B(C 6 F 5) 3 -Catalyzed Reduction of Sulfoxides and Sulfones to Sulfides with Hydrosilanes

Article abstract of DOI:10.1055/s-0036-1588476

B(C ₆ F ₅) ₃ is shown to catalyze the deoxygenation of sulfoxides and sulfones to the corresponding sulfides with Et ₃ SiH as the stoichiometric hydride source. While the method is limited in terms of functional group tolerance, it is applicable to the reduction of alkyl/aryl-, aryl/aryl-, and alkyl/alkyl-substituted sulfoxides, including the benzyl/benzyl-substituted derivative. The same protocol converts alkyl/aryl- but not aryl/aryl-substituted sulfones into sulfides.

Full text of DOI:10.1055/s-0036-1588476

SYNTHESIS
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©
Georg Thieme Verlag Stuttgart · New York
2017, 49, A–E
paper
A
en
Synthesis
D. Porwal, M. Oestreich
Paper
B(C F ) -Catalyzed Reduction of Sulfoxides and Sulfones to
6
5 3
Sulfides with Hydrosilanes
Digvijay Porwal
0
0
0
0
0-
0
0
3
1-
2
9
2
0-
3
5
X
B(C6F5)3 (10 mol%)
Et3SiH (10 equiv)
Martin Oestreich*
0
0
0
0
0-
0
0
2
1-
4
8
7
9-
2
1
8
O
S
O O
S
or
3
1
2
4
S
R /R
8–90% for sulfoxides
0–95% for sulfones
R /R
R1
R2
R3
R4
neat
100 °C
4
6
Institut für Chemie, Technische Universität Berlin,
Strasse des 17. Juni 115, 10623 Berlin, Germany
martin.oestreich@tu-berlin.de
8
h
R1 and R2 = alkyl and/or aryl for sulfoxides
R3 = aryl and R4 = alkyl for sulfones
Received: 27.05.2017
Accepted: 31.05.2017
Published online: 05.07.2017
Recently, we demonstrated that the electron-deficient
boron Lewis acid tris(pentafluorophenyl)borane [B(C F ) ]
6
5 3
DOI: 10.1055/s-0036-1588476; Art ID: ss-2017-z0361-op
in combination with a stoichiometric amount of a hydrosi-
lane19 as the reductant promotes the reduction of phos-
Abstract B(C F ) is shown to catalyze the deoxygenation of sulfoxides
20
6
5 3
phine oxides as well as aliphatic and aromatic nitro com-
and sulfones to the corresponding sulfides with Et SiH as the stoichio-
3
21
pounds.
This prompted us to also consider the
metric hydride source. While the method is limited in terms of function-
al group tolerance, it is applicable to the reduction of alkyl/aryl-,
aryl/aryl-, and alkyl/alkyl-substituted sulfoxides, including the ben-
zyl/benzyl-substituted derivative. The same protocol converts al-
kyl/aryl- but not aryl/aryl-substituted sulfones into sulfides.
B(C F ) /hydrosilane system for the reduction of sulfox-
6
5 3
ides18 and sulfones. Herein, we report the B(C F ) -cata-
6
5 3
lyzed reduction of these sulfur functional groups to sulfides
with Et SiH as the stoichiometric hydride source.
3
The reduction of methyl phenyl sulfoxide was chosen as
the model reaction to study the effect of catalyst loading,
hydrosilane equivalents, and other important parameters
(1a → 2a, Table 1). A blank reaction without the catalyst and
Key words boron, Lewis acids, reduction, Si–H activation, silicon
The reduction of compounds containing oxidized sulfur
groups, especially sulfoxides and sulfones, is a widely used
Et SiH as the reductant furnished only trace amounts of
3
1
reaction. Sulfoxides are frequently employed as chiral aux-
methyl phenyl sulfide (entry 1). Promising conversion was
obtained by adding 10 mol% of B(C F ) with 2.0 equiv of
iliaries in stereoselective synthesis, also because of their
facile removal by desulfurization after sulfoxide-to-sulfide
6
5 3
Et SiH at elevated temperature (entry 2). To our delight, on
3
2
reduction. The more oxidized sulfones have been exten-
increasing the amount of Et SiH to 10 equiv and completely
3
sively used in key transformations such as the Ramberg–
Bäcklund reaction and the Julia olefination. The sulfonyl
group thereby generated is usually further converted into
other functional groups by reduction or elimination.3 Over
the years, various procedures have been reported for the
catalytic reduction of sulfoxides where transition-metal
catalysts are combined with reducing agents such as phos-
omitting the solvent, quantitative conversion was detected
after 18 h (entry 3). It is important to note that the same
catalytic setup was also found optimal in our aforemen-
,4
21
tioned reduction of nitro compounds. However, the sulf-
oxide reduction was faster, reaching completion within 8 h
(entry 4). Reducing either the amount of catalyst or the
temperature led to diminished conversions (entries 5 and
6). Other hydrosilanes were then routinely tested at 100 °C
for 8 h. Ph SiH was found to be ineffective (entry 7). In
phines, hydrosilanes, hydroboranes, and thiols.8 These
5
6
7
methods represent improvements over stoichiometric pro-
2
2
8b,9–12
tocols
but still rely on the use of transition metals. Un-
turn, PhSiH afforded complete conversion (entry 8) but the
3
like sulfoxides, the reduction of sulfones is much more chal-
lenging owing to the high thermodynamic stability of the
separation of the non-polar sulfide from unreacted PhSiH3
was difficult. Hence, the use of PhSiH was abandoned. Con-
3
3
sulfonyl group, and just a handful of procedures are avail-
versely, (EtO) SiH, which had afforded complete reduction
3
13–17
20
able.
Hence, efficient transition-metal-free, catalytic
in the case of phosphine oxides, did not participate in the
reductions of both sulfoxides and sulfones are highly desir-
able.
catalysis (entry 9). The use of polymethylhydrosiloxane
(PMHS) and tetramethyldisiloxane (TMDS) yielded moder-
ate conversions (entries 10 and 11); analytical thin layer
18
©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

B
Synthesis
D. Porwal, M. Oestreich
Paper
Table 1 Optimization of the B(C F ) -Catalyzed Reduction of Sulfoxide
with Hydrosilanes
6 5 3
B(C F ) (10 mol%)
Et3SiH (10 equiv)
6
5
3
a
O
S
1
S
R1
R2
R1
R2
neat
100 °C
2
B(C6F5)3 (10 mol%)
hydrosilane
8
h
O
S
S
Ph
Me
solvent
temperature
Ph
Me
S
S
1a
2a
Me
X
X
X
Entry
Hydrosilane (equiv) Solvent
Temp (°C)
Conv. (%)b
2
a (X = H): 90% (>99% conv)
2d (X = H): 78% (90% conv)
tracec
2b (X = Me): 76% (>99% conv)
2e (X = Me): 66% (80% conv)
2f (X = Cl): 80% (90% conv)
1
2
3
4
5
6
7
8
9
Et
Et
Et
Et
Et
Et
3
3
3
3
3
3
SiH (2.0)
SiH (2.0)
SiH (10)
SiH (10)
SiH (10)
SiH (10)
toluene
toluene
neat
100
100
100
100
100
50
2c (X = Br): 86% (>99% conv)
43
S
S
S
quant.
Bn
Bn
X
X
quant.d
2g: 75% (>99% conv)
2h (X = Butyl): 48%
i (X = Dodecyl): 63%
2j: 80% (95% conv)
neat
2
neat
78e
Scheme 1 B(C F ) -catalyzed reduction of various sulfoxides with
6
5 3
neat
30
Et SiH
3
Ph
PhSiH
(EtO)
2
SiH
2
(10)
(10)
SiH (10)
neat
100
100
100
100
100
trace
quant.
trace
47
3
neat
mental to the reactivity, showing low conversion of the sul-
fone (3k → 2k). This result is similar to what we had ob-
served for 1-fluoro-4-nitrobenzene. An example with an
electron-donating MeO group in the para position was also
included but was unreactive (3l; no decomposition). In con-
trast, diaryl sulfones did not react at all, e.g., 3d did not af-
ford 2d.
3
neat
1
0
1
PMHS (10)
TMDS (10)
neat
21
1
neat
59
a
Reactions performed on a 0.20-mmol scale.
b
Determined by GLC analysis with reference to the starting material after
1
8 h.
c
No catalyst.
d
e
8
5
h reaction time.
.0 mol% of B(C
6
F
5
)
3
used.
B(C6F5)3 (10 mol%)
Et3SiH (10 equiv)
O
O
S
R1
R2
S
R1
R2
neat
3
100 °C
2
8
h
chromatography indicated the formation of several byprod-
ucts, probably stemming from the thermal degradation of
these hydrosilanes.
S
S
Me
X
To explore the scope of the new method, we subjected
various sulfoxides to our optimized setup (Scheme 1). The
model substrate afforded the corresponding sulfide in high
isolated yield (1a → 2a). Me and Br groups in the para posi-
tion were tolerated (1b,c → 2b,c). Various diaryl sulfoxides
were also tested. Diphenyl sulfoxide afforded excellent con-
version and isolated yield (1d → 2d). Me and Cl groups on
both phenyl rings were again compatible (1e,f → 2e,f). Grat-
ifyingly, dibenzyl sulfoxide, which had displayed poor reac-
2
2
a (X = H): 95% (>99% conv)
b (X = Me): 75% (89% conv)
2c (X = Br): 60% (93% conv)
2d: not observed
2
2
k (X = F): 11% conv
l (X = OMe): not observed
Scheme 2 B(C F ) -catalyzed reduction of various sulfoxides with
6
5 3
Et SiH
3
Finally, to demonstrate the practicality of the new pro-
cedure, we performed the reduction of methyl phenyl sulf-
oxide on a gram scale, affording the desired sulfide in 71%
isolated yield (1a → 2a, Scheme 3).
6a
tivity with most other methods, underwent smooth deox-
ygenation (1g → 2g). Linear alkyl chains as well as cyclic ali-
phatic sulfoxides were also investigated, and moderate to
good yields were obtained for representative sulfoxides
B(C6F5)3 (10 mol%)
Et3SiH (10 equiv)
(
1h–j → 2h–j).
O
S
S
To expand the scope of the present methodology, we
Ph
Me
neat
00 °C
8
Ph
1a (1.00 g)
Me
1
next turned toward the reduction of sulfones (Scheme 2).
We began with the reduction of methyl phenyl sulfone and
were glad to observe quantitative conversion with 95% iso-
lated yields of the desired sulfide (3a → 2a). Motivated by
this initial success, we then subjected various alkyl aryl sul-
fones electronically modified in the para position of the
phenyl ring to the same setup. Me and Br substitution were
again fine (3b,c → 2b,c) while F as substituent was detri-
2a (0.63 g)
h
Scheme 3 Gram-scale experiment
To recap, we have disclosed here a transition-metal-
free, B(C F ) -catalyzed reduction of sulfoxides and sulfones
to sulfides with Et SiH as the stoichiometric reductant. The
6
5 3
3
method is applicable to a limited set of sulfoxides and sul-
©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

C
Synthesis
D. Porwal, M. Oestreich
Paper
fones as a consequence of the high reaction temperature
and the presence of excess hydrosilane. However, the ex-
haustive deoxygenation of sulfones is noteworthy.
Methyl p-Tolyl Sulfide (2b) (From Methyl p-Tolyl Sulfoxide)
Colorless oil; yield: 21 mg (76%); GLC (SE-54): t = 11.6 min. The spec-
troscopic data are in accordance with those reported.
R
22a
1
H NMR (500 MHz, CDCl ): δ = 2.35 (s, 3 H), 2.44 (s, 3 H), 7.26 (d, J =
3
7.6 Hz, 2 H), 7.45 (d, J = 7.7 Hz, 2 H).
1
3
Reactions were performed in flame-dried glassware using an MBraun
glovebox or conventional Schlenk techniques under a static pressure
of N2 gas unless otherwise stated. Liquids and solutions were trans-
ferred with syringes. All sulfur compounds were purchased from
commercial suppliers and used without further purification. Flash
C NMR (125 MHz, CDCl ): δ = 21.5, 44.0, 123.7, 130.2, 141.7, 142.5.
3
HRMS (APCI): m/z [M + H]+ calcd for C H11S: 139.0576; found:
1
8
39.0582.
Methyl p-Tolyl Sulfide (2b) (From Methyl p-Tolyl Sulfone)
column chromatography was performed on silica gel 60 (40–63 μm,
1
Colorless oil; yield: 21 mg (75%); GLC (SE-54): t = 11.6 min. The spec-
troscopic data are in accordance with those reported.
2
30–400 mesh, ASTM) by Merck using the indicated solvents. H and
R
22a
1
3
C NMR spectra were recorded in CDCl3 on Bruker AV500 instru-
1
ments referenced to the residual solvent resonance as the internal
H NMR (500 MHz, CDCl ): δ = 2.34 (s, 3 H), 2.64 (s, 3 H), 7.25 (d, J =
3
1
13
standard (CHCl : δ = 7.26 for H NMR and CDCl : δ = 77.16 for
C
7.6 Hz, 2 H), 7.46 (d, J = 7.7 Hz, 2 H).
3
3
NMR). Gas-liquid chromatography (GLC) was performed on an Agilent
Technologies 7820A gas chromatograph equipped with an FS-SE-54
capillary column (30 m × 0.32 mm, 0.25 μm film thickness) by CS-
13
C NMR (125 MHz, CDCl ): δ = 21.5, 44.0, 123.7, 130.2, 141.7, 142.5.
3
HRMS (APCI): m/z [M + H]+ calcd for C H11S: 139.0576; found:
8
139.0580.
Chromatographie Service using the following program: N carrier gas,
2
injection temperature 240 °C, detector temperature 300 °C, flow rate:
4
-Bromophenyl Methyl Sulfide (2c) (From 4-Bromophenyl Methyl
1.74 mL/min; temperature program: start temperature 40 °C, heating
Sulfoxide)
rate 10 °C/min, end temperature 280 °C for 10 min. Mass spectra (MS)
were obtained from the Analytical Facility at the Institut für Chemie
of the Technische Universität Berlin.
Colorless oil; yield: 35 mg (86%); GLC (SE-54): t = 16.2 min. The spec-
troscopic data are in accordance with those reported.
R
22b
1
H NMR (500 MHz, CDCl ): δ = 2.26 (s, 3 H), 7.15 (d, J = 8.5 Hz, 2 H),
3
Reduction of Sulfoxides and Sulfones; General Procedure
7.40 (d, J = 8.2 Hz, 2 H).
In a glovebox, an oven-dried 1-mL screw-capped sealed tube with a
magnetic stir bar was charged with B(C F ) (10 mol%), Et SiH (10
equiv), and the indicated oxidized sulfur compound (0.20 mmol). The
tube was sealed properly and transferred to an oil bath preheated at
13
C NMR (125 MHz, CDCl ): δ = 21.5, 125.0, 130.1, 141.6, 142.8.
3
6
5
3
3
HRMS (APCI): m/z [M + H]+ calcd for C H BrS: 202.9525; found:
7
8
202.9500.
1
00 °C. After 8 h, the reaction was cooled to r.t. and passed through a
4
-Bromophenyl Methyl Sulfide (2c) (From 4-Bromophenyl Methyl
small plug of silica gel using Et O. The crude material was collected in
a glass vial and subjected to GLC analysis to determine the conversion
with respect to starting material. The ethereal solution was dried
2
Sulfone)
Colorless oil; yield: 25 mg (60%); GLC (SE-54): t
troscopic data are in accordance with those reported.
= 16.2 min. The spec-
R
22b
(Na SO ) and filtered, and the solvent was removed under reduced
2
4
pressure. The mixture was then subjected to high vacuum at 70 °C
until the unreacted hydrosilane was removed from the system. If
needed, the residue was purified further by flash column chromatog-
1
H NMR (500 MHz, CDCl ): δ = 2.33 (s, 3 H), 7.25 (d, J = 8.5 Hz, 2 H),
.45 (d, J = 8.2 Hz, 2 H).
3
7
13
C NMR (125 MHz, CDCl ): δ = 21.6, 127.5, 129.7, 141.7, 141.9.
3
raphy (silica gel, cyclohexane/Et O 9:1) to afford the desired sulfides.
2
HRMS (APCI): m/z [M + H]+ calcd for C H BrS: 202.9525; found:
7
8
202.9543.
Methyl Phenyl Sulfide (2a) (From Methyl Phenyl Sulfoxide)
Colorless oil; yield: 22 mg (90%); GLC (SE-54): t = 8.1 min. The spec-
troscopic data are in accordance with those reported.
R
22a
Diphenyl Sulfide (2d) (From Diphenyl Sulfoxide)
Colorless oil; yield: 28 mg (78%); GLC (SE-54): t = 17.8 min. The spec-
1
R
H NMR (500 MHz, CDCl ): δ = 2.41 (s, 3 H), 7.04–7.08 (m, 1 H), 7.17–
3
22a
troscopic data are in accordance with those reported.
7.23 (m, 4 H).
1
H NMR (500 MHz, CDCl ): δ = 7.25–7.28 (m, 2 H), 7.31–7.34 (m, 4 H),
13
3
C NMR (125 MHz, CDCl ): δ = 16.0, 125.2, 126.8, 128.9, 138.5.
3
7.36–7.38 (m, 4 H).
HRMS (APCI): m/z [M + H]+ calcd for C H S: 125.0419; found:
7
9
13
C NMR (125 MHz, CDCl ): δ = 127.2, 129.3, 131.2, 136.0.
3
125.0426.
HRMS (APCI): m/z [M + H]+ calcd for C12H11S: 187.0576; found:
87.0581.
1
Methyl Phenyl Sulfide (2a) (From Methyl Phenyl Sulfone)
Colorless oil; yield: 24 mg (95%); GLC (SE-54): t = 8.1 min. The spec-
troscopic data are in accordance with those reported.
R
22a
Di-p-Tolyl Sulfide (2e)
Colorless oil; yield: 28 mg (66%); GLC (SE-54): t = 21.6 min. The spec-
troscopic data are in accordance with those reported.
1
R
H NMR (500 MHz, CDCl ): δ = 2.43 (s, 3 H), 7.06–7.10 (m, 1 H), 7.19–
.25 (m, 4 H).
3
22a
7
1
H NMR (500 MHz, CDCl ): δ = 2.32 (s, 6 H), 7.11 (d, J = 8.1 Hz, 4 H),
13
3
C NMR (125 MHz, CDCl ): δ = 16.0, 125.2, 126.8, 129.0, 138.6.
3
7.24 (d, J = 7.7 Hz, 4 H).
HRMS (APCI): m/z [M + H]+ calcd for C H S: 125.0419; found:
1
7
9
13
C NMR (125 MHz, CDCl ): δ = 21.1, 129.9, 131.1, 132.7, 136.9.
3
25.0423.
HRMS (APCI): m/z [M + H]+ calcd for C14H15S: 215.0889; found:
25.0893.
1
©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

D
Synthesis
D. Porwal, M. Oestreich
Paper
Bis(4-chlorophenyl) Sulfide (2f)
Supporting Information
Colorless oil; yield: 41 mg (80%); GLC (SE-54): t = 22.7 min. The spec-
troscopic data are in accordance with those reported.
R
22a
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1588476.
S
u
p
p
o
nrtogI
i
f
rm oaitn
S
u
p
p
ortioIgnfmr oaitn
1
H NMR (500 MHz, CDCl ): δ = 7.45 (d, J = 8.7 Hz, 4 H), 7.24 (d, J = 8.6
3
Hz, 4 H).
1
3
C NMR (125 MHz, CDCl ): δ = 126.2, 129.9, 137.9, 144.0.
References and Notes
3
HRMS (APCI): m/z [M + H]+ calcd for C12H Cl S: 254.9797; found:
2
9
2
(1) (a) Kunieda, N.; Nokami, J.; Kinoshita, M. Chem. Lett. 1974, 369.
54.9750.
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Demailly, G.; Greck, C. Tetrahedron Lett. 1985, 26, 435.
(e) Kosugi, H.; Watanabe, Y.; Uda, H. Chem. Lett. 1989, 1865.
Dibenzyl Sulfide (2g)
Colorless oil; yield: 32 mg (75%); GLC (SE-54): t = 20.7 min. The spec-
troscopic data are in accordance with those reported.
R
22a
(2) (a) Carreno, M. C. Chem. Rev. 1995, 95, 1717. (b) Madesclaire, M.
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1
H NMR (500 MHz, CDCl ): δ = 3.86 (m, J = 13.3 Hz, 4 H), 7.23–7.35 (m,
3
(
(
10 H).
4) Desrosiers, J. N.; Charette, A. B. Angew. Chem. Int. Ed. 2007, 46,
13
C NMR (125 MHz, CDCl ): δ = 57.4, 128.5, 129.1, 130.2, 130.2.
3
5955.
HRMS (APCI): m/z [M + H]+ calcd for C14H15S: 215.0889; found:
15.0889.
(5) (a) Zhu, Z.; Espenson, J. H. J. Mol. Catal. A: Chem. 1995, 103, 87.
2
(b) Arterburn, J. B.; Perry, M. C. Tetrahedron Lett. 1996, 37, 7941.
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(6) (a) Sousa, S. C. A.; Fernandes, A. C. Tetrahedron Lett. 2009, 50,
Colorless oil; yield: 14 mg (48%). The spectroscopic data are in accor-
dance with those reported.
22a
6
9
872. (b) Fernandes, A. C.; Romão, C. C. Tetrahedron 2006, 62,
650. (c) Cabrita, I.; Sousa, S. C. A.; Fernandes, A. C. Tetrahedron
1
H NMR (500 MHz, CDCl ): δ = 0.95 (t, J = 7.3 Hz, 6 H), 1.41–1.56 (m, 4
3
H), 1.72–1.79 (m, 4 H), 2.62–2.76 (m, 4 H).
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Company, A.; Enthaler, S.; Driess, M. ChemCatChem 2011, 3,
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13
C NMR (125 MHz, CDCl ): δ = 13.8, 22.2, 24.8, 52.1.
3
HRMS (APCI): m/z [M + H]+ calcd for C H19S: 147.1202; found:
8
147.1206.
(h) Mikami, Y.; Noujima, A.; Mitsudome, T.; Mizugaki, T.;
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K.; Zehbe, R.; Roeser, J.; Strauch, P.; Enthaler, S.; Thomas, A.
Polym. Chem. 2013, 4, 1848.
Didodecyl Sulfide (2i)
Colorless oil; yield: 40 mg (63%); together with an inseparable impu-
rity. The spectroscopic data are in accordance with those reported.
22b
(
7) (a) Fernandes, A. C.; Romão, C. C. Tetrahedron Lett. 2007, 48,
9176. (b) Enthaler, S.; Krackl, S.; Irran, E.; Inoue, S. Catal. Lett.
1
H NMR (500 MHz, CDCl ): δ = 0.88 (t, J = 7.1 Hz, 6 H), 1.26–1.30 (m,
3
20 H), 1.31–1.35 (m, 4 H), 1.39–1.49 (m, 4 H), 1.71–1.79 (m, 4 H),
2012, 142, 1003. (c) Enthaler, S. Catal. Lett. 2012, 142, 1306.
2.63–2.79 (m, 4 H).
(
8) (a) Wallace, T. J.; Mahon, J. J. J. Am. Chem. Soc. 1964, 86, 4099.
(b) Karimi, B.; Zareyee, D. Synthesis 2003, 1875.
13
C NMR (125 MHz, CDCl ): δ = 14.2, 22.7, 28.9, 29.3, 29.4, 29.5, 29.7,
3
2
9.7, 32.0, 38.5.
(9) (a) Chasar, D. W. J. Org. Chem. 1971, 36, 613. (b) Drabowicz, J.;
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Sugino, E.; Hibino, S. Synthesis 1980, 695. (d) Harrison, D. J.;
Tam, N. C.; Vogels, C. M.; Langler, R. F.; Baker, R. T.; Decken, A.;
Westcott, S. A. Tetrahedron Lett. 2004, 45, 8493. (e) Zhang, J.;
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(c) Landini, D.; Maia, A. M.; Rolla, F. J. Chem. Soc., Perkin Trans. 2
HRMS (APCI): m/z [M + H]+ calcd for C20H43S: 315.3080; found:
15.3089.
3
Tetramethylene Sulfide (2j)
Colorless oil; yield: 14 mg (80%); GLC (SE-54): t = 18.8 min. The spec-
troscopic data are in accordance with those reported.
R
22a
(
(
1
H NMR (500 MHz, CDCl ): δ = 1.86-1.88 (m, 4 H), 2.75-2.77 (m, 4 H).
3
13
C NMR (125 MHz, CDCl ): δ = 31.2, 32.0.
3
+
HRMS (APCI): m/z [M + H] calcd for C H S: 89.0419; found: 89.0461.
4
9
1976, 1288. (d) Doi, J. T.; Musker, W. K. J. Am. Chem. Soc. 1981,
103, 1159.
Acknowledgment
(12) (a) Castrillón, J. P. A.; Szmant, H. H. J. Org. Chem. 1965, 30, 1338.
b) Still, I. W. J.; Hasan, S. K.; Turnbull, K. Can. J. Chem. 1978, 56,
(
This research was supported by the German Academic Exchange Ser-
vice (predoctoral fellowship to D.P., 2014–2018) and the Cluster of Ex-
cellence ‘Unifying Concepts in Catalysis’ of the Deutsche
Forschungsgemeinschaft (EXC 314/2). M.O. is indebted to the Einstein
Foundation (Berlin) for an endowed professorship.
1
2
423. (c) Kikuchi, S.; Konishi, H.; Hashimoto, Y. Tetrahedron
005, 61, 3587. (d) Iranpoor, N.; Firouzabadi, H.; Jamalian, A.
Synlett 2005, 1447. (e) Hua, G.; Woollins, J. D. Tetrahedron Lett.
007, 48, 3677. (f) Bahrami, K.; Khodaei, M. M.; Khedri, M.
2
Chem. Lett. 2007, 36, 1324. (g) Pandey, L. K.; Pathak, U.; Rao,
A. N. Synth. Commun. 2007, 37, 4105. (h) Jang, Y.; Kim, K. T.;
Jeon, H. B. J. Org. Chem. 2013, 78, 6328.
©
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D. Porwal, M. Oestreich
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13) Akgün, E.; Mahmood, K.; Mathis, C. A. J. Chem. Soc., Chem.
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6
5 3
(
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(
1
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(
1
(
(
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©
Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E