Unprecedented oxidative addition of highly active manganese into the oxygen-sulfur bond of coumarin and pyrone 4-tosylates

Full text of DOI:10.1002/bkcs.10777

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DOI: 10.1002/bkcs.10777
BULLETIN OF THE
U. S. Shin et al.
KOREAN CHEMICAL SOCIETY
Unprecedented Oxidative Addition of Highly Active Manganese into
the Oxygen–Sulfur Bond of Coumarin and Pyrone 4-Tosylates
Ueon Sang Shin,† Seong-Ryu Joo,‡ and Seung-Hoi Kim‡,
*
†Graduate School of Nanobiomedical Science, Dankook University, Cheonan 330-714, Republic of Korea
‡Department of Chemistry, Dankook University, Cheonan 330-714, Republic of Korea.
*E-mail: kimsemail@dankook.ac.kr
Received January 8, 2016, Accepted February 19, 2016, Published online May 18, 2016
Keywords: Organomanganese, Active manganese, Direct insertion, Oxygen–sulfur bond, Coupling
The efficient construction of carbon–carbon bonds is of the
utmost importance in contemporary synthetic organic
chemistry. For the successful performance of this proce-
dure, current methods typically involve the use of organo-
metallic reagent and/or transition metal (pre)catalysts.1
Among many organometallics, organomanganese reagents
have been consistently prepared and used in cross-coupling
reactions,2 but are utilized relatively less than the Grignard,
organozinc, and organoborane reagents. Because of the var-
ious benefits of organomanganese reagents such as low tox-
icity, versatile availability, excellent functional group
tolerance, and high chemoselectivity, they are utilized in a
wide range of synthetic organic chemistry. To date, regard-
ing the synthetic approaches, there is a need for more
diverse synthetic tools to develop facile routes for the prep-
aration of organomanganese complexes. In particular, the
direct manganation methodology is more valuable because
the corresponding organometallic reagents such as organo-
lithium or organomagnesium reagents are available in lim-
ited scope in terms of diverse functionalities.
During ongoing investigations into the preparation and
application of highly active manganese (Mn*), we became
interested in the direct oxidative addition of highly active
Mn* into the carbon–oxygen bond. In 1999, the first pio-
neering example of this methodology was reported by
Rieke et al. with the successful insertion of highly active
Mn* into the carbon–oxygen bonds of benzylic mesylates
and tosylate. The resulting organomanganese reagents were
efficiently coupled with various carbonyl compounds such
as acid chlorides, aldehydes, and ketones.3 Considering the
high reactivity of the active manganese, we decided to fur-
ther explore the possibility of the oxidative addition of
active manganese into the carbon–oxygen bond using some-
what different tosylates. To accomplish the goal of this
study, two tosylates, 2-oxo-2H-chromen-4-yl-4-methyl-
benzenesulfonate (I) and 6-methyl-2-oxo-2H-pyran-4-yl-4-
methylbenzenesulfonate (II) were selected as suitable
substrates. We assumed that this approach could provide
an alternative route for the preparation of 4-coumarinyl and
4-pyronylmanganese reagents. Most of the organometallic
reagents containing a coumarin and/or pyrone moiety have
been prepared by use of the corresponding halogenated deri-
vatives.4 In most cases, coumarin and pyrone tosylates
derived from 4-hydroxy coumarins and pyrones were uti-
lized as coupling partners in cross-coupling reactions to gen-
erate 4-substituted coumarin/pyrone derivatives.5
To accomplish our goal, we tested the novel reactivity of
active manganese with tosylate derivatives and discovered
unprecedented oxidative addition of active manganese.
Herein, we present the first outcomes obtained by the direct
insertion of active manganese into oxygen–sulfur bonds.
As described in Scheme 1, we assumed that the oxida-
tive addition of active manganese would occur between the
carbon (Csp2)–oxygen bond, and could result in the forma-
tion of reagent B. However, the first coupling reaction of
the resulting organomanganese reagent (A1) with benzoyl
chloride under
a
typical copper-catalyst condition
(CuI/LiCl) gave rise to a somewhat unexpected product, an
ester. Therefore, we decided to conduct a close investiga-
tion into the formation of the organomanganese reagent.
This was performed by GC–MS analysis of the reaction ali-
quot after an acidic quenching. According to the GC–MS
chromatogram, compound I (or II) was completely con-
sumed after being stirred at room temperature for 3 h, and
a major peak (m/z 155) was clearly observed, correspond-
ing to the p-toluene sulfonyl fragment, along with several
unidentified minor peaks. Unfortunately, no peaks corre-
sponding to the 4-hydroxy coumarin (or 6-methyl-4-
hydroxy-2-pyrone) fragment were observed, probably
because of the instability of hydroxyl ring compounds. This
observation was fully confirmed by the full characterization
Scheme 1. Schematic procedure for the preparation of organoman-
ganese complexes.
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Table 1. Coupling reactions with acid chlorides.
of the final coupling products using NMRs and high-
resolution mass spectrometer (HRMS). Thus, based on
these outcomes, it could be concluded that the oxidative
addition of active manganese occurred in the oxygen–sulfur
bond, yielding the corresponding organomanganese
reagents,
(2-oxo-2H-chromen-4-yloxy)(tosyl)manganese
(A1), and (6-methyl-2-oxo-2H-pyran-4-yloxy)(tosyl)manga-
nese (A2), in Scheme 1.
Further coupling reactions with a variety of acid chlor-
ides were carried out with A1 and A2, and the correspond-
ing ester products were consistently obtained. The results
summarized in Table 1 confirm the structure of the organo-
manganese reagents depicted above. Even though the cou-
pling conditions were not optimized, in general, most of the
aryl acid chlorides including heteroaryl acid chlorides were
smoothly reacted with the organomanganese reagents (A1
and A2) under mild conditions to provide esters in moder-
ate yields. No ester product, however, was obtained from
the coupling reaction with cyclohexane carbonyl chloride
in both cases (using A1 and A2). Instead, a mixture of uni-
dentified compounds was observed.
As we have seen so far, the majority of the experiments
resulted in the formation of O-substituted coumarins and
pyrones from organomanganeses (A1 and A2) under the
conditions used. With these results in hand, our next attempt
was to expand the scope of this strategy by applying an
alternative electrophile. Because the alkoxide (or enolate) is
a good nucleophile in typical nucleophilic substitution reac-
tions, we anticipated that the organomanganese reagents
(A1 and A2) would be suitable for the preparation of ether
derivatives through simple reaction with benzyl halides.
With this aim, A1 and A2 were reacted with various benzyl
halides under the CuI/LiCl-catalytic system. Surprisingly,
this approach yielded another unexpected outcome. Against
our expectation, the organomanganese reagents reacted with
benzyl halides to afford sulfones (3a–3f, Table 2) as the
major products instead of ether compounds. GC–MS analy-
sis of the reaction mixture also clearly showed that there
was no ether compound even in the presence of excess
amounts of benzyl halide. This outcome can be easily ratio-
nalized by the nucleophilic attack of the manganese sulfinate
in A1 (or A2) to benzyl halide.6 In general, sulfones (3a–3d,
Table 2) were isolated in moderate yields. However, slightly
lower yields were obtained when benzyl chlorides were
utilized (3e–3f, Table 2). Once again, the coupling product
1
sulfones were fully characterized by H-, 13C NMR spec-
troscopy and HRMS.7
obtained by the reaction of highly active Mn* with 2-oxo-
2H-chromen-4-yl 4-methylbenzenesulfonate (I) and 6-
methyl-2-oxo-2H-pyran-4-yl-4-methylbenzenesulfonate (II),
respectively. This was accomplished by the insertion of
Mn* into the oxygen–sulfur bond. Of interest, subsequent
cross-coupling reactions of the thus-obtained organomanga-
nese reagents afforded two different products, esters and
sulfones, depending on the electrophile used under mild
conditions. Optimization including mechanistic studies and
more efficient catalytic systems is currently under way.
In addition, coupling reactions of A2 with heterobenzylic
halide (entry 1, Table 3) and α-bromocarbonyl compounds
(entries 2 and 3, Table 3) were also examined under the
same conditions. The coupling reactions progressed
smoothly to produce the corresponding products (4a–4c,
Table 3) in moderate isolated yield.
In conclusion, novel organomanganese reagents, 2-oxo-
2H-chromen-4-yloxy tosylmanganese (A1), and 6-methyl-
2-oxo-2H-pyran-4-yloxy tosylmanganese (A2), were
Bull. Korean Chem. Soc. 2016, Vol. 37, 950–953
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Table 2. Coupling reactions with benzyl halides.
Table 3. More couplings with active bromo-compounds.
Yield
(%)b
Entry
1c
Halidea
Product
Yield
(%)a
37 (53)b
Entry
1
FG-ArCH2X
Product
40
2
36
27
2
58 (45)
3
a
Reacted with 2.2 equiv of A2, otherwise mentioned.
Isolated yield (based on A2).
3.0 equiv of A2 used.
3
4
51 (54)
43 (60)
b
c
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5
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L. Zhang, H.-G. Xia, Tetrahedron Lett. 2006, 47, 1525;
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Isolated yield (based on organomanganese).
Number in parentheses: obtained from using A2.
b
Acknowledgment. This research was financially supported
by a fund from Dankook University in 2015.
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Bull. Korean Chem. Soc. 2016, Vol. 37, 950–953
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7. (a)Representative procedures: (a) Preparation of 2-oxo-2H-
chromen-4-yloxy tosylmanganese (A1); In an oven-dried
250 mL round-bottomed flask equipped with a stir bar was
added 3.57g of active manganese (Mn*, 40.0 mmol) in 40 mL
of THF. Next, 2-oxo-2H-chromen-4-yl 4-methylbenzenesuo-
fonate (I) (6.33 g, 20.0 mmol) was cannulated into the flask at
room temperature. The resulting mixture was allowed to stir at
room temperature for 3 h. After the mixture was settled down,
the supernatant was taken and used for the subsequent coupling
reactions; (b)Representative coupling reaction of A1 with an
acid chloride; preparation of 6-methyl-2-oxo-2H-pyran-4-yl 2-
iodobenzoate (2f): Into a 25 mL round-bottomed flask were
placed CuI (0.04 g, 0.20 mmol), and LiCl (0.04 g, 0.40 mmol)
under argon atmosphere, then 2.0 mL of THF was added into
the flask. The mixture was cooled down to 0 C. Next, 6.0 mL
of 6-methyl-2-oxo-2H-pyran-4-yloxy tosylmanganese (A2)
(0.5 M in THF, 3.0 mmol) was cannulated into the flask at
0 C. Then, 2-iodobenzoyl chloride (0.53 g, 2.0 mmol) dis-
solved in 5.0 mL of THF was added. The whole mixture was
allowed to warm up to room temperature over 12 h while being
stirred. The reaction mixture was quenched with 3 M HCl solu-
tion, and extracted with EtOAc (10 mL × 3). The combined
organic solution was washed with saturated NaHCO3 (aq),
brine, dried over anhydrous MgSO4 and purified by column
chromatography (90% hexanes/10% EtOAc) to afford 0.33 g
of a white solid (2f, m.p. = 146–148 C) in 49% isolated yield.
1H NMR (500 MHz, CDCl3): δ = 8.08 (d, J = 8.0 Hz, 1H),
7.96 (d, J = 8.0 Hz, 1H), 7.50 (t, J = 7.5 Hz, 1H), 7.27 (t,
J = 7.5 Hz, 1H), 6.19 (s, 1H), 6.15 (s, 1H), 2.31 (s, 3H). 13C
NMR (125 MHz, CDCl3): δ = 163.6, 163.1, 162.3, 142.1,
134.1, 132.3, 131.9, 128.3, 101.4, 101.2, 95.0, 20.2. HRMS;
calcd for C13H9O4I: 355.9546; found: 355.9553; (c)Represen-
tative Cu-catalyzed cross-coupling reaction with benzyl halide;
preparation of 1-(benzylsulfonyl)-4-methylbenzene (3a); Into a
25 mL round-bottomed flask were placed CuI (0.04 g,
0.20 mmol) and LiCl (0.04 g, 0.40 mmol) under argon atmos-
phere, then 2.0 mL of THF was added into the flask. The mix-
ture was cooled down to 0 C. Then, 4.0 mL of 6-methyl-2-
oxo-2H-pyran-4-yloxy tosylmanganese (A2, 0.5 M in THF,
2.0 mmol) was added into the flask under an argon atmos-
phere. Next, benzyl bromide (0.75 g, 4.40 mmol) was added at
0 C. The whole mixture was allowed to warm up to room tem-
perature, then heated under reflux for 24 h while being stirred.
The reaction mixture was quenched with 3 M HCl solution,
and extracted with ethyl ether (10 mL × 3). The combined
organic solution was washed with saturated NaHCO3 (aq),
Na2S2O3 (aq) solution and brine, then dried over anhydrous
MgSO4. Purification by column chromatography on silica gel
(10% ethyl acetate/90% heptane) afforded 0.26 g of 3a in 53%
isolated yield as an off-white solid (m.p. = 115–116 C). 1H
NMR (500 MHz, CDCl3): δ = 7.49 (d, J = 8.0 Hz, 2H),
7.31–7.30 (m, 1H), 7.26–7.23 (m, 4H), 7.08 (d, J = 7.5 Hz,
2H), 4.29 (s, 2H), 2.42 (s, 3H). 13C NMR (125 MHz, CDCl3):
δ = 144.7, 134.9, 130.8, 129.5, 128.7, 128.6, 128.5, 128.2,
62.9, 21.6. HRMS; calcd for C14H14O2S: 246.0715; found:
246.0714.
Bull. Korean Chem. Soc. 2016, Vol. 37, 950–953
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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