Photosensitizer-Free Visible-Light-Mediated Gold-Catalyzed 1,2-Difunctionalization of Alkynes

Article abstract of DOI:10.1002/anie.201511487

Under visible-light irradiation, the gold-catalyzed intermolecular difunctionalization of alkynes with aryl diazonium salts in methanol affords a variety of α-aryl ketones in moderate to good yields. In contrast to previous reports on gold-catalyzed reactions that involve redox cycles, no external oxidants or photosensitizers are required. The reaction proceeds smoothly under mild reaction conditions and shows broad functional-group tolerance. Further applications of this method demonstrate the general applicability of the arylation of a vinyl gold intermediate instead of the commonly used protodemetalation step. This step provides facile access to functionalized products in one-pot processes. With a P,N-bidentate ligand, a stable aryl gold(III) species was obtained, which constitutes the first direct experimental evidence for the commonly postulated direct oxidative addition of an aryl diazonium salt to a pyridine phosphine gold(I) complex.

Full text of DOI:10.1002/anie.201511487

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International Edition: DOI: 10.1002/anie.201511487
German Edition: DOI: 10.1002/ange.201511487
Photochemistry Very Important Paper
Photosensitizer-Free Visible-Light-Mediated Gold-Catalyzed
1,2-Difunctionalization of Alkynes
Long Huang, Matthias Rudolph, Frank Rominger, and A. Stephen K. Hashmi*
Abstract: Under visible-light irradiation, the gold-catalyzed
intermolecular difunctionalization of alkynes with aryl diazo-
nium salts in methanol affords a variety of a-aryl ketones in
moderate to good yields. In contrast to previous reports on
gold-catalyzed reactions that involve redox cycles, no external
oxidants or photosensitizers are required. The reaction pro-
ceeds smoothly under mild reaction conditions and shows
broad functional-group tolerance. Further applications of this
method demonstrate the general applicability of the arylation
of a vinyl gold intermediate instead of the commonly used
protodemetalation step. This step provides facile access to
functionalized products in one-pot processes. With a P,N-
bidentate ligand, a stable aryl gold(III) species was obtained,
which constitutes the first direct experimental evidence for the
Scheme 1. Known strategies for the synthesis of a-aryl ketones and our
alternative approach.
commonly postulated direct oxidative addition of an aryl
diazonium salt to a pyridine phosphine gold(I) complex.
T
he synthesis of a-aryl ketone derivatives is of great
importance in organic synthesis because of their utility as
a core structural component of numerous pharmaceuticals,
bioactive natural products, and functional materials.[1] Thus,
considerable attention has been devoted to the construction
of this moiety. Over the last two decades, the transition-metal-
catalyzed a-arylation of carbonyl compounds and their
equivalents with aryl halides or pseudohalides was studied
particularly well, and both the substrate scope and the
functional-group tolerance were substantially broadened.
However, the vast majority of a-arylation methods still
suffer from some drawbacks, such as harsh reaction con-
ditions (elevated temperatures or the use of strong bases).
Alternatively, visible-light- and base-mediated couplings of
prefunctionalized enol ether derivatives with aryl diazonium
salts as the aryl radical source have been developed.[2] Despite
this progress, the development of a new efficient synthetic
method that utilizes readily available starting materials to
afford the desired products with high selectivity under mild
conditions is still highly desirable (Scheme 1).
Homogeneous gold catalysis has made significant prog-
ress in the past decade.[3] In contrast to the frequently applied
concept of using gold as a mild carbophilic p Lewis acid, fewer
examples of reactions based on gold(I)/gold(III) redox cycles
have been reported. Strong external oxidants, such as hyper-
valent iodine reagents or F+ donors, are required in stoichio-
metric amounts owing to the high redox potential of gold.[4,5]
Recently, work by the groups of Glorius and Toste demon-
strated that the oxidation of gold can also be achieved by
combining the use of photosensitizers and aryl radical sources
(aryl diazonium or diaryl iodonium salts) with visible-light
irradiation (Scheme 2).[6]
The gold-catalyzed hydration of alkynes represents an
important method for C O bond formation.[7] Previous work
À
by the groups of Utimoto and Laguna has demonstrated that
organometallic AuIII complexes can efficiently catalyze the
hydration of alkynes.[7e,h] A beautiful expansion of this
method was published by Hammond and co-workers, who
reported on the gold-catalyzed trifunctionalization of alkynes.
In this case, a gold(III) intermediate is generated by oxidation
with Selectfluor as the external oxidant, and the functional-
ization proceeds by transmetalation from a boronic acid
followed by reductive elimination. The third functionalization
is then induced by the electrophilic fluorinating reagent.[4e]
Inspired by this success and based on the recent work of our
group in the context of photocatalysis,[8] we envisioned that
highly electrophilic aryl gold(III) intermediates, generated by
photoredox processes, could also serve as efficient activators
[*] L. Huang, Dr. M. Rudolph, Dr. F. Rominger, Prof. Dr. A. S. K. Hashmi
Organisch-Chemisches Institut, Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: hashmi@hashmi.de
Homepage: http://www.hashmi.de
Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University
ꢀ
of C C bonds for nucleophilic additions. The generated
mixed aryl/vinyl gold(III) intermediates could then deliver
the desired a-aryl ketones after reductive elimination and
hydrolysis. Thus, in contrast to known methods, a major
Jeddah 21589 (Saudi Arabia)
Supporting information for this article can be found under http://dx.
doi.org/10.1002/anie.201511487.
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3a in 27% after 24 h (Table 1, entry 1). Using
[Ir{dF(CF3)ppy}2(dtbbpy)]PF6 as the photo-
sensitizer improved the yield to 57%
(entry 2). The successful use of dimeric gold
complexes as photosensitizers was recently
demonstrated by Barriault and co-workers
and by our group.[8] Thus we decided to explore
the feasibility of a process in which gold not
only acts as a p-activator but also serves as
a photosensitizer. Indeed, the employment of
dinuclear gold photocatalysts also produced 3a
in 47% and 65% yield (depending on the
counterion; entries 3 and 4). Control experi-
ments confirmed that the transformation does
not occur in the absence of a catalyst, and only
a trace amount of the product was observed in
the presence of [Au2(m-dppm)2](NTf2)2
(entries 5 and 6). To our surprise, the yield of
3a dramatically increased to 76% when simple
Ph3PAuCl was used (entry 7). Further control
experiments showed that light was essential for
Scheme 2. AuI/AuIII redox cycles under different conditions.
drawback would be avoided: The additional functionalization
would not require an external oxidant.
the success of this reaction (entry 8). Next, different light
sources were examined, and blue LEDs turned out to be most
effective, affording 3a in 79% yield (entries 9–11). The
reaction also proceeds smoothly under sunlight irradiation,
albeit in a slightly reduced yield (entry 11). A survey of
different basic and acidic additives failed to give better results
(entries 12–14). Other transition-metal salts, such as
Pd(OAc)2 or Cu(OAc)2, could not afford the desired product
To test this hypothesis, we initially performed an experi-
ment with 1-phenyl-1-hexyne (1a) and phenyldiazonium
tetrafluoroborate (2a) in the presence of 5 mol% [Ru-
(bpy)3]Cl2·6H2O and 10 mol% Ph3PAuCl as the catalytic
system at room temperature under irradiation with UVA light
in methanol. These conditions afforded the desired product
Table 1: Optimization of the reaction conditions.[a]
Entry
Photocatalyst (5 mol%)
Light source
Au (10 mol%)
Additive
Yield of 3a[b,c]
Ratio 3a/3a’
1
2
3
4
[Ru(bpy)3]Cl2·6H2O
Ir[dF(CF3)ppy]2(dtbbpy)PF6
[Au2(m-dppm)2]Cl2
[Au2(m-dppm)2](NTf2)2
UVA
UVA
UVA
UVA
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
–
–
–
–
–
27%
57%
47%
65%
–
5:1
9:1
4:1
11:1
–
5
–
UVA
–
6
7
8
9
[Au2(m-dppm)2](NTf2)2
UVA
UVA
–
–
–
–
–
–
–
–
trace
76%
–
–
14:1
–
6:1
>20:1
16:1
–
6:1
>20:1
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
Pd(OAc)2
Cu(OAc)2
(4-CH3C6H4)3PAuCl
(4-FC6H4)3PAuCl
(4-CF3C6H4)3PAuCl
AuCl
20 W CFL
blue LEDs
sunlight
blue LEDs
blue LEDs
blue LEDs
blue LEDs
blue LEDs
blue LEDs
blue LEDs
blue LEDs
blue LEDs
blue LEDs
9%
10
11
12
13
14
15
16
17
18
19
20
21[d]
79%
71%
trace
62%
71%
–
Na2CO3
NaOMe
MsOH
–
–
–
–
–
–
–
–
–
71%
83%
87% (81%)
trace
–
13:1
16:1
15:1
–
Ph3PAuCl
–
[a] Reaction conditions: 1-Phenyl-1-hexyne (1a, 0.1 mmol), phenyldiazonium salt 2a (0.4 mmol), photocatalyst (5 mol%), and gold catalyst
(10 mol%) in MeOH (0.25 mL) were reacted at room temperature under irradiation with various light sources. [b] The yields and ratios were
determined by 1H NMR analysis of the crude product using 1,3,5-trimethoxybenzene as the internal standard. [c] Yield of isolated product given in
parentheses. [d] Ph2IOTf was used instead of 2a.
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(entries 15 and 16). Variations of the ligand at the gold center
revealed that electron-poor ligands provided better results.
The yield of 3a improved to 87% with (4-CF3C6H4)3P,
whereas the electron-rich ligand (4-CH3C6H4)3P gave the
worst result (entries 17–19). When simple AuCl was
employed, only a trace amount of the product was observed
(entry 20). Finally, diphenyliodonium triflate was tested as the
aryl source as well, but no reaction was observed (entry 21).
Under the optimized reaction conditions (Table 1,
entry 19), we next examined the scope of the difunctionaliza-
tion reaction with respect to the alkyne substrate. As shown in
Table 2, 1-aryl-1-hexynes with various substituents on the aryl
ring underwent the difunctionalization smoothly, affording
the desired a-aryl ketones 3a–3i in moderate to good yields,
3q in 46–54% yield. Encouraged by these results, we
extended this catalytic process to 5-decyne; the arylation
was successful, and afforded the product 3r in 45% yield.
We then turned our attention to the use of various aryl
diazonium salts 2b–2i in combination with alkyne 1l under
the standard reaction conditions (Table 3). With 4-methyl-
Table 3: Reaction scope with respect to various aryl diazonium tetra-
fluoroborate salts.[a]
Entry
Aryl diazonium salt
Product
Yield [%]
1
2
3
4
5
6
7
8
4-CH3C6H4, 2b
4-tBuC6H4, 2c
4-FC6H4, 2d
4-ClC6H4, 2e
4-BrC6H4, 2 f
4-(CO2Me)C6H4, 2g
4-CF3C6H4, 2h
3-CH3C6H4, 2i
3s
3t
61
40
61
41
55
57
–
Table 2: Reaction scope with respect to various alkynes.[a]
3u
3v
3w
3x
3y
3z
65
[a] All reactions were carried out using 1l (0.2 mmol) and 2 (0.8 mmol)
in the presence of (4-CF3C6H4)3PAuCl (10 mol%) in MeOH (0.5 mL).
phenyldiazonium tetrafluoroborate and 4-tert-butylphenyl-
diazonium tetrafluoroborate, the difunctionalized products 3s
and 3t were obtained in 61% and 40% yield, respectively
(entries 1 and 2). A variety of functional groups attached to
the aromatic ring of the aryl diazonium substrate, namely
fluoro, chloro, bromo, and ester moieties, were also tolerated,
providing a-aryl ketones 3u–3x in moderate yields (entries 3–
6). Among the tested starting materials, 4-trifluoromethyl-
phenyldiazonium tetrafluoroborate failed to produce any
product, probably because of its tendency to decompose
(entry 7). The aryl diazonium salt with a methyl substituent at
the meta position produced 3z in 65% yield (entry 8).
A recent report showed that the a-arylation of carbonyl
compounds can be enabled through a radical process.[1k] To
exclude the possibility that our transformation proceeds by
hydration followed by ketone arylation, we examined the
reaction of 3a’ with phenyldiazonium tetrafluoroborate 2a
under the standard reaction conditions, but a-aryl ketone 3a
was not obtained (Scheme 3a). During the optimization
studies, we had observed the formation of an arylated enol
ether by GC-MS analysis, which is believed to be transformed
into the corresponding ketal upon hydrolysis to produce the
final product. To gain insight into the reaction mechanism, we
thus further investigated the reaction by isotope labeling
experiments. With H218O (10 equiv) as an additive, the 18O-
labeled a-aryl ketone 3a was obtained in 61% yield under the
standard reaction conditions (Scheme 3b). The reduced yield
was attributed to the decomposition of phenyldiazonium
tetrafluoroborate in the presence of excess water. In a pre-
vious report,[6d] a dual Au and photoredox catalytic system
was used for the intramolecular oxyarylation of alkenes with
aryl diazonium salts. The photocatalyst was claimed to be
essential in these reactions for the generation of an electro-
[a] All reactions were carried out using 1 (0.2 mmol) and 2a (0.8 mmol)
in the presence of 10 mol% (4-CF3C6H4)3PAuCl in MeOH (0.5 mL).
independent of their electronic nature. Encouraged by these
results, we further explored the scope of this transformation
by using a series of alkynes bearing different R’ groups.
Different alkyl, cycloalkyl, and aryl substituents were exam-
ined first, and the corresponding a-aryl ketones 3j–3n were
formed in 51–75% yield. Remarkably, this method also
exhibits good functional-group tolerance without affecting
a methyl ether, benzoyl ether, or sulfonamide, providing 3o–
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gold(II) center, which oxidizes the latter to gold(III) species
B. However, the direct addition of the diazonium species to
the gold(I) center is also conceivable. Efforts to isolate this
species failed as it is too sensitive. Then, assisted by light
irradiation, N2 is extruded, and the aryl gold(III) species C is
formed. In the next step, methanol adds to the p-activated
alkyne to form vinyl gold species D. Reductive elimination
then provides arylated enol ether E, which delivers the final
products upon hydrolysis. Methanol as the solvent might also
play a crucial role in the reaction mechanism. It has been
reported that under basic conditions, aryl diazonium salts can
form azo ethers that absorb light in the visible range.[10] The
observation of these species as key intermediates in photo-
catalyzed reactions was recently reported by Rueping and co-
À
workers for a titanium dioxide catalyzed C H arylation of
Scheme 3. Mechanistic investigation.
heteroarenes.[11] However, despite the fact that these species
might also be formed in the gold-catalyzed process, their role
in the reaction remains unclear.
philic aryl gold(III) species by a photoredox process. Even in
the absence of a photocatalyst under our optimized reaction
conditions, the reaction of 4-penten-1-ol (4a) with phenyl-
diazonium salt 2a provided tetrahydrofuran 5a in 54% yield.
This finding underlines that these types of reactions can be
performed in the absence of a photocatalyst (Scheme 3c).[6e]
The UV/Vis absorption spectrum of (4-CF3C6H4)3PAuCl
showed a very weak absorption band at 450 nm (e = 17),
which excludes the possibility of (4-CF3C6H4)3PAuCl being
a photosensitizer itself. A light on/off experiment showed that
the transformation required continuous irradiation with blue
LEDs (see the Supporting Information), excluding the
possibility that an efficient radical-chain propagation mech-
anism is operating.
Attempts to isolate intermediates directly from the
catalytic reactions failed. Therefore, we used P, N-bidentate
ligands as alternatives that can improve the thermal stability
of transition-metal complexes.[12] The stoichiometric reaction
between pyridine phosphine gold(I) complex 6 and phenyl-
diazonium tetrafluoroborate (2a) provided the stabilized
chelated gold(III) complex 7 in 76% yield. The structure of 7
was confirmed unequivocally by X-ray crystallography[13]
(Scheme 5, right). Compared to previously reported proce-
Based on this experimental evidence in combination with
literature reports,[7,9] a highly speculative mechanism that
accounts for the a-aryl ketone synthesis is depicted in
Scheme 4. We assume that the reaction is initiated by gold-
induced single electron transfer (SET) to the aryl diazonium
salt, generating an aryl diazo radical and gold(II) species A.
Then, the phenyldiazonium radical recombines with the
Scheme 5. Synthesis of the P,N-chelate-stabilized gold(III) complex by
direct oxidative addition of the phenyldiazonium salt to gold.
dures, which are based on the combination of oxidative
addition and transmetalation under harsh conditions,[14] this is
a rather facile and efficient way to synthesize aryl gold(III)
complexes from gold(I) precursors. Moreover, it is the first
direct experimental evidence of a gold(III) species being
formed in gold-mediated reactions with aryl diazonium salts.
Next, we explored further applications of this light-
assisted gold-catalyzed intermolecular arylation step. Under
the standard reaction conditions, propargylic alcohol 6a was
converted into the a-arylated, a,b-unsaturated carbonyl
compound 7a in 64% yield (Scheme 6a). This reaction can
be rationalized by a Meyer–Schuster rearrangement[15] that is
enabled by the electrophilic aryl AuIII intermediate acting as
the catalyst followed by reductive elimination to form the
À
additional C C bond. On the basis of a previous report on
gold(III)-catalyzed Sonogashira-type cross-coupling reac-
tions,[4c] we next envisioned the development of a tandem
reaction that involves a cross-coupling reaction followed by
the difunctionalization using a terminal alkyne as the starting
Scheme 4. Proposed mechanism.
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Acknowledgements
L.H. is grateful for a scholarship of the “Heinz Goetze
Memorial Fellowship Program” of the Athenaeum Founda-
tion. Gold salts were generously donated by Umicore AG &
Co. KG.
Keywords: alkynes · arylations · gold · photochemistry ·
a-aryl ketones
How to cite: Angew. Chem. Int. Ed. 2016, 55, 4808–4813
Angew. Chem. 2016, 128, 4888–4893
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Received: December 10, 2015
Published online: March 11, 2016
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