Catalytic enantioselective alkylation of sulfenate anions to chiral heterocyclic sulfoxides using halogenated pentanidium salts

Article abstract of DOI:10.1002/anie.201407512

We report halogenated pentanidiums as phasetransfer catalysts for the asymmetric alkylation of sulfenate anions to various sulfoxides with high enantio selectivities (up to 99% ee) and yields (up to 99%). This approach gives access to enantioenriched heterocyclic sulfoxides that might not be compatible with strong oxidants or organometallic reagents. Computational studies have revealed that the multiple non-covalent interactions such as halogen bonds and nonclassical hydrogen bonds are involved.

Full text of DOI:10.1002/anie.201407512

Angewandte
Chemie
DOI: 10.1002/anie.201407512
Organocatalysis
Catalytic Enantioselective Alkylation of Sulfenate Anions to Chiral
Heterocyclic Sulfoxides Using Halogenated Pentanidium Salts**
Lili Zong, Xu Ban, Choon Wee Kee,* and Choon-Hong Tan*
Abstract: We report halogenated pentanidiums as phase-
transfer catalysts for the asymmetric alkylation of sulfenate
anions to various sulfoxides with high enantioselectivities (up
to 99% ee) and yields (up to 99%). This approach gives access
to enantioenriched heterocyclic sulfoxides that might not be
compatible with strong oxidants or organometallic reagents.
Computational studies have revealed that the multiple non-
covalent interactions such as halogen bonds and nonclassical
hydrogen bonds are involved.
thesis of sulfoxides remain scant.[7a,c] Recent attempts with
cinchona alkaloids as phase-transfer catalysts were reported
by Perrio and co-workers; despite the high yields, the ee
values did not exceed 60%.[8]
Asymmetric phase-transfer catalysis is a convenient, scal-
able and environmentally benign method to prepare com-
pounds in high enantiopurity.[9] Despite the success of several
reported phase-transfer catalysts (PTCs) in the literature, it is
still meaningful to design new ones with a high level of
stereocontrol.[10] Recently, our group has developed structur-
ally novel pentanidiums[11] based on quaternized sp2-hybrid-
ized N-atoms[12] as PTCs (Figure 1). The pentanidiums are
highly amenable to variation due to the ease of changing the
R group on the catalyst to vary electronic and steric proper-
ties. Herein, we introduce novel pentanidiums with halogen-
ated benzyl R groups (1b, 1c, and 1d) and their application in
the enantioselective synthesis of heterocyclic sulfoxides from
sulfenate anions.
Optically active sulfoxides are extensively used as chiral
auxiliaries, chiral ligands for metal complexes, and organo-
catalysts.[1] This unique moiety is also found in several
marketed drugs such as esomeprazole (proton pump inhib-
itor), armodafinil (eugeroic), and sulindac (anti-inflamma-
tion).[2] Currently, there are two main strategies to synthesize
enantioenriched sulfoxides: the nucleophilic substitution of
nonracemic sulfinates using organometallic reagents (Ander-
sen method)[3] and the direct oxidation of prochiral sulfides
catalyzed by metal complexes (Kagan and Modena meth-
ods).[4] Recently, a metal-free approach using chiral imidodi-
phosphoric acid catalysts for highly enantioselective sulfide
oxidation employing aqueous H2O2 was demonstrated by List
and co-workers.[5]
Figure 1. Various chiral pentanidium salts.
Due to the importance of such compounds, the develop-
ment of catalytic and enantioselective methodologies with
broad substrate scope is still highly desirable. In the classical
Andersen method and its variants, the reactive sulfur center is
electrophilic and a stoichiometric quantity of chiral auxiliary
is required (although the recycling of auxiliary is possible in
some cases[3b]). Recently, a complementary approach based
on sulfenate anions (RSOÀ)[6] with a nucleophilic sulfur center
has emerged as a viable method for the enantio- and
diastereoselective synthesis of sulfoxides.[7] However, reports
with sulfenate anions in the catalytic enantioselective syn-
Thiophene derivatives[13] are important heteroaromatic
compounds, which have been used prevalently in functional
materials and pharmaceutical industry due to their distinct
electronic and biological activities (see the Supporting
Information, SI, Figure S1). Based on currently available
methodologies to synthesize optically active sulfoxides, sulf-
oxides which contain thienyl group pose significant challenges
(SI, Scheme S4), such as potential incompatibility with strong
oxidants[14] or organometallic reagents.[15] Herein, we report
the nucleophilic displacement of alkyl halides by sulfenate
anions using pentanidiums as catalysts to synthesize various
heterocyclic sulfoxides such as thienyl sulfoxides with high
enantioselectivity.
We began by investigating the enantioselective benzyla-
tion of 2-thienyl sulfenate anion generated in situ with
pentanidiums as PTCs by using benzyl bromide as the
electrophile. Preliminary variations of reaction parameters
such as pentanidiums, sulfenate anion precursors, solvent,
inorganic base, and temperature were performed (SI).
Representative variations are presented in Table 1. The b-
sulfinyl methyl ester 2a was found to be the appropriate
sulfenate anion precursor. Halogenated pentanidiums have
significant effects on both the course and enantioselectivity of
the reaction. We found that the level of enantioselectivity
[*] L. Zong, X. Ban, C. W. Kee, Prof. Dr. C.-H. Tan
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
21 Nanyang Link, Singapore 637371 (Singapore)
E-mail: CWKee@ntu.edu.sg
choonhong@ntu.edu.sg
[**] We gratefully acknowledge the financial support by grants from the
NTU (M4080946.110 and RG 6/12 M40110018.110) and a scholar-
ship (to L.Z.) from the National University of Singapore (NUS).
Generous support from the NUS high performance computing
center for providing free computing resources (to C.W.K) is
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407512.
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Table 1: Benzylation of the 2-thienyl sulfenate anion catalyzed by
pentanidiums.
with various alkyl halides produced the corresponding sulf-
oxides in good yields and excellent enantioselectivities.
Benzyl bromides with electron-withdrawing and electron-
donating substituents are well tolerated (entries 1–6). It
should be noted that benzyl bromides caused an erosion of
the enantioselectivity in the previous report of a related
reaction by Perrio group.[8] Moreover, commercially available
PTCs were also tested and low enantioselectivity was
obtained (below 15% ee, SI, Table S5). The construction of
stereogenic sulfoxides bearing allylic and propargylic sub-
stituents could also be achieved in a similar manner
(entries 7–9). Moreover, the alkylation of 2-theinyl sulfenate
anion with alkyl iodides provided sulfoxides 3j–3l with good
enantioselectivities (77–81% ee, entries 10–12). These results
indicated that both alkyl and benzyl groups can be installed
using this methodology. For specific benzyl bromide deriva-
tives with electron-withdrawing substituents (entries 3–4) and
alkyl iodides (entries 10–12), brominated pentanidium 1c
gave marginally better enantioselectivities.
Entry
Catalyst
X
t [h]
Product
Yield [%][a]
ee [%][b]
1
2
1a
1c
1c
1d
–
1a
1a
1b
1d
Br
Br
Br
Br
Br
–
Cl
Cl
Cl
48
48
48
48
1
24
48
48
24
3a
3a
3a
3a
–
8
8
76
72
86
72
–
40
27
9
61
81
88
91
–
–
–
83
90
3[c]
4[c]
5[d,e]
6[e]
7[e]
8[e]
9[e]
3a/8 (1:2)[f]
3a
29
[a] Yield of isolated product. [b] Determined by HPLC analysis. [c] The
reaction was conducted at À708C with saturated aqueous CsOH
solution (24 mL) in a solvent mixture of CPME (0.2 mL)/Et2O (0.6 mL).
[d] 2a decomposed under basic conditions without PTCs at RT.
[e] Saturated aqueous CsOH solution (24 mL) was used. [f] The ratio was
The scope of this reaction was further examined under the
optimized conditions with various b-heteroaryl and aryl
sulfinyl methyl esters (4a–e, 5a–f) as sulfenate anion pre-
cursors (Scheme 2). They participated in the reaction effi-
ciently to provide the anticipated sulfoxides in high yields
with good to excellent enantioselectivities. The methyl
substituted 2-thienyl sulfinyl ester 4a was transformed to
the corresponding highly enantioenriched sulfoxides 6a–f.
Interestingly, the reaction between the sulfenate anion
generated from b-sulfinyl ester 4a and m-xylene dibromide
produced the bis-sulfoxide 6g in good yield (87%, dl/meso =
4.75:1) and excellent enantioselectivity (> 99% ee). Sulfox-
ides 6k–v containing benzothiophene were also obtained with
excellent enantioselectivities. Benzimidazole sulfoxide 7a,
a reminiscence to the drug esomeprazole, was obtained in
high enantioselectivity (90% ee). Sulfenate anions containing
furyl, benzofuryl, and pyridyl moieties can be tolerated and
converted into the corresponding sulfoxides 7c–f with a slight
decrease of the enantioselectivity. To the best of our knowl-
edge, this methodology allows the widest range of hetero-
cyclic sulfoxides to be obtained in high optical purity and
yield.[16] The efficiency of pentanidium 1d was further tested
by reducing the catalyst loading to 0.25 mol% in a 1 mmol
scale reaction (Scheme 1).The sulfoxide 6q was obtained in
85% yield with an excellent enantioselectivity of 91% ee.
Based on experiments that were conducted to provide
more insight into the reaction mechanism (Table 1, entries 5–
9), a working model was proposed (Scheme 3).[17] The
inorganic base promotes the deprotonation of b-sulfinyl
methyl ester 2a to give the corresponding cesium enolate,
which leads to the generation of the sulfenate anion and the
release of methyl acrylate through a retro-Michael process
even without PTC. However, the sulfenate anion will undergo
alkaline hydrolysis under basic conditions in the absence of
PTC (Table 1, entry 5).[6b] In the presence of the chiral PTC,
cationic exchange and retro-Michael processes lead to a chiral
ion pair A, which can readily react with electrophile RX (X =
Br) to afford the product 3a (Scheme 3, path a). When the
reaction was performed in the presence of PTC but not the
alkyl halides, 2a produced thiophene sulfenate 8 through
1
determined by H NMR analysis. The absolute configuration of 3a was
assigned to be R by single-crystal X-ray diffraction of 6q and DFT-
calculated specific optical rotation of 3a and 6q.
significantly increases with the introduction of halogen to
pentanidiums (entries 1–4). Pentanidium 1d can promote the
reaction with an excellent enantioselectivity of 91% ee
(Table 1, entry 4). When a less reactive electrophile benzyl
chloride (entries 5–9) was used, the ratio of desired chiral
product 3a to 8 increases as the substituent is varied from H to
I (from 1a to 1d).
A variety of alkyl halides was examined as electrophile
under optimized conditions (Table 2). With 1 mol% of
pentanidium 1d, the alkylation of 2-thienyl sulfenate anion
Table 2: Asymmetric alkylation of 2-thienyl sulfenate anion catalyzed
with iodo-pentanidium 1d.
Entry
R
Product
t [h]
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
PhCH2
3a
3b
3c
3d
3e
3 f
12
12
12
12
12
12
87
88
84
83
84
84
92
94
4-MeC6H4CH2
4-ClC6H4CH2
4-CF3C6H4CH2
3-MeOC6H4CH2
2-naphthylCH2
90[c]
94[c]
90
92
7
8
9
3g
3h
3i
36
21
21
82
79
77
92
92
82
10[d]
11[d]
12[d]
Me
3j
3k
3l
40
28
20
65
66
69
77[c]
81[c]
81[c]
CH3CH2CH2
CH3(CH2)6CH2
[a] Yield of isolated product. [b] Determined by HPLC analysis. [c] Data
was obtained in the presence of 1 mol% of brominated pentanidium 1c.
[d] 2.5 Equivalents of alkyl iodides was used.
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Chemie
ated pentanidium 1a and benzyl chloride,
a less active electrophile, only sulfenate 8
instead of sulfoxide 3a was observed (Table 1,
entry 7; Scheme 3, path b).[7a] However, it is
noteworthy that sulfoxide 3a can be formed
Scheme 1. Further reduction of catalyst loading of pentanidium 1d.
Scheme 3. Working model of sulfenate anion intermediate.
when halogenated pentanidiums were used as catalysts
(Table 1, entry 9; Scheme 3, path a). These results demon-
strate the ability of halogenated pentanidiums to activate
both the electrophile and the sulfenate anion nucleophile.
To obtain structural features of relevant transition state
(TS) structures, we performed theoretical studies with the
ONIOM method[19] as implemented in Gaussian09.[20] M06[21]
was used as the high level and UFF molecular mechanics
method[22] as the low level (tert-butyl group and phenyl ring of
catalyst).
The most stable TS structure for the R- and S-sets of the
TS, in terms of the Gibbs free energy, is shown in Figure 2.
The R-TS is more stable than the S-TS by 1.2 kcalmolÀ1,
Figure 2. Optimized most stable (in terms of G) R- and S-TS at M06/
BS1:UFF calculated with ONIOM method in Gaussian09A2 (see SI for
ONIOM partition and definition of BS1). Most of the hydrogen atoms
at carbon are hidden. CYLview was used for preparation of the
graphics.[23]
which is in good agreement with the experimental e.r. of
[24]
À
Scheme 2. Scope of pentanidium-catalyzed alkylation to enantioen-
riched heterocyclic sulfoxides. [a] Ratio of dl/meso=4.75:1, determined
by 1H NMR analysis. [b] The reaction was conducted at À408C.
[c] Data was obtained in the presence of brominated pentanidium 1c.
The absolute configuration was assigned as R by analogy to 6q.
CPME=cyclopentyl methyl ether.
95.5:4.5 (R/S). The Br I halogen bond (XB) between the
leaving Br and the aryl iodide donor of the side chain of the
iodinated pentanidium catalyst 1d is evident in both R- and S-
TS structures (Figure 2). We also applied the noncovalent
interaction (NCI) index[25] to analyze the NCI between
catalyst 1d and the substrates leading to product 3a (see
SI). This computational study revealed that the halogen bonds
and nonclassical hydrogen bonds (NCHB)[26] stabilize the
substrates in the TS.
oxygen-Michael addition of the sulfenate anion to methyl
acrylate released during the retro-Michael process (Table 1,
entry 6; Scheme 3, path b).[18] Moreover, using nonhalogen-
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In summary, we have described a highly enantioselective
alkylation of sulfenate anions to prepare enantioenriched
sulfoxides using halogenated pentanidiums. These heterocy-
clic sulfoxides could potentially be used as chiral ligands or
organocatalyst and bioactive molecules.[27] Moreover, this
work demonstrates the possibility of incorporating halogen
bonding as primary noncovalent interaction in catalyst design.
Received: July 23, 2014
Published online: September 10, 2014
Keywords: Brønsted bases · density functional calculations ·
.
guanidinium · organocatalysis · pentanidium
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