Enantioselective organocatalyzed α sulfenylation of aldehydes

Article abstract of DOI:10.1002/anie.200462101

The facile elaboration of optically active α-sulfanyl-substituted aldehydes makes their direct preparation from unmodified aldehydes and 1-benzylsulfanyl-1,2,4-triazole particularly valuable. The substituted aldehydes are formed with excellent enantiosele

Full text of DOI:10.1002/anie.200462101

Communications
aldehydes and the merits of organocatalytic processes, which
circumvent the undesired association of sulfur reagents with
metal catalysts. To date, all practical methods for the
preparation of chiral a-sulfenylated aldehydes have been
multistep procedures that involve chiral auxiliaries.[5] Accord-
ing to our knowledge, no catalytic processes are available for
the preparation of these useful optically active building
blocks. Herein, we report the first direct organocatalyzed
enantioselective a sulfenylation of aldehydes [Eq. (1)].
In analogy to organocatalyzed halogenation reactions,
sulfenylations are substitution reactions, which are inherently
more difficult to perform enantioselectively than addition
processes as a result of the more flexible nature of the
transition state. Therefore, the design of a suitable leaving
group (Lg) is crucial. Furthermore, a second substituent that
can serve as a protecting group (Pg) needs to be chosen for
sulfenylation reactions. To provide for facile product elabo-
ration, S-benzyl-protected a-sulfenylated aldehydes were
chosen as synthetic targets, as there are well-established
methods for the cleavage of this protecting group.[6] Similarly,
the development of a practical sulfenylation process called for
a leaving group that could be separated readily from the
product after the reaction. Additionally, the protonated
nucleofuge should be a neutral species that does not affect
the equilibrium of enamine formation or deactivate the amine
catalyst. In line with these considerations, initial experiments
were carried out for the a sulfenylation of isovaleraldehyde
(1a) with the reagents shown in Scheme 1, all of which
Asymmetric Catalysis
Enantioselective Organocatalyzed a Sulfenylation
of Aldehydes**
Mauro Marigo, Tobias C. Wabnitz, Doris Fielenbach,
and Karl Anker Jørgensen*
Optically active a-heterosubstituted aldehydes are versatile
building blocks for the synthesis of chiral molecules that bear
heteroatom functionalities. Recent advances in the synthesis
of these molecules have been focused on the development of
direct organocatalytic procedures[1] that avoid metal catalysts
and reagents. Organocatalyzed additions of simple carbonyl
compounds to diazocarboxylates and nitrosobenzene allow
the incorporation of nitrogen-[2] and oxygen-containing[3]
a substituents into aldehydes and ketones with excellent
levels of stereoselectivity. Very recently, organocatalyzed
substitution reactions of N-halosuccinimides and related
electrophiles have been developed for the a halogenation of
aldehydes and ketones.[4] In contrast, the analogous asym-
metric introduction of sulfur-based substituents has not been
reported, in spite of the synthetic potential of a-sulfenylated
Scheme 1. Sulfenylation reagents with different leaving groups tested
in the organocatalyzed enantioselective a sulfenylation of isovaleralde-
hyde (1a).
[*] M. Marigo, Dr. T. C. Wabnitz, Dr. D. Fielenbach,
Prof. Dr. K. A. Jørgensen
The Danish National Research Foundation: Center for Catalysis
Department of Chemistry, Aarhus University
8000 Aarhus C (Denmark)
Fax: (+45)8919-6199
E-mail: kaj@chem.au.dk
contain weakly basic heterocyclic nitrogen-centered nucleo-
fuges, in the presence of different pyrrolidine derivatives.
Whereas the phthalimide and succinimide reagents 2a and 2b
underwent only sluggish conversion, and the imidazole-
derived electrophile 2c turned out to be unstable, the desired
a-sulfenylated product was obtained from isovaleraldehyde
in good yield with the reagents 2d and 2e. Finally, the novel
triazole derivative 1-benzylsulfanyl-1,2,4-triazole (2e) was
[**] This work was made possible by a grant from The Danish National
Research Foundation. M.M. and T.C.W. thank the EU (HMPT-CT-
2001-00317) for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200462101
Angew. Chem. Int. Ed. 2005, 44, 794 –797

Angewandte
Chemie
chosen as the electrophilic sulfur source,[7] as it exhibited the
highest level of reactivity.
Systematic studies of a model system comprising isovaler-
aldehyde (1a), the sulfur electrophile 2e, and 10 mol% of a
chiral secondary amine organocatalyst at ambient temper-
ature are summarized in Table 1. With the l-proline-derived
or formation of an unreactive hemiaminal by reaction with
the aldehyde. However, trimethylsilyl protection of the free
hydroxy moiety of 4d restored reactivity and enhanced
selectivity (Table 1, entry 8). Further improvements were
made through variation of the aryl substituents in the catalyst
structure. The silylated l-prolinol derivatives 4 f–h with
sterically demanding aryl substituents furnished the product
with high enantiomeric excess (Table 1, entries 9–11). The
fluorinated derivative 4h was identified as the best of these
catalysts, as it gave 3a in 90% yield and with 98% ee, with
only traces of concomitant a,a-disulfenylation and no product
racemization (Table 1, entry 11). However, the reaction rate
was found to be strongly dependent on the purity of the
reagent 2e, which slowly degraded upon storage. Neverthe-
less, in cases of slow conversion the turnover could be
increased by adding salts such as LiClO4 to the reaction
mixture, with only a minor decrease in enantiomeric excess
(Table 1, entry 12). Protic acids also accelerated the reaction,
but led to a more pronounced loss of enantiomeric excess
(Table 1, entry 13).[8]
Table 1: Organocatalyzed enantioselective a sulfenylation of isovaleral-
dehyde (1a).[a]
Entry
4
Solvent
Yield [%][b]
ee [%][c]
[d,e]
Under the optimized conditions, a series of aldehydes 1a–
g underwent enantioselective a sulfenylation in the presence
of 2e as the S-benzyl-protected sulfur donor and 4h as the
catalyst. To facilitate workup, the reaction products were
isolated as the alcohols 5 after in situ reduction of the
aldehyde moiety with NaBH4.[9] The reduction of the a-
sulfenylated aldehyde 3a to the alcohol 5a showed that this
process occurs without loss of enantiomeric excess. Simple
aliphatic aldehydes 1a–c (Table 2, entries 1–3), as well as
those containing a phenyl group or an additional double bond
(Table 2, entries 4 and 5), underwent the desired reaction, and
the optically active a-sulfenylated alcohols 5a–e were
obtained in good yields and with excellent enantioselectiv-
ities. Similarly, the sterically encumbered aldehyde 1 f was
transformed smoothly into the corresponding chiral alcohol
5 f with very high enantioselectivity (Table 2, entry 6). Fur-
thermore, the method could be extended to the construction
1
2
3
4
5
6
7
8
9
10
11
12[g]
13[h]
a
a
a
a
b
c
d
e
f
g
h
h
h
DMSO
Et2O
–
5
7
–
18
22
25
0
CH2Cl2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
30
16
56[e]
52[f]
–
[d]
–
90
77
84
90
98
96
90
75[e]
73[e]
90
90
90
[a] Compound 2e (0.27 mmol) was added to 1a (0.25 mmol) and 4
(0.025 mmol) in the solvent (0.5 mL), and the mixture was stirred at
room temperature. [b] Based on NMR spectroscopic analysis of the
crude reaction mixture after 3 h. [c] The ee value was determined by GC
on a chiral phase (chrompak CP-chiralsil dex CB column) and verified by
HPLC (chiralpak AD column) after reduction of 3a to the corresponding
alcohol. [d] Compound 3a was not formed. [e] The a,a-disulfenylated
aldehyde by-product was detected in the crude reaction mixture in
>10% yield. [f] After 24 h at room temperature: 25% ee. [g] After 1 h in
the presence of LiClO4 (30 mol%). [h] After 1 h in the presence of
o-nitrobenzoic acid (10 mol%). DMSO=dimethyl sulfoxide, Naph=
naphthyl, TMS=trimethylsilyl.
Table 2: Organocatalyzed enantioselective a sulfenylation of alde-
hydes.[a]
amide 4a, only traces of the product 3a, but large amounts of
the a,a-disulfenylated aldehyde, were observed in DMSO
(Table 1, entry 1). In less polar solvents, such as toluene, the
desired product 3a was generated in low yield with low
enantioselectivity in the presence of 4a (Table 1, entries 2–4).
Whereas l-proline (4b) was ineffective (Table 1, entry 5), the
chiral pyrrolidine derivative 4c increased the rate and
enantioselectivity of the reaction (Table 1, entry 6), although
it slowly racemized the product upon prolonged reaction
times and led to a,a disulfenylation. To minimize such
undesired interactions between the organocatalyst and the
reaction product, we attempted the reaction in the presence
of a catalyst with increased steric bulk. No reaction occurred
with a,a-diphenyl-l-prolinol (4d; Table 1, entry 7), probably
as a result of the reaction of 2e with the hydroxy group[7] and/
Entry
1/5
R
R’
Yield of 5 [%][b]
ee [%][c]
1
2
3
4
a
b
c
d
e
f
iPr
Me
Et
Bn
allyl
tBu
Ph
H
H
H
H
H
H
Me
81
60
85
94
64
83
84
98
95
96
97
96
95
61
5
6
7[d]
g
[a] Compound 2e (0.33 mmol) was added to 1 (0.25 mmol) and 4h
(0.025 mmol) in toluene (0.5 mL), and the mixture was stirred at room
temperature for 3 h. [b] Yield of the isolated product after column
chromatography. [c] The ee value was determined by HPLC of the
alcohols 5 on a chiral phase (see Supporting Information). [d] After 16 h
with catalyst 4e (10 mol%) and o-nitrobenzoic acid (10 mol%). Bn=
benzyl.
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795

Communications
of quaternary stereocenters starting from a,a-disubstituted
In summary, a novel class of organocatalysts in the form of
sterically encumbered chiral pyrrolidine derivatives without
additional free heteroatom functionalities has been devel-
oped. These compounds were found to be highly efficient
organocatalysts for the direct enantioselective a sulfenylation
of aldehydes, which is one of the first examples of an
asymmetric intermolecular substitution reaction mediated by
a secondary amine. This procedure constitutes the first
enantioselective catalytic preparation of a-sulfenylated alde-
hydes and, to the best of our knowledge, the first successful
use of electrophilic sulfur sources in asymmetric catalysis. The
optically active products were obtained in high yields with
excellent enantioselectivities and underwent further facile
modifications. Further exploration of the new class of chiral
organocatalysts described are now in progress in our labo-
ratory.
aldehydes, such as 2-phenyl propanal (1g). In this case, the
optically active a-sulfenylated alcohol 5g was obtained in
high yield and with good enantioselectivity when 4e was used
as the catalyst (Table 2, entry 7).
As well as reduction to the a-sulfenylated alcohols 5, the
optically active a-sulfenylated aldehydes 3 also undergo
reductive amination with dibenzylamine and sodium triace-
toxyborohydride. By using this procedure, the chiral a-
sulfenylated amine 6 was obtained directly from the aldehyde
3a with only a minor loss of enantiomeric excess [Eq. (2)].
Moreover, after protection of the hydroxy group as a tert-
butyldimethylsilyl (TBDMS) ether, the benzyl sulfide moiety
could be cleaved reductively with Na/NH3(l). Thus, the free
thiol 8 was formed from 7 in good yield [Eq. (3)]. These
transformations underline the synthetic versatility of optically
active a-sulfenylated aldehydes in the preparation of chiral
sulfur-containing compounds.
Received: September 24, 2004
Keywords: aldehydes · asymmetric catalysis · enantioselectivity ·
.
organocatalysis · sulfenylation
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796
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Chemie
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