Gold Redox Catalysis with a Selenium Cation as a Mild Oxidant

Article abstract of DOI:10.1002/chem.202000166

Gold-catalyzed alkyne and allene diselenations were developed. Excellent regioselectivity (trans) and good to excellent yields were achieved (up to 98 % with 2 % catalyst loading) with a wide range of substrates. Mechanistic investigation revealed the formation of a vinyl gold(I) intermediate followed by an intermolecular selenium cation migration, suggesting that a gold(I/III) redox process was successfully implemented under mild conditions.

Full text of DOI:10.1002/chem.202000166

A Journal of
Accepted Article
Title: Gold Redox Catalysis with a Selenium Cation as Mild Oxidant
Authors: Xiaodong Shi, Jin Wang, Chiyu Wei, Xuming Li, Pengyi Zhao,
Chuan Shan, Lukasz Wojtas, and Hao Chen
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To be cited as: Chem. Eur. J. 10.1002/chem.202000166
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COMMUNICATION
Gold Redox Catalysis with a Selenium Cation as Mild Oxidant
Jin Wang‡, Chiyu Wei‡, Xuming Li, Pengyi Zhao, Chuan Shan, Lukasz Wojtas, Hao Chen and Xiaodong
Shi*
Abstract:
important for enhancing the basic understanding of this rapidly-
growing field, but also holds promise for generating synthetically
valuable building blocks.
Gold-catalyzed alkyne and allene diselenations were developed.
Excellent regioselectivity (trans) and good to excellent yields were
achieved (up to 98% with 2% catalyst loading) with a wide range of
substrates. Mechanistic investigation revealed the formation of a vinyl
gold(I) intermediate followed by an intermolecular selenium cation
migration, suggesting that a gold (I/III) redox process was successfully
implemented under mild conditions.
Recently, we reported a highly efficient and stereoselective
thioallylation of electron-deficient alkynes (Scheme 1B).[8]
Mechanistic study suggested that this reaction was initiated by
sulfur nucleophilic addition to form a vinyl gold intermediate, which
underwent a sequential allyl transfer via Au(I/III) cycle. Notably,
we for the first time propose the allyl sulfonium cation functions as
a mild internal oxidant to accomplish the Au(I/III) oxidation, which
represents a powerful strategy to combine the excellent p-acid
reactivity of Au(I) complexes with gold redox catalysis.
The past two decades have witnessed tremendous growth in
homogeneous gold catalysis.[1] The unique capability of the gold
cation as a carbophilic Lewis acid[2] has rendered it a superior
catalyst in nucleophilic addition reactions towards alkenes,
allenes, and alkynes.[3] On the other hand, the redox chemistry of
gold(I) catalyst is largely neglected, mainly due to the high
oxidation potential (AuI/III = 1.4 eV).[4] Thanks to the pioneering
work from Zhang, Hashmi, Toste and other groups, this challenge
has been successfully overcome by utilizing external strong
oxidants such as selecfluor, hypervalent iodine reagents,
aryldiazonium salts and even alkynyl bromides.[5] Another
significant observation in this regard was reported by Glorius’
group, who first indentified that diazonium salt could be used as a
mild oxidant for gold (I) oxidation in the presence of an additional
ruthenium-based photoredox catalyst(Scheme 1A).[6] Despite of
these progresses, the necessity of using an external oxidant
impedes further development of this methodology with only
limited reaction scope. More recently, direct oxidative addition at
a gold(I) complex emerged.[7] However, the reported examples
so far either requires highly-strained starting materials in the case
of X-X oxidative addition (X=Si or C) or a delicate catalyst design
to activate allyl or aryl halides. Thus, developing a novel and
effective approach to accomplish gold redox catalysis is not only
A) General strategies in achieving gold redox catalysis
X
AuIII
strong oxidant
oxidative addition
AuIII
Ln
R
Ln
X
selectfluor or
PhI(OAc)2
Ar-X or Allyl-Br
ref. 7
ref. 5
AuI
Ln
new and mild
oxidants?
hv or base activation
challenging
transformations
AuIII
Ln
Ar
-
ArN2+BF4
,
ref. 6
[Ru] photocatalyst
B) Recent example from our lab: alkyne thioallylation
SPh
PhS
PhS
[L-Au]+
3,3-rearrang.
or AuIII
CO2Me
H
CO2Me
+
H
?
AuI-L
CO2Me
Concerns: 1) only works for EWG activated alkynes;
2) plausible 3,3-rearrangement, not gold redox catalysis?
C) This work: Exploring RX-XR as plausible Nu and oxidant for gold redox catalysis
RX-XR
XR
R2
R2
R2
H
RX
R1
RX
R1
RX
R2
R
X
or
R1
R2
AuIII
R1
XR
XR
R1
AuI-L
AuI-L
L
Tasks: 1) developing new alkyne/allene difunctionalization under mild conditions;
2)evaluating choice of RXXR (X=O,N,S etc.); 3) confirming gold redox process.
Scheme 1. Recent development on gold redox catalysis.
Although we have successfully observed Au(III) species via
Mass Spectrometry in the thioallylation of alkynes, we could not
completely rule out a possible [3,3]-rearrangement pathway[9]
which doesn’t involve Au(I) oxidation. Moreover, only electron
deficient alkynes (EWG modified alkynes) successfully
participated in this transformation, which greatly limited the
reaction scope. Other alkynes and allenes, which are typical
substrates in gold catalysis, did not give the desired products.
Therefore, it is important to conduct a more detailed investigations
regarding detailed reaction mechanism and further extend the
reaction to a broader scope. Herein, we report our efforts on
facilitating RXXR addition toward alkynes and allenes under gold-
catalyzed conditions (Scheme 1C). Notably, regioselective gold-
catalyzed alkyne and allene diselenations were successfully
achieved. The transformation took place under mild conditions
with high efficiency, giving desired products in good to excellent
yields and excellent regioselectivity. Mechanistic investigations
confirmed Au(I/III) redox cycle, which validates the great potential
[*]
Dr. J. Wang
College of Chemistry, Chemical Engineering and Material Science
Shandong Normal University
Jinan, Shandong 250014, China
Dr. J. Wang, C. Wei, X. Li, C. Shan, Prof. Dr. X. Shi
Department of Chemistry
University of South Florida
Tampa, FL 33620, USA
E-mail: xmshi@usf.edu
Homepage: http://xmshi.myweb.usf.edu/index.htm
P. Zhao, Prof. Dr. H. Chen
Department of Chemistry and Environmental Science
New Jersey Institute of Technology
Newark, NJ 07102, USA
[‡]
These authors contribute equally to this work.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
This article is protected by copyright. All rights reserved.

10.1002/chem.202000166
Chemistry - A European Journal
COMMUNICATION
of applying selenium cation as an effective mild oxidant to
promote gold redox catalysis in future. What’s more, the
diselenation product can be further derived into value-added
synthetic intermediates.[10]
As suggested in Scheme 1C, compounds with RXXR moiety
could be of interest in performing sequential nucleophilic addition
observed when using PhTeTePh (3c), which is likely due to a
stronger coordination between Te and gold catalyst, comparing to
Se or S, that leads to slower reaction. Comparisons of the
optimal conditions with some alternative conditions are
summarized in Table 1.
Table 1. Gold-catalyzed alkyne diselenation: condition screening
2% IPrAuNTf2
PhSe
CO2Me
+
and redox chemistry with a single Au(I) catalyst.
Unlike
CO2Me
PhSeSePh
DCE, ꢀꢁꢂꢃꢄ
60 oC, 24 h
thioallylation, [3,3]-rearrangement path is not available in this
alkyne difuntionalization reaction, which provides a simple model
for studying gold redox catalytic cycle. Notably, with a relatively
weak X-X bond, it has been reported in literature that RXXR type
compounds could form RX radical and proceed aforementioned
SePh
1a
2b
3b
3b (E/Z)
100% 92% (>100:1)
entry Variation from standard conditions
conv.
3b’
5%
-
1
2
none
5% Ph3PAuNTf2
60%
46%
3
4
5
6
7
8
9
10
11
12
5% (PhO)3PAuNTf2
5% XPhosAuNTf2
2% IPrAuNTf2 (0.5 M)
1% IPrAuNTf2 (0.5 M)
Other [Au] catalyst
10% PtCl2
60%
31%
-
100% 72% (40:1)
100% 85% (32:1)
100% 85% (20:1)
14%
7%
8%
-
transformation via
a a result, poor
radical path.[11] As
regioselectivity was often observed. The proposed gold catalytic
route, if successful, will not only provide an alternative approach
for RXXR activation, but also likely accomplish the reaction with
much better stereo-outcome. Potential challenges can be 1)
reduced [LAu]+ reactivity caused by X-Au coordination and 2) slow
C-X reductive elimination (relative to C-C), which could lead to
competing protodeauration as the major byproduct. To initiate our
investigation, we first attempted using ROOR, RNNR and RSSR
-
-
80%
67%
<60%
<80%
nd
-
other metal catalysts (Ag, Cu, Fe, etc)
other solvents
10% HOTf
no catalyst
<5%
<5%
-
-
nd
Reaction conditions: the catalyst (2%) was added into a DCE solution of alkyne (0.3 mmol)
and diselenide (0.2 mmol), and reacted at 60 oC for 24 h. Conversion and yield were
determined by 1H NMR spectroscopy with dimethyl sulfone as the internal standard.
Table 2 Substrate scope for diselenation of alkynes
as nucleophiles to react with alkynes.
The results are
’’R
Se
summarized in Figure 1.
2-5% IPrAuNTf2
R’
R
R’
+
R’’Se-SeR’’
R
DCE, 0.2 M, temp., 24 h
5% IPrAuNTf2
Se
RX
CO2Me
XR
RXXR conv. of 1a
RX
CO2Me
+
R’’
CO2Me
1a
RXXR
or
1
2
3
12 h, CDCl3, 50 o
C
H
2
3
3’
carbonyl-activated alkynes, 2% IPrAuNTf2, 60 oC:
RXXR
3
3’
conv. of 1a
3
3’
3b, R = H, 94%
Me
H
Bn
R
Se
O
Se
O
3d, R = 4-OMe, 92%
(t-BuO)2
(PhC(O)O)2
(Me2N)2
100%
(PhS)2
(PhSe)2
(PhTe)2
<5% <5%
<5% <5%
n.d. n.d.
80%
100%
100%
3a, 58% 22%
3b, 92% <5%
3c, 61% <5%
Se
O
3eꢀꢁꢂ = 4-Me, 88%
3f, R = 4-Br, 86%
3g, R = 4-F, 86%
3h, R = 4-CF3, 66%
3i, R = 3-F, 80%
100%
<5%
OMe
Me
3j, 86%
H
OMe
Bn
3k, 85%
H
OMe
Se
Se
Se
EꢀZ > 100:1 for both 3 and 3’
R
Figure 1. Gold-catalyzed RXXR addition to alkynes
Ph
Ph
H
Ph
Ph
Treating alkyne 1a with peroxide or peracid gave 100%
conversion. However, desired product 3 or byproduct 3’ was not
obtained based on crude NMR. Reaction with tetramethyl
hydrazine gave almost no alkyne conversion, likely due to catalyst
poisoning. Interestingly, reaction of disulfide 2a (PhSSPh) gave
the desired dithiolation product 3a in 58% yield (80% conversion).
As expected, hydrothiolation product 3a’ was observed (22%) as
the byproduct. It is important to note that both 3a and 3a’ were
observed as single isomer (E/Z > 100:1). The alkene geometry
was later confirmed by X-ray crystallography (vide infra). These
results are exciting, since they suggested that the proposed
combination of gold p-acid activation and redox catalysis is valid.
In theory, protodeauration could occur form both gold(I)
intermediate (vinyl gold) or gold(III) intermediate.[12] Considering
gold(III) reductive elimination is a rather rapid process, we
postulate that stronger oxidants may help to avoid
protodeauruation by promoting the reaction with faster gold (I)
oxidation. Thus, we turned our attention to diselenides based on
1) Se as better nucleophile over S and 2) selenium cation as a
stronger oxidant. To our delight, the desired diselenation product
3b was formed in excellent yield (90%). Further catalyst
screening revealed that electron-rich gold catalyst IPrAuNTf2
gave the best yield and DCE was identified as the optimal solvent.
The catalyst loading can be reduced to 2% without sacrificing the
yield or regioselectivity. Other metal catalysts including Pt, Ag, Cu,
Fe gave inferior results (see SI for details). Reduced yield was
Se
O
Se
O
Se
O
Se
O
Ph
Ph
H
OBn
Ot-Bu
H
N
H
H
N
H
Se
Se
Se
Se
Ph
Ph
Ph
Ph
3l, 87%[a]
3m, 95%[a]
3n, 82%[a]
3o, 48%[a]
Ph
H
Ph
Ph
H
Ph
O
Se
O
Se
O
Se
O
MeO
N
CO2Me
Me
OMe
Ph
H
Se
Se
Se
Ph
3p, 55%[a]
Ph
3r, 93%[b]
3q, 85%
haloalkynes, ynamides and simple alkynes, 5% IPrAuNTf2, rt:
Ph
Ph
3s, R = H, 82%
Se
Ph
Br
Se
Se
3t, R = 4-Br, 86%
3u, R = 4-F, 94%
3v, R = 3-Me, 65%
3w, R = 3-F, 69%
3x, R = 2-Me, 60%
R
Br
Br
Cl
Se
Se
Ph
Se
Ph
Ph
3y, 92%
3z, 62%
R
Bn
Br
Ph
Cl
Se
Se
Se
3aa, R = 4-OMe, 90%
3ab, R = 4-Me, 96%
3ac, R = 4-F, 84%
Ph
Ph
Ph
Br
Se
Se
Se
3ad, R = 4-CF3, 72%
Bn
Ph
R
3ae, 98%
3af, 92%
Ph
Ph
Me
Ph
Se
Se
O
Se
Ph
Ph
N
O
H
Me
Se
Ph
Se
Se
Ph
Ph
3t
3ah, 69%
3ai, 50%
3ag, 64%
Reaction conditions: the catalyst was added into a DCE solution of alkyne (0.3 mmol) and
diselenide (0.2 mmol), and reacted at given temperature for 24 h. Isolated yield. [a] 0.2 mmol
alkyne and 0.3 mmol diselenide were used for the ease of purification. [b] React at 80 oC.
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10.1002/chem.202000166
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With the optimal conditions in hand, we next explored the
reaction scope as shown in Table 2. Most aryl diselenides with
either electron-donating or electron-withdrawing substituents
proved to be suitable for this transformation, giving good to
excellent yields when reacting with methyl propiolate (3a-3g).
One exception is para-CF3 substituted aryl deselenides, which
only provided 3h with moderate yield due to sluggish nucleophilic
addition step. Alkyl diselenides participated in this reaction
smoothly as well, realizing 3j and 3k in satisfactory yields. Other
carbonyl-activated alkynes, including esters (3l and 3m), ketone
(3q) and acetylene diester (3r) all underwent diselenation
successfully with excellent yields; amides (3n-3p) on the other
hand gave lower yields presumably due to substrate coordination.
We next turned our attention to a broader scope of alkynes. First,
various bromoalkynes with either aryl or alkyl group connecting to
the C-C triple bond reacted with phenyl diselenide and provided
desired products in moderate to excellent yields (3s-3z); other
diselenides were also compatible with bromoalkynes (3aa-3ae).
The fact that no cyclopropyl ring opening product was observed
ruled out a radical reaction pathway (3z). Chloroalkyne was a
valid substrate for this reaction as expected (3af). We were
pleased to find out that electron-rich alkynes such as ynamides
underwent diselenation successfully, giving desired products in
moderate yields (3aq). Encouraged by this result, we tested both
terminal and internal alkynes without electronic biases. We are
delighted to find out that desired products 3ah and 3ai were
formed in moderate yield under gold-catalyzed conditions, which
further showcased the broad scope of this transformation. It is
worthwhile to mention that in all cases diselenation products were
formed as essentially one alkene isomer, and their E-geometry
were ambiguously identified by X-ray crystallography of product
3t.
find out that allene diselenation products could also be formed
smoothly via this protocol. As shown in Table 3, when treating
1,1-diphenyl allene with different diselenides, desired products
were formed in good to excellent yields (5a-5e). 1,1-dimethyl
allene could also give the desired products in excellent yield (5f).
Trisubstituted
allenes
underwent
diselenation
reaction
uneventfully, providing desired product 5g in moderate yield.
Monosubstituted allenes also provided excellent yields, although
only moderate E/Z selectivity were observed (5i-5k). The
structure of E/Z isomers were unambigiously assigned by X-ray
crystallography (E-5i).
To better elucidate the mechanism,[13] a series of experiments
were carried out. First, cross-over experiment clearly supported
an intermolecular selenium transfer mechanism (Figure 2A). In
addition, direct oxidative addition of phenyl diselenide with gold(I)
catalyst was not observed (see SI). When using more electron-
rich styrene, such reaction didn’t occur, which ruled out a Au(I)-
assisted selenium cation formation pathway. Based on these
results, we propose a mechanism which includes gold-catalyzed
nucleophilic addition of diselenides and subsequent selenium
transfer via gold redox catalysis, and attempted to trap the key
intermediates with Mass spectrometry (Figure 2B). A mass
corresponding to vinyl gold intermediate A was clearly detected.
In addition, CID experiment of m/z of intermediate A suggested a
Au(II)-SePh+ fragment, which most likely came from a Au(III)
intermediate B that possesses the same mass as intermediate A.
Another Au(III) intermediate
C which contains a similar
composition as in the thioallylation case was also observed, and
CID confirmed the existence of a Au(III) fragment too. The Mass
study thus strongly supported that Au(I/III) redox cycle is plausible
for this transformation.
A) Cross-over coupling confirmed intermolecular reaction
PhSe
CO2Me
TolSe
H
CO2Me
SeTol
Table 3. Substrate scope for diselenation of allenes
CO2Me
’’’R
PhSe-SePh
1 equiv
H
SePh
1:1:1:1 by NMR
R’
Se R’
Se
5% [Au]
2% IPrAuNTf2
+
+
DCE, 60 o
C
R’’’Se-SeR’’’
R
R’’
R
R’’
R’’’
DCE, 0.4 M, rt, in dark
TolSe
CO2Me
SePh
PhSe
H
CO2Me
H
TolSe-SeTol
1 equiv.
3 equiv.
H
SeTol
4
2
5
B) In-situ MS-MS confirmed Au(III) mechanism
SePh
Bn
R
Se Ph
Se
CO2Me
5a, R = H, 82%
5b, R = 4-OMe, 94%
5c, R = 4-F, 82%
COOMe
+
PhSe-SePh
H
Se Ph
Se
Ph
Au(I)-IPr
SePh
SePh
R.E.
Ph
5d, R = 4-Me, 91%
Bn
SePh
SePh
PhSe
H
5e, 67%
PhSe
H
R
ox.
CO2Me
CO2Me
- H
CO2Me
Au(I)-IPr
H
AND
Au(III)
Au(III)
Ph
Ph
Ph
IPr
Se Me
Me
Se Me
Me
Se
IPr
SePh
SePh
OTBS
m/z = 983 detected;
consistent with CID
m/z = 1139 detected;
consistent with CID
Ph
m/z = 983 detected
Se
Se
Se
Ph
Ph
5g, 40%
Ph
C) Suzuki Coupling conversion of 3r
5f, 84%
5h, 96%, Z:E = 1ꢀꢁ:1
PhSe
10% Pd(PPh3)4
1.2 eq. Cu(OAc)2.H2O
PhSe
COOMe
Ar
COOMe
+
MeOOC
ArB(OH)2
Se
MeOOC
DMF, 80 oC, 2 h, 75%
SePh
3r
5i, Ar = 4-Methylphenyl
Ar
6
94%, E:Z = 2:1
Ar=4-COOMe-C6H4
Se
single isomer, confirmed by X-ray
5i
Ar
Br
Figure 2. Mechanistic investigations and 3r conversion
Reaction conditions: the catalyst (2%) was added into a DCE solution of allene (0.6 mmol)
and diselenide (0.2 mmol) and reacted at rt for 24 h. Isolated yield.
While 1,2-diselenide products 3 are interesting building
blocks, they can be readily converted into other compounds
through simple transformations. As shown in Figure 2C, treating
3r under standard Suzuki coupling conditions gave desired
couping product 6 in 75% yield. Interestingly, although excess
amount ArB(OH)2 were used, only single isomer with mono-
Encouraged by the successful diselenation of alkynes, we
wondered if this transformation can be further extended to allenes,
which were not suitable substrates in previous thioallylation
studies. After detailed reaction optimization, we were pleased to
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10.1002/chem.202000166
Chemistry - A European Journal
COMMUNICATION
substitution product was obtained with structure confirmed by X-
ray crystallography (see SI). This result suggested a Heck-type
mechanism might occurred with this substrate instead of direct C-
Se bond oxidative addition as proposed in literature.[10a] This
excellent chemo and stereoselectivity greatly highlighted the
potential synthetic utility of these 1,2-diselenide compounds
under this new and mild gold redox conditions.
Jones, C. A. Russell, J. Am. Chem. Soc. 2014, 136, 254-264; k) Y. Yu, W.
Yang, D. Pflästerer, A. S. K. Hashmi, Angew. Chem. Int. Ed. 2014, 53,
1144-1147; l) M. Pažický, A. Loos, M. J. Ferreira, D. Serra, N. Vinokurov,
F. Rominger, C. Jäkel, A. S. K. Hashmi, M. Limbach, Organometallics 2010,
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4813.d
In conclusion, we herein reported
a
gold-catalyzed
diselenation reaction of alkynes and allenes. Excellent yields and
stereoselectivity as well as broad reaction scope were achieved
for this transformation. Au(I/III) mechanism was proposed based
on the cross-over experiment, control experiments and in-situ MS
results. Other mild oxidants are currently under investigation in
our lab.
[6] a) B. Sahoo, M. N. Hopkinson, F. Glorius, J. Am. Chem. Soc. 2013, 135,
5505-5508; b) A. Tlahuext-Aca, M. N. Hopkinson, R. A. Garza-Sanchez, F.
Glorius, Chem. Eur. J. 2016, 22, 5909-5913; c) R. Cai, M. Lu, E. Y. Aguilera,
Y. Xi, N. G. Akhmedov, J. L. Petersen, H. Chen, X. Shi, Angew. Chem. Int.
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Acknowledgements
We are grateful to NSF (CHE-1665122 and CHE-1915878) and
NIH (1R01GM120240-01) for financial supports.
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Keywords: gold redox catalysis • deselenation • mild oxidant •
alkyne • allene
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This article is protected by copyright. All rights reserved.

10.1002/chem.202000166
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Jin Wang‡, Chiyu Wei‡, Xuming Li,
Pengyi Zhao, Chuan Shan, Lukasz
Wojtas, Hao Chen and Xiaodong Shi*
SePh
2-5% IPrAuNTf2
DCE, rt or 60 o
R‘
SeR
R’’
PhSe
H
+
R‘
R’’
RSe-SeR
CO2Me
Au(III)
C
RSe
IPr
SePh
Page No. – Page No.
33 examples, up to 98% yield, excellent E/Z selectivity
Broad alkyne scope
Compatible with allenes
Intermediate confirmed by HRMS
Au (I/III) redox cycle
Gold Redox Catalysis with a
Selenium Cation as Mild Oxidant
Gold-catalyzed alkyne and allene diselenations were developed.
Excellent
regioselectivity (trans) and good to excellent yields were achieved (up to 98% with
2% catalyst loading) with a wide range of substrates. Mechanistic investigation
revealed the formation of a vinyl gold(I) intermediate followed by an intermolecular
selenium cation migration, suggesting that a gold (I/III) redox process was
successfully implemented under mild conditions.
This article is protected by copyright. All rights reserved.