Alkyne Difunctionalization by Dual Gold/Photoredox Catalysis

Article abstract of DOI:10.1002/chem.201600710

Highly selective tandem nucleophilic addition/cross-coupling reactions of alkynes have been developed using visible-light-promoted dual gold/photoredox catalysis. The simultaneous oxidation of Au^I and coordination of the coupling partner by photo-generated aryl radicals, and the use of catalytically inactive gold precatalysts allows for high levels of selectivity for the cross-coupled products without competing hydrofunctionalization or homocoupling. As demonstrated in representative arylative Meyer-Schuster and hydration reactions, this work expands the scope of dual gold/photoredox catalysis to the largest class of substrates for gold catalysts and benefits from the mild and environmentally attractive nature of visible-light activation. United we stand! Tandem nucleophilic addition/cross-coupling reactions have been developed with challenging alkynes using a visible-light-promoted dual gold/photoredox catalytic system (see scheme). High levels of selectivity for the cross-coupled products were obtained without competition from the homocoupling or conventional hydrofunctionalization.

Full text of DOI:10.1002/chem.201600710

DOI: 10.1002/chem.201600710
Communication
&
Photoredox Catalysis
Alkyne Difunctionalization by Dual Gold/Photoredox Catalysis
Adrian Tlahuext-Aca, Matthew N. Hopkinson+, R. Aleyda Garza-Sanchez+, and
Frank Glorius*[a]
conditions to a coordinatively unsaturated cationic species,
which then activates the p-system towards nucleophilic attack.
Abstract: Highly selective tandem nucleophilic addition/
cross-coupling reactions of alkynes have been developed
In order for the resulting organogold(I) species (A) to selective-
ly engage in coupling, the three-stage sequence of oxidation
using visible-light-promoted dual gold/photoredox cataly-
to AuIII, transmetalation of the coupling partner, and reductive
elimination must outcompete conventional proto-demetalation
(Scheme 1b). As a result, most selective gold-catalyzed cou-
pling reactions reported to date have focused on specific al-
kene[5a] or allenoate[5b] substrates, for which the corresponding
organogold(I) species A are comparatively less prone to proto-
demetalation.[5c,d] In particular, coupling reactions of alkynes,
which represent the largest class of substrates for gold cataly-
sis, are especially challenging with conventional hydrofunction-
alization often dominating. Furthermore, independent oxida-
tion and coordination steps can lead to low selectivity for
cross-coupling over competing homodimerization.
sis. The simultaneous oxidation of AuI and coordination of
the coupling partner by photo-generated aryl radicals,
and the use of catalytically inactive gold precatalysts
allows for high levels of selectivity for the cross-coupled
products without competing hydrofunctionalization or ho-
mocoupling. As demonstrated in representative arylative
Meyer–Schuster and hydration reactions, this work ex-
pands the scope of dual gold/photoredox catalysis to the
largest class of substrates for gold catalysts and benefits
from the mild and environmentally attractive nature of
visible-light activation.
As complementary strategy to access AuI/AuIII redox cycles
without strong external oxidants, our group disclosed a redox-
neutral approach by merging gold and visible-light photore-
dox catalysis.[6] In our initial studies, aryldiazonium or diarylio-
donium salts were employed as arylating reagents in intra- and
intermolecular oxyarylation reactions of unactivated al-
kenes,[7a,b] while impressive applications of the concept to re-
lated arylative ring expansion and allenoate cyclization pro-
cesses have since been reported by the groups of Toste[7c] and
Shin,[7d] respectively. The same visible-light-promoted strategy
has also led to the development of gold-catalyzed cross-cou-
pling methodologies involving PÀH[8a] and C(sp)ÀH bond func-
tionalization.[8b,c]
Homogeneous gold catalysis has emerged as a powerful tool
in organic synthesis during the last 16 years. This chemistry
mostly relies on the use of AuI or AuIII complexes as ligand-tun-
able soft, carbophilic Lewis acids, which are able to selectively
activate unsaturated molecules, such as allenes, alkenes and,
especially, alkynes towards nucleophilic attack.[1] Whereas
a wide range of different intra- and intermolecular nucleo-
philes may be employed in these processes, in the vast majori-
ty of cases, the organogold species generated upon nucleo-
philic attack undergoes proto-demetalation leading to hydro-
functionalized products (Scheme 1a).[2] A promising strategy to
expand the scope of such reactions involves engaging the or-
ganogold intermediates in a cross-coupling process mediated
by a AuI/AuIII redox cycle.[3] The high potential of the AuI/AuIII
couple (E0 = +1.41 V),[4] however, means that strong external
oxidants, such as Selectfluorꢀ, tBuOOH or hypervalent iodine
species are generally required.
Several features of dual gold/photoredox catalysis indicate
that this approach may be inherently better suited to impart-
ing high selectivities for difunctionalized, cross-coupled prod-
ucts in gold-catalyzed tandem nucleophilic addition reactions
(Scheme 1c). In this system, AuI is oxidized in a stepwise fash-
ion by photo-generated aryl radicals leading to cationic AuIII
species B, which already bears the aryl coupling partner. Coor-
dination of the p-system then leads selectively to the cross-
coupling product via intermediate C, without competing ho-
modimerization. Furthermore, coordinatively-saturated AuI
complexes, such as [Ph3PAuCl], which themselves are not cata-
lytically-active, can be successfully employed without preacti-
vation. As such, organogold(I) species A, which are prone to
proto-demetalation, are not formed with coordination to the
p-system taking place only once photo-mediated oxidation to
the arylgold(III) species B has occurred. Indeed, previous stud-
ies by Toste and co-workers have shown that arylative ring ex-
pansion can be favored with reactive allenes,[7c] whereas no
diyne homocoupling products were observed during our stud-
Ensuring high selectivity for the desired cross-coupled prod-
ucts in these reactions, however, remains a major challenge. In
most cases, a AuI precatalyst is converted under the reaction
[a] A. Tlahuext-Aca, Dr. M. N. Hopkinson,+ R. A. Garza-Sanchez,+
Prof. Dr. F. Glorius
NRW Graduate School of Chemistry, Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster (Germany)
E-mail: glorius@uni-muenster.de
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number for the
author of this article can be found under http://dx.doi.org/10.1002/
chem.201600710.
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Scheme 1. Mechanistic pathways accessible to gold catalysts during tandem nucleophilic addition/cross-coupling reactions with p-systems.
ies on the dual gold/photoredox-catalyzed Sonogashira–Hagi-
hara reaction.[8b] Herein, we report the successful application of
these concepts in challenging tandem nucleophilic addition/ar-
ylation reactions of alkynes. Focusing on two benchmark gold-
catalyzed transformations, the Meyer–Schuster rearrangement
of propargyl alcohols and the hydration of simple alkynes, dual
gold/photoredox catalysis delivered the cross-coupled, arylated
products with high selectivity without competing homodimeri-
zation or hydrofunctionalization typically observed with these
substrates.
3aa was observed using this AuI complex catalyst and enone
4a, resulting from the conventional Meyer–Schuster rearrange-
ment, was instead obtained in 20% NMR yield. The weakly co-
ordinating ÀNTf2 counterion in [Ph3PAu]NTf2 is thought to dis-
sociate in solution and release catalytically active cationic AuI,
Table 1. Reaction optimization.[a]
As an initial study, the well-established Meyer–Schuster rear-
rangement of easily prepared propargyl alcohols 1 was select-
ed as a representative alkyne nucleophilic addition reaction.
This formal 1,3-hydroxyl migration process has attracted wide-
spread attention as a powerful method to access enone com-
pounds, which are feedstocks for synthetically important pro-
cesses, such as Diels–Alder cycloadditions, Michael additions,
and catalytic hydrogenations.[9] An arylative version of a related
rearrangement of propargyl acetates with arylboronic acids
was disclosed in a seminal report by Zhang and co-workers in
2009 using Selectfluorꢀ as an external oxidant. However, in this
study, the competing formation of enones, enone dimers, and
fluoroenones, resulting from conventional hydrofunctionaliza-
tion, homocoupling, and fluorination, respectively, was observ-
ed.[9c] Taking the conditions from our previous studies,[7a] sub-
strate 1a was reacted with benzenediazonium tetrafluorobo-
rate (2a) in the presence of the photoredox catalyst [Ru-
(bpy)3](PF6)2 (2.5 mol%; bpy=2,2’-bipyridine) and the AuI com-
plex [Ph3PAu]NTf2 (10 mol%; Tf=trifluoromethanesulfonyl)
under visible-light irradiation in MeOH for 4 h (Table 1, entry 1).
As predicted, no formation of the desired a-arylenone product
Entry
AuI Complex
Photocatalyst
Yield [%] (E/Z)[b]
1[c]
2[c]
3
4
5
[Ph3PAu]NTf2
[Ph3PAuCl]
[Ph3PAuCl]
[((4-MeO)C6H4)3PAuCl]
[Cy3PAuCl]
[IPrAuCl]
[Ph3PAuCl]
[Ph3PAuCl]
[Ph3PAuCl]
[Ph3PAuCl]
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
eosin Y
fluorescein
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
3aa: 0; 4a: 20
78 (88:12)
78 (71) (>95:5)
56 (>95:5)
79 (>95:5)
–
72 (>95:5)
68 (>95:5)
78 (>95:5)
71 (84:16)
6
7[d]
8[e]
9[f]
10[g]
[a] General conditions: 1a (0.10 mmol), 2a (0.40 mmol), photocatalyst
(2.5 mol%), gold complex (10 mol%), and MeOH (1.0 mL) under argon.
[b] NMR yields using CH2Br2 as internal standard; isolated yield in bold;
unless otherwise stated, enone 4a was not observed in any case. [c] No
base. [d] Eosin Y (5 mol%), 12 h. [e] Fluorescein (5 mol%), 12 h. [f] 5 W
Blue LEDs or 23 W CFL, 4 h. [g] Sunlight, 3 h. See the Supporting Informa-
tion for details. IPr=1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene.
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which is capable of reacting directly with 1a without first en-
gaging with the aryl coupling partner. Switching to the coordi-
natively saturated AuI complex [Ph3PAuCl], which can only
react with the alkyne after having first undergone activation to
form an arylgold(III) species (B; Scheme 1), led to much more
satisfactory results. Using 10 mol% of the precatalyst
[Ph3PAuCl] under the same conditions, the cross-coupled prod-
uct 3aa was delivered in 78% NMR yield in a respectable E/Z
ratio of 88:12 with formation of 4a being completely sup-
pressed (entry 2).
propargyl alcohols 1b–l (Scheme 2). Electron-rich aromatic
groups and alkyl chains at the acetylenic position (R1) were
found to be well tolerated in this transformation, giving access
to the corresponding a-arylenones 3ba, 3ca and 3ea in good
isolated yields and perfect selectivity for the E-isomer. On the
other hand, poor reactivity was observed when electron-with-
drawing groups were installed at this position. We then turned
our attention to the propargylic moiety. Electron-poor and
-rich aromatic groups performed well for this transformation
affording good yields of the chalcone derivatives 3ha–ka up
to 82%, again exclusively as the E-isomers. 2-Thiophene
groups and tertiary alkyl propargyl alcohols were also suitable,
giving moderate to good yields of the enones 3 fa, 3ga and
3la, although the latter thiophene-substituted compound was
obtained as a 3:1 diastereoisomeric mixture.
At this stage, an optimization study was carried out aimed
at maximizing both the yield and E/Z selectivity of 3aa (see
the Supporting Information for full details). As shown in
Table 1, the basic additive KH2PO4 (3 equiv) was found to great-
ly improve the diastereoselectivity of the process with the E-
isomer being exclusively delivered with the same overall yield
(78% NMR, 71% isolated yield; Table 1, entry 3). The prefer-
ence for the E-isomer is consistent with the previously report-
ed gold-catalyzed rearrangement reactions with propargyl al-
cohols.[9] Various coordinatively saturated gold(I) chloride pre-
catalysts could be successfully employed in this process, albeit
with efficiency not exceeding that of [Ph3PAuCl] (entries 4–6).
A selection of photoredox catalysts, including the inexpensive
organic dyes eosin Y and fluorescein, was also suitable, al-
though these latter species required longer reaction times (en-
tries 7 and 8). Finally, the tolerance of the reaction to different
The scope of the arylative Meyer–Schuster rearrangement
with respect to different aryldiazonium salts 2b–i was then as-
sessed. As shown in Scheme 3, a selection of derivatives bear-
ing electron-withdrawing or -neutral groups were successfully
coupled with 1a, delivering the corresponding arylenones
3ab–ai in moderate to good yields (up to 76%). As with previ-
ously reported gold-catalyzed coupling reactions, halogen sub-
stituents were well tolerated and products 3ae and 3af could
be further functionalized using standard cross-coupling meth-
odologies. In all cases, no enone products, resulting from con-
ventional hydrofunctionalization, or homocoupled enone de-
rivatives were observed in the reaction mixtures, highlighting
the selectivity of the method.
sources of visible light was evaluated. 5 W Blue LEDs (lmax
=
465 nm) or a 23 W CFL gave the same performance to that ob-
tained under 5 W green LED (lmax =525 nm) irradiation
(entry 9). Interestingly, even sunlight was suitable for this trans-
formation, giving 3aa in 71% NMR yield in good selectivity to-
wards the E-isomer (entry 10).[10]
As the next stage of our study, we sought to investigate
whether dual gold/photoredox catalysis could be applied to
one of the simplest gold-catalyzed transformations of alkynes
in an arylative hydration reaction.[11] This process would deliver
a-arylketone derivatives, which are useful as building blocks in
organic synthesis. Applying the set of optimized reaction con-
Having the optimized conditions in hand, the substrate
scope of the transformation was assessed using a range of
Scheme 2. Scope of propargyl alcohols 1b–l in the dual gold/photoredox-catalyzed arylative Meyer–Schuster rearrangement. General conditions: propargyl
alcohol (1b–l, 0.30 mmol), 2a (1.2 mmol), [Ru(bpy)3](PF6)2 (2.5 mol%), [Ph3PAuCl] (10 mol%), and KH2PO4 (0.90 mmol) under argon. [a] 2a (1.8 mmol) for 16 h.
[b] 3:1 E/Z ratio. Isolated yields.
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catalysts.[12] Finally, intramolecular oxyarylation could be per-
formed using the 2-alkynylphenol derivative 5d. This reaction
delivered the diaryl-substituted benzofuran heterocycle 6da as
the only product in 53% isolated yield.
In accordance with previous studies on dual gold/photore-
dox catalysis,[7,8] we propose a mechanism for the arylative
Meyer–Schuster rearrangement of propargyl alcohols analo-
gous to that outlined in Scheme 1. Whereas [Ph3PAu]NTf2 likely
leads to a vinylgold(I) complex of type A, and thus is suscepti-
ble to fast proto-demetalation, [Ph3PAuCl] must first undergo
oxidative arylation with an aryl radical generated upon oxida-
tive quenching of the visible-light-activated photocatalyst by
the aryldiazonium salt 2 (Scheme 5).[13,14] Subsequent single-
Scheme 3. Scope of aryldiazonium salts 2b–i in the dual gold/photoredox-
catalyzed arylative Meyer–Schuster rearrangement. General conditions: 1a
(0.30 mmol), 2b–2i (1.2 mmol), [Ru(bpy)3](PF6)2 (2.5 mol%), [Ph3PAuCl]
(10 mol%), and KH2PO4 (0.90 mmol) under argon. Isolated yields.
ditions from the Meyer–Schuster rearrangement, a fast aryl-
ative hydration reaction was observed with 6-dodecyne (5a)
using 4-(ethoxycarbonyl)benzenediazonium tetrafluoroborate
(2g) as the coupling partner. a-Arylketone 6ag was obtained
in a moderate 41% yield in this reaction while no trace of the
simple hydration product was observed (Scheme 4). This latter
compound, resulting from standard nucleophilic addition
proto-demetalation, was again obtained as the only product
using [Ph3PAu]NTf2 as catalyst. Diphenylacetylene 5b was
found to be a more suitable substrate for the arylative hydra-
tion reaction with 6bg being isolated in 76% yield. A further
interesting observation was made with the unsymmetrical
alkyne 5c. Applying the standard dual gold/photoredox cataly-
sis conditions to this compound resulted in the formation of
the phenone 6cg as a single regioisomer. To the best our
knowledge, this reaction represents one of the only examples
reported to date where perfect selectivity is achieved for inter-
nal alkyne hydration-type reactions using gold complexes as
Scheme 5. Mechanistic proposal.
electron oxidation of the resulting arylgold(II) complex either
by the oxidized photocatalyst [Ru(bpy)3]3+ or in a radical chain
with another equivalent of 2 would lead to the arylgold(III)
species B.[15] This coordinatively unsaturated intermediate is
now able to activate the alkyne towards the Meyer–Schuster
rearrangement and deliver complex C, which bears both cou-
pling partners and is set up for reductive elimination, selective-
ly affording the cross-coupled product and regenerating
[Ph3PAuCl].
In conclusion, we have shown that the unique mechanistic
features of dual gold/photoredox catalysis can allow for al-
kynes to be employed as substrates in highly selective tandem
nucleophilic addition/cross-coupling reactions, overcoming the
conventional hydrofunctionalization and avoiding homocou-
pling of the alkyne. Using easily prepared propargyl alcohols
as substrates, efficient access to useful a-arylenone com-
pounds could be achieved in an arylative version of the
Meyer–Schuster rearrangement, whereas simple alkyne hydra-
tion could be intercepted with cross-coupling to selectively
afford a-arylketones. Both reactions proceed at room tempera-
ture under irradiation with visible light from readily available
sources or even sunlight. These processes expand the scope of
dual gold/photoredox catalysis to the largest class of sub-
Scheme 4. Dual gold/photoredox arylative hydration of alkynes 5. General
conditions: 5a–d (0.30 mmol), 2g, 2a (1.2 mmol), [Ru(bpy)3](PF6)2
(2.5 mol%), and [Ph3PAuCl] (10 mol%) under argon. Isolated yields.
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Acknowledgements
We thank Dr. A. Gómez-Suµrez, Dr. B. Sahoo, and M. Becker (all
WWU Münster) for experimental assistance and helpful discus-
sions. Generous financial support by the NRW Graduate School
of Chemistry (A.T.A.), the Alexander von Humboldt Foundation
(M.N.H.) and the DFG (Leibniz Award) is gratefully acknowl-
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Received: February 16, 2016
Published online on March 8, 2016
Chem. Eur. J. 2016, 22, 5909 – 5913
www.chemeurj.org
5913
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