Photoredox-Assisted Gold-Catalyzed Arylative Alkoxycyclization of 1,6-Enynes

Article abstract of DOI:10.1021/acs.orglett.0c00799

The photoredox-assisted gold-catalyzed arylative cyclization of 1,6-enynes with aryldiazonium salts gives rise to cyclization products with the opposite configuration at the alkene than that obtained by gold(I)-catalyzed alkoxycyclization. The reaction oc

Full text of DOI:10.1021/acs.orglett.0c00799

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Letter
Photoredox-Assisted Gold-Catalyzed Arylative Alkoxycyclization of
1,6-Enynes
Joan G. Mayans, Jean-Simon Suppo, and Antonio M. Echavarren*
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ABSTRACT: The photoredox-assisted gold-catalyzed arylative
cyclization of 1,6-enynes with aryldiazonium salts gives rise to
cyclization products with the opposite configuration at the alkene
than that obtained by gold(I)-catalyzed alkoxycyclization. The
reaction occurs under mild conditions and shows high functional
group tolerance.
Scheme 1. Gold-Catalyzed Functionalization of Alkynes
he carbophilic character of gold(I) complexes allows the
T
selective activation of π-systems toward the attack of
nucleophiles.1 This reactivity has been widely studied and
successfully applied for the construction of complex polycyclic
structures including core scaffolds of natural products2 and
relevant organic materials.3
In the past decade, an increasing interest has emerged in
employing gold complexes as catalysts for cross-coupling
reactions.4 However, the direct oxidative addition of
commonly used electrophiles to gold(I) complexes is difficult,
because of the relatively high redox potential of the couple
(E0(Au(I)/Au(III)) = 1.41 V).5 This transformation can be
promoted by using external oxidants, such as hypervalent
iodine reagents or selectfluor, which limits the substrate scope
to substrates that are stable under the oxidative conditions.6
The Bourissou group discovered that gold(I) complexes with
small-bite-angle bidentate ligands readily undergo oxidative
addition of aryl halides.7 An alternative approach was
developed by the groups of Glorius and Toste, using the
combination of aryldiazonium salts and a photocatalyst in the
presence of visible light (Scheme 1a).8 This catalytic system
has allowed the development of several gold-catalyzed arylative
reactions such as tandem rearrangement/arylations, nucleo-
philic addition/arylations, and cross-coupling reactions.9
Recently, the group of Fensterbank discovered that the
oxidative addition of iodoalkynes can readily happen to
photosensitized gold(I) catalysts.10
The gold(I)-catalyzed alkoxycyclization of 1,6-enynes
proceeds via the activation of the alkyne I the formation of a
cyclopropyl gold(I) carbene intermediate II, the nucleophilic
attack of an alcohol to form an alkenyl gold(I) complex III,
and protodeauration to give the final product IV (Scheme
1b).11 We wondered whether it would be possible to design a
dual gold-catalyzed/photoredox-initiated process in which a
Au(III) species generated oxidatively from Au(I) and an aryl
diazonium salt, in the presence of an alcohol, would be able to
activate the 1,6-enyne to form alkenyl gold(III) complex V,
which would finally furnish product VI after reductive
elimination (Scheme 1c).12 To be successful, the 1,6-enyne
cyclization should be faster than the previously described
Sonogashira-type coupling13 and the reductive elimination of V
should be faster than the protodeauration.
This strategy would expand the scope of gold-catalyzed
enyne cyclizations, giving access to product VI with the
opposite configuration at the alkene to that obtained via metal-
catalyzed alkoxycyclization11,14 and would complement other
arylative cyclizations of enynes.15
We first screened different gold(I) catalysts for the
transformation of enyne 1a and PhN2BF4 into the desired
Received: March 2, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00799
© XXXX American Chemical Society
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a
arylated product 2a by irradiation with a 23 W fluorescent bulb
in MeOH (see Table 1). Gold(I) complexes with arylphos-
Scheme 2. Arylative Cyclization of 1,6-Enynes 1a−1i
Table 1. Formation of 2a from 1,6-Enyne 1a with Different
a
Gold(I) Catalysts and Ru(II) Photocatalysts
b
c
c
entry
catalyst
photocatalyst
2a (%)
3a (%)
1
2
3
4
5
6
7
8
9
Ph3PAuCl
[Ru(bpm)3]Cl2
[Ru(bpm)3]Cl2
[Ru(bpm)3]Cl2
[Ru(bpm)3]Cl2
[Ru(bpm)3]Cl2
[Ru(bpm)3]Cl2
[Ru(bpm)3]Cl2
[Ru(bpm)3]Cl2
[Ru(bpy)3]Cl2
[Ru(bpy)3](PF6)2
[Ru(bpz)3](PF6)2
[Ru(bpz)3](PF6)2
Ru(bpz)3](PF6)2
21
24
−
28
−
54
68
70
61
59
72
46
51
42
10
8
20
15
11
5
Ph3PAuNTs2
JohnphosAuCl
(MeO)3PAuCl
IPrAuCl
Cy3PAuCl
Et3PAuCl
Me3PAuCl
Me3PAuCl
Me3PAuCl
Me3PAuCl
Me3PAuCl
Me3PAuCl
10
11
14
10
−
d
e
12
d,f
90 (83)
74
13
3
a
1
The product of Sonogashira coupling was observed by H NMR
b
(<10% yield). bpm = 2,2′-bipyrimidine; bpy = 2,2′-bipyridine; bpz =
c
2,2′-bipyrazine. Yields determined by 1H NMR (3,5-dimethylpyr-
d
azole as internal standard). Reaction under optimized conditions:
e
f
0.04 M, −15 °C, MeOH/MeCN (1:1) as a solvent. Isolated yield. 5
mol % of Me3PAuCl.
phines in combination with [Ru(bpm)3]Cl2 as a photoredox
catalyst (where bpm = 2,2′-bipyrimidine) led mainly to
product 3a of direct methoxycyclization (Table 1, entries 1−
3). The selectivity toward 2a improved by using a phosphite
gold(I) complex (Table 1, entry 4), whereas IPrAuCl was
ineffective (Table 1, entry 5). Interestingly, electron-rich
trialkylphosphine gold(I) complexes proved to be the best
catalysts for this reaction (Table 1, entries 6−13). Then, a
screening of photocatalysts showed that commonly used
Ru(bpy)3 (E1/2 III/*II = −0.81 V vs SCE) (Table 1, entries 9
a
b
Z = C(CO2Me)2. Reaction at 30 °C.
donating OMe substituent, we observed a decrease of the yield
when placed at the para-position (2t, 40%), compared to its
meta and ortho analogues (2v, 62% yield and 2z, 68% yield,
respectively).
and 10) worked less efficiently than the more oxidizing
Ru(bpm)3 (E *II = −0.21 V vs SCE) (Table 1, entry 8) or
III/
1/2
16
Ru(bpz)3 (E *II = −0.26 V vs SCE) (Table 1, entry 11).
III/
1/2
The optimal results were finally obtained when Me3PAuCl and
[Ru(bpz)3](PF6)2 were used in a mixture of MeOH/ACN (1/
1), at −15 °C, affording 2a in 83% isolated yield (Table 1,
entry 12).17 Decreasing the amount of catalyst to 5 mol % led
to lower yield (Table 1, entry 13).
Different alcohols could be used in the arylative cyclization
under the optimized conditions to form products 2a−2f
(Scheme 2). Interestingly, the addition of propargylic alcohol
led to 2d in 60% yield, without the formation of other products
from the activation of the new terminal alkyne. Reaction in the
presence of water led to alcohol 2g. The reaction is sensitive to
the steric hindrance of the alcohol since iPrOH led to product
2c in 61% yield, whereas only traces of 2h could be obtained in
the presence of t-BuOH. A range of aryldiazonium salts with
electronically different substituents at the ortho-, meta-, and
para-positions led to the corresponding products of arylation
2i−2ab in 38%−75% yields. In the case of the electron-
Other 1,6-enynes 1b−1h with differently substituted alkenes
also led to the expected products 2ac−2ai in 61%−79% yields
(Scheme 2). However, 1,6-enyne 1i with a phenyl-substituted
internal alkyne failed to give 2aj, even at 30 °C.
Several experiments were performed to elucidate the
mechanism of the arylative cyclization (Scheme 3). First, an
experiment in darkness was conducted with and without
Ru(II) photocatalyst (reactions fully covered with aluminum
foil) (Scheme 3a). To our surprise, product 2a was obtained in
19% and 49% yields, respectively, showing that gold catalytic
turnover could happen in the absence of the photocatalytic
cycle. The thermal decomposition of diazonium salts into the
corresponding aryl radicals or the direct interaction between
Au(I) catalyst and the radical precursor18 could explain these
results, which are consistent with the different reports of
photocatalyst-free visible-light-mediated gold-catalyzed aryla-
tions of alkynes.19 However, all our attempts at developing a
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2a and was recovered quantitatively after being subjected to
the optimized reaction conditions (Scheme 3e).23
Scheme 3. Control Experiments and Formation of Au(III)
Complex 5
Based on these control experiments and previous re-
ports,21,24 a mechanistic proposal for the gold-catalyzed
arylative cyclization of enynes is depicted in Scheme 4. Thus,
Scheme 4. Proposed Mechanism for the Arylative
Cyclization
the aryl radical generated upon reduction of the diazonium salt
in the photoredox cycle adds to Au(I) complex to form Au(II)
intermediate VII, which is further oxidized to Au(III) complex
VIII through SET from the photocatalyst or another
diazonium salt (radical chain pathway). Next, coordination
of the 1,6-enyne 1 provides IX, which undergoes a 5-exo-dig
cyclization to form X, followed by addition of the alcohol to
generate intermediate V′. Finally, reductive elimination
delivers the desired product 2 and regenerates the initial
Au(I) complex. A similar mechanism in which the Au(III)
intermediate is obtained without the need of the photocatalyst
could also be considered.
In conclusion, we have developed a photoredox-initiated
gold-catalyzed arylative alkoxycyclization of 1,6-enynes with
aryldiazonium salts in the presence of alcohols. This three-
component reaction leads to five-membered ring compounds
bearing an exocyclic alkene with the opposite configuration to
that obtained by gold(I)-catalyzed cyclizations. Mechanistic
investigations suggest that the catalytic cycle starts with the
stepwise oxidative formation of a gold(III) species, which
triggers the 5-exo-alkoxycyclization of the 1,6-enyne.
photosensitizer-free version of the reaction led to lower yields
and complex reactions mixtures,17 showing the importance of
this latter concept. Furthermore, the presence of photocatalyst
lowers the performance of the reaction, which could be
rationalized by catalyst inactivation by coordination of gold
with the basic 2,2′-bipyrazine (bpz) ligand.
Next, we examined at which stage the oxidation of gold(I) to
gold(III) occurs.20 In our system, Me3PAuCl was not able to
activate the alkyne, leading to recovered starting enyne 1a in
95% (Scheme 3b), and, as expected, no reaction occurred in
the absence of gold(I) complex (Scheme 3c). These
experiments suggest that the oxidation of gold(I) precedes
enyne cyclization, pointing toward the involvement of an Ar−
Au(III) species as the actual catalyst of the cyclization. Indeed,
reaction of Me3PAuCl with diazonium salt 4 led to the
formation of gold(III) complex 5, whose structure was
determined by X-ray diffraction (Scheme 3d).21 This result
reinforces the idea of a direct interaction between the gold
catalyst and the diazo compound.
The alkoxycyclization of 1,6-enynes bearing internal alkynes
occurs with Pt(II)22 or Au(I).11 On the other hand, the
arylation of terminal alkynes with diazonium salts by dual
gold/photoredox-catalyzed is a known process.13 Therefore,
we considered the possibility that our system proceeds via a
Sonogashira-type coupling of the alkyne of enynes 1, followed
by a gold-catalyzed enyne alkoxycyclization and isomerization
of the aryl-substituted alkene. However, compound 6 with a Z-
configured alkene, prepared using our previously reported
procedures,11,17 did not undergo Z to E isomerization to form
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00799.
Experimental procedures and characterization data for
compounds (PDF)
Accession Codes
CCDC 1987486 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
C
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Letter
(5) Bratsch, S. G. Standard Potentials and Temperature. Coefficients
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data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Antonio M. Echavarren − Institute of Chemical Research of
Catalonia (ICIQ), Barcelona Institute of Science and
Technology, 43007 Tarragona, Spain; Departament de
́
(7) (a) Joost, M.; Zeineddine, A.; Estevez, L.; Mallet-Ladeira, S.;
Miqueu, K.; Amgoune, A.; Bourissou, D. Facile Oxidative Addition of
Aryl Iodides to Gold(I) by Ligand Design: Bending Turns on
Reactivity. J. Am. Chem. Soc. 2014, 136, 14654−14657. (b) Joost, M.;
́
̀
́
Quımica Organica i Analıtica, Universitat Rovira i Virgili,
43007 Tarragona, Spain; orcid.org/0000-0001-6808-
3007; Email: aechavarren@iciq.es
́
Estevez, L.; Miqueu, K.; Amgoune, A.; Bourissou, D. Oxidative
Addition of Carbon−Carbon Bonds to Gold. Angew. Chem., Int. Ed.
́
2015, 54, 5236−5240. (c) Zeineddine, A.; Estevez, L.; Mallet-Ladeira,
Authors
S.; Miqueu, K.; Amgoune, A.; Bourissou, D. Rational Development of
Catalytic Au(I)/Au(III) Arylation Involving Mild Oxidative Addition
of Aryl Halides. Nat. Commun. 2017, 8, 565−568. (d) Rodriguez, J.;
Zeineddine, A.; Sosa Carrizo, E. D.; Miqueu, K.; Saffon-Merceron, N.;
Amgoune, A.; Bourissou, D. atalytic Au(I)/Au(III) arylation with the
hemilabile MeDalphos ligand: unusual selectivity for electron- rich
iodoarenes and efficient application to indoles. Chem. Sci. 2019, 10,
7183−7192.
Joan G. Mayans − Institute of Chemical Research of Catalonia
(ICIQ), Barcelona Institute of Science and Technology, 43007
́
̀
Tarragona, Spain; Departament de Quımica Organica i
́
Analıtica, Universitat Rovira i Virgili, 43007 Tarragona, Spain
Jean-Simon Suppo − Institute of Chemical Research of
Catalonia (ICIQ), Barcelona Institute of Science and
Technology, 43007 Tarragona, Spain; Departament de
(8) (a) Sahoo, B.; Hopkinson, M. N.; Glorius, F. Combining Gold
and Photoredox Catalysis: Visible Light-Mediated Oxy- and Amino-
arylation of Alkenes. J. Am. Chem. Soc. 2013, 135, 5505−5508.
(b) Shu, X.-Z.; Zhang, M.; He, Y.; Frei, H.; Toste, F. D. Dual Visible
Light photoredox and Gold-Catalyzed Arylative Ring Expansion. J.
Am. Chem. Soc. 2014, 136, 5844−5847.
́
̀
́
Quımica Organica i Analıtica, Universitat Rovira i Virgili,
43007 Tarragona, Spain
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00799
(9) For selected reviews on the oxidative addition of gold(I) with
diazonium salts, see: (a) Hopkinson, M. N.; Tlahuext-Aca, A.;
Glorius, F. Merging Visible Light Photoredox and Gold Catalysis. Acc.
Chem. Res. 2016, 49, 2261−2272. (b) Zhang, M.; Zhu, C.; Ye, L.-W.
Recent Advances in Dual Visible Light Photoredox and Gold-
Catalyzed Reactions. Synthesis 2017, 49, 1150−1157.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
(10) Xia, Z.; Corce, V.; Zhao, F.; Przybylski, C.; Espagne, A.; Jullien,
́
We thank the Agencia Estatal de Investigacion (AEI)/FEDER,
̀
L.; Le Saux, T.; Gimbert, Y.; Dossmann, H.; Mouries-Mansuy, V.;
UE (No. CTQ2016-75960-P), the AGAUR (No. 2017 SGR
1257), and CERCA Program/Generalitat de Catalunya for
financial support. We also thank Dr. Fedor M. Miloserdov for
additional experiments and fruitful discussions (experiments
performed at the Institute of Chemical Research of Catalonia).
Ollivier, C.; Fensterbank, L. Photosensitized Oxidative Addition to
Gold(I) Enables Alkynylative Cyclization of o-Alkynylphenols with
Iodoalkynes. Nat. Chem. 2019, 11, 797−805.
(11) Nieto-Oberhuber, C.; Munoz, M. P.; Bunuel, E.; Nevado, C.;
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Cardenas, D. J.; Echavarren, A. M. Cationic Gold(I) Complexes:
Highly Alkynophilic Catalysts for the exo- and endo-Cyclization of
Enynes. Angew. Chem., Int. Ed. 2004, 43, 2402−2406.
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E
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