Gold-Catalyzed C(sp2)?C(sp) Coupling by Alkynylation through Oxidative Addition of Bromoalkynes

Article abstract of DOI:10.1002/chem.201902213

A gold(I)-catalyzed cascade cyclization–alkynylation of allenoates using alkynyl bromide to generate β-alkynyl-γ-butenolides was investigated. Whereas alkynyl iodides afforded significant amounts of the homo-coupling of two lactone units, alkynyl bromides led to a selective reaction, and a broad functional group tolerance was observed. Under the optimized reaction conditions, it was possible to directly synthesize a large range of β-alkynyl-γ-butenolides in moderate to good yields without the need for any external oxidant.

Full text of DOI:10.1002/chem.201902213

A Journal of
Accepted Article
Title: Gold-Catalyzed sp2-sp C-C Coupling by Alkynylation via
Oxidative Addition of Bromoalkynes
Authors: Yangyang Yang, Jasmin Schießl, Sirine Zallouz, Verena
Göker, Jürgen Gross, Matthias Rudolph, Frank Rominger,
and A. Stephen K. Hashmi
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201902213
Link to VoR: http://dx.doi.org/10.1002/chem.201902213
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10.1002/chem.201902213
Chemistry - A European Journal
Gold-Catalyzed sp2-sp C-C Coupling by Alkynylation via Oxidative Addition of
Bromoalkynes
Yangyang Yang,a Jasmin Schießl,a Sirine Zallouz,a Verena Göker,a Jürgen Gross,a
Matthias Rudolph,a Frank Rominger,a A. Stephen K. Hashmi*,a,b
a Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany.
E-mail: hashmi@hashmi.de
b Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah
21589, Saudi Arabia.
Abstract
A gold(I)-catalyzed cascade cyclization/alkynylation of allenoates using alkynyl
bromide to generate -alkynyl--butenolides was investigated. While alkynyl iodides
afforded significant amounts of the homo-coupling of two lactone units, alkynyl
bromides led to a selective reaction, and a broad functional group tolerance was
observed. Under optimized reaction conditions it was possible to directly synthesize a
large range of -alkynyl--butenolides in moderate to good yields without the need for
any external oxidant.
Keywords
Allenes; cross-coupling; gold catalysis; heterocycles; oxidative addition
Due to the high redox potential of the Au(I)/ Au(III) (1.41 V) compared to those of
Pd(0)/Pd(II) (0.91 V) or Pt(0)/Pt(II) (1.18 V),[1] the development of methodologies on
the oxidative addition of Au(I) has attracted much attention.[2] In the last years, some
general oxidizing agent were used in the gold-catalyzed redox coupling involving
oxidation of Au(I) to Au(III). First, in 2009, Waser et al. developed the gold-catalyzed
C-H alkynylation of indole with ethynyl-benziodoxolone (EBX) reagents, followed by a
series of methodologies for alkynylation of (hetero)arenes (Scheme 1a).[3] Second, in
2013, Glorious and coworkers expanded Au(I) oxidation with aryldiazonium salt
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combined with a Ru photoredox catalyst, leading to the explosion of new
methodologies for gold-catalyzed arylation (Scheme 1b).[4] Third, in 2017, in
Bourissou’s group, aryl halides was used in the gold-catalyzed arylation of electron-
rich arenes by hemilabile (P,N) ligand-promoted oxidative addition of gold(I) (Scheme
1c).[5] However, these strategies suffered from some drawbacks, for example the steps
for the synthesis of the hypervalent iodine regents, the low atom efficiency (only a small
part of the reagent is transferred or more than one equivalent of silver is needed)[5] and
the need of complicated bidentate ligands.
a) Gold-catalyzed redox alkynylation with EBX reagents: Waser, Liu, Hashmi[4]
L
Y
low atom efficiency
R1
IIII
Au
functionalizaiton
R1
R2
difficult access to
EBX reagents
LAuX
X
R1
b) Gold-catalyzed redox arylation with aryl diazonium salts: Glorious, Toste, Shi, Hashmi[3]
N2Y
Ar
L
Y
fuctionalizaiton
Au
R
LAuX
Ar
X
Ar
c) Gold-catalyzed redox arylation with aryl halides: Bourissou[5]
N
halogen scavenger
(1 equiv. silver salt)
I
I
Ar
N
fuctionalizaiton
Au
R
Ar
P
P
AuX
complicated bidentate
ligand
Ar
d) This work:
R1
R2
R2
readily accessible
alkynyl bromide
Br R2
Ph3P
O
R3
Br
R3
O
Au
O
halogen scavenger-free
simple gold catalyst
O
R3
O
Ph3PAuNTf2
R1
O
R1
Scheme 1. a), b) and c) previous work on the gold-catalyzed redox coupling with
general oxidizing agents; d) this work for the gold-catalyzed cyclization/ alkynylation
with alkynyl bromides.
Haloalkynes, readily accessible oxidizing agents in transition metal-catalyzed cross-
couping, are widely used in synthetic chemistry.[6] However, the development of gold
catalysis with haloalkynes is hampered by some challenges. First, the lower oxidative
potential compared to EBX reagents makes an oxidative addition of the haloalkynes to
the gold(I) center difficult. Second, simple gold catalysts such as Ph3AuX and IPrAuX
(X = halogen) are sluggish in the oxidative addition of Csp/Csp2-X bond.[7] Finally, the
halogen could block the active gold catalyst by the formation of thermodynamically
stable LAuX.[8] The common solution for halogen poisoning is by the addition of more
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than one equivalent of halogen scavenger, which however, largely decreases the atom
efficiency of these gold catalyses.[5]
Due to the strong Lewis acidity of gold catalyst, a sequential cyclization and
functionalization is an advantage for gold catalysis.[3,4] In 2010, Gouverneur et al.
developed a cascade cyclization – oxidative alkynylation of allenoates with terminal
alkynes by using Selectfluor as external oxidant.[9] It provided an efficient method for
the synthesis of -alkynyl--butenolides, which was previously conducted by a two-step
procedure based on the stoichiometric cyclization of the allenoate with gold(I) complex
and a subsequent palladium-catalyzed cross-coupling.[10] However, this strategy still
suffered from the use of a strong external oxidant. Herein, we sought to combine the
gold(I)-induced cyclization of allenoates with a mild oxidative addition of gold(I), a gold-
catalyzed cyclization/cross-coupling alkynylation reaction with haloalkynes (Scheme
1d).
In initial experiments, stoichiometric reactions of lactonyl gold(I) complex 1a with
phenylacetylene and the corresponding halogenated derivatives were examined
(Table 1). The organogold(I) complexes 1a-c, which were first reported by Hammond
et al. in 2008,[11] are characterized by a remarkable stability and the feature that they
can conveniently be obtained by a cyclization reaction.[9,10,12] One equivalent of the
organogold(I) complex 1a was treated with 1.1 equivalents of the alkynes 2a-d in THF
at 50 °C. After stirring for 4 days, no reaction occurred using the non-halogenated
phenylacetylene 2a (Table 1, entry 1). But the brominated phenylacetylene 2c yielded
74 % of the desired -alkynyl--butenolide 3a without significant amounts of homo-
coupling product 4 (Table 1, entry 3) while in case of the iodinated alkyne 2d only 47
% of the desired product 3a were gained (Table 1, entry 4) accompanied by 37 % of
the homo-coupled lactone 4. A low reactivity was observed with the chlorinated
phenylacetylene 2b (Table 1, entry 2).
Table 1. Stoichiometric reactions.[a]
Yield 3a [%][b]
Alkyne
2a
Yield 4 [%]
Entry
X
H
1
2
no reaction
3[c]
-
2b
Cl
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2c
2d
3
Br
74
47[c]
traces
4
I
37
[a] Reaction conditions: Organogold(I) complex 1a (55 μmol, 0.55 mmol/ml), alkyne 2a-
[b]
1
d (60 μmol), THF at T = 50 °C. Yield determined by H NMR spectroscopy of the
[c]
crude reaction mixture using 1,4-dinitrobenzene as an internal standard.
defined decomposition was observed, too.
An ill-
Based on these results, studies concerning the gold-catalyzed cascade
cyclization/alkynylating cross-coupling were conducted with brominated alkynes (See
supporting information Table S1). The tert-butyl allenoate 5a was treated with 1.1
equivalents of brominated phenylacetylene 2c, 2 equivalents of base and a gold(I)
catalyst (10 mol%) in a mixture of THF/H2O (1/2) at 50 °C. Pleasingly, the gold(I)
complex Ph3PAuNTf2 in combination with NaHCO3 led to the desired -alkynyl--
butenolide 3a in 72 % yield after 4 days (Table S1, entry 3). Compared to the
stoichiometric experiment (Table 1, entry 3) the gold-catalyzed reaction (Table S1,
entry 3) delivered a similarly high yield. The reaction of the gold(I) catalyst Ph3PAuNTf2
under exclusion of base delivers a 44 % yield (Table S1, entry 16). This emphasizes
the importance of the usage of base. Further efforts for an optimization of the gold(I)
catalyst only led to significantly decreased yields (Table S1, entries 1, 2, 4 – 12). This
included various counter anions such as Cl-, Br-, OTf-, SbF6 , BF4 and OTs- and
different ligands such as other phosphines (Mes3P, (p-CF3Ph)3P), Buchwald ligands
(SPhos, BrettPhos) and one NHC ligand (IPr). Especially, in case of Buchwald ligands,
an unselective reaction was observed. Alternative bases (Table S1, entries 13 – 15)
such as K2CO3, K3PO4 and Cs2CO3 reduced the turnover number up to 1/4. The control
experiment with only NaHCO3 showed no conversion (Table S1, entry 17).
-
-
Under the optimized reaction conditions the compatibility of the domino cyclization –
alkynylation and the effects on the reaction efficiency were investigated with a wide
range of substituted allenoates 5a-d and arylacetylenes including para-, meta- and
ortho-substituted derivatives 2c,e-m as well as one alkylacetylene 2n (Table 2).
Allenoate 5b with a methyl-substitutent in R2-position almost afforded a full conversion
of the starting material within a reaction time of 4 days. The cross-coupled butenolide
3b was obtained in 80 %, the highest yield in the study of the scope of the reaction. A
variation of the side chain R1 to butyl (5c) or benzyl (5d) delivered the products 3c and
3d in only moderate yields. The effect of different substitution patterns of chlorinated
phenyl groups in 2 on the reactivity was examined on the basis of the alkynes 2e-g.
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Obviously, there is a significant effect on the yield. Here, the p-Cl substituent on the
benzene ring provided 3e in 53 % isolated yield, whereas, the m-Cl-substituted
arylalkyne delivered a NMR yield of 3f of 9 % after a reaction time of 4 days. Especially
electron-poor arylbromoalkynes, bearing p-CF3, p-NO2 and even p-COOCH3 on the
aryl group, produced the corresponding butenolides 3i,j,l in low yields. On the other
hand, the electron-donating p-CH3O-Ph-substituted alkyne 2m afforded 3m in 69 %
yield and thus showed a slightly higher reactivity than the unsubstituted alkyne 2c. The
reaction with 1-bromohex-1-yne 2n as alkylalkyne was extremely slow, however, the
isolation of 3n succeeded after an extended reaction time of 12 days. A trityl-
substituted bromo alkyne did not show any conversion, after 4 days allenoate 5a could
be completely recovered. In the conversion of all these substrates no homo-coupling
could be detected. For the highly crystalline cross-coupled butenolide 3h a single
crystal X-ray structure analysis could be obained.[13]
Table 2. Scope of the reaction.[a]
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R1
R2
Ph3PAuNTf2 (10 mol%)
NaHCO3 (2 equiv)
THF/H2O (1/2)
R3
R3
O
O
Br
O
O
R1
50 °C, 4 d
R2
5a-d
2c,e-n
3a-n
O
O
O
O
O
O
O
O
3a
3b
3c
3d
64 % (72 %)
80 % (95 %)
37 % (45 %)
25 % (31 %)
Cl
Cl
O
O
O
O
Cl
O
O
O
3e
3f
3g
32 % (9 %)[b]
53 % (56 %)
17 % (30 %)
O
O
O
O
O
Br
F3C
O2N
O
O
3h
3i
3j
38 % [c]
48 % (53 %)
5 % (8 %)
O
O
O
O
O
3l
3m
69 % (77 %)
3h[13]
10 % (10 %)
O
O
O
O
3k
3n
28 % (34 %)
23 % (2 %)[b]
[a] Reaction conditions: Allenoate 5a-d (110 μmol, 1.1 mmol/ml THF), bromoalkyne
2c,e-n (121 μmol), NaHCO3 (219 μmol), Ph3PAuNTf2 (10 mol%), THF/H2O (1/2) at T =
50 °C. Yield of isolated product. Yield as determined by 1H NMR spectroscopy of the
crude reaction mixture using 1,4-dinitrobenzene as an internal standard is given within
parentheses. [b] Purification and isolation of the product after a reaction time of 12 d.
[c] No yield determined by 1H NMR due to its poor solubility.
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The mechanism of this reaction can be rationalized by four alternative pathways
including: a) carbometalation[14], b) halogen metal exchange and c/d) two alternative
AuI/AuIII redox cycles (Scheme 3, for details, see Scheme S1). Path I is a sequential
Au-catalyzed cyclization, followed by carbometalation and anti-β-bromide elimination.
In path II, a -bond metathesis was proposed. Furthermore, two other paths III and IV,
involving a sequential Au-catalyzed cyclization/oxidative addition/reductive elimination
or and oxidative addition/cyclization/reductive elimination are conceivable. A series of
experiments were carried out to investigate these four pathways. In the event that this
cross-coupling proceeds via carbometalation to form intermediate vinylgold(I) B,
followed by anti-β-bromide elimination, a homo-coupling product (like 4) should not be
fromed. However, 4 was experimetally observed in the stoichiometric reaction,
opposing path I. Path II, involving transition state C, was excluded by the absence of
the cross-coupling product in a stoichiometric experiment with alkynylgold(I) substrate
7 and 1-bromo-4-methoxybenzene (Scheme 2a), in which the cross-coupling product
was expected. Oppositely, mixing 1-(bromoethynyl)-4-benzene 2c and 4-
methoxyphenylgold(I) 6 afforded the cross-coupling product in 35% yield, along with
17% of homo-coupling product (Scheme 2b).
Scheme 2. Stoichiometric experiments to exclude bromo-gold exchange mechanism.
Alternatively, the reaction mechanism of AuI/AuIII redox cycles are in agreement with
the presence of the homo-coupling product from group scrambling.[15] In path III,
initially, the oxidative addition with 1-(bromoethynyl)-4-benzene 2c yields the gold(III)
complex E. The subsequent cyclization of the allenoate 5a leads to the gold(III)
complex D, followed by a reductive elimination, regenerating the active catalyst
species and releasing -alkynyl--butenolide 3a. An alternative pathway is shown in
path IV (Scheme 3). First, the gold(I) species coordinates to the allenoate 5a, and in
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the presence of base and water the cyclization leads to the organogold(I) complex A.
The oxidative addition of 1-(bromoethynyl)-4-benzene 2c affords the gold(III) complex
D, and a subsequent reductive elimination regenerates the active catalyst and sets
free the -alkynyl--butenolide 3a by the formation of the new C–C bond.
Br
Ph
2c
oxidative addition
O
L
AuI
L
Br
Au
O
A
O
Path IV
Ph
O
D
O
cyclisation
reductive
elimination
O
5a
Br
NTf2
LAuINTf2
LAuIBr
O
Ph
O
3a
Scheme 3. The mechanism of path IV.
The mechanistic research on the Au(I)/Au(III) cycle is hampered by some challenges.
First, precedents on the fast reductive elimination of alkynylgold(III) complex
suggested that the life of the gold(III) intermediate E is expected to live extremely short,
preventing the isolation of the gold(III) intermediate or detection by NMR
spectrometry.[16] Secondly, the slow oxidative addition (4 days needed in the
stoichiometric reaction) also fundamentally increases the difficulty for the detection of
gold(III) intermediate. Indeed, efforts to isolate the gold(III) complex in combination with
the ligands IPr, Ph3P and (2-(2-(diphenylphosphanyl)phenyl)pyridine) all failed
(Scheme S2). Moreover, the gold intermediate being uncharged, prevents the
detection by ESI-MS spectrometry, a method fast and sensitive enough to detect of
the gold intermediate.[17] By introducing a pyrrolidinium to a remote position of the
alkynyl bromide, the intermediates become accessible to ESI(+) MS. As shown in
Scheme 4, in the mixture of the alkynyl bromide 2o with organogold(I) complexes 1a
in THF/ H2O, an ion with m/z 906.1984 was detected in high-resolution mass
spectrometry (electrospray ionization, positive mode), which is in agreement with the
calculated m/z for the gold(III) cation 8a (906.1981). In the catalytic reaction with 10
mol% of Ph3PAuNTf2, an ion of the gold(III) intermediate 8b from oxidative addition
was also observed (calcd. 920.2137, found 920.2146). The gold(III) compound 8c from
the oxidative addition of arylgold(I) complex with alkynyl bromide was also found in the
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ESI-MS spectrum (calcd. 888.1875, found 888.1878). These data strongly support the
Au(I)/Au(III) cycle in this gold-catalyzed cross-coupling by alkynylation with
bromoalkynes. Moreover, in the ESI-MS spectrometry, the absence of the
dibromogold(III) intermediate F in the mixture of Ph3PAuBr with alkynyl bromide 2o
excludes the path III with a sequential oxidative addition, followed by gold(III)-induced
cyclization.
Scheme 4. Exploring the reaction intermediates with ESI-MS (LIFDI)
Next, the potential halogen poisoning by the formation of the Ph3PAuBr (followed by
the first catalytic cycle), which is inactive in the cyclization, was investigated. Due to
the specificity of Ph3PAuNTf2 (see Table S1) in the catalytic reaction and the stronger
binding of NTf2 to gold than other counter anions,[5] we think there might be an
equilibrium between Ph3PAuBr and Ph3PAuNTf2, which enables the followed catalytic
cycles. This assumption was firstly supported by the addition of NaBr to the reaction
mixture (Table S2). Obviously, the addition of NaBr to the reaction mixture reduces the
catalytic efficiency stepwise but, as expected because the catalytic reaction works,
more than one equivalent of bromide with respect to the catalyst is tolerated.
-
Additionally, the catalytic reaction with the combination of Ph3PAuBr and NTf2 gave
the desired product in 49% yield (Scheme 5). This strongly supports the existence of
an equilibrium between Ph3PAuBr and Ph3PAuNTf2.
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-
Scheme 5. Effect of NTf2 on the catalytic reaction.
Overall, a novel cascade cyclization/alkynylation reaction of allenoates was
established for various alkynyl bromides. It was demonstrated that the use of simple,
easily accessible alkynyl bromides, ideal for organic synthesis, shows a broader
functional group tolerance than hypervalent iodine reagents or other related oxidants.
In consideration of the optimized reaction procedure, the -alkynyl--butenolide could
be isolated depending on the electronic properties of the substituents in low to good
yields (5 – 80 %). Moreover, the mechanistic investigation strengths the proposed
reaction mechanism of an AuI/AuIII redox cycle even. This unique reactivity may open
new opportunities in gold catalysis.
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TOC Graphic
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