Stereoselective substitution of (R)-2-(sulfonyloxy)nitriles with sulfur nucleophiles

Article abstract of DOI:10.1016/S0957-4166(99)00154-8

Optically active 2-(4-toluenesulfonyloxy)-and (4- nitrobenzenesulfonyloxy)nitriles (R)-3 and (R)-4, which were obtained from (R)-cyanohydrins (R)-2 by sulfonylation, react with sulfur nucleophiles such as potassium thioacetate, potassium ethylxanthogenate, potassium thiocyanate, as well as thioalcohols and thiophenol in a typical S(N02 manner to give the (S)-2-sulfanyl nitriles (S)-7-9 and (S)-11-13 in good chemical yields and enantiomeric excesses. Even (R)-2-methyl-2-(methanesulfonyloxy)hexanenitrile (R)-6, derived from ketone cyanohydrin (R)-5, reacts with potassium thioacetate to yield (S)-2-acetylthio-2-methylhexanenitrile (S)-10 with 97% ee.

Full text of DOI:10.1016/S0957-4166(99)00154-8

Pergamon
Tetrahedron: Asymmetry 10 (1999) 1765–1775
Stereoselective substitution of (R)-2-(sulfonyloxy)nitriles with
sulfur nucleophiles1
Franz Effenberger ∗ and Sebastian Gaupp †
Institut für Organische Chemie der Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
Received 12 April 1999; accepted 21 April 1999
Abstract
Optically active 2-(4-toluenesulfonyloxy)- and (4-nitrobenzenesulfonyloxy)nitriles (R)-3 and (R)-4, which were
obtained from (R)-cyanohydrins (R)-2 by sulfonylation, react with sulfur nucleophiles such as potassium thioace-
tate, potassium ethylxanthogenate, potassium thiocyanate, as well as thioalcohols and thiophenol in a typical SN2
manner to give the (S)-2-sulfanyl nitriles (S)-7–9 and (S)-11–13 in good chemical yields and enantiomeric excesses.
Even (R)-2-methyl-2-(methanesulfonyloxy)hexanenitrile(R)-6, derived from ketone cyanohydrin (R)-5, reacts with
potassium thioacetate to yield (S)-2-acetylthio-2-methylhexanenitrile(S)-10 with 97% ee. © 1999 Elsevier Science
Ltd. All rights reserved.
1. Introduction
Nucleophilic substitutions of activated α-hydroxycarboxylic acids and esters with O-, N- and S-
nucleophiles are well established,2 but little is known about analogous reactions of cyanohydrins.3
Optically active 2-(sulfonyloxy)nitriles, easily accessible from the corresponding chiral cyanohydrins
by sulfonylation,4 have a high configurational stability and can therefore be applied as activated cyano-
hydrins for stereoselective reactions with nucleophiles. With potassium acetate, for example, sulfonylated
cyanohydrins, derived from aliphatic aldehydes, react under very mild conditions (room temperature,
DMF as solvent) with complete inversion of configuration to yield the corresponding substitution
products.4 Besides reactions with O-nucleophiles, (R)-2-(sulfonyloxy)nitriles have also been reacted
with various N-nucleophiles such as potassium azide, potassium phthalimide, and amines to give the
corresponding substitution products with (S)-configuration.5
In the present publication we describe substitution reactions of (R)-2-(sulfonyloxy)nitriles with various
sulfur nucleophiles, especially with respect to the stereochemical outcome of the reactions.
∗
†
Corresponding author. E-mail: franz.effenberger@po.uni-stuttgart.de
Part of dissertation, Universität Stuttgart, 1998.
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00154-8

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2. Preparation of optically active 2-(sulfonyloxy)nitriles (R)-4 and (R)-6
While optically active 2-(4-toluenesulfonyloxy)nitriles (R)-3 are known,4 2-(4-nitrobenzenesulfonyl-
oxy) nitriles (R)-4 have not been described so far. The starting cyanohydrins (R)-2 were prepared by (R)-
hydroxynitrile lyase (PaHNL) catalyzed addition of HCN to aldehydes 1 using a slightly modified method
developed by Brussee6 with almond meal as enzyme source. HCN was generated in situ from KCN with
acetic acid. Analogously, the ketone cyanohydrin (R)-5 was obtained from butyl methyl ketone. Based
on the published procedure4,7 aldehyde cyanohydrins (R)-2 were reacted with 4-nitrobenzenesulfonyl
chloride in the presence of pyridine as a base to yield the corresponding sulfonyloxynitriles (R)-4
(Scheme 1, Table 1).
Scheme 1.
2-(4-Nitrobenzenesulfonyloxy)nitriles (R)-4, which are solid at room temperature, can be recrystal-
lized for improving enantiomeric excesses.
In contrast to aldehyde cyanohydrins (R)-2 ketone cyanohydrin (R)-5 (98% ee) did not react with
toluene- and nitrobenzenesulfonyl chloride, respectively. Only the reaction with methanesulfonyl chlo-
ride in pyridine was successful (Scheme 1). After chromatographic purification, mesyl-activated (R)-6
was isolated in 79% yield. Whereas the determination of ee-values of sulfonyloxynitriles (R)-4 requires
a follow-up reaction (see Table 1), the enantiomeric excess of (R)-6 could be determined directly by gas
chromatography to be 98% ([α]20=+18.9 (c 1.0, CH2Cl2)). This result indicates that mesylation also
D
proceeds without racemization.
Table 1
2-(4-Nitrobenzenesulfonyloxy)nitriles (R)-4 by reaction of cyanohydrins (R)-2 with 4-nitrobenzene-
sulfonyl chloride in the presence of pyridine

F. Effenberger, S. Gaupp / Tetrahedron: Asymmetry 10 (1999) 1765–1775
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3. Reactions of 2-(sulfonyloxy)nitriles (R)-3, (R)-4 and (R)-6 with various sulfur nucleophiles
Many sulfur nucleophiles are known to react in a typical SN2 manner with alkylhalides or sulfonates
to give the corresponding thio-derivatives.8 With respect to possible follow-up reactions we have first
investigated conversions of the tosyl-activated racemic cyanohydrin (RS)-3a with various S-nucleophiles
under SN2 reaction conditions.8a All attempts failed to react thiourea, hydrogensulfide, sulfite, and
thiosulfate with 3a under reasonable reaction conditions. Thioacetate, xanthogenate, thiocyanate, and
some thiols, however, were found to undergo very easily reactions with 2-(sulfonyloxy)nitriles 3 and 4,
respectively.
3.1. Reactions with thioacetate, ethylxanthogenate and thiocyanate
In Scheme 2 the performed nucleophilic substitution reactions of sulfonyl-activated (R)-cyanohydrins
3 and 4, respectively, with sulfur nucleophiles are summarized.
Scheme 2.
Reactions of optically active 2-(sulfonyloxy)nitriles with potassium acetate yielding cyanohydrin
acetates with complete inversion of configuration have been investigated intensively.4 The higher
nucleophilicity of thioacetate in comparison to acetate, which is easy to handle9 and readily soluble in
an organic solvent such as DMF, should make SN reactions possible under mild conditions. Indeed, the
reaction conditions used earlier4b could be applied successfully to the reactions of 2-(sulfonyloxy)nitriles
(RS)-3 and (R)-3, respectively, with potassium thioacetate (KSAc) (Scheme 2). Due to good solubility
of KSAc in DMF, addition of crown ether is not necessary. The conversion was complete after 2 h
at room temperature in all cases, and the desired (S)-2-acetylthionitriles (S)-7a–c were obtained after
chromatographic purification in good chemical yields and high enantiomeric excesses (Table 2). A
comparison of the enantiomeric excesses of starting compounds (R)-3 and products (S)-7 shows that
the reaction proceeds almost without racemization (Table 2) under complete inversion of configuration
as could be proved by follow-up reaction of (S)-7.10
Because of their good solubility in polar aprotic solvents and their high nucleophilicity xanthoge-
nates are excellent sulfur nucleophiles. As expected, the reaction of (R)-3a,b with potassium ethyl-
xanthogenate in DMF as solvent was already completed after 1 h yielding the corresponding (S)-2-
ethylxanthogennitriles (S)-8a,b, but with at least 20% racemization (Table 2). Their purification was

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Table 2
(S)-2-Acetylthio- and 2-ethylxanthogennitriles (S)-7 and (S)-8 by reaction of (R)-2-(4-toluene-
sulfonyloxy)nitriles (R)-3 with potassium thioacetate and potassium ethylxanthogenate, respectively
difficult and resulted in chemical yields between 66–70% only (Table 2). Due to low thermal stability,
xanthogennitriles 8 are not distillable. After chromatography on silica gel compounds 8 contain traces
of impurities (GC purity 97%) and decompose at room temperature. The chromatography of small
amounts finally afforded pure 2-ethylxanthogennitriles 8 (GC purity >99%) which could be stored at
low temperature.
The nucleophilicity of thiocyanate is lower than that of thioacetate under comparable conditions.
Consequently, the tosyl-activated cyanohydrin (R)-3a reacts with potassium thiocyanate (KSCN) in
DMF only at temperatures above 60°C. The racemization observed thereby can be explained by an
exchange of thiocyanate, which, as pseudohalide, represents both a good leaving group and a relatively
good nucleophile. At lower temperatures racemization could be suppressed but the conversion was
unsatisfactory.
(R)-2-(4-Nitrobenzenesulfonyloxy)nitriles (R)-4, which are more reactive than (R)-2-(4-toluene-
sulfonyloxy)nitriles (R)-3 were therefore applied. The reaction of (R)-4 with KSCN in DMF at 40°C
provided the corresponding (S)-2-thiocyanatonitriles (S)-9 after chromatography on silica gel in good
chemical yields and enantiomeric excesses (Scheme 2, Table 3). Table 3 shows that the reactions proceed
almost with complete inversion. A reaction with DMF as nucleophile, as previously described for SN2
reactions of sulfonyloxynitriles with acetate,4b was not observed.
Table 3
(S)-2-Thiocyanatonitriles (S)-9 by reaction of (R)-2-(4-nitrobenzenesulfonyloxy)nitriles (R)-4 with
potassium thiocyanate
The reaction of (R)-2-methyl-2-(methanesulfonyloxy)hexanenitrile (R)-6, a ketone cyanohydrin deri-
vative, with thioacetate as nucleophile has also been investigated, as outlined in Scheme 3.

F. Effenberger, S. Gaupp / Tetrahedron: Asymmetry 10 (1999) 1765–1775
1769
Scheme 3.
Owing to the more bulky methyl group compared to hydrogen and the mesyl group with a lower
leaving ability in comparison to the tosyl moiety, (R)-6 should be less reactive than compounds (R)-3 in
SN2 reactions. The conversion of (RS)-6 with potassium thioacetate in DMF was, as expected, very slow,
and after 72 h only 35% of the product (RS)-10 was obtained. The reaction was accompanied by side-
reactions, resulting in numerous by-products which were not characterized. Comparable results were
achieved replacing DMF by toluene as solvent in the presence of crown ether. Thereby the formation
of by-products increased at temperatures above 50°C. Satisfying results were obtained, however, by
using thioacetic acid and collidine as nucleophilic reagent (Scheme 3). After a reaction time of 14 h
at 55°C, (R)-6 (97% ee) was converted without racemization to (S)-2-acetylthio-2-methylhexanenitrile
(S)-10 (97% ee, [α]20=−42.5 (c 1.0, CH2Cl2)) in 84% yield.
D
3.2. Reactions with thioalcohols and thiophenol
Scheme 4 illustrates substitution reactions of (R)-2-(4-toluenesulfonyloxy)nitriles (R)-3 with thioalco-
hols and thiophenol.
Scheme 4.
Optically active sulfonylated α-hydroxycarboxylic acid esters are known to undergo nucleophilic
substitution reactions with thiophenolate.2a,11 The published reaction conditions2a,11 could be applied
to the reaction of tosylated cyanohydrins (R)-3 with thiophenol as nucleophile (Scheme 4). The reaction
was performed inversely, in which (R)-3 was added to a small excess of thiophenol in a suspension of
potassium carbonate/acetonitrile at 0°C. After 14 h reaction time at room temperature, the corresponding
(S)-2-phenylthionitriles (S)-11 could be isolated in good chemical yields (Table 4). Complete exclusion
of oxygen during the reaction is not necessary.
All attempts have failed so far to separate the enantiomers of 11 by gas chromatography. The specific
rotations, however, are reproducible, indicating a reaction without racemization.

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Table 4
(S)-2-Phenyl- and benzylthionitriles (S)-11 and (S)-12 by reaction of tosyloxynitriles (R)-3 with
thiophenol and benzyl thiol, respectively
Under the optimized reaction conditions (R)-3 reacted with benzyl thiol to give (S)-2-benzylthionitriles
(S)-12 (Scheme 4, Table 4). Table 4 shows that the yields of (S)-12 are comparable with those of (S)-11.
The reaction times, however, differ markedly. With benzyl thiol, reaction times of ≥40 h are required
compared with 14 h using thiophenol. This result is in agreement with experimental data published
for reactions of activated α-hydroxycarboxylic acid esters.2a,11 Possibly, in the heterogeneous system
K2CO3/acetonitrile, the concentration of deprotonated thiophenol could be higher than that of benzyl
thiolate, resulting in an increased total reaction rate. The separation of the enantiomers of 12 by gas
chromatography has also failed so far. Follow-up reactions of (S)-12 to (S)-5-alkyl-2-thiazolidinones10
confirm that the substitution of (R)-3 proceeded with complete inversion.
2-Mercaptoethanol has been chosen as sulfur nucleophile since it allows the introduction of an
additional functional group. Also 2-mercaptoethanol as primary aliphatic thiol reacts easily under the
optimized reaction conditions with (R)-3 to yield (S)-2-(2-hydroxyethylthio)nitriles (S)-13 (Scheme 4,
Table 5).
Table 5
(S)-2-(2-Hydroxyethylthio)nitriles (S)-13 by reaction of (R)-3 with 2-mercaptoethanol
A reaction of the hydroxy group is very unlikely since both nucleophilicity and acidity of the alcohol
group is lower than that of the thiol function. Indeed, only substitution by the thiolate function has been
found yielding nitriles 13. As can be seen from Table 5, the substitution products (S)-13 were obtained in
good yields. Owing to the results obtained for 2-phenylthio- and 2-benzylthionitriles (S)-11 and (S)-12, a
substitution reaction with complete inversion of configuration is assumed also in the case of the reaction
of mercaptoethanol.
4. Experimental
4.1. Materials and methods
Racemic cyanohydrins were prepared according to a known procedure,12 racemic 2-(toluenesulfonyl-
oxy)nitriles (RS)-3 are described in the literature.13 Melting points were determined on a Büchi SMP-20

F. Effenberger, S. Gaupp / Tetrahedron: Asymmetry 10 (1999) 1765–1775
1771
1
and are uncorrected. H NMR spectra were recorded on a Bruker AC 250 F (250 MHz) with TMS as an
internal standard. Column chromatography was performed with glass columns of different sizes packed
with silica gel S (Riedel–de Haen) or silica gel 60 (Merck), grain size 0.032–0.063 mm. Optical rotations
were determined on a Perkin–Elmer polarimeter 241 LC. GC for determination of enantiomeric excess:
Hewlett–Packard 6890 Series with FID, 0.9 bar hydrogen, column 30 m×0.32 mm, phase Chiraldex
B-TA (ICT).
4.2. Preparation of (R)-cyanohydrins 2 and 5; general procedure
Defatted almond meal (100 mg/mmol substrate) was soaked in 0.1 mL citrate buffer (20 mM, pH 4.0)
for 15 min. A 2 M solution of aldehyde or ketone in tert-butyl methyl ether was added, and the reaction
mixture was cooled to 0°C. Then a ca. 1 M solution of HCN in tert-butyl methyl ether was added portion-
wise to the vigorously stirred reaction mixture. After being stirred for a further 5 h (aldehydes) or 60 h
(ketone), almond meal was filtered off, and the filtrate was dried (MgSO4), and concentrated. The residue
was distilled in vacuo.
HCN was generated from KCN as follows: a solution of KCN (2 equiv. with respect to substrate) in
water was cooled to 0°C and overlaid with tert-butyl methyl ether. After addition of an equimolar amount
of glacial acetic acid (with respect to KCN), the aqueous phase was extracted with the tert-butyl methyl
ether layer to yield a ca. 1 M HCN solution.
4.3. Preparation of 2-(4-nitrobenzenesulfonyloxy)nitriles (R)-4; general procedure
To a stirred 1 M solution of (R)-2 and pyridine in dichloromethane at −10°C a solution of 4-
nitrobenzenesulfonyl chloride in dichloromethane (0.4 M) was added dropwise within 1 h, and the
reaction mixture was allowed to warm to room temperature (14 h). Dichloromethane was removed in
vacuo, and the residue was taken up in diethyl ether. After being stirred for 1 h, precipitated pyridinium
hydrochloride was filtered off. The filtrate was concentrated, and the residue was chromatographed on
silica gel with petroleum ether/ethyl acetate.
4.4. Preparation of 2-methyl-2-(methanesulfonyloxy)hexanenitrile (R)-6
To a solution of (R)-5 (3.2 g, 25.3 mmol) in pyridine (50 mL) at 0°C methanesulfonyl chloride (3.3
g, 29 mmol) was added dropwise. After being stirred for 18 h, the reaction mixture was poured onto ice
and extracted with diethyl ether. The combined extracts were washed with 10% HCl, dried (MgSO4), and
concentrated. The residue was chromatographed on silica gel with petroleum ether/ethyl acetate to give
4.1 g (79%) of (R)-6; 98% ee, [α]20=+18.9 (c 1.0, CH2Cl2). 1H NMR (CDCl3): δ 0.96 (t, J=7.3 Hz, 3H,
D
CH3CH2), 1.37–1.44 (m, 2H, CH3CH2), 1.51–1.67 (m, 2H, CH2CH2CH2), 1.92 (s, 3H, CH3), 1.94–2.11
(m, 2H, CCH2), 3.18 (s, 3H, SO2CH3). Anal. calcd for C8H15NO3S (205.3): C, 46.81; H, 7.37; N, 6.82;
S, 15.62. Found: C, 46.86; H, 7.36; N, 6.78; S, 15.38.

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4.5. (S)-2-Acetylthionitriles (S)-7 and (S)-2-ethylxanthogennitriles (S)-8 by reaction of (R)-3 with
potassium thioacetate and potassium ethylxanthogenate, respectively; general procedure
To a solution of (R)-3 in DMF (0.1–0.5 M) at room temperature a solution of KSAc or EtOCSSK
(1.0–1.5 equiv.) in DMF was slowly added dropwise. After being stirred for 2 h (for 7) or 1 h (for 8),
the reaction mixture was hydrolyzed with ice-water, and extracted with diethyl ether. The combined
extracts were dried (MgSO4), and concentrated. The residue was either distilled in vacuo ((RS)-7) or
chromatographed on silica gel with petroleum ether/ethyl acetate ((S)-7 and (S)-8).
4.6. (S)-2-Thiocyanatonitriles (S)-9 by reaction of (R)-4 with potassium thiocyanate; general procedure
To a solution of (R)-4 in DMF (5 M) at room temperature a solution of KSCN (1.0–1.4 equiv.) in
DMF (ca. 0.05 M) was added dropwise. After being stirred for 16–20 h at 40°C, the reaction mixture was
hydrolyzed with ice-water, and extracted with diethyl ether. The combined extracts were dried (MgSO4),
and concentrated. The residue was chromatographed on silica gel with petroleum ether/ethyl acetate.
4.7. Preparation of (S)-2-acetylthio-2-methylhexanenitrile (S)-10
To a solution of (R)-6 (1.0 g, 4.87 mmol) and collidine (1 mL) in toluene (15 mL) at room temperature
a solution of thioacetic acid (1 mL) in toluene (10 mL) was added dropwise. After being stirred for 14 h
at 55°C, the reaction mixture was hydrolyzed with ice-water, and the aqueous phase was extracted with
diethyl ether. The combined organic phases were dried (MgSO4), and concentrated. The residue was
chromatographed on silica gel with petroleum ether/ethyl acetate to give 762 mg (84%) (S)-10; 97% ee,
[α]20=−42.5 (c 1.0, CH2Cl2). 1H NMR (CDCl3): δ 0.94 (t, J=7.3 Hz, 3H, CH2CH3), 1.34–1.43 (m, 2H,
D
CH2CH3), 1.50–1.57 (m, 2H, CH2CH2CH2), 1.75 (s, 3H, CH3), 1.79–1.98 (m, 2H, CCH2), 2.35 (s, 3H,
COCH3). C9H15NOS (185.3): calcd for MH+ 186.09526. Found: 186.09460. GC–MS (CI, 100 eV) m/z
(%): 186 (12) [MH+], 142 (7) [C7H12NS+], 43 (100) [C2H3O+].

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4.8. Determination of enantiomeric excesses
(i) The enantiomeric excesses of 7, 8, 9, 11 and 12 were determined directly without further reaction
(see Tables 2 and 3).
(ii) A solution of 3 or 4 (10 mg) and potassium thioacetate (10 mg) in DMF (100 µL) was reacted for 1 h
at room temperature. Water (2 mL) was added, and the reaction mixture extracted with diethyl ether
(2 mL). The extract was either dried (MgSO4) or directly used for the determination of enantiomeric
excesses.
4.9. (S)-2-Phenylthio-, (S)-2-benzylthio- and (S)-2-(2-hydroxyethylthio)nitriles (S)-11, 12 and 13 by
reaction of (R)-3 with thiophenol, benzyl thiol and 2-mercaptoethanol, respectively; general procedure
To a suspension of thiophenol, benzyl thiol or 2-mercaptoethanol (1.1 equiv.) and K2CO3 (2 equiv.) in
acetonitrile at 0°C a solution of (R)-3 (1 equiv.) in acetonitrile (0.4 M) was added dropwise. The reaction
mixture was allowed to warm to room temperature and stirred for the times given in Tables 4 and 5. Then
it was hydrolyzed with ice-water and extracted with diethyl ether. The combined extracts, which contain

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water because of acetonitrile, were concentrated in vacuo until phase separation. Diethyl ether was added,
and the aqueous phase was separated. The organic phase was dried (MgSO4), and concentrated in vacuo.
The residue was chromatographed on silica gel with petroleum ether/ethyl acetate.
Acknowledgements
We would like to thank the Bundesministerium für Bildung und Forschung (Zentrales Schwerpunkt-
programm Bioverfahrenstechnik, Stuttgart) and the Fonds der Chemischen Industrie for financial support.

F. Effenberger, S. Gaupp / Tetrahedron: Asymmetry 10 (1999) 1765–1775
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