α-Benzoyloxylation of β-keto sulfides at ambient temperature

Article abstract of DOI:10.1039/c7ra10888e

A facile and efficient protocol for the α-benzoyloxylation of β-keto sulfides is presented. This methodology provides a step-economical, mild and practical access to highly functionalized α-O-benzoyloxy substituted β-keto sulfides, including those with quaternary carbons, which are not easily obtained through currently available methods.

Full text of DOI:10.1039/c7ra10888e

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a-Benzoyloxylation of b-keto sulfides at ambient
temperature†
Cite this: RSC Adv., 2017, 7, 49215
a
Enrico Piras,a Francesco Secci,a Pierluigi Caboni,b Maria Francesca Casula
a
*
and Angelo Frongia
Received 2nd October 2017
Accepted 14th October 2017
A facile and efficient protocol for the a-benzoyloxylation of b-keto sulfides is presented. This methodology
provides a step-economical, mild and practical access to highly functionalized a-O-benzoyloxy substituted
b-keto sulfides, including those with quaternary carbons, which are not easily obtained through currently
available methods.
DOI: 10.1039/c7ra10888e
rsc.li/rsc-advances
yield and/or with low selectivity in combination with signicant
amounts of the corresponding sulfoxides.
Introduction
In this work we address a general protocol for the synthesis
of a-O-benzoyl substituted b-keto suldes by the reaction
strategy shown in Scheme 1. Based on literature, we speculated
that the nucleophilic attack of the sulfur atom of a b-keto sulde
1 on the O–O bond of benzoyl peroxide 2 would form an inter-
mediate sulfonium salt A bearing fairly acidic hydrogens, due to
the inductive effect of the positive sulfur and the presence of the
carbonyl function. We hypothesize that intermediate A would
be deprotonated at ambient temperature by the benzoate anion
(Scheme 1, path a), previously formed during the course of the
reaction, leading to a sulfur-stabilized carbonium ion B poised
Sulfur-containing compounds are key structural cores of
bioactive products, medicinally important compounds, and
organic materials and thus, of great chemical relevance.1 In
particular, a-acyloxy suldes and their analogues are frequently
used in various synthetic applications: as valuable precursors in
the preparation of complex synthetic intermediates2 and
bioactive natural products.3 Well-established strategies to
prepare these compounds include: (i) Pummerer-type rear-
rangement of alkyl sulfoxides;4 (ii) nucleophilic reaction of a-
substituted alkyl suldes;5 (iii) anti-Markovnikov addition of
vinyl esters with arylthiols;6 (iv) addition of aryl disuldes with
(acyloxymethyl)magnesium chlorides;7 (v) anodic oxidation of
suldes.8 Recent advances, by Shi et al. report that alkyl suldes
can be converted to a-acyloxy suldes in the presence of
hypervalent iodine reagents and tetra-n-butylammonium
bromide.9 Alternatively, Yuan developed an elegant gold-
catalyzed iodine(III)-mediated direct oxidative acyloxylation of
C(sp3)–H bonds of methyl suldes.10 It is interesting to point out
that it is known that benzoyl peroxide could undergo decom-
position in the presence of organic suldes11 to form several
products including: sulfoxides, sulfenic acid, olens, disuldes
and a-benzoyloxy substitution adducts. Surprisingly, despite
some scattered procedures,12 typically requiring high reaction
temperatures, the relevance of this latter transformation in
organic synthesis has been highly overlooked so far. It is likely
that this transformation is scarcely described from the point of
view of preparation, due to the fact that a-benzoyloxy
substituted products are generally obtained in poor chemical
a
`
Dipartimento di Scienze Chimiche e Geologiche, Universita degli Studi di Cagliari,
Complesso Universitario di Monserrato, S.S. 554, Bivio per Sestu, I-09042,
Monserrato, Cagliari, Italy. E-mail: afrongia@unica.it
b
`
Dipartimento di Scienze della Vita e dell’Ambiente, Universita degli Studi di Cagliari,
Via Ospedale 72, 09124, Cagliari, Italy
† Electronic supplementary information (ESI) available: Detailed synthetic
procedures and characterization of new compounds. See DOI: 10.1039/c7ra10888e
Scheme 1 Uncatalyzed synthesis of a-O-benzoyl substituted b-keto
sulfides 3.
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Table 1 Optimization of the reaction conditionsa
Entry
Solvent
3ab (%)
4ab (%)
1
2
3
4
CH2Cl2
1,4-Dioxane
Toluene
CH3COOEt
CH3CN
EtOH
n-Hexane
DMF
THF
78
79
82
76
71
46
67
74
12
12
8
Scheme
2 a-O-Acyl-and benzoyl substituted b-keto sulfides in
organic synthesis.
13
21
13
23
14
12
13
11
9
5
6c
7
to undergo a Pummerer-like rearrangement via intermediate C.
Considering that the alternative path b and c (in the case of
disubstituted b-keto suldes), leading to sulfoxide 4 and a-sul-
fanyl enone 6 respectively, should be less-favored as a result of
both the enhanced a-proton acidity of A, as well as the increased
electrophilicity of intermediate C, provided by the carbonyl
group, the preferential formation of 3 with high level of selec-
tivity can be reasonably expected. If successful, the proposed
method would provide a general, straightforward, and mild
synthetic route to O-benzoyloxy b-keto suldes, avoiding the use
of catalysts and saving energy as well as allowing trans-
formation to be carried out at ambient temperature in response
to the principle of sustainable chemistry.13
a-Acyloxy b-keto suldes are important molecules due to
their applications in different synthetic elds. Investigations
into the reactivity of monosubstituted a-acyloxy b-keto suldes
have established them as valuable intermediates for the
synthesis of a-acyloxy-and a-amino-thioester derivatives
(Scheme 2; gure a) which can be easily transformed into the
corresponding sulfur-free biologically active products.14 More-
over, disubstituted a-acyloxy b-keto suldes (Scheme 2; gure b)
play a key role as starting materials for the generation of mono-
enolized diketones,15 1,2-diketones,16 dicarboxylic acids17 as
well as carbocyclic a-hydroxy carboxylic acids.18 Surprisingly,
synthetic strategies for the construction of these latter
compounds are scarce and exclusively rely on the use of the
Pummerer rearrangement19 and the acetoxylation reaction of
phenyl thio ketones with lead tetraacetate.20
8
9
10
11
12d
75
45
DMSO
Acetone
Toluene
79
87 (78)e
a
Reaction conditions: 1a (333 mmol), benzoyl peroxide 2 (366 mmol),
b
c
solvent (0.5 mL). GC-MS yield. 23% of 1-ethoxy-1-(phenylthio)
propan-2-one was also observed (the formation of this product during
d
the reaction is consistent with our mechanistic proposal). 0.1 mL of
solvent was used. e Isolated yield.
mechanism, selected control experiments were carried out
(Scheme 3). In particular, the reaction was unaffected by the
presence of radical scavenger such as butylated hydroxy-toluene
(BHT). This result indicate that, as expected, the reaction likely
proceeds through a nonradical pathway as illustrated in previ-
ously reported studies.11 Furthermore, sulfoxide 7 failed to
afford the corresponding O-benzoyloxylation products suggest-
ing the requirement of a nucleophilic sulfur atom other than
sufficiently acidic protons in alpha position for the success of
this transformation at room temperature.
To further improve the control over the reaction and the
conversion, the effect of solvent was investigated. All solvents
tested in this study can provide good conversions except EtOH
(46%) and DMSO (45%). The best result was obtained with
toluene. The expected adduct 3a can be obtained in 82%
Results and discussion
As a proof of concept for our study, the reaction of phenythio
acetone 1a and benzoyl peroxide 2 was carried out in CH2Cl2 at
room temperature and monitored by GC-MS (Table 1, entry 1).
The investigated reaction took place extraordinarily
smoothly without any kind of catalysts to afford 3a in good
conversion (78%) along with only a small amount of the ex-
pected by-product sulfoxide 4a (12%, via path b, see: Scheme 1),
benzoic acid 6 and benzoic anhydride 5. These preliminary data
indicated that the reaction is spontaneous, efficient and rela-
tively rapid at room temperature. To conrm the reaction Scheme 3 Control experiments.a aGC-MS yield.
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conversion (Table 1, entry 3). Finally, in more concentrated was very well tolerated in the case of ortho-; and meta-
solution the conversion of 3a could be further increased to 87% substituted substrates such as 1i and 1h that gave the corre-
(78% isolated yield, entry 12). With the optimal conditions for sponding O-benzoyl substituted compounds 3i (81%; 3 : 4 ratio
the a-benzoylation of phenylthioacetone in hand, the scope of 86 : 14) and 3h (85%; 3 : 4 ratio 87 : 13) in high chemical yield.
the transformation was evaluated with respect to the presence
Furthermore, naphthylthio ketone 1j also reacted well,
of ring substituents on the phenylthio group (Scheme 4). affording the corresponding product 3j in 84% yield (3 : 4 ratio
Various O-benzoyl substituted substrates bearing electron- 96 : 4). Under the optimized conditions, phenyl selenide ketone
withdrawing (1f, 1g); electron-donating groups (1b–c) and 1k was well tolerated and the product 3k was obtained in 88%
alogens (1d, 1e) on the phenyl ring were synthesized using this yield and excellent selectivity (3 : 4 ratio > 99 : 1). Linear (1l) and
methodology with high efficiency (69–78% yields; 3 : 4 ratio branched (1m) 3-alkyl-substituted b-keto suldes, despite the
84 : 16–91 : 9). Moreover, only a slightly decreased chemical relatively greater steric hindrance, were both effective in the
yield (62%; 3 : 4 ratio 91 : 9) was observed in the case of para- reaction leading to the desired products in good to high yields
nitro-substituted b-keto sulde 1f despite the relatively lower (79–82%; 3 : 4 ratio 93 : 7–97 : 3). To our delight, a representa-
nucleophilicity of the sulfur atom. Notably, the steric hindrance tive aromatic a-acyloxy b-keto sulde 1n, subjected to the same
reaction conditions, was smoothly converted into the corre-
sponding product with a good chemical yield (76%; 3 : 4 ratio
82 : 18). On the other hand, 2-(phenylthio)acetonitrile 1p and
methyl 2-(phenylthio)acetate 1o also reacted with benzoyl
peroxide to yield O-benzoyl substituted derivatives 3p (78%;
3 : 4 ratio 93 : 7) and 3o (68%; 3 : 4 ratio 85 : 15) respectively,
albeit much more slowly than the corresponding ketone deriv-
atives, requiring longer reaction times in order to obtain good
yields. This seemed to be due to the higher pKa value of the
hydrogens adjacent to the ester and cyano group with respect to
the corresponding ketone derivatives which disfavor the
formation of the required intermediate B (see: Scheme 1). Next,
encouraged by the above-mentioned positive results, the
applicability of the reaction protocol to other substituted
benzoyl peroxides was then preliminary investigated.
As a result, a decreased yield of 3t (68%), and the forma-
tion of a signicant amount of sulfoxide 4 (3 : 4 ratio 74 : 26)
was observed in the case of benzoyl peroxides with a strong
electron-withdrawing substituent such as –NO2 on the
aromatic ring compared to those with electron-donating
group such as 3q (77% yield; 3 : 4 ratio 90 : 10), 3r (77%;
3 : 4 ratio 88 : 12) and alogens for example 3s (81%; 3 : 4 ratio
84 : 16). This behavior may be related to the signicantly
more electrophilic character of the carbonyl of the benzoyl
group (provided by the strong electron-withdrawing group) in
intermediate sulfonium A which increases the reaction rate
of path b (Scheme 1). b-Keto suldes 1u and 1v, bearing an
aliphatic group on sulfur performed very well, and delivered
regioselectively the corresponding a-benzoyloxy b-keto
sulde 3u (80%; 3 : 4 ratio 94 : 6) and 3v (66%; 3 : 4 ratio
91 : 9) in good to high yields. Similarly, 1w also underwent
efficient reaction (75% yield; 3 : 4 ratio 97 : 3) to afford only
the mono benzoyloxylation product 3w.
Finally, the substrate scope was extended to include the
synthesis of some challenging a-branched O-benzoyloxy b-
keto suldes. As summarized in Scheme 5, gratifyingly
acyclic disubstituted b-keto suldes 1aa–1ad performed well
and delivered the corresponding products 3aa–3ad in
moderate to good yields (63–77%; 3 : 4 ratio 80 : 20–94 : 6).
The reaction proved to be rather general; in addition to
Scheme 4 Substrate scope.a,b,c aReaction conditions: 1 (333 mmol),
acyclic b-keto suldes, cyclic b-keto suldes 1ae–1ai also
could be used giving the corresponding quaternary adducts21
in moderate to high yields (63–80%; 3 : 4 ratio 73 : 27–96 : 4).
benzoyl peroxide (366 mmol), toluene (0.1 mL). bIsolated yield. c3 : 4
ratio determined by GC-MS. d3 : 4 ratio determined by 1H NMR. eThe
reaction was carried out for 5 days.
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Acknowledgements
Financial support from the MIUR, by the University of Cagliari
(FIR 2016-FIR 2017) and from RAS and Fondazione Banco di
Sardegna is acknowledged. Sardinia Scientic Park Polaris is
thanked for free access to IR spectrophotometer. We thank Dr
Danilo Loche for aid in analyses and for useful discussions.
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In this work a practical and versatile a-benzoyloxylation of b-
keto suldes under mild conditions is demonstrated.
Through this procedure, a series of a-benzoyloxy-b-keto
suldes were obtained in moderate to high yields. Besides, a-
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compatible with the current protocol and provide the corre-
sponding O-benzoyloxy b-keto suldes in moderate to high
yields. The wide functional group tolerance highlights the
potential of this transformation as an effective method for
the operationally simple installation of a benzoyloxy group in
the a-position of a sulfur atom in the presence of a carbonyl
group. We prospect that the impact of our ndings will be
beyond the research on sustainable organic synthetic
approaches, due to the role of sulfur-bearing compounds in
bioactive and medicinally relevant compounds.
Conflicts of interest
There are no conicts to declare.
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