Halogen and chalcogen cation pools stabilized by DMSO. Versatile reagents for alkene difunctionalization

Article abstract of DOI:10.1021/ja4092648

Halogen and chalcogen cations (X⁺ = Br⁺, I ⁺, ArS⁺, and ArSe⁺) were generated by low-temperature electrochemical oxidation in the presence of dimethyl sulfoxide (DMSO) and were accumulated in the solution. DFT calculations indicated that DMSO stabilizes these cations by coordination. The complexes of I⁺ with one and two DMSO molecules were observed by cold-spray-ionization MS analyses. The stability of the resulting cation pools of X⁺ increased in the order of Br⁺ < I⁺ < ArS⁺ < ArSe⁺, which could be explained in terms of the electronegativity of X. The cation pools served as versatile reagents for organic synthesis; the reactions with alkenes gave β-X-substituted alkoxysulfonium ions, which were converted to the corresponding carbonyl compounds by the treatment with triethylamine, whereas the treatment with methanol gave the corresponding alcohols. The reactions with aminoalkenes and 1,6-dienes gave the cyclized products.

Full text of DOI:10.1021/ja4092648

Communication
pubs.acs.org/JACS
Halogen and Chalcogen Cation Pools Stabilized by DMSO.
Versatile Reagents for Alkene Difunctionalization
Yosuke Ashikari, Akihiro Shimizu, Toshiki Nokami, and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University,
Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
Scheme 1. Halogen and Chalcogen Cations Stabilized by
DMSO
ABSTRACT: Halogen and chalcogen cations (X+ = Br+,
I+, ArS+, and ArSe+) were generated by low-temperature
electrochemical oxidation in the presence of dimethyl
sulfoxide (DMSO) and were accumulated in the solution.
DFT calculations indicated that DMSO stabilizes these
cations by coordination. The complexes of I+ with one and
two DMSO molecules were observed by cold-spray-
ionization MS analyses. The stability of the resulting
cation pools of X+ increased in the order of Br+ < I+ < ArS+
< ArSe+, which could be explained in terms of the
electronegativity of X. The cation pools served as versatile
reagents for organic synthesis; the reactions with alkenes
gave β-X-substituted alkoxysulfonium ions, which were
converted to the corresponding carbonyl compounds by
the treatment with triethylamine, whereas the treatment
with methanol gave the corresponding alcohols. The
reactions with aminoalkenes and 1,6-dienes gave the
cyclized products.
Figure 1. Structures of (a) DMSO−I+, (b) (DMSO)2−I+, and (c)
(DMSO)3−I+ indicated by DFT calculation on the level of B3LYP/6-
31G(d) and LANL2DZ (with ECP) for I.
spectroscopy.11 Shono and co-workers reported that “I+” could
be accumulated in trimethyl orthoformate (TMOF), and its
reactivity was different from one generated in CH3CN.12
Moreover, we reported that arylsulfenium ions (ArS+) could be
accumulated in the presence of diaryl disulfide (ArSSAr).13 We
have been searching for a more versatile stabilizing agent for a
wide variety of reactive cations, and herein we report that
dimethyl sulfoxide (DMSO) can be generally used as a stabilizing
agent for halogen and chalcogen cations (Scheme 1) and that the
pools of the stabilized cations serve as powerful reagents for
alkene difunctionalization. Also, the use of DMSO leads to a
useful synthetic transformation, because the alkoxysulfonium ion
intermediates can be converted to carbonyl compounds via
Swern−Moffatt type oxidation14,15 by taking advantage of
reaction integration using reactive intermediates.16,17
rganic molecules bearing halogens and chalcogens not
Oonly serve as powerful intermediates in organic syntheses
but also functional materials and biologically active compounds.1
Although a wide variety of methods for introducing such
elements into organic molecules have been developed so far, one
of the most straightforward and powerful methods would be the
use of highly reactive halogen cations and monovalent chalcogen
cations.2 However, such species are usually too unstable to
accumulate in the solution, and therefore they are difficult to use
as reagents.
The electrochemical oxidation3,4 provides a powerful method
for generating and accumulating highly reactive cationic species
in the solution (the “cation pool” method),3c,5 whereas the
chemical methods are not effective because of reversibility of
cation generation.6,7 Although some carbocations and onium
ions8 can be accumulated as cation pools at low temperatures,
halogen and chalcogen cations are difficult to accumulate in the
solution as “cation pools” because they are too unstable.
One potential method to solve this problem would be the use
of a stabilizing agent that coordinates the cation being generated.
The crucial point for the success of this approach is choosing an
appropriate stabilizing agent. To date, only a few stabilizing
agents have been reported. For example, Miller and co-workers
reported the generation and accumulation of “I+” cation in
acetonitrile (CH3CN),9,10 and later we reported the detection of
CH3CN−I+ and (CH3CN)2I+ by cold-spray-ionization mass
We began by studying the nature of I+/DMSO18 by
computational calculations (Figure 1).19,20 For comparison, I+/
CH3CN and I+/TMOF systems were also studied. The DFT
calculations indicated that I+ forms a complex with DMSO in the
gas phase. The oxygen atom of DMSO interacts with I+ (I−O
distance: 2.09 Å), and the stabilization energy is 118.9 kcal/mol.
The coordination by the second DMSO also causes further
stabilization (36.1 kcal/mol). The interaction with the third
DMSO somewhat stabilizes I+ (7.5 kcal/mol), but the I−O
distance is much longer (3.98 Å), indicating the third DMSO
does not directly coordinate to I+. In the case of CH3CN, the
calculations indicated that I+ forms a complex with CH3CN in
Received: September 11, 2013
Published: October 10, 2013
© 2013 American Chemical Society
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Table 1. Reactions of Alkenes with Halogen and Chalcogen
a
Cation Pools Stabilized by DMSO (X+/DMSO)
Figure 2. Generation of I+/DMSO 1-I, and its reaction with an alkene.
CSI-MS and 1H NMR analyses.
gas phase, but stabilization energy (101.5 kcal/mol) is smaller
than that for DMSO. The further stabilization energy by the
second CH3CN (31.6 kcal/mol) is also smaller than that for
DMSO. It is interesting that I+ does not form a stable complex
with a single TMOF molecule in gas phase whereas it forms a
stable complex with two molecules of TMOF. However, total
stabilization energy with two TMOF (130 kcal/mol) is smaller
than those for two DMSO and two CH3CN. Therefore, the
computational studies indicated that stabilizing ability of DMSO
to I+ is stronger than those of CH3CN and TMOF.
With the information obtained by the computational studies in
hand, we next performed the experimental studies on the
generation of I+/DMSO (1-I) cation pool. The electrochemical
oxidation of Bu4NI was carried out in a divided cell in DMSO/
CH2Cl2 (1:9 v/v) using Bu4NBF4 as a supporting electrolyte at
−78 °C. After 2.1 F/mol of electricity was consumed, the
resulting solution was analyzed by the cold-spray-ionization mass
spectroscopy (CSI-MS)21 at 0 °C, which showed the peaks due
to DMSO−I+ and (DMSO)2−I+ species (Figure 2, top). (Z)-5-
Decene (2a) was added to the solution, and the mixture was
stirred at 0 °C. Although 1-I could not be characterized by NMR,
the resulting β-iodo alkoxysulfonium ion 3a-I could be well
characterized by NMR (Figure 2, bottom). The cross peak of the
HMQC spectrum indicated that a signal at 4.18 ppm (1H NMR)
is due to the proton (Ha) attached to the carbon (δ = 91.4 ppm in
13C NMR) adjacent to the oxygen atom derived from DMSO. A
signal at 4.14 ppm could be assigned to the proton attached to the
carbon adjacent to the iodine atom. The HMQC spectrum also
indicated that two singlet signals at 3.26 and 3.30 ppm (1H
NMR) are due to the protons attached to the methyl carbons
derived from DMSO. The formation of 3a-I indicated that 1-I
having the interelement bond22 served as both a nucleophile and
an electrophile toward the carbon−carbon double bond.
Treatment of 3a-I with triethylamine gave α-iodo ketone 4a-I.
It is also noteworthy that the electrochemical oxidation of Bu4NI
in the absence of DMSO at −78 °C followed by the addition of
2a and DMSO and subsequent treatment with triethylamine did
not give 4a-I, indicating that I+ could not be accumulated in the
solution in the absence of DMSO.
a
The electrolysis of Bu4NI and Bu4NBr was carried out using 1.3 equiv
of Bu4NX (based on the alkene which was added after electrolysis)
with 2.1 F/mol of electricity based on Bu4NX. The electrolysis of
ArSSAr and ArSeSeAr was carried out using 0.65 equiv of ArXXAr
(based on the alkene which was added after electrolysis) with 2.1 F/
b
c
mol of electricity based on ArXXAr. Isolated yield. The electrolysis
was carried out using 0.65 equiv of Br2 (based on the alkene which was
added after electrolysis) with 2.1 F/mol of electricity based on Br2.
d
The electrolysis was carried out using 0.65 equiv of I2 (based on the
amount of the alkene which was added after electrolysis) with 2.1 F/
mol of electricity based on I2.
electrochemical oxidation of I2 instead of Bu4NI was also effective
for generating 1-I (entry 8).
It is noteworthy that electrochemically generated I+/CH3CN
and I+/TMOF are not effective for the present transformation.
For example, the reaction of I+/CH3CN with 2b in DMSO/
CH2Cl2 followed by the treatment with triethylamine gave 4b-I
only in a low yield (39%). The use of I+/TMOF did not give 4b-I
at all. The reaction of 2b with N-iodosuccinimide (NIS) in
DMSO/CH2Cl2 followed by the treatment with triethylamine
resulted in recovery of 2b. I2, NIS, and NBS with IBX are well-
known systems for transforming alkenes to α-halocarbonyl
compounds.23 However, in addition to the advantage of
providing mild, easily scalable reaction conditions, the present
electrochemical approach seems to be particularly more
attractive from safety and environmental standpoints, because
the use of hazardous and explosive24 IBX can be avoided.
As shown in Table 1, 1-I reacted with various alkenes to give
the corresponding α-iodo ketones. It is noteworthy that the
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of two products (entries 16 and 17). This remarkable contrast in
regioselectivity might be explained in terms of the difference in
electronegativity of X, although the detailed mechanism is not yet
clear. The reaction of 1-Br with 2d would give the three-
membered ring bromonium ion. However, a positive charge
seems to be located mainly on the benzylic carbon rather than the
electronegative Br. Therefore, DMSO attacks the benzylic
carbon to give phenyl ketone 4d-Br after treatment with
triethylamine. The natural bond orbital (NBO) analysis31 is
consistent with this idea.32 The charge on X of the three-
membered ring onium ion intermediate decreases in the order of
X = Se > S > I > Br, whereas the charge on the benzylic carbon
increases in the order of X = Se < S < I < Br. The bond order
between X and the benzylic carbon also decreases in the order of
X = Se > S > I > Br.
Stereochemistry of the addition to C−C double bonds is
another intriguing aspect of the chemistry of X+/DMSO,
although the stereochemistry is destroyed during Swern−
Moffatt-type oxidation. To gain the information of the
stereochemistry, an alkoxysulfonium ion intermediate was
treated with methanol instead of triethylamine. Thus, both E
and Z isomers of 2d were reacted with 1-Br, and the resulting
solution was treated with methanol.33 (E)-2d gave 5d-Br (eq 1),
Figure 3. Thermal stability of DMSO−X+ species.
We next examined the generation and reactions of Br+/DMSO
(1-Br). DFT calculations indicated that Br+ is also stabilized by
coordination of one or two DMSO molecules like 1-I. The
electrochemical oxidation of Bu4NBr was carried out in the
presence of DMSO at −78 °C. Although 1-Br was not detected
by NMR and CSI-MS analyses, the resulting solution reacted
with 2a to give β-bromo alkoxysulfonium ion 3a-Br, which was
1
characterized by H and 13C NMR. The subsequent treatment
with triethylamine gave α-bromo ketone 4a-Br (Table 1, entry
1). As shown in Table 1, 1-Br also reacted with various alkenes to
give the corresponding α-bromo ketones. Br2 is also effective as a
precursor of 1-Br (entry 6).
Next, we examined the generation and accumulation of
chalcogenium ions stabilized by DMSO. DFT calculations
indicated that the ArS and ArSe are also stabilized by
coordination of one or two DMSO molecules. The electro-
chemical oxidation of bis(4-fluorophenyl) disulfide (ArSSAr, Ar
= 4-FC6H4)25 and that of diphenyl diselenide (ArSeSeAr, Ar =
Ph)26 at −78 °C followed by the addition of 2a and subsequent
treatment with triethylamine gave α-thio ketone 4a-S (75%) and
α-seleno ketone 4a-Se (60%), respectively. The results indicated
that ArS+/DMSO27 and ArSe+/DMSO could be generated and
accumulated in the solution as cation pools.
We next examined the relative stability of these species. Thus,
the electrochemical oxidation reactions of Bu4NBr, Bu4NI,
PhSSPh, and PhSeSePh were carried out at −78 °C, and the
resulting solutions were warmed to a second temperature. After
being kept there for 30 min, the solutions were cooled at −78 °C
and were reacted with 2b. After treatment with triethylamine, the
yields of the corresponding ketones were determined. Relative
stabilities were evaluated by the relative yields based on those not
kept at the second temperature. As shown in Figure 3, Br+ is the
least stable. The stability increases in the order of Br+ < I+ < PhS+
< PhSe+. This tendency can be explained in terms of
electronegativity of the cationic elements (Br: 2.96, I: 2.66, S:
2.58, and Se: 2.55).28,29
whereas (Z)-2d gave its diastereomer 5d′-Br (eq 2), suggesting
the addition of Br+ and DMSO across the C−C double bond in
an anti fashion, because it is known that methanol attacks the
sulfur atom to cleave the O−S bond.33 The anti-selectivity
suggests a mechanism involving the back-side attack of DMSO in
the bulk solution on the three-membered ring bromonium ion
generated from Br+ and an alkene.
1,3-Dienes reacted with 1-S and 1-Se to give α,β-unsaturated
carbonyl compounds in moderate yields after treatment with
triethylamine (entries 19 and 20), although the reactions with 1-
Br and 1-I gave complex mixtures. The regiochemistry is
interesting. DMSO attacked the inner carbon of the 1,3-diene
selectively to give the 1,2-addition intermediate. The product
derived from 1,4-addition was not obtained. Allenes also reacted
with 1-S and 1-Se to give α,β-unsaturated carbonyl compounds
(entries 21 and 22). In this case, DMSO attacked the terminal
carbon of the allene selectively.
It is also interesting that the nature of the cationic elements
also affected the regioselectivity of the addition to unsymmetri-
cally substituted alkenes. The reaction of 1-Br with terminal
alkene 2c gave ketone 4c-Br as a major product and aldehyde
4c′-Br as a minor product (entry 11). The amount of aldehyde
decreased when 1-I and 1-Se were used (entries 12 and 13). The
reaction of 1-Se with 2c gave ketone 4c-Se exclusively. Aldehyde
4c′-Se was not observed (entry 14). Low selectivity of 1-Br
might be explained in terms of higher reactivity of the three-
membered ring bromonium ion intermediate than others,
leading to less regioselective nucleophilic attack by DMSO.30
The regioselectivity for 1-phenyl-1-propene (2d) was more
interesting. The reaction of 1-Br with 2d gave 2-bromo-1-
phenylpropan-1-one (4d-Br) selectively (entry 15), whereas the
reaction of 1-Se with 2d gave 1-phenyl-1-(phenylselanyl)propan-
2-one (4d′-Se) selectively (entry 18). 1-I and 1-S gave mixtures
It is also interesting that halogen and chalcogen cation pools
initiate alkene-cyclization reactions. Reactions of 1-X with an
alkene having a nucleophilic tosylamide group (2g) gave the
cyclized pyrrolidine derivatives (entries 23−26), which are useful
building blocks of biologically interesting compounds.34
The reaction of 1-X with 1,6-diene 2h is more interesting. The
cyclized products 4h-X, piperidine derivatives34 having both the
X substituent and a carbonyl group, were obtained as a single
diastereomer after treatment with triethylamine (entries 27−30).
In conclusion, halogen cations (Br+ and I+) and monovalent
chalcogen cations (ArS+ and ArSe+) stabilized by DMSO can be
generated and accumulated as cation pools by low temperature
electrochemical oxidations. The cations have both sufficient
stability and marked reactivity toward alkenes. The reactions
with alkenes gave the corresponding carbonyl compounds
bearing a halogen or chalcogen substituent at the α-carbon
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Yoshida, J. Asian J. Org. Chem. 2013, 2, 325.
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after treatment with triethylamine. This new tactic will provide
access to a wide range of chemical processes for making synthetic
intermediates having halogen and chalcogen atoms.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and spectroscopic data of compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
yoshida@sbchem.kyoto-u.ac.jp
■
(15) (a) Ashikari, Y.; Nokami, T.; Yoshida, J. J. Am. Chem. Soc. 2011,
133, 11840. (b) Ashikari, Y.; Nokami, T.; Yoshida, J. Org. Biomol. Chem.
2013, 11, 3322.
Notes
The authors declare no competing financial interest.
(16) (a) Schmidt, B. Pure Appl. Chem. 2006, 78, 469. (b) Enders, D.;
Huttl, M. R. M.; Grondal, C.; Raabe, G. Nature 2006, 441, 861.
̈
ACKNOWLEDGMENTS
■
(c) Yoshida, J.; Saito, K.; Nokami, T.; Nagaki, A. Synlett 2011, 1189.
(17) Suga, S.; Yamada, D.; Yoshida, J. Chem. Lett. 2010, 39, 404.
(18) Cationic halogen−DMSO complexes in the troposphere have
been only predicted by computational calculation; see: Sayin, H.;
McKee, M. L. J. Phys. Chem. A. 2004, 108, 7613.
(19) The DFT calculations (B3LYP/6-31G(d), and LANL2DZ (with
ECP) for I) were performed with the Gaussian 09 program.
(20) The example of computational calculation of stabilized halogen
We thank the Ministry of Education, Culture, Sports, Science &
Technology, Japan, for a Grant-in-Aid for Scientific Research on
Innovative Areas, 2105. We also grateful to Dr. Keiko Kuwata of
Kyoto University (present Nagoya University) for the CSI-MS
analyses.
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