Coupling of Sulfoxonium Ylides with Arynes: A Direct Synthesis of Pro-Chiral Aryl Ketosulfoxonium Ylides and Its Application in the Preparation of α-Aryl Ketones

Article abstract of DOI:10.1021/acs.orglett.8b03126

A general, mild, and versatile synthesis of the challenging α-aryl-β-ketosulfoxonium ylides has been developed for the first time, substituting traditional methods starting from diazo compounds. The arylation of easily accessible β-ketosulfoxonium ylides using aryne chemistry allowed the preparation of a large scope of the pro-chiral ylides in very good yields (40 examples; up to 85%). As applications, these ylides were smoothly converted into α-aryl ketones after desulfurization in good yields (up to 98%) as well as in other important derivatives.

Full text of DOI:10.1021/acs.orglett.8b03126

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Coupling of Sulfoxonium Ylides with Arynes: A Direct Synthesis of
Prochiral Aryl Ketosulfoxonium Ylides and Its Application in the
Preparation of α‑Aryl Ketones
es Martins, and Antonio C. B. Burtoloso*,a
́
Alexander Garay Talero, Bruna Simo
̃
Instituto de Química de Sao Carlos, Universidade de Sao Paulo, CEP 13560-970 Sao Carlos, SP, Brazil
̃ ̃ ̃
S
* Supporting Information
ABSTRACT: A general, mild, and versatile synthesis of the challenging α-aryl-β-ketosulfoxonium ylides has been developed for
the first time, substituting traditional methods starting from diazo compounds. The arylation of easily accessible β-
ketosulfoxonium ylides using aryne chemistry allowed the preparation of a large scope of the pro-chiral ylides in very good
yields (40 examples; up to 85%). As applications, these ylides were smoothly converted into α-aryl ketones after desulfurization
in good yields (up to 98%) as well as in other important derivatives.
ulfonium and sulfoxonium ylides1,2 have played significant
Sroles in organic synthesis as important building blocks in
total synthesis,3−9 chemical materials,10−12 and pharmaceut-
icals,13−15 among others.16,17 These ylides have also proven to
be alternative metallocarbene surrogates to diazo com-
pounds,18 having many reactions in common such as
insertion,19−21 cyclopropanation,21 epoxidation,22 aziridina-
tion,23 dimerizations,24 and Wolff24,25 and Stevens rearrange-
ments.26,27 Furthermore, when compared with diazo com-
pounds, these ylides are generally crystalline solids, more
stable, have already been used in industrial scales,15 and are
easier/safer to prepare (since they do not involve the use of
potentially explosive compounds such as azides, diazomethane,
and its derivatives).28
Over many years, sulfoxonium ylides have received less
attention compared to sulfonium ylides. However, in the past
few years, this class of compounds has resurfaced as a powerful
Figure 1. Sulfur ylides as diazo compound surrogates in different
and versatile synthetic tool in the arsenal of organic chemists.
Two examples are the seminal work of Baldwin20 in metal-
catalyzed N−H insertion reactions and from Mangion19,29 at
Merck, where β-ketosulfoxonium ylides were applied as diazo
carbonyl surrogates. Since these contributions, this type of
ylide has been employed in several important and new
transformations that explore its ambiphilic behavior. For-
mation of C−C, C−N, C−O, and C−halogen bonds, synthesis
of naphthols,30 indoles,13 and pyrroles,13 cross-coupling
reactions,31,32 and synthesis of difunctionalized haloketones,33
among others,34−43 are some of the very recent examples
(Figure 1).
reactions with limited substitution patterns.
Importantly, many of these reactions are fruitless when the
equivalent sulfonium ylide or diazo carbonyl are employed in
place of sulfoxonium ylides, showing that these three classes of
compounds (though similar) can also have distinct behavior.
Regardless of the appearance of these new methodologies, the
limitation of the substitution pattern in the ylide center (only
Received: October 1, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03126
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
H in C1) (Figure 1) is still a drawback. For example, it reduces
the product scope of the method and does not permit
asymmetric versions in many cases.31,32 These limitations are
directly associated with the absence of specific methodologies
that offer substitution in the α-position of ylides (direct
synthesis of pro-chiral ylides), and that is why the respective
diazo equivalents have gained far more attention in organic
synthesis.44−47 To the best of our knowledge, prochiral
ketosulfoxonium ylides, containing either alkyl or aryl groups
in the α-position, can be better synthesized with the
intermediacy of diazo compounds (sulfoxide insertions
catalyzed by metals, light or heat)48,49 (Scheme 1, eq 1).
This is in some way a paradox since the idea to employ sulfur
ylides is to avoid the use of diazo compounds in similar
transformations.19,29
Herein, we describe the direct synthesis of α-aryl-β-
ketosulfoxonium ylides through the coupling of simple and
easily prepared β-ketosulfoxonium ylides with arynes. The
arylated ylides are also rapidly transformed into α-aryl ketones
after desulfurization as well as in some α-carbonyl derivatives
(Scheme 1, eq 4). Our optimization studies began with the use
of ylide 1a and the benzyne precursor 2 as standard substrates
under various conditions (Table 1). Initially, equimolar
a
Table 1. Reaction Optimization
Scheme 1. Methodologies for the Synthesis of Prochiral β-
Ketosulfoxonium Ylides
entry
solvent
F− source
additives
none
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
18-crown-6
Cs2CO3
Cs2CO3, MS 4 Å
Cs2CO3, MS 4 Å
Cs2CO3, MS 4 Å
Cs2CO3, MS 4 Å
Cs2CO3, MS 4 Å
MS 4 Å
yield (%)
1
2
3
4
5
6
7
8
9
CH3CN
CH3CN
THF
CsF
CsF
CsF
CsF
CsF
CsF
KF
TBAF
CsF
CsF
CsF
CsF
CsF
CsF
55
61
42
39
traces
40
54
32%
63
traces
57
30%
61
DME
b
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
c
d
e
f
10 ,
f g
11 ,
f h
12 ,
f i
13 ,
f j
f j
14 ,
70
a
All reactions were performed using 0.127 mmol of 1a and 0.127
mmol of 2 in 1 mL of solvent, 4 equiv of CsF, and 3 equiv of Cs2CO3
b
and at a temperature of 65 °C, unless otherwise indicated. At 25 °C.
c
d
At 110 °C in microwave reactor. Using 2 equiv of 18-crown-6 and 3
e
f
g
equiv of KF. Using 2 equiv of TBAF. 25 mg of MS 4 Å. Using 1.5
equiv of 2 and addition of 1a over 3 h. Using 1.5 equiv of 2 over 24 h
addition. 0.127 mmol of 3a as starting ylide and 1.5 equiv of 2. This
reaction led to a complex mixture. 1.5 equiv of 2 added in three
h
i
j
Prochiral α-alkylketosulfoxonium ylides could be prepared
via a multistep protocol involving alkylation of thioethers,
followed by oxidation50 (that fails in many substrates,
including the ones with benzyl substituents that could provide
the desired α-aryl-ketosulfoxonium ylides) and acylation
(Scheme 1, eq 2). However, only one example is
demonstrated.31 Another less general method that also leads
to this kind of sulfoxonium ylide is the reaction of the
expensive 2- or 4-chloropyrimidines with dimethylsulfoxonium
methylide to afford, after nucleophilic substitution and
acylation, α-pyrimidinyl-β-ketosulfoxonium ylides51,52
(Scheme 1, eq 3). Unfortunately, this method is restricted
only to pyrimidines as substituents. In view of the comments
above, a method that permits the direct alkylation or arylation
of easily accessible β-ketosulfoxonium ylides is highly desired.
In the case of the challenging arylation of β-ketosulfoxonium
ylides, one of the many possible applications is the synthesis of
α-arylated ketones.51,52 Monoarylation of ketones in a regio-
and chemoselective fashion is still challenging and is limited to
ketone substrates with no competitive enolization or
functionality.53,54 In general, ketones with bulky substituents
at one of the α-carbons are employed to guarantee high
degrees of regioselectivity.55−58
portions with 3 h of reaction.
proportions of starting materials were used under classic
conditions to generate arynes.59,60 This includes acetonitrile
(CH3CN) as the solvent, CsF as the fluoride source, and a
temperature of 65 °C. This first condition was enough to
provide our desired product 3a in 55% yield (Table 1, entry 1).
However, total consumption of ylide 1a was not observed,
which made the purification of compound 3a difficult. Efforts
were then focused to optimize the yield of reaction and the
total consumption of 1a.
Considering that intermediate 3a′ is formed after ylide
attack to the aryne, we thought that the addition of base could
help the 1,3-hydrogen migration step (see the scheme of Table
1). Therefore, when cesium carbonate was used as base, a
slight increase in yield was observed (Table 1, entry 2).
Subsequently, other solvents such as tetrahydrofuran (THF)
and dimethoxyethane (DME) were tested without increasing
the yield (Table 1, entries 3 and 4). Lower and higher reaction
temperatures from the optimum 65 °C provided compound 3a
in traces and 40% yield, respectively (Table 1, entries 5 and 6).
Other sources of fluoride ions were also explored without any
B
DOI: 10.1021/acs.orglett.8b03126
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Letter
improvement in the reaction performance (Table 1, entries 7
and 8) along with difficulty in the purification process. Next,
the use of molecular sieves (4 Å) was evaluated, observing a
slight increase in the yield (Table 1, entry 9). Surprisingly, the
use of excess of 2 (1.5 equiv) furnished only a trace amount of
the product with the slow addition of ylide 1a (Table 1, entry
10). Contrary to the previous entry, the slow addition of 2 to
ylide 1a (Table 1, entry 11) furnished a similar yield when
compared to our best conditions (see entry 9). Results in
Table 1, entries 10 and 11, indicate that the product might
undergo degradation in the presence of excess aryne left in the
reaction medium. To prove it, a control experiment using ylide
3a (our product) was carried out in the presence of 2
(conditions of entry 9), resulting in a complex mixture of
products (Table 1, entry 12). Compound 2 was then added in
three portions, providing 3a in 61% yield with total
consumption of 1a (Table 1, entry 13). As controlled and
slow generation of benzyne is the key to reaction success and
believing that Cs2CO3 also helps in promoting its in situ
generation,61 we next repeated our best conditions in the
absence of the base (aiming a slower benzyne formation). In
this case, the desired product 3a was obtained in 70% yield
(Table 1, entry 14).
We next investigated the scope of the reaction by varying the
aryne precursors (Figure 2).
Symmetrical aryne precursors such as 4 and 11 led to the
formation of a single product (11, as a racemic mixture of
atropisomers 12−14).63 Furthermore, ortho-substituted aryne
precursors 15, 17, 21, and 26 (having methoxy, fused aromatic
ring and Br substituents) led to the formation of single
regioisomers with addition at the meta carbon. This agrees
with predictive models.64−67 On the other hand, aryne
precursor 18 afforded regioisomers in equal proportions,
showing a weak or no effect of the distant methoxy groups as
well as a little effect in terms of the volume of the nucleophile
used. In the case of the N-tosyl-3-azacyclohexene68 33,
pyridinium precursor69 31, and the new type of aryne
precursor70 9, only the latter two provided the desired product
(although in low yield: 32 and 10, with 5% and 26% yield,
respectively). For these cases, the decomposition of the
respective aryne precursors and recovery of the starting ylide
were observed.
It is important to highlight that the reactions described in
this work employing sulfoxonium ylides cannot be performed
efficiently using sulfonium ylides or diazo ketones. For
example, the first results in the formation of thioanisole
derivatives by insertion into the C−S σ-bond.71 The second
gives indazoles by a [3 + 2]-cycloaddition reaction on the
diazo moiety.72
After the optimization studies, we next evaluated the scope
of the reaction using 2 in the presence of different ylides
(Scheme 2). Ylides with various aromatic substituents
As an application of this methodology, some of these ylides
were subjected to a desulfurization reaction (Scheme 3) using
Raney-Ni in refluxing 2-propanol.73,74 This provided the
respective monoarylated ketones (35−45) in good yields (up
to 98%). In general, all of the substrates are well tolerated
except for ylide 3g where an inseparable mixture of desired
product along with dehalogenation product of 35 was obtained
in approximately a 1:1 ratio. This two-step approach for the
synthesis of monoarylated ketones from β-ketosulfoxonium
ylides not only competes with the methodologies already
reported in the literature but also constitutes one of the few
that involves arynes in its synthesis.75,76
Scheme 2. Scope of Ylide Starting Material
To illustrate the synthetic utilities of these new types of ylide
substrates in the α-heterofunctionalization of carbonyl
compounds, we performed some known transformations
(that are carried out with diazo compounds77 as well as with
simpler sulfoxonium ylides)13,19,29,34,78 (Scheme 4). For
example, O−H, S−H, N−H, and Cl−H insertions are some
of the classic transformations that can be made with diazo
compounds and some sulfur ylides and were extrapolated for
ylide 3a. The acyloin, α-keto thio, amino, and chloro
derivatives 46−50 were obtained in good yields, showing the
possible applications in the formation of new stereocenters
with great potential in asymmetric versions for this kind of
substrate.
In conclusion, we have developed the first general and
versatile synthesis of a range of α-aryl-β-ketosulfoxonium
ylides, starting from the readily available β-ketosulfoxonium
ylides that is operationally simple and does not need to pass
through the intermediacy of diazo compounds. Additionally,
the desulfurization of representative arylated ylides successfully
afforded the respective α-aryl ketones. Moreover, we also
demonstrated that these ylides can be readily diversified to
other important carbonyl compounds of choice, using classical
reactions from the literature, and thus exhibiting its great
potential as substitute for diazo compounds not only in
reactions involving metallocarbenes but also in polar reactions.
containing electron-donating and -withdrawing groups are
well tolerated (3a−h). It is worth mentioning that ylide 3h,
obtained in 68% yield, is the same ylide used by Vaitla in the
formal synthesis of atorvastatin (prepared through a rhodium-
catalyzed sulfoxide insertion from a α-diazoketone).13
Although without success, the preparation of ylide 3h was
also investigated from iodonium ylide chemistry, showing the
importance of these types of prochiral arylated sulfoxonium
ylides.62 Alkyls groups, including t-Bu and benzyl substituents,
smoothly led to desired ylides (3i−n) in good yields. Similarly,
amino acid derivatives with different protecting groups were
well tolerated (3o−q) without loss of the respective stereo-
genic center (3o). Derivatives of esters and amides also led to
the formation of the respective ylides 3r−t in good yields.
Synthesis of 3a on a 10-fold scale (1.3 mmol) did not change
the yield (69%).
C
DOI: 10.1021/acs.orglett.8b03126
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Organic Letters
Letter
Figure 2. Aryne scope in β-ketosulfoxonium arylation.
We believe that this methodology will open new paths to the
reactivity and use of arylketosulfoxonium ylides in stereo-
selective functionalization of α-carbonyl compounds.
Scheme 3. Scope of Desulfurization Reaction of α-Aryl-β-
ketosulfoxonium Ylides
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03126.
Experimental procedures, cNMR spectra, and other
analysis data for the compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: antonio@iqsc.usp.br.
ORCID
́
Alexander Garay Talero: 0000-0002-9734-826X
Antonio C. B. Burtoloso: 0000-0003-2203-1556
Scheme 4. Different α-Carbonyl Heterofunctionalizations
for the Ylide 3a
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the FAPESP for financial support (2013/18009-4;
2017/23329-9), the postdoctoral fellowship to B.S.M. (2017/
02092-0), and the HRMS multiuser equipment (2004/09498-
2) and the CNPQ for the fellowship to A.G.T. (140584/2015-
́
3). We also thank the Instituto de Quimica de Sao
Universidade de Sao Paulo (IQSC-USP), for the facilities.
̃
Carlos,
̃
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