Catalytic Cross-Coupling of Vinyl Golds with Diazonium Salts under Photoredox and Thermal Conditions

Article abstract of DOI:10.1002/adsc.201500525

Catalytically generated vinyl gold complexes from tert-butyl allenoates were found to undergo an efficient cross-coupling with arenediazonium salts. The gold(I)-gold(III) redox cycle can be accessed under two different conditions, i.e., visible-light photoredox as well as a thermally induced radical chain pathway. The current C(sp²)-C(sp²) cross-coupling protocol that is catalytic in gold, would make available desirable structural diversity to the traditional cross-coupling chemistry.

Full text of DOI:10.1002/adsc.201500525

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DOI: 10.1002/adsc.201500525
Catalytic Cross-Coupling of Vinyl Golds with Diazonium Salts
under Photoredox and Thermal Conditions
Dilip V. Patil,+a Hokeun Yun,+a and Senghoon Shina,*
a
Institute for Natural Sciences, CNOS and Department of Chemistry, Hanyang University, 17
Haengdang-dong,Seongdong-gu, 133-791 Seoul, Republic of Korea
Fax: (+82)-2-2299-0762; phone: (+82)-2-2220-0948; e-mail: sshin@hanyang.ac.kr
+
DVP and HY contributed equally to this work.
Received: May 31, 2015; Revised: June 18, 2015; Published online: August 19, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500525.
Abstract: Catalytically generated vinyl gold com-
plexes from tert-butyl allenoates were found to un-
dergo an efficient cross-coupling with arenediazoni-
um salts. The gold(I)-gold(III) redox cycle can be
accessed under two different conditions, i.e., visible-
light photoredox as well as a thermally induced rad-
ical chain pathway. The current C(sp ) C(sp )
cross-coupling protocol that is catalytic in gold,
would make available desirable structural diversity
to the traditional cross-coupling chemistry.
though use of strong external oxidants such as Select-
fluor, PhI(OAc)2 and NFSI has recently shown some
promise in accessing Au(I)/Au(III) cycles, furnishing
homo- and hetero-coupling.[4,5] However, such exter-
nal oxidants are generally expensive and/or atom-in-
efficient and thus there is a clear demand for cleaner
oxidants or coupling partners.
2
2
À
Recently, the redox property of gold complexes has
gained renewed interest in combination with visible
light photoredox catalysis.[6] Pioneering works in the
groups of Glorius and Toste showed that aryl radicals
from diazonium salts[7] can oxidize Au(I), enabling
Keywords: cross-coupling; diazonium salts; gold
catalysis; radical chain reaction; visible light photo-
catalysis
3
2
[8]
À
C(sp ) C(sp ) coupling.
Instead of two-electron
redox cycles,[5h] these processes are proposed to oper-
ate via stepwise one-electron transfers, which likely
lowers the oxidation barrier of Au(I). Surprisingly,
2
À
however, cross-coupling of more ubiquitous C(sp )
Au intermediates with arenediazonium salts has so far
Over the past decade, gold catalysis has evolved into only rarely been studied,[8b,9] presumably due to com-
a powerful tool for organic transformations, enabling
À
À
the formation of C C and C X (heteroatom) bonds
through diverse cycloisomerization, rearrangements
and cycloadditions under mild conditions.[1] In majori-
ty of these transformations, formation of C(sp ) Au
intermediates obtained from the p-activation of al-
kynes or allenes, is followed by simple protodeaura-
tion or halogenation. Combining the complexity-gen-
2
À
À
erating power of gold catalysis with C C cross-cou-
pling in a catalytic fashion would provide a fine pros-
pect for bis-functionalization of p-bonds.[2] While con-
À
ventional C H functionalization/cross-coupling of
arenes relies on directing groups to control regioselec-
tivity (Scheme 1a), vinyl metal species formed from
alkynes can couple regiospecifically and also have in-
herently more diverse structures than the preformed
cyclic motifs (Scheme 1b). Toward this goal, reluc-
tance of Au(I) to undergo two-electron oxidation
[Eo (Au3+/Au+1)=1.36 V] has been a hurdle,[3] al- Scheme 1. Cross-coupling of in-situ generated organogolds.
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Catalytic Cross-Coupling of Vinyl Golds with Diazonium Salts
Table 1. Optimization study.[a]
Entry
Catalyst
Conditions
3a (4a) Yield [%][b]
1
Ph3PAuCl, Ru(bpy)3(PF6)2
MeOH, 4 h
79
2
Ph3PAuCl, Ru(bpy)3(PF6)2
CH3CN, 4 h
31
3
4
5
6
7
8
9
10
11
12
13[d]
Ph3PAuCl, Ru(bpy)3(PF6)2
Ph3PAuCl, Ir(ppy)2(dtbbpy)(PF6)
Ph3PAuCl, Ru(bpz)3(PF6)2
MeOH:CH3CN (20:1), 4.5 h
MeOH:CH3CN (20:1), 4.5 h
MeOH:CH3CN (20:1), 4.5 h
MeOH:CH3CN (20:1), 12 h
MeOH:CH3CN (20:1), 4 h
MeOH:CH3CN (20:1), 4 h
MeOH:CH3CN (20:1), 1.5 h
MeOH:CH3CN (20:1), 1.5 h
MeOH:CH3CN (20:1), 12 h
MeOH:CH3CN (20:1), 12 h
MeOH:CH3CN (20:1), 24 h
82
65
25 (trace)
(31)
27
47
92
34
trace
NR
Au(JohnPhos)Cl, Ru(bpy)3(PF6)2
Au(IMes)Cl, Ru(bpy)3(PF6)2
[Ph3PAu]NTf2, Ru(bpy)3(PF6)2
[Ph3PAu]OTf,[c] Ru(bpy)3(PF6)2
AuCl3, Ru(bpy)3(PF6)2
[Ph3PAu]OTf,[c] no photocatalyst
Ru(bpy)3(PF6)2, no Au catalyst
[Ph3PAu]OTf,[c] Ru(bpy)3(PF6)2
37 (trace)
[a]
Reactions were conducted with 1a (0.1 mmol) and 2a (0.4 mmol) in the presence of 10 mol% of gold complex and
2.5 mol% of photocatalyst in thoroughly degassed solvents (0.1M) under blue LED irradiation.
NMR yields with CH2Br2 as an internal standard; yields of 4a in parenthesis.
Prepared in-situ from (Ph3P)AuCl and AgOTf.
[b]
[c]
[d]
23 W CFL (hood light) instead of blue LED.
peting protodeauration. Given different propensity of (entries 6 and 7). We were delighted to find that there
Au(I) and Au(III) complexes toward protodeaura- was substantial improvement in the yield and rate
tion,[10] the timing of Au(I) oxidation may affect the when a cationic Au(I) complex, generated in-situ
competing protodeauration. We therefore investigated from Ph3PAuCl and AgOTf, was used, leading to 92%
2
À
photoredox cross-coupling of C(sp ) Au intermedi- yield of 3a in only 1.5 h (entry 9). AuCl3, although
ates from tert-butyl allenoates,[11] for which stable competent, led to an inferior yield (entry 10). Control
vinyl Au(I) intermediates can be isolated.[11b] We experiments indicated that little progress was ob-
report an efficient oxidative tandem cyclization/aryla- served at room temperature in the absence of photo-
tion of tert-butyl allenoates, leading to 3-arylated bu- catalyst or gold catalyst (entries 11 and 12). Blue
tenolides for which diverse biological activities have LED light was significantly more effective than fluo-
been reported. We found that the Au(I)-Au(III) cycle rescent hood light (entry 13). No reaction was ob-
can be accessed under two different conditions, served in the dark under otherwise identical condi-
namely a visible-light photoredox and a thermally in- tions to entry 9.
duced radical chain pathway and the details are de-
Having established mild conditions for the current
scribed herein. Such a catalytic process may open up cyclization/sp2-sp2 coupling, we set out to examine the
À
a new, general and mechanistically distinct C C bond scope. Initial testing was conducted with regard to
forming processes.
substitutions on the allenoates 1 (Table 2). Various
The initial test of tandem cyclization of tert-butyl alkyl groups at R1 were widely tolerated, giving good
allenoate/cross-coupling was encouraging. The reac- to excellent yields of the corresponding butenolides
tion of 1a with 4-MeO-C6H4N2BF4 (2a) in the pres- 3a–3e. Functional groups such as Ph, alkynes and
ence of Ph3PAuCl (10 mol%) and [Ru(bpy)3](PF6)2 imides and alcohols that can potentially interfere with
(2.5 mol%) in thoroughly degassed MeOH under irra- the formation of 3 were well tolerated to provide 3f–
diation of blue LED provided 79% of 3a. Use of the 3i. Substrates having both a,g-substituents (3k and 3l,
mixed solvent MeOH/CH3CN (20:1) somewhat im- 3n and 3o) gave somewhat lower yields of the coupled
proved the yield to 82% (entry 3). Photocatalysts that products because of the competing protodeauration.
are stronger or weaker reductants [E1/2(M*/M+)] led In these cases, use of NaHCO3 as an additive effec-
to poorer yields (entries 4 and 5), indicating that tively suppressed this side pathway.
2+
Ru(bpy)3 is optimal.[6a] Change of Au catalysts to
Variations on the structure of arenediazonium salts
those having a bulky and/or a strong s-donor ligand were subsequently tested (Table 3). Both electron-do-
turned out to be ineffective or led to inferior results nating as well as electron-withdrawing arenes turned
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Dilip V. Patil et al.
Table 2. Variation of allene substituents.[a]
Table 3. Variation of the arenediazonium salt.[a]
[a]
[b]
Reactions were performed under identical conditions to
Table 2, unless otherwise noted; isolated yields after
chromatography.
Eosin Y (2.5 mol%) was used as a photocatalyst.
Table 4. Cross-coupling under thermal (non-photoredox)
conditions.[a]
[a]
Reactions were performed with 1 (0.2 mmol) and 2a
(0.8 mmol), Ph3PAuCl (10 mol%), AgOTf (10 mol%)
and
Ru(bpy)3(PF6)2
(2.5 mol%)
in
degassed
MeOH:CH3CN (20:1, 2 mL) under blue LED irradiation
for 1–4 h at room temperature; isolated yields after chro-
matography.
Without AgOTf under otherwise standard conditions.
An O-TBS protected allenoate was used as substrate.
NaHCO3 (1 equiv.) as an additive.
Entry
Ar
Time
3, Yield [%][b]
[b]
[c]
[d]
1
2
3
4
5
6
7
8
9
4-MeO-C6H4
2,5-di-MeO-C6H3
Ph
4-CH3-C6H4
3-CH3-C6H4
2-CH3-C6H4
4-Cl-C6H4
4 h
3a, 67
3aa, 70
3v, 0
3w, 20
3y, <2
3x, 5
3r, 84
3t, 82
3u, 73
1.5 h
1.5 h
2 h
h
2 h
1.5 h
2.5 h
1.5 h
out to be equally good. Halogen, nitro and ester sub-
stituents were completely compatible (3p–3u). The
steric environment in the arenediazonium salts did
not adversely affect the efficiency as in the formation
of 3x–3aa. Polycyclic (3ab) and heteroarenes (3ac)
was also tested, but in the latter case, only a poor
yield was obtained, indicating a limitation of the cur-
rent coupling.
4-NO2-C6H4
4-CO2Me
[a]
1a (0.2 mmol), 2 (4 equiv.) in the presence of (Ph3P)AuCl
(10 mol%); no photocatalyst and in the dark.
Isolated yields after chromatography.
[b]
During the course of our investigation, we were sur-
prised to find that the activation of arenediazonium
salts do not necessarily involve the photocatalyst or
light source. At room temperature, omitting photoca- (entry 11, Table 1). In sharp contrast, the reaction run
talyst led to a complete shutdown of the reaction at 608C with Ph3PAuCl as a sole catalyst in the dark,
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Catalytic Cross-Coupling of Vinyl Golds with Diazonium Salts
Scheme 2. Control experiments.
proceeded to give 3a in 67% yield (entry 1, Table 4). mation of cationic gold [Eq. (2), Scheme 2]. Thus, the
In this case, a cationic Au(I) complex formed from cyclization of 1a in the absence of a silver salt
Ph3PAuCl and AgOTf gave extensive proto-demetal- (entry 1, Table 1) is attributed to an initial oxidation
lation product 4a. This raised an intriguing possibility of Ph3PAuCl by photoredox-generated aryl radical,[8b]
of thermally induced oxidative addition of arenedia- followed by the cyclization of the allenoate by the
zonium salts to Ph3PAuCl.[12] We then promptly exam- Au(III) species. Thus it appears that, in the case of
ined the scope under these thermal conditions with cationic gold catalysis (Ph3PAu+), initial cyclization
various arenediazonium salts (Table 4). In contrast to then photoredox oxidation is the predominant mecha-
facile reactions of alkoxy-substituted arenes (entries 1 nistic route, although possibly not exclusive.[15]
and 2), phenyl- or methyl-substituted arenes per-
From the above experiments, a plausible mecha-
formed poorly under these conditions, providing only nism is delineated as in Scheme 3. The reactions using
extensive biaryl or arylmethyl ether side-products cationic [LAu]+ start with cyclization of 1 to form I.
+
from ArN2 (GC-MS; entries 3–6). In contrast, elec- An aryl radical generated from photoredox reaction
tron-deficient arenes turned out to be general and ex- between excited state Ru(II)* and diazonium salts
cellent coupling partners (entries 7–9). The observed would combine with vinyl gold I to form an open-
reactivity profile was strikingly consistent to what was shell Au(II) intermediate (II) which then undergo fur-
observed in the thermal reduction of arenediazonium ther oxidation by Ru(III) into III.[8a] Subsequent re-
salts in alcoholic solvents, in which the free-radical ductive elimination closes the Au(I)-Au(III) catalytic
chain process is favored in the order of p-MeO>p- cycle. When LAuCl that is incapable of cyclizing 1 is
NO2, p-Br>p-CH3 >p-H in the diazonium salts.[16b] used, oxidation by the photoredox-generated Ar radi-
Clearly, these results indicated that a different mecha- cal should occur first to form Au(II) (IV) which is fur-
nism is operating, possibly involving a radical chain ther oxidized by Ru(III) to Au(III) (V). At 608C, oxi-
mechanism or two-electron oxidative addition of are- dation of IV into V may be coupled to a radical chain
nediazonium salts to LAuCl.[13,16]
propagation step via generation of Ar radical from di-
A series of control experiments was conducted to azonium salts, which explains the formation of 3 in
further probe the mechanism. Vinyl gold complex 5j the absence of the photocatalyst in the dark (Table 4).
prepared according to the literature[11b] was coupled In the latter case, radical initiation may involve SET
with 2a affording 3j [Eq. (1), Scheme 2], with a virtu- from solvents.[16] Finally, cyclization of 1 by Au(III)
ally identical efficiency as the catalytic version (V)[11a] and reductive elimination would form 3.
(Table 2). This supports an initial, fast cyclization by
The oxidation state of gold catalyst in the reaction
the cationic [Ph3PAu]OTf into 5j, followed by oxida- with diazonium salts under photoredox and thermal
tion into Au(III) species.[8a] Formation of vinyl gold conditions was probed with XPS (X-ray photoelectron
complexes and their protodeauration in MeOH was spectroscopy) to support such a mechanism. A mix-
also studied. A cationic [Ph3PAu]OTf (1 equiv-; pre- ture of Ph3PAuCl (1 equiv.) and PhN2BF4 (3 equiv.)
formed) immediately (<5 min) provided the vinyl heated at 608C for 1 h in CH3OH/CH3CN (20:1) in
gold 5d in a quantitative yield and it remained intact the absence of photocatalyst, showed predominantly
without any protodeauration in CD3OD for 24 h at two peaks at 87.1 eV and 90.9 eV, from 4f7/2 and 4f5/2
room temperature.[14] However, Ph3PAuCl (1 equiv.) respectively, in good agreement with a reference
without AgOTf failed to cyclize 1d at room tempera- sample of NaAuCl3 (87.0 and 90.4 eV), which indicat-
ture even in the presence of excess NaBF4 salt, ex- ed that an Au(III) species is the predominant form of
cluding the possibility of anion metathesis for the for- gold in the mixture.[17,18] The corresponding mixture
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Dilip V. Patil et al.
Scheme 3. Proposed mechanism of cyclization/cross-coupling via dual Au(PPh3)OTf/photocatalysis or thermal Au catalysis.
under photocatalytic conditions using Ru(bpy)3(PF6)2 Experimental Section
(2.5 mol%) and blue LED, although less characteris-
tic, also indicated that Au(III) species is prevalent. General Procedure (Photocatalytic Conditions)
Moreover, the quantum yield was measured by means
In a flame-dried test tube was placed [Ru(bpy)3](PF6)2
of chemical actinometry.[17,19] The quantum yield of
(4.3 mg, 0.005 mmol), diazonium salts
(Ph3P)AuCl (9.9 mg, 0.02 mmol), AgOTf
2
(0.8 mmol),
(5.1 mg,
the photocatalytic reaction using neutral gold com-
plex (Ph3PAuCl) was measured to be 0.31, which is
consistent with the dual Au/photoredox catalysis
(Scheme 3, left). However, in the photocatalytic reac-
tion with a cationic gold complex generated from
Ph3PAuCl and AgOTf, the apparent quantum yield
was measured to be 1.21, which suggests that a radical
chain process contributes to some extent to the oxida-
tion of II to III where the photocatalyst functions as
a radical initiator.
0.02 mmol) and tert-butyl allenoates 1 (0.2 mmol). The mix-
ture was suspended in mixture of anhydrous
a
MeOH:CH3CN (20:1, 2 mL) and was degassed by freeze-
pump-thaw cycles (”3) under argon. The mixture was then
allowed to stir at room temperatrure under blue LED irradi-
ation. Upon completion (TLC), the reaction mixture was
quenched with water (2 mL) and was extracted with ether
(3”3 mL). The combined organic extracts were dried over
anhydrous MgSO4, filtered and concentrated under vacuum.
The crude products were purified by column chromatogra-
phy over silica gel (ethyl acetate/hexane).
In summary, we have developed an efficient cross-
coupling of in-situ generated vinyl gold complexes for
2
2
À
the formation of C(sp ) C(sp ) bonds. Replacing ex-
ternal oxidants with readily available arenediazonium
salts and a thermal reaction not requiring a photocata-
lyst[20] are two distinct advantages that offer an eco-
nomically and mechanistically distinct Au-catalyzed
General Procedure (Thermal Conditions)
To a flame-dried test tube was added diazonium salt 2
(0.8 mmol, 4 equiv.), (Ph3P)AuCl (9.9 mg, 0.02 mmol) and
tert-butyl allenoates 1 (0.2 mmol, 1.0 equiv.) and the tube
was wrapped with aluminum foil. In the absence of light, an-
hydrous MeOH:CH3CN (20:1, 2 mL) was added and the
mixture was heated at 608C under argon. Similar work-up
and purification led to the isolation of 3.
2
2
À
C(sp ) C(sp ) cross-coupling. The current tandem
cyclization/cross-coupling strategy will be particularly
valuable since it can combine the structural diversity
enabled by gold catalysis with cross-coupling chemis-
try. Further investigations to demonstrate the general-
ity of the cross-coupling of vinyl gold intermediates
with electrophilic partners are currently underway
and will be reported in due course.
Acknowledgements
The authors thank the National Research Foundation of
Korea (NRF-2012M3A7B4049653 and NRF-2014-011165)
for generous financial support. We also thank Professor
Sung-Hwan Han for fruitful discussions and XPS measure-
ments.
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Catalytic Cross-Coupling of Vinyl Golds with Diazonium Salts
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[12] In dual Pd-photoredox catalysis, it was noted that the
desired coupling with arenediazonium salts proceeded
smoothly in the absence of photocatalyst [Ru(bpy)32+],
but poor conversion was observed upon exclusion of
light (ref.[7c]).
[13] The radical chain mechanism has been proposed for
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[14] Compound 5d was stable at room temperature in spite
of the fact that one equivalent of HBF4 should formed
in the solution from the cleaved tert-butyl carbocation.
For a related study, see: L.-P. Liu, G. B. Hammond,
Chem. Asian J. 2009, 4, 1230, and ref.[11b]
[15] A butenolide 4d failed to undergo cross-coupling with
a photoredox-generated aryl radical in the presence or
absence of [Ph3PAu]+, which ruled out an auration of
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Dilip V. Patil et al.
4d followed by photoredox or gold-independent path-
ways.
[17] See the Supporting Information for details.
[18] For related XPS studies in gold catalysis, see: M.
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[16] The chain initiation possibly occurs by thermal homoly-
sis of aryldiazenyl radicals formed from electron trans-
fer from MeOH or via arenediazo methyl ethers
(inner-sphere mechanism): a) P. S. J. Canning, H. Mas-
kill, K. McCrudden, B. Sexton, Bull. Chem. Soc. Jpn.
2002, 75, 789; b) D. F. DeTar, T. Kosuge, J. Am. Chem.
Soc. 1958, 80, 6072; c) R. M. Paton, R. U. Weber, J.
Chem. Soc. Chem. Commun. 1977, 769. However, at
[19] H. J. Kuhn, S. E. Braslavsky, R. Schmidt, Pure Appl.
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[20] During the submission of the manuscript, a paper de-
scribing oxidation of Au(I) by arenediazonium salt
without a photocatalyst appeared. R. Cai, M. Lu, E. Y.
Aguilera, Y. Xi, N. G. Akhmedov, J. L. Petersen, H.
Chen, X. Shi, Angew. Chem. 2015, 127, 8896–8900;
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+
this point, three-centered oxidative addition of ArN2
with (L)AuCl cannot be excluded.
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