Synthesis of allyl sulfones from potassium allyltrifluoroborates

Article abstract of DOI:10.1016/j.tetlet.2017.05.097

Potassium allyltrifluoroborates underwent a bora-ene reaction with sulfur dioxide in the absence of Lewis acid catalysts to give sulfinyloxy-trifluoroborates, which subsequently undergo alkylation with electrophiles to produce sulfones in up to 91% yield. Benzyl halides and haloacetic acid derivatives can be used as the alkylation reagents while the Sanger reagent undergoes a S_NAr reaction with sulfinyloxy-trifluoroborates to produce the corresponding 2,4-dinitrophenylsulfone. The developed method allows the transformation of potassium allyltrifluoroborates into allyl sulfones.

Full text of DOI:10.1016/j.tetlet.2017.05.097

Accepted Manuscript
Synthesis of allyl sulfones from potassium allyltrifluoroborates
Agnese Stikute, Jevgeņija Lugiņina, Māris Turks
PII:
S0040-4039(17)30708-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.05.097
TETL 48990
Reference:
Tetrahedron Letters
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Accepted Date:
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24 May 2017
31 May 2017
Please cite this article as: Stikute, A., Lugiņina, J., Turks, M., Synthesis of allyl sulfones from potassium
allyltrifluoroborates, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.05.097
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Tetrahedron Letters
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Synthesis of allyl sulfones from potassium allyltrifluoroborates
Agnese Stikute, Jevgeņija Lugiņina, Māris Turks*1
Institute of Technology of Organic Chemistry, Faculty of Materials Science and Applied Chemistry, Riga Technical University, Paula Valdena Str. 3, Riga, LV-
1048, Latvia
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Potassium allyltrifluoroborates underwent a bora-ene reaction with sulfur dioxide in the absence
of Lewis acid catalysts to give sulfinyloxy-trifluoroborates, which subsequently undergo
alkylation with electrophiles to produce sulfones in up to 91% yield. Benzyl halides and
haloacetic acid derivatives can be used as the alkylation reagents while the Sanger reagent
undergoes a SNAr reaction with sulfinyloxy-trifluoroborates to produce the corresponding 2,4-
dinitrophenylsulfone. The developed method allows the transformation of potassium
allyltrifluoroborates into allyl sulfones.
Available online
Keywords:
Bora-ene reaction
Sulfur dioxide
Potassium allyltrifluoroborates
Potassium 3-((allylsulfinyl)oxy)trifluoroborate
Sulfones
———
* Corresponding author. Tel.: +371 67089251; fax: +37167615765; e-mail address: maris.turks@rtu.lv
1

2
Tetrahedron Letters
Sulfones are regarded as a very useful class of compounds1
allylplatinum,21 and allylpalladium22 derivatives are known. Also,
less reactive allylsilanes react with SO2 under Lewis acid
catalysis.23 On the other hand, allylic systems possessing intrinsic
Lewis acidity such as allylboronates24 and allylstananes25 react in
the absence of any catalyst. Besides the discussed metallo-ene
reactions,26 H-ene reactions of simple allylic systems with SO2
are also known. These require activation with BCl3 and produce
sulfinic acid–BCl3 complexes.27 These achievements are due to a
renaissance of the use of sulfur dioxide and its solid surrogates as
reagents28 and liquid sulfur dioxide as a solvent29 in organic
synthesis.
that have many important applications in medicinal chemistry,2
agrochemistry,3 and materials science.4 Sulfones also possess
distinct chemical reactivity and as such have been successfully
used as synthetic intermediates. Named reactions including the
Julia-Kocienski olefination5 and Ramberg-Bäcklund reaction,6
the use of vinyl sulfones as Michael acceptors,7 and the
applications of sulfone-stabilized carbanions,8 are representative
illustrations for the synthetic utility of sulfones.
Recent advances in the synthesis of sulfones have been
reviewed.8b,9 A well-accepted method is alkylation of the
corresponding sulfinate salts, which can be obtained by reacting
various organometallic reagents with sulfur dioxide or its solid
surrogates. Contemporary synthetic pathways developed until
2015 and dealing with the insertion of SO2 into carbon-
heteroatom bonds, including reactions of aryl and vinyl halides,
alkoxysilanes, and boronic acids in the presence of transition
metal catalysts, have also been reviewed (Fig. 1.1).9a In 2016,
aryl nonaflates were also reported to yield sulfinates in a
palladium catalyzed reaction with sulfur dioxide.10 Other very
recent achievements regarding the fixation of SO2 into small
organic molecules are sulfone synthesis via copper(I) oxide11 and
cobalt(II) oxide12 catalyzed SO2 insertion into aryl
triethoxysilanes. Also, copper(I)13 and palladium(II)14 catalyzed
processes were developed for the transformation of aryl and
alkenyl boronic acids into sulfinates and subsequently into
sulfones. The fixation of SO2 into cyclic sulfones,
benzo[b]thiophene 1,1-dioxides, was achieved by reacting the
former with 2-alkynylaryldiazonium in the presence of catalytic
CuBr.15 On the other hand, advancing the field of late transition
metal free processes has led to the iron-catalyzed synthesis of
arylsulfinates through a radical coupling reaction16 and photo-
induced fixation of sulfur dioxide into aryl/alkyl halides.17
To the best of our knowledge, the reactions of potassium
allyltrifluoroborates with sulfur dioxide have not been described.
Herein, we report the synthesis of potassium 3-
((allylsulfinyl)oxy)trifluoroborates via the bora-ene reactions
between potassium allyltrifluoroborates and SO2, and their
application in sulfone synthesis.
Commercially available potassium allyltrifluoroborate (1)
readily reacts with sulfur dioxide at -78 °C to give potassium 3-
((allylsulfinyl)oxy)trifluoroborate (2) (Scheme 1). Sulfur dioxide
serves as both the reagent and solvent, and the addition of a
catalyst to promote the reaction is not required. After the removal
of SO2, product 2 is obtained as a white paste in quantitative
1
yield, which was determined by H NMR analysis of the crude
product in the presence of sodium acetate as an internal standard.
Similar results were obtained by adding either D2O saturated with
gaseous SO2 or the 1,4-diazabicyclo[2.2.2]octane bis(sulfur
dioxide) adduct (DABSO) to a solution of starting material 1 and
sodium acetate (internal standard) in D2O. H NMR analysis of
the latter solutions revealed instant and quantitative formation of
1
product 2.
1) insertion of SO2 into C(sp2)-Q bond:
O
Q
S
synthetic applications:
e.g. sulfone, sulfoxide
sulfonamide synthesis
[SO2]
O
transition metal
catalysis
+ 15
Q = halogen,8,9,16,17 O-SO2-R,10 Si(OR)3,11,12 B(OH)2,13,14 N2
.
2) reaction modes of SO2 with allylic systems:
known approaches
this work
MLA = BF3K
MLA
H
M
Scheme 1. Synthesis and stability studies of potassium 3-
((allylsulfinyl)oxy)trifluoroborate (2).
O
S
O
S
O
S
O
LA
O
O
LA
BF2
H-
M = metaloid without
ene
reaction27
Lewis acidic
During the characterization of compound 2 we observed its
variable stability which was dependent on the solvent used. Tests
were carried out using 1H-NMR and 2-bromomesitylene (for
organic solvents) or sodium acetate (for aqueous solutions) as
internal standards. Sulfinyloxy trifluoroborate 2 is reasonably
soluble in water and is stable for at least 12 hours at ambient
temperature. In contrast, it is less soluble in wet DMSO, but still
stable at ambient temperature. However, it is less stable in wet
DMSO at elevated temperatures (80 °C) and degrades to propene
properties: SiR3.23
MLA = metal/metaloid with Lewis acidic
properties: Mn,19 Fe,19,20 B(OR)2,24 SnR3.25
O
synthetic
applications
S
OM(H)
1
(4, observed by H-NMR) and SO2, apparently through a retro-
ene elimination from the intermediate allylsulfinic acid (3).30,31
Finally, it was found that the addition of DMF to the aqueous
solution of 1 does not influence its stability and such solutions
are stable for at least 24 h, even at slightly elevated temperatures
(60 °C).
Figure 1. Examples of SO2 incorporation into organic compounds.
From the above mentioned methods for the synthesis of
sulfinate intermediates,18 one can separate the reactions of allylic
systems with SO2 (Fig. 1.2). Thus, procedures involving
allylmagnesium
halides,
allylmanganese,19
allyliron,19,20

With these results in hand we turned our attention to the
alkylation of in situ generated potassium 3-
bromoacetate (5h), α-bromo acetophenone (5i) and propargyl
bromide (5j), provided the expected sulfones in 62-69% yield
(Entries 7-9). It is interesting to note that both iodomethane and
dimethylsulfate (Entries 10-11) provided the S-methylated
product 6k in moderate yield. On the other hand,
((allylsulfinyl)oxy)trifluoroborate (2). Benzyl bromide (5a) was
chosen as a model electrophile and several experimental
conditions were screened by varying solvent systems, bases and
temperatures (Table 1). It was found that the alkylation
proceeded poorly under non-aqueous conditions (Table 1, entries
1-3), most probably due to the low solubility of intermediate 2.
Use of the biphasic systems H2O/DCM and H2O/MeCN also did
not improve the yields (Entries 4-5). This information regarding
the stability of intermediate 2 and its solubility led to use of the
H2O/DMF (1:1) solvent system (Entries 6-11). Initially the
alkylation was attempted at 20 °C and 40 °C in the absence of
additional base since intermediate 2 formally represents a
sulfinate salt (Entries 6-7). Improved yields for the synthesis of
sulfone 6a was achieved after the examination of various bases as
additives. It was found that KF slightly improved the yield of
sulfone 6a to 32% (Entry 8), while the addition of either NaOAc
or K2CO3 resulted in 64% and 62% yield, respectively (Entries
9,10). The best result (77%) was achieved when NaOH (0.4
equiv.) was used as the basic additive in the presence of
tetrabutylammonium iodide (TBAI, 10 mol%) (Entry 11).
chloroacetonitrile (5m),
chloroacetone (5n) and p-
chlorobenzylchloride (5o) gave the corresponding sulfones in
less than 10% yield. The ethylation of sulfinate 2 with EtI (5p)
and its allylation with allylbromide (5q) also gave sulfones, albeit
with yields below 40%. It is interesting to note that the mixed
anhydride of sulfinic and boric acid, freshly generated from
allylboronic acid pinacol ester and SO2 according to the
previously reported method,24 did not provide reasonable yields
of sulfones when submitted to the aforementioned optimized
alkylation reaction conditions (NaOH or K2CO3, TBAI,
DMF/H2O, RX).
Table 2. Synthesis of sulfones 6b-p from in situ generated potassium 3-
((allylsulfinyl)oxy)trifluoroborate 2.a,b
Table 1. Optimization of the reaction conditions for the synthesis of sulfone
6a using in situ generated potassium 3-((allylsulfinyl)oxy)trifluoroborate 2.a
RX (5b-q)
Base
(0.4
equiv.)
Yield
DMF/
H2O
ratio
Entry
6b-p (%)
X
R
1:1
3:1
6b, 19
6b, 70
1
2
5b: Br
NaOH
NaOH
Temp.
(°C)
20
Time
(h)
28
Product
6a (%)
16b
Entry
Base
Solvent
DMF
1:1
3:1
6c, 52
6c, 77
5c: Br
5d: Br
1
2
3
-
-
13b
25b
4b
1:1
3:1
3:1
NaOH
NaOH
K2CO3
6d, 29
6d, 41
6d, 70
DMF
40
20
20
20
20
40
20
20
20
20
23
22
20
7
3
K2CO3
DMSO
4
5
NaOH
H2O/DCM
H2O/MeCN
H2O/DMF
H2O/DMF
H2O/DMF
H2O/DMF
H2O/DMF
H2O/DMF
4
5
5e: Br
5f: Br
1:1
NaOH
6e, 78
-
-
7b
1:1
3:1
3:1
NaOH
NaOH
K2CO3
6f, 46
6f, 39
6f, 70
6
23
22
27
27
24
25
23b
28b
32d
62d
64d
77d
7
-
8c
9c
10c
11c
KF
6
7
5g: F
1:1
NaOH
6g, 91
K2CO3
NaOAc
NaOH
NaOH
K2CO3
6h, 35
6h, 62
5h: Br
5i: Br
1:1
1:1
a
Reagents and conditions: 1 (0.22 mmol), RX (0.27 mmol, 1.2 equiv.), base
NaOH
K2CO3
6i, 34
6i, 64
(0.4 equiv.), solvent (2.0 mL, 1:1 ratio for entries 4-11), rt, 24 h; 1H-NMR
b
8
9
yield, internal standard: diphenylmethane; c TBAI (10 mol%); d isolated yield.
5j: Br
5k: I
3:1
1:1
K2CO3
NaOH
6j, 69
6k, 37
With the optimized reaction conditions in hand, the scope and
limitations
of
the
reaction
between
3-
10
11
12
13
14
15
16
Me
Me
((allylsulfinyl)oxy)trifluoroborate 2 and various electrophiles was
examined (Table 2). These experiments revealed that the
reaction outcome fluctuated depending on the electrophile used
and a minor adjustment of the base was necessary in order to
obtain a compromise between the reactivity of the sulfinate ion
and the hydrolysis rate of the electrophile. In the cases when
NaOH caused too fast hydrolysis of the corresponding bromides,
K2CO3 was the base of choice. Benzyl bromides with both
electron withdrawing and electron donating substituents provided
the expected products 6b-f in 70-78% isolated yield (Entries 1-5).
The best yield was achieved for sulfone 6g (91%) in the
nucleophilic aromatic substitution reaction with the Sanger
reagent (5g, entry 6). Several other electrophiles, ethyl
5l: MeSO4
5m:Cl
5n: Cl
5o: Cl
5p: I
1:1
1:1
1:1
1:1
1:1
1:1
NaOH
NaOH
NaOH
NaOH
6k, 53
6l, 5
6m, 9
6m, 7
NaOH
K2CO3
6o, 27
6o, 32
Et
5q: Br
NaOH
6p, 37

4
Tetrahedron Letters
a Reagents and conditions: 1 (0.22 mmol), RX (0.27 mmol, 1.2 equiv.), base
(0.4 equiv.), TBAI (10 mol%), solvent (2.0 mL), 20 °C, 24 h; b Isolated yield
In conclusion, we have uncovered a novel reactivity of
potassium allyltrifluoroborates towards sulfur dioxide. This
reaction does not require Lewis acid activation and proceeds with
characteristic regioselectivity for bora-ene reactions. The
obtained potassium 3-((allylsulfinyl)oxy)trifluoroborates can be
S-alkylated and S-arylated under basic conditions and thus
provide a novel entry to the synthesis of sulfones.
over two-steps from starting material 1.
Next, we examined the crotyl and prenyl systems in order to
determine whether formation of the potassium 3-
((allylsulfinyl)oxy)trifluoroborate system follows the bora-ene
pathway. Thus, potassium (Z)-but-2-en-1-yltrifluoroborate 7a32
and potassium trifluoro(3-methylbut-2-en-1-yl)borate 7b32 were
reacted with sulfur dioxide (Fig. 2, Scheme 2). The reaction of 7a
+ SO2 → 8a resulted in quantitative conversion in 30 min, but
required a reaction temperature of –10 °C (bp of SO2) due to the
additional steric hindrance in substrate 7a compared with
compound 1. Introduction of another methyl group in substrate
7b resulted in an extended time (2 h) for the full conversion to
sterically hindered sulfinate 8b at –10 °C. In both cases the
reaction progress was monitored by 1H NMR spectroscopy (Fig.
2). The latter clearly showed that compounds 8a and 8b
originated from reaction of the γ-center of the crotyl and prenyl
systems with the S-center of sulfur dioxide.
Acknowledgments
This work was financed by the Latvian Council of Science
Grant No 12.0291. A.S. thanks the Latvian Academy of Sciences
for scholarships. J.L. thanks L`ORÉAL Latvia with the support
of the Latvian National Commission for UNESCO and the
Latvian Academy of Sciences for “For Women in Science”
scholarship.
O
References and notes
R1
BF3K
7a,b
S
BF3K
SO2
- 10 o
O
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→5.77 ppm
→5.80
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Supplementary Material
Supplementary data associated with this article can be found,
in the online version.

6
Tetrahedron Letters
Synthesis of allyl sulfones from
potassium allyltrifluoroborates
Agnese Stikute, Jevgeņija Lugiņina, Māris Turks*
Institute of Technology of Organic Chemistry, Faculty
of Materials Science and Applied Chemistry, Riga
Technical University, Paula Valdena Str. 3, Riga, LV-
1048, Latvia
HIGHLIGHTS
•
Sulfur dioxide inserts into potassium
allyltrifluoroborates
•
Novel compound class -
((allylsulfinyl)oxy)trifluoroborates - was
discovered
•
Allyltrifluoroborates can be transformed into
allyl sulfones

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Synthesis of allyl sulfones from potassium
allyltrifluoroborates
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Agnese Stikute, Jevgeņija Lugiņina, Māris Turks
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