Gold-Catalyzed Oxidation of Thioalkynes to Form Phenylthio Ketene Derivatives via a Noncarbene Route

Article abstract of DOI:10.1021/acs.orglett.9b01768

Gold-catalyzed oxidations of thioalkynes with 8-methylquinoline oxides afford 2-phenylthioketenes that can be trapped efficiently with alcohols. The synthetic utility is manifested by terminal and internal thioalkynes over a wide scope, bearing esters, ke

Full text of DOI:10.1021/acs.orglett.9b01768

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Gold-Catalyzed Oxidation of Thioalkynes To Form Phenylthio
Ketene Derivatives via a Noncarbene Route
Pankaj Sharma,†,∇ Rahulkumar Rajmani Singh,†,∇ Sovan Sundar Giri,† Liang-Yu Chen,‡
Mu-Jeng Cheng,*,‡ and Rai-Shung Liu*,†
†Frontier Research Center on Fundamental and Applied Sciences of Matters, Department of Chemistry, National Tsing-Hua
University, Hsinchu, Taiwan, Republic of China
‡Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan, Republic of China
S
* Supporting Information
ABSTRACT: Gold-catalyzed oxidations of thioalkynes with 8-methylquinoline
oxides afford 2-phenylthioketenes that can be trapped efficiently with alcohols.
The synthetic utility is manifested by terminal and internal thioalkynes over a
wide scope, bearing esters, ketones, alkyl, and oxime substituents. Our density
functional theory calculations suggest that gold-catalyzed oxidations of terminal
and internal thioalkynes with 8-methylquinoline oxides generate gold-bound
ketene intermediates without the intermediacy of α-oxo gold carbene.
he Wolff rearrangement refers to the transformation of α-
diazocarbonyl species into ketene intermediates via a 1,2-
the sulfur atom can enhance the electrophilicity of gold-π-
alkynes toward nucleophiles.9 We report the development of
this new ketene synthesis on thioalkynes; astonishingly, this
alkyne oxidation generates ketene intermediates without prior
formation of α-oxo gold carbenes for both terminal and
internal alkynes. A weak S···Au interaction in intermediate IV
facilitates this 1,2-sulfur shift. A notable feature of this catalysis
is the wide scope of thioalkynes bearing hydrogen, alkyl, esters,
ketones, and oxime ethers. In contrast, the use of α-diazo thiol
esters6c for the Wolff rearrangement is severely limited to the
unsubstituted type (R = H), as noted in eq 5. We selected
thioalkynes as a target because α-diazo thiol ester (I) can
generate a ketene species (II) efficiently with thermolysis or by
Rh(II) catalysis,6c as depicted in eq 5; however, these diazo
species could not be prepared for an alkyl- or aryl-substituted
form (R = alkyl or aryl).
T
group migration of α-oxo carbenes I (X = alkyl, aryl, and
heteroatom); the reactions were first reported by Wolff one
century ago.1 The synthetic utility of this rearrangement is
commonly manifested by trapping these ketene species with
nucleophiles (Nu-H) to give ketene addition products (eq 1).
The high reliability of this rearrangement is typically achieved
by thermolysis,2a,b photolysis2c−e or Ag(I)-catalyzed3 diazo
decompositions (eq 1). In the presence of Au(I),4a−d
Cu(I),4e−g Pd(II),5a Rh(II),5b−f and Fe(II)5 catalysts, these
g
α-diazocarbonyl precursors typically yield carbene insertion
products (eq 2). Notably, Rh(II) catalysts can implement the
Wolff rearrangement of α-diazo esters of special types.6
One recent advance in gold catalysis is the generation of α-
oxo gold carbenes (III) from the catalytic oxidations of alkynes
with pyridine-based oxides;7a−c the significance of this gold
catalysis is to employ readily available alkynes as α-diazo
carbonyl surrogates. These gold carbenes are generally trapped
with nucleophiles (Nu-H) to yield carbene Nu-H insertion
products exclusively (eq 3).7d−k Alkynes are inexpensive and
readily available, compared to α-diazo carbonyl reagents. We
sought to use alkyne8 as ketene precursors via gold-catalyzed
oxidations of alkynes. In contrast with ynamides, the utility of
thioalkynes have been little explored in gold catalysis, although
Initial tests of terminal thioalkyne 1a with LAuCl/AgNTf2
[L= P(t-Bu)2(o-biphenyl)] with 8-methylquinoline oxide 2a in
Received: May 20, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01768
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Letter
1:2.5 and 1:1.2 proportions in DCE/MeOH (10/1, 25 °C)
yielded methyl 2-(phenylthio)acetate 3a in 65% and 84%
yields, respectively (see Table 1, entries 1 and 2). Other
Table 3. Oxidative Rearrangement with Various Thioalkynyl
Esters
Table 1. Oxidative Rearrangement of Thioalkyne 1a with
Gold Catalysts
b
Yield (%)
entry
catalyst
x
time (h)
3a
1a
1
2
3
4
5
6
7
8
9
LAuCl/AgNTf2
LAuCl/AgNTf2
IPrAuCl/AgNTf2
(PhO)3PAuCl/AgNTf2
PPh3AuCl/AgNTf2
LAuCl/AgOTf
LAuCl/AgSbF6
AgNTf2
2.5
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
8
8
20
13
8
8
8
24
24
65
84
32
65
84
85
84
14
−
−
−
55
−
−
−
−
66
22
a
b
[4] = 0.13 M. Product yields are reported after isolation from a
silica column.
alternations of 4-phenyl substituents of thioalkynes 4a−4e (X
= H, Me, OMe, Cl, and F) to afford the desired 2-
phenylthiomalonates 5a−5e in 82−88% yields (Table 3,
entries 1−5). For 3-methoxyphenyl, 1- or 2-naphthyl
thioalkynes (4f−4h), their resulting compounds (5f−5h)
were produced in satisfactory yields (84%−88%; see Table 3,
entries 6−8). This rearrangement is compatible with various
alcohols Nu-H = i-PrOH and BnOH, yielding the rearrange-
ment products 5i and 5j in yields of 85%−88% (Table 3,
entries 9 and 10).
This oxidative rearrangement is compatible also with
thioalkynyl ketones 6 to form 2-phenylthio α-ketoesters 7/7′
in a keto/enol mixture efficiently; herein, enol forms 7 are the
dominant species (seeTable 4). Entries 1−3 in Table 4 show
HOTf
a
b
[1a] = 0.18 M. Product yields are reported after purification from a
silica column, L = P(t-Bu)2(o-biphenyl), IPr = 1,3-bis-
(diisopropylphenyl)-imidazol-2-ylidene.
catalysts including L′AuCl/AgNTf2 (L′ = IPr, P(OPh)3, PPh3)
gave compound 3a in yields of 32%−84% with PPh3AuCl
being the most efficient (Table 1, entries 3−5). A variation of
the silver salts10 such as LAuCl/AgX (X = OTf and SbF6)
maintained the high efficiency, affording compound 3a in
yields of 84%−85% (Table 1, entries 6 and 7). In contrast,
AgNTf2 was an inefficient catalyst to deliver compound 3a in
only 14% yield over a 24 h period (Table 1, entry 8). The
spectral data of compound 3a are identical to those reported in
the literature.11
Table 4. Oxidative Rearrangement with Various Thioalkynyl
Ketones
We assessed the scope of this oxidative rearrangement using
various terminal thioalkynes and alcohols ROH (R = alkyl and
phenyl; see Table 2). In the oxidations of thioalkyne 1a with 8-
Table 2. Oxidative Rearrangement with Various Terminal
Thioalkynes
a
b
[6] = 0.14 M. Product yields are reported after isolation from a
silica gel column.
a
b
[1] = 0.18 M. Product yields are reported after purification from a
the variations of ketones (R = Me, i-Pr, and n-butyl) of
thioalkynes 6a−6c, affording the desired products 7a/7a′−7c/
7c′ in yields of 71%−83% (Table 4, entries 1−3). Thioalkynes
bearing various 4-phenylthio moieties 6d−6g (X = Cl, F, Me,
and OMe) were also compatible with this new rearrangement
to deliver products 7d/7d′−7g/7g′ in 87%−92% yields (Table
4, entries 4−7). For 3-methoxyphenyl thioalkyne (6h), its
gold-catalyzed reaction led to the desired product 7h/7h′ in
91% yield (Table 4, entry 8). We also tested the reactions on
1- and 2-naphthyl thioalkyne analogues 6i−6j, further yielding
β-keto esters 7i/7i′ and 7j/7j′ in yields of 92%−94% (Table 4,
entries 9 and 10, respectively). We also achieved new oxidative
rearrangement of thioalkynyl oxime ethers 8 to yield 2-
(phenylthio)-2H-azirine-2-carboxylates 9, of which the pro-
silica gel column.
methylquinoline oxide 2a with various ROH (R = Et, i-Pr, Ph
and CH2Ph), rearrangement products 3b-3e were obtained in
yields of 79%−90% (Table 2, entries 1−4). For various 4-
phenyl-substituted thioalkynes 1b−1e (X = Me, OMe, F, and
Cl), their oxidative rearrangement with methanol yielded the
desired products 3f−3i in yields of 83%−94% (Table 2, entries
5−8). The scope of these new catalytic oxidations is
significantly expanded with its applicability to thioalkynyl
esters 4 (Table 3). This new substrate scope is synthetically
significant because α-diazo thiol esters can only be prepared in
unsubstituted form (eq 5). Entries 1−5 in Table 3 show the
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DOI: 10.1021/acs.orglett.9b01768
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posed structures were inferred by X-ray diffraction of one
representative 9h (Table 5, entry 8).
mechanism. For terminal thioalkyne 1a, the formation of
gold-containing enol ether lnt-3 from gold π-thioalkyne (Int-
2) has ΔG⧧ = 16.9 kcal/mol and ΔG = 7.5 kcal/mol (Figure
1). Notably, we are unable to find a feasible route to yield α-
Table 5. Catalytic Oxidative Cyclization via Wolff
Rearrangement
a
b
[8] = 0.10 M. Product yields are reported after isolation from a
silica gel column.
Figure 1. Gibbs free-energy surface for the conversion of the gold π-
thioalkyne intermediate to gold-bound ketenes. The structure of TS-
34 is shown below the energy profile along with the Newman project
for the CC bond. Values are given in units of kcal/mol.
In entry 3 in Table 5, we obtained byproduct 9′′c, arising
from a decarboxylation of β-oxime acid 9′. Entries 1−4 in
Table 5 show the compatibility of this new arrangement with
alkynyl oxime ethers 8a−8d bearing various 4-phenylthio
moieties, yielding 2H-azirine derivatives 9a−9d in yields of
62%−68% (Table 5, entries 1−4); compound 9d was also
characterized by 2D HMBC NMR spectra, indicating a linkage
of n-butyl to the iminoyl carbon. This oxidative rearrangement
is operable also with 1- and 2-naphthylthio derivatives 8e and
8f, further yielding 2H-azirine products 9e and 9f in yields of
69%−71% (Table 5, entries 5 and 6). For 3-methoxyphenylth-
io alkyne 8g, its resulting 2H-azirine 9g was obtained in 67%
yield (Table 5, entry 7). We varied the oxime ethers with R =
Ph and i-Pr as in species 8h and 8i that afforded the desired
2H-azirine derivatives 9h and 9i in yields of 61%−65% (Table
5, entries 8 and 9).
oxo gold carbenes (lnt-1); removing 8-methyl quinoline leads
directly to the formation of gold-bound ketenes lnt-4 with
ΔG⧧ = 4.8 kcal/mol and ΔG = −66.7 kcal/mol. This is
probably due to the strong affinity of PhS toward 1,2-
migration, as evidenced by the very negative reaction free
energy. Note that, in TS-34, the four atoms bound to CC
are not coplanar; therefore, the lone pair of S is able to interact
with the unoccupied CC π*-orbital, causing the migration of
−S-Ph to form the ketene intermediate. In the case of n-butyl-
substituted thioalkyne, the formation of gold-containing enol
ether lnt-3′ from lnt-2′ has ΔG⧧ = 22.8 kcal/mol and ΔG =
14.7 kcal/mol (Figure 2). Dissociation of 8-methyl quinoline
To our pleasure, this oxidative rearrangement was applicable
even to alkyl-substituted thioalkynes 10a−10c (R = n-butyl,
cyclopropyl, and isopropyl), yielding the Wolff rearrangement
product 11a−11c in a major proportion (58%−65%), whereas
an unsaturated thioester (11a′) was isolated in only 10% yield
for R = n-butyl (see eqs 6 and 7). Notably, our DFT
calculations suggest that compounds 11a and 11a′ are
produced from the same gold enol ether intermediates Int-3′
other than α-oxo gold carbenes (see Figure 2, vide infra).
Treatment of α-diazo thiol ester 12a with a gold catalyst gave a
Wolff rearrangement product 3a very efficiently (eq 8). We
failed to prepare n-butyl-derived α-diazo thiol ester 12b, which
remains unknown in the literature6c (eq 8).
Figure 2. Gibbs free-energy surface for the conversion of the gold-
bounded π-thioalkyne n-butyl substituted to the corresponding gold-
bound ketene intermediate. Values are given in units of kcal/mol.
results in the formation of gold-bound ketene (Int-4′) with
ΔG⧧ = 3.1 kcal/mol and ΔG = −72.4 kcal/mol. Thus, as
compared to the terminal thioalkyne case, we find that the
replacement of H by n-butyl makes C−O coupling less
favorable but 8-methyl quinoline dissociation more favorable.
Interestingly, besides the ketene formation pathway, we find an
additional route, in which 8-methyl quinoline dissociation from
Int-3′ is accompanied by n-butyl α- proton migration to a base.
Density functional theory (DFT) calculations were
performed to provide more insights into the reaction
C
DOI: 10.1021/acs.orglett.9b01768
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
When another 8-methyl quinoline is used as the base, ΔG⧧ and
ΔG were calculated to be 18.7 and −83.2 kcal/mol (Int-3′ →
TS-35′ → Int-5′), respectively.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: rsliu@mx.nthu.edu.tw (R.-S. Liu).
*E-mail: mjcheng@mail.ncku.edu.tw (M.-J. Cheng).
ORCID
The higher ΔG⧧ value than that for Int-3′ → TS-34′ → Int-
4′, which is due to the entropic cost involved in bringing two
molecules together, suggests that this pathway only plays a
minor role.
Mu-Jeng Cheng: 0000-0002-8121-0485
Rai-Shung Liu: 0000-0002-2011-8124
Author Contributions
We postulate a mechanism12 depicted in Scheme 1 to
rationalize the Wolff rearrangement products. A notable
∇These authors contributed equally.
Notes
Scheme 1. Plausible Mechanism
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Education (No. MOE
106N506CE1) and the Ministry of Science and Technology
(No. MOST 107-3017-F-007-002), Taiwan, for financial
support of this work.
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* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01768.
Experimental procedures, characterization data, crystal-
lography data, and 1H NMR and 13C NMR for
representative compounds (PDF)
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Accession Codes
CCDC 1888301 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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DOI: 10.1021/acs.orglett.9b01768
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