Oxidative hydroxylation mediated by alkoxysulfonium ions

Article abstract of DOI:10.1021/ol203467v

Oxidative hydroxylation of toluene derivatives via alkoxysulfonium ion intermediates was achieved by integration of anodic oxidation and hydrolysis to give benzyl alcohols which are also susceptible to oxidation. Alkenes were also oxidized to give 1,2-diols without overoxidation. The integration of electrochemical oxidative cyclization and hydrolysis was achieved using alkenes bearing a nitrogen atom in an appropriate position to give cyclic β-amino-substituted alcohols.

Full text of DOI:10.1021/ol203467v

ORGANIC
LETTERS
2012
Vol. 14, No. 3
938–941
Oxidative Hydroxylation Mediated by
Alkoxysulfonium Ions
Yosuke Ashikari, Toshiki Nokami, and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto University, Katsura, Nishikyou-ku, Kyoto 615-8510, Japan
yoshida@sbchem.kyoto-u.ac.jp
Received December 28, 2011
ABSTRACT
Oxidative hydroxylation of toluene derivatives via alkoxysulfonium ion intermediates was achieved by integration of anodic oxidation and
hydrolysis to give benzyl alcohols which are also susceptible to oxidation. Alkenes were also oxidized to give 1,2-diols without overoxidation. The
integration of electrochemical oxidative cyclization and hydrolysis was achieved using alkenes bearing a nitrogen atom in an appropriate position
to give cyclic β-amino-substituted alcohols.
Combining multiple steps without isolating intermediates is
important to enhance the power and efficiency of organic
synthesis.1,2 The integration of chemical reactions enables syn-
thetic transformations that would be otherwise very difficult
or impossible, such as transformations involving the genera-
tion of unstable highly reactive species that are swiftly utilized
for a subsequent reaction before they decompose.3
synthesis. Because products are exposed to the oxidation
conditions, significant amounts of undesired overoxida-
tion products are formed during the course of the reac-
tion.4 Thus, access to a general oxidation method which
prevents overoxidation is highly desirable.
Electrochemical reactions using electron transfer on
the surface of the electrode serve as a powerful means
of selective oxidation.5,6 Electrochemistry allows for the
selective removal of electrons under mild conditions. How-
ever, the electrochemical oxidation also often suffers from
overoxidation.7 For example, anodic oxidation of alkenes
and that of 1,2-diols often lead to carbonÀcarbon bond
cleavage.7a,b
Oxidation of organic compounds to products that are
susceptible to oxidation provides a challenge for organic
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r
10.1021/ol203467v
Published on Web 01/24/2012
2012 American Chemical Society

We have developed the “cation pool” method using low-
temperature electrochemical oxidation.8À10 Based on the
“cation pool” method, we have recently reported alkoxy-
sulfonium ion mediated integrated electrochemicalÀchemical
oxidation, which solves the formidable overoxidation
problem (Scheme 1a).11 Electrochemically generated car-
bocations are converted to the alkoxysulfonium ions,
which give the corresponding carbonyl compounds by
treatment with amines. We envisioned that hydrolysis of
the electrochemically generated alkoxysulfonium ions
would give the corresponding alcohols (Scheme 1b). Be-
cause the oxidation step (electrolysis) and the alcohol-
forming step (hydrolysis) are separated, the products
are not exposed to the oxidation conditions and overoxi-
dation should not occur. This integrated hydroxylation
reaction and previously reported integrated electroche-
micalÀchemical oxidation to carbonyl compounds are
complementary. Herein we report on the aforementioned
oxidation reaction via electrochemical oxidation to give
alkoxysulfonium ions and their hydrolysis.
First, we began by studying the oxidation of toluenes to
benzyl alcohols.12 Toluenes were electrochemically oxi-
dized in the presence of DMSO in CH2Cl2 using Bu4NBF4
as a supporting electrolyte at rt. A divided cell equipped
with a C-felt anode and a Pt-plate cathode was used. After
electrolysis, the resulting solution was hydrolyzed by aqu-
eous NaOH or MeOH/H2O at 0 °C to give the correspond-
ing benzyl alcohols in good yields (Table 1).
Table 1. Integrated OxidationÀHydrolysis of Toluenesa
Scheme 1. Selective Oxidation Mediated by Alkoxysulfonium
Ion
a Reactions were carried out on a 0.25 mmol scale. b Isolated yield
after chromatography. c Hydrolyzed by aqueous NaOH.
Ingeneral, electrochemical benzylic monohydroxylation
is quite difficult because the products are also oxidized to
give benzaldehyde and benzoic acid.7c,13 In fact, the oxida-
tion potential of 4-methoxybenzyl alcohol (1.42 V vs SCE)
lies close enough to that of 4-methoxytoluene (1.38 V vs
SCE)14 to make selective oxidation of the toluene near
impossible. However, the present transformation enables
direct oxidation of toluenes to benzyl alcohols with high
selectivity. Presumably a positive charge in the alkoxysul-
fonium ion retards further oxidation by raising the oxida-
tion potential.15 Alternatively, overoxidation is prevented
because a positively charged alkoxysulfonium ion cannot
readily approach a positively charged anode. Interestingly,
toluenes having more than two methyl groups were selec-
tively oxidized to monoalcohols (Table 1, entries 3 and 4).
Preventionofthe oxidationofthe second methylgroupcan
also be attributed to the positive charge of the alkoxysul-
fonium ion intermediate.
(7) (a) Raoult, E.; Sarrazin, J.; Tallec, A. J. Appl. Electrochem. 1984,
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Fry, A. J. Electrochim. Acta 2003, 48, 1837.
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Kinomura, N.; Yamamoto, A.; Fujiwara, K. J. Am. Chem. Soc. 1999,
121, 9546. (b) Suga, S.; Okajima, M.; Fujiwara, K.; Yoshida, J. J. Am.
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2008, 37, 1008.
(9) Alkoxycarbenium ion pools: (a) Suga, S.; Suzuki, S.; Yamamoto,
A.; Yoshida, J. J. Am. Chem. Soc. 2000, 122, 10244. (b) Okajima, M.;
Suga, S.; Itami, K.; Yoshida, J. J. Am. Chem. Soc. 2005, 127, 6930. (c)
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2006, 128, 7710.
(10) Diarylcarbenium ion pools: (a) Okajima, M.; Soga, K.; Nokami,
T.; Suga, S.; Yoshida, J. Org. Lett. 2006, 8, 5005. (b) Nokami, T.; Ohata,
K.; Inoue, M.; Tsuyama, H.; Shibuya, A.; Soga, K.; Okajima, M.; Suga,
S.; Yoshida, J. J. Am. Chem. Soc. 2008, 130, 10864. (c) Okajima, M.;Soga,
K.; Watanabe, T.; Terao, K.; Nokami, T.; Suga, S.; Yoshida, J. Bull.
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Morofuji, T.; Tahara, K.; Tobe, Y.; Yoshida, J. Chem. Commun. 2011,
47, 5575.
Next, we examined oxidative dihydroxylation of alkenes.16
trans-Stilbene was electrochemically oxidized in the presence
of DMSO to give the bisalkoxysulfonium ion, whose struc-
ture was determined by 1H NMR spectroscopy. Hydrolysis
(13) Anodic acetoxylation usually gives monoacetolylation products;
see: (a) Srivastav, M. K.; Saraswat, A.; Singh, R. K. P. Orient. J. Chem.
2010, 26, 61. (b) Tajima, T.; Kishi, Y.; Nakajima, A. Electrochim. Acta
2009, 54, 5959.
(11) Ashikari, Y.; Nokami, T.; Yoshida, J. J. Am. Chem. Soc. 2011,
133, 11840.
(12) For recent advances within this area: (a) Wang, S.; Lie, F.; Lim,
E.; Li, K.; Li, Z. Adv. Synth. Catal. 2009, 351, 1849. (b) Monfared, H. H.;
Kheirabadi, S.; Lalami, N. A.; Mayer, P. Polyhedron 2011, 30, 1375.
(14) The oxidation potentials were measured by RDE in LiClO4/
CH3CN using an SCE as a reference electrode. See Supporting
Information.
(15) In the presence of DMSO, a substrate having a higher oxidation
potential than that of DMSO is not electrolyzed. See ref 11.
Org. Lett., Vol. 14, No. 3, 2012
939

with aqueous NaOH gave the corresponding 1,2-diol,
hydrobenzoin, in 75% yield with high diastereoselectivity
(R*R*/R*S* = 93:7) (Table 2, entry 1). cis-Stilbene gave
the same stereoisomer preferentially, although the diastereo-
selectivity was lower (R*R*/R*S* = 66:34) (entry 2).
variety of methods for dihydroxylation of alkenes have
been reported, a highly toxic osmium catalyst18 or explosive
peroxides19 are required in many cases. This present
method, however, serves in a mild and environmentally
benign manner in the dihydroxylation of alkenes.
Based on the diastereoselectivity observed, we propose a
mechanism shown in Scheme 2, although the details are
not clarified as yet. Presumably intermediate I is formed by
two-electron oxidation. A conformation in which two
phenyl groups are apart is favorable. The sulfonium ion
moiety in I directs the attack of the second DMSO to give
bisalkoxysulfonium ion II,20 which gives the diol upon
hydrolysis.21
Table 2. Integrated OxidationÀHydrolysis of Alkenesa
Scheme 2. Proposed Mechanism for Stereoselective Dihydrox-
ylation of Alkenes
Finally, we applied this present oxidation method to
intramolecular alkene cyclization, which assists a powerful
synthetic method for the construction of heterocyclic
compounds.22,23 Electrolysis and subsequent hydrolysis
of β-alkyl styrenes having a nitrogen nucleophilic moiety
in an appropriate position elicited cyclization to give the
corresponding alcohols having a pyrrolidine ring in high
yields (Table 3). Interestingly, the reaction of (E)-alkenes
gave the anti-addition product selectively (Table 3, entries
1 and 2), although the reaction of the (Z)-alkene gave a 1:1
mixture (Table 3, entry 3).
a Reactions were carried out on a 0.25 mmol scale. b Isolated yield
after chromatography. c Determined by 1H NMR. d cis-Stilbene was used.
Other stilbene derivatives underwent dihydroxylation
under the same conditions with similar diastereoselectivity
(entries 3 and 4). A β-methylstyrene derivative was also
oxidized to give the corresponding diol, although the yield
and diastereoselectivity were lower (entry 5). To the best of
our knowledge, this is the first example of the direct
electrochemical dihydroxylation of alkenes.17 Although a
Table 3. Integrated OxidationÀHydrolysis of Alkenes Bearing a
Nitrogen Nucleophilic Moietya
(16) For recent advances within the area: (a) Plietker, B.; Niggemann,
A.; Pollrich, A. Org. Biomol. Chem. 2004, 2, 1116. (b) Emmanuvel, L.;
Shaikh, T. M. A.; Sudalai, A. Org. Lett. 2005, 7, 5071. (c) Plietker, B.;
Niggemann, A. J. Org. Chem. 2005, 70, 2402. (d) de Boer, J. W.;
Brinksma, J.; Browne, W. R.; Meetsma, A.; Alsters, P. L.; Hage, R.;
Feringa, B. L. J. Am. Chem. Soc. 2005, 127, 7990. (e) Oldenburg, P. D.;
Que, L., Jr. Catal. Today 2006, 117, 15. (f) Li, Y.; Song, D.; Dong, V. M.
J. Am. Chem. Soc. 2008, 130, 2962. (g) Plietker, B.; Neisius, N. M. J. Org.
Chem. 2008, 73, 3218. (h) de Boer, J. W.; Brinksma, J.; Browne, W. R.;
Harutyunyan, S. R.; Bini, L.; Tiemersma-Weyman, T. D.; Alsters, P. L.;
Hage, R.; Feringa, B. L. Chem. Commun. 2008, 3747. (i) Wang, A.; Jiang,
H.; Chen, H. J. Am. Chem. Soc. 2009, 131, 3846. (j) Wang, W.; Wang, F.;
Shi, M. Organometallics2010, 29, 928. (k) Seayad, J.;Seayad, A. M.;Chai,
C. L. L. Org. Lett. 2010, 12, 1412.
(17) Dihydroxylation of alkenes by an electrochemically regenerated
Os catalyst was reported: Torii, S.; Liu, P.; Tanaka, H. Chem. Lett. 1995,
24, 319.
(18) (a) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem.
Rev. 1994, 94, 2483. (b) Dobler, C.; Mehltretter, G. M.; Sundermeier, U.;
Beller, M. J. Organomet. Chem. 2001, 621, 70.
(19) (a) Adam, W., Ed. Peroxide Chemistry: Mechanistic and Pre-
parative Aspects of Oxygen Transfer; Wiley-VCH: Weinheim, Germany,
2000. (b) Rappoport, Z., Ed. The Chemistry of Peroxides; Wiley: Chiche-
ster, U.K., 2006; Vol. 2.
a Reactions were carried out on a 0.13 mmol scale. b Isolated yield
after chromatography. c Determined by 1H NMR. d The product was
isolated by recrystallization. e A small amount of the other diastereomer
seemed to be formed as a byproduct, although it was not fully identified.
f cis-Isomer of starting material was used.
940
Org. Lett., Vol. 14, No. 3, 2012

To gain deeper insight into the mechanism, an experi-
ment using 18O-labeled DMSO (containing 70% 18O) was
conducted.24 The product containing 18O (70% 18O) was
obtained in 81% yield as shown in Scheme 3. It indicates
that the oxygen atom of the hydroxyl group in the product
is derived from the oxygen atom of DMSO. Therefore, the
CÀO bond in the alkoxysulfonium ion intermediate was
not cleaved. Instead, the OÀS bond was cleaved upon
hydrolysis. This means that the inversion of configuration
at the benzylic carbon is not plausible.
apart from eachother. The backside attackofDMSOgives
the alkoxysulfonium ion intermediate IV, which gives the
product upon hydrolysis.
Scheme 4. Proposed Mechanism for Stereoselective Cyclization
Scheme 3. Mechanistic Study Using 18O-DMSO
In conclusion, we have developed a novel oxidative
hydroxylation mediated by electrochemically generated
alkoxysulfonium ions. Toluenes are oxidized to benzyl
alcohols, and alkenes to 1,2-diols stereoselectively. In
addition, the method can be applied to intramolecular
alkene cyclization to give alcohols bearing a pyrrolidine
ring. The intermediacy of alkoxysulfonium ions allows
selective oxidation of substrates to alcohols which are
easily oxidized under conventional conditions. The inte-
grated hydroxylation reaction and previously reported
integrated electrochemicalÀchemical oxidation to carbo-
nyl compounds can be used in a complementary way.
Further work to explore the full scope of the present
approach and its applications to the synthesis of molecules
with interesting functions is in progress.
A mechanism involving formation of a bisalkoxysulfo-
nium ion followed by internal SN2 displacement by the
amino group seems to be a reasonable explanation for the
stereoselectivity. However, a mechanism involving the
cationic intermediate stabilized by the intramolecular in-
teraction with the amino group25 seems to be more plau-
sible (Scheme 4), because the oxidation potential of the
substrate is lower than those of simple alkyl-substituted
styrenes.26 Thus, the cation radical generated by one-
electron oxidation undergoes an intramolecular nucleo-
philic reaction with a nitrogen atom. After deprotonation
an additional one-electron oxidation takes place to givethe
benzyl cation intermediate III having a conformation in
which the benzylic cation center interacts with the amino
group as well as the phenyl group and the alkyl chain are
Acknowledgment. This work was supported in part by
the Grant-in-Aid for Scientific Research on Innovative
Areas (No. 2015) and a Grant-in-Aid for Global COE
program from MEXT, Japan.
(20) 1H NMR analysis indicated that the bisalkoxysulfonium ion was
obtained as a single stereoisomer. See ref 11 and its Supporting
Information.
(21) It was reported that an optically pure alcohol was obtained by
hydrolysis of an optically pure alkoxysulfonium ion. See: Ray, D. G.,
III; Koser, G. F. J. Am. Chem. Soc. 1990, 112, 5672.
Supporting Information Available. Experimental pro-
cedures and special data for key compounds. This mate-
rial is available free of charge via Internet at http://pubs.
acs.org.
(22) Some recent examples of oxidative alkene cyclization reactions:
(a) Jensen, K. H.; Pathak, T. P.; Zhang, Y.; Sigman, M. S. J. Am. Chem.
Soc. 2009, 131, 17074. (b) Pathak, T. P.; Gligorich, K. M.; Welm, B. E.;
Sigman, M. S. J. Am. Chem. Soc. 2010, 132, 7870. (c) McDonald, R. I.;
White, P. B.; Weinstein, A. B.; Tam, C. P.; Stahl, S. S. Org. Lett. 2011, 13,
2830. (d) McDonald, R. I.; Liu, G.; Stahl, S. S. Chem. Rev. 2011, 111, 2981.
(23) For electron-transfer-mediatedcyclizationreactions: (a) Tokuda,
M.; Miyamoto, T.; Fujita, H.; Suginome, H. Tetrahedron 1991, 47, 747.
(b) Tokuda, M.; Fujita, H.; Miyamoto, T.; Suginome, H. Tetrahedron
1993, 49, 2413. (c) Xu, H.-C.; Moeller, K. D. J. Am. Chem. Soc. 2008, 130,
13542. (d) Liu, L.; Floreancig, P. E. Angew. Chem., Int. Ed. 2010, 49, 3069.
(e) Xu, H.-C.; Moeller, K. D. Angew. Chem., Int. Ed. 2010, 49, 8004. (f)
Redden, A.; Moeller, K. D. Org. Lett. 2011, 13, 1678.
(25) Some recent examples of reactions involving a cation intermedi-
ate stabilized by an intramolecular nitrogen atom interaction: (a) Ori,
M.; Toda, N.; Takami, K.; Tago, K.; Kogen, H. Angew. Chem., Int. Ed.
2003, 42, 2540. (b) Krow, G. R.; Gandla, D.; Guo, W.; Centafont, R. A.;
Lin, G.; DeBrosse, C.; Sonnet, P. E.; Ross, C. W., III; Ramjit, H. G.;
Carroll, P. J.; Cannon, K. C. J. Org. Chem. 2008, 73, 2114. (c) Lovick,
H. M.; Michael, F. E. J. Am. Chem. Soc. 2010, 132, 1249.
(26) The oxidation potentials of the substrates (R = CH3: 1.34 V vs
SCE, R = H: 1.31 V) are less positive than that of 1-phenylpropene
(1.42 V). See ref 11 and references cited therein.
(24) Fenselau, A. M.; Moffatt, J. G. J. Am. Chem. Soc. 1966, 88, 1762.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 3, 2012
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