased from Aldrich and used as received without further
purification. Gold complexes were stored under a N2 atmo-
sphere. All reactions were monitored by thin layer chro-
matography using TLC Alugram G/UV254, 0.20 mm.
Chromatography purifications were performed using flash
grade silica gel (SDS Chromatogel 60 Acc, 40–60 μm). The
NMR spectra were recorded at 25 °C on a Jeol Eclipse
300 MHz, Bruker Avance 400 MHz and Varian Unity Inova
500 MHz spectrometers. Chemical shifts are reported in ppm.
High resolution mass spectra (HRMS) were recorded on a Jeol
AccuTOF JMS-T100LC spectrometer using polyethylene glycol
as the internal standard. Melting points were determined
using a Reichert microscope apparatus and were uncorrected.
General methods for arylative cyclization
Method A (with the preformation of the arylAu(III) inter-
mediate). To a solution of p-nitroaniline (0.10 mmol) in THF
(0.70 mL), HCl·Et2O was added (1 M, 0.20 mmol, 0.20 mL) at
0 °C. The mixture was stirred until a white precipitate
appeared, then was cooled to −15 °C and tert-butyl nitrite
(0.12 mmol, 0.018 mL) was added dropwise. The reaction
mixture was stirred from −15 °C to 0 °C over a period of
20 min; thereafter the solution was removed under vacuum.
Next, the diazonium salt was dissolved in DMSO (2 mL), and
[AuCl(SMe2)] (0.10 mmol) was added under nitrogen. The
temperature was raised up to 50 °C and the mixture was
heated for 4 h. Later, Li2CO3 (0.20 mmol) and 1a (0.10 mmol)
were added and the reaction was further heated at 50 °C for
another 4 h. Finally, the solvent was removed under vacuum
and the residue obtained was purified by silica gel chromato-
graphy using mixtures of hexane/EtOAc as eluents.
Method B (one-pot). To a solution of the corresponding
aniline (0.10 mmol) in THF (0.70 mL), HCl·Et2O was added
(1 M, 0.20 mmol, 0.20 mL) at 0 °C. The mixture was stirred
until a white precipitate appeared, then was cooled to −15 °C
and tert-butyl nitrite (0.12 mmol, 0.018 mL) was added drop-
wise. The reaction mixture was stirred from −15 °C to 0 °C
over a period of 20 min; thereafter the solution was removed
Scheme 4 Trapping experiments with TEMPO.
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under vacuum. Next, the diazonium salt was dissolved in
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3 Due to their high electrophilicity, arenediazonium salts
have efficiently been used as aryl halides and aryl triflate
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degassed MeOH/MeCN (3 : 1,
3 mL), and [AuCl(SMe2)]
(0.10 mmol) and Li2CO3 (0.20 mmol) along with the corres-
ponding alkynoic acid (0.10 mmol) were added under nitro-
gen. The reaction mixture was heated at 50 °C for 2.5 h.
Finally, the solvent was removed under vacuum and the
residue obtained was purified by silica gel chromatography
using mixtures of hexane/EtOAc as eluents.
1
1b/1c: White solid. H NMR (500 MHz, CDCl3) δ 8.31–8.28
(m, 1H 1c), 8.21–8.17 (m, 2H 1c), 8.16–8.11 (m, 2H 1b), 7.97
(dd, J = 8.0, 1.7 Hz, 1H 1c), 7.74–7.69 (m, 1H 1c), 7.50–7.43 (m,
1H 1c), 7.41–7.35 (m, 2H 1c), 7.32 (dd, J = 7.8, 1.6 Hz, 1H 1b),
7.25–7.20 (m, 2H 1b), 7.07 (ddd, J = 8.2, 7.2, 1.1 Hz, 1H 1b),
7.04–6.99 (m, 1H 1b), 6.91 (dd, J = 8.4, 0.8 Hz, 1H 1b), 4.90 (s,
2H 1b), 4.58 (s, 2H 1c), 2.10 (s, 3H 1c), 1.83 (s, 3H 1b).
13C NMR (125 MHz, CDCl3) δ 167.75 (C), 165.73 (C), 156.73 (C),
153.43 (C), 151.05 (C), 147.66 (C), 147.51 (C), 145.66 (C), 143.99
(C), 141.99 (C), 135.33 (CH), 134.74 (CH), 133.20 (CH), 131.02
(C), 130.12 (CH), 129.48 (CH), 129.22 (CH), 128.94 (CH), 128.48
(CH), 128.30 (C), 124.54 (CH), 124.05 (CH), 123.93 (CH), 122.40
(CH), 122.06 (CH), 120.36 (CH), 119.98 (C), 119.91 (CH), 117.20
(C), 68.18 (CH2), 66.49 (CH2), 17.87 (CH3), 17.65 (CH3).
IR (film): 3072.41, 2918.77, 2852.39, 1735.60, 1675.08, 1598.86,
1515.40, 1477.63, 1440.59 cm−1. HRMS-DART calculated for
C17H14NO5 [M + H]+: 312.08720; found: 312.08652.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by DGAPA (IN202017) and
I. Química. Ulises A. Carrillo-Arcos thanks CONACyT (583677)
for an undergraduate fellowship. The authors would like to
thank M. A. Peña-González, E. Huerta-Salazar, B. Quiroz-
García, I. Chávez-Uribe, H. Ríos-Olivares, M. R. Patiño-Maya,
L. Velasco-Ibarra, F. Javier Pérez-Flores and M. C. García-
González for technical support.
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M. Rudolph, F. Rominger and A. S. K. Hashmi, Angew.
9 In the same way, when 1a, [Au(SMe2)Cl] and p-nitrobenzene-
diazonium tetrafluoroborate were stirred in DMSO at 25 °C
in the darkness, a mixture of lactones 1b and 1c was
obtained in 43% yield, confirming that also under these
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Org. Biomol. Chem.
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