Noncatalyzed addition of trimethylsilyl isothiocyanate to aziridines and cyclohexene oxide

Article abstract of DOI:10.1080/00397910802691890

Ring opening of N-substituted aziridines and cyclohexene oxide with trimethylsilyl isothiocyanate proceeds without the use of any catalyst under mild conditions to give N-substituted trans-2-amino-1-isothiocyanates, trans-2-amino-1-thiocyanates, or trans-

Full text of DOI:10.1080/00397910802691890

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Noncatalyzed Addition of
Trimethylsilyl Isothiocyanate
to Aziridines and Cyclohexene
Oxide
Natalia Prusinowska a & Jacek Gawronski a
a Department of Chemistry , A. Mickiewicz
University , Poznan, Poland
Published online: 26 Jun 2009.
To cite this article: Natalia Prusinowska & Jacek Gawronski (2009) Noncatalyzed
Addition of Trimethylsilyl Isothiocyanate to Aziridines and Cyclohexene Oxide,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 39:15, 2795-2803, DOI: 10.1080/00397910802691890
To link to this article: http://dx.doi.org/10.1080/00397910802691890
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Synthetic Communications1, 39: 2795–2803, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802691890
Noncatalyzed Addition of Trimethylsilyl
Isothiocyanate to Aziridines
and Cyclohexene Oxide
Natalia Prusinowska and Jacek Gawronski
Department of Chemistry, A. Mickiewicz University, Poznan, Poland
Abstract: Ring opening of N-substituted aziridines and cyclohexene oxide with
trimethylsilyl isothiocyanate proceeds without the use of any catalyst under mild
conditions to give N-substituted trans-2-amino-1-isothiocyanates, trans-2-amino-
1-thiocyanates, or trans-2-hydroxy-1-isothiocyanates.
Keywords: Aziridines, isothiocyanates, thiocyanates, trimethylsilyl isothiocyanate
INTRODUCTION
Oxiranes and aziridines are excellent substrates for the addition of various
nucleophiles. This constitutes a general method for the synthesis of various
2-substituted amines and alcohols.[1–4] The reaction is highly trans-stereo-
selective, but it usually requires the use of either a Lewis acid or a Lewis
base as a catalyst. Trimethylsilyl (TMS) nucleophiles, such as trimethylsilyl
cyanide (TMSCN), trimethylsilyl azide (TMSN3), or trimethylsilyl chloride
(TMSCl), are commercially available alternatives for protonated nucleo-
philes. Under activation with a suitable Lewis base, such as fluoride ion
or various compounds with polar C¼O, N¼O, P¼O or S¼O bonds, the
TMS nucleophile adds to electrophiles, such as aldehydes, ketones, imines,
epoxides, aziridines, enones, and nitroolefins. These reactions were
widely studied[5] with the emphasis on developing conditions for optimal
asymmetric induction.[6] Sulfur-centered nucleophiles[7] are among the most
Received November 5, 2008.
Address correspondence to Jacek Gawronski, Department of Chemistry,
A. Mickiewicz University, 60740 Poznan, Poland. E-mail: gawronsk@amu.edu.pl
2795

2796
N. Prusinowska and J. Gawronski
reactive species and, in combination with aziridines as electrophiles, their
addition reactions provide access to substituted trans-1,2-aminothiols.
Alkali metal or ammonium thiocyanates undergo addition to N-tosyl
aziridines under catalysis by lithium perchlorate,[8] sufated zirconia,[9]
ammonium 12-molybdophosphate,[10] and macrocyclic diamides in reflux-
ing acetonitrile[11] or in the presence of b-cyclodextrin in water.[12] The
reaction of N-tosylaziridnes with an ionic liquid, 1-(1-carboxyethyl)-
3-methyl-3H-imidazolium thiocyanate, has recently been reported.[13]
N-Tosyl aziridines react with alkyl and aryl isothiocyanates under catalysis
with sodium iodide to give derivatives of 2-imidazolidinethione.[14] In the
case of phenanthrene 9,10-imine, the thiocyanate addition product cyclizes
to a derivative of phenanthro[9,10-d]-thiazol-2-amine.[15]
All of these reactions require activation of the aziridine substrate. The
reaction of aziridines with silylated sulfur nucleophiles has not received
much attention. As a silylated nucleophile, trimethylsilyl isothiocyanate
(TMSNCS, 1) has found use in substitution reactions of alkyl halides,
mediated by tetra-n-butylammonium fluoride (TBAF) and leading to
alkyl thiocyanates.[16] Peracetylated hexoses react with 1 in the presence
of SnCl4 to give glycosyl isothiocyanates in one step.[17] The reaction of
1 with aldehydes, catalyzed by ZnCl2, leads to a,a0-diisothiocyanato ethers
in good yields.[18] In the presence of TBAF, 1 reacts regioselectively with
oxiranes to give 2-hydroxythiocyanates and=or their O-TMS deriva-
tives.[19] The selena analog of 1, TMSNCSe, reacts with aldehydes with
extrusion of Se to give predominantly silylated cyanohydrins.[20]
RESULTS AND DISCUSSION
In this communication, we demonstrate that TMSNCS (1) reacts with
N-substituted aziridines and cyclohexene oxide without the action of
any activator to give the ring-opening products with good to high yields,
even at temperatures as cold as ꢀ50ꢁC.
The reactions of 1 with N-substituted aziridines 2a–2e[21] and oxirane
2f were initially carried out in three different solvents, tetrahydrofuran
(THF), dichloromethane (DCM), and toluene, at 20ꢁC to determine
the conditions for asymmetric catalysis of the addition. To our surprise,
in all cases the reaction quenched after 24 h and gave the products
according to Scheme 1. TMSNCS is an ambiphilic reagent: however, in
the reported additions, it behaved preferentially as a sulfur-centered
nucleophile. Thus, it gave nearly exclusively the thiocyanate addition
products 3 with N-benzyl (2a) and N-tosyl (2e) aziridines, as well as with
oxirane 2f. With N-benzoylated aziridines 2b–d, we isolated substantial
amounts of isothiocyanates 4b–d, in a ratio 3:4 depending on the

Addition of Trimethylsilyl Isothiocyanate
2797
Scheme 1. Noncatalyzed addition of TMSNCS to aziridines 2a–e and to cyclo-
hexene oxide (2f).
substrate and the reaction conditions. Thus, 2b gave products 3b and 4b
in a ratio of 4:1 at 20ꢁC and 5.2:1 at ꢀ50ꢁC in DCM solution, whereas in
the case of 2c under identical conditions the ratio of 3c to 4c was 7:1 to
8:1. Changing DCM for toluene did not result in a significant difference
in the proportion of the products. The addition was less selective in THF
solution at 20ꢁC (3c to 4c ¼1.7:1).
The isolated products were identified by 1H NMR, electron ¼
impact–mass spectrometry (EIMS), and infrared (IR) spectra. For
example, products 3 and 4 can be readily distinguished by the position of
the signals (characteristic ddd) of protons attached to the heteroatom-
substituted cyclohexane carbon atoms. Using the published data[22] and a
Dd increment for the –N¼C¼S substituent (Ra 1.2, Rb 0.6) the positions
1
of the CHN signals in the H NMR spectra of 4b–d could be calculated
and compared to the experimental data. In thiocyanates 3, the signal appears
at higher field than in isocyanates 4. Thiocyanates 3 can be further identified
by an IR band at 2150–2160 cmꢀ1, whereas IR absorption due to the
isothiocyanate group in 4 appears at less than 2100 cmꢀ1
.
The isolated yields of the products of addition are collected in Fig. 1.
In general, the yields are the greatest for the reactions run in THF solu-
tions, and the reactivity of N-nitrobenzoylated aziridines 2c and 2d
apparently exceeds the reactivity of other electrophiles studied here.
Interestingly, the reactivity of 2d–f decreases significantly in toluene solu-
tion. Lowering the reaction temperature to ꢀ50ꢁC did not significantly
decrease the yield of the products isolated after 24 h, with the exception
of the reaction of N-tosyl aziridine 2e in DCM or toluene solution, in
which case no addition product could be isolated (Fig. 2).
With ‘‘activated’’ aziridines 2b–e, the role of the initiator is played by
a polar C¼O (or S¼O) bond. Transient silylation of such a bond releases
the isothiocyanate anion, which can further act as an S or N nucleophile,
affording products 3 and 4. N-Benzyl aziridine 2a is considered less

2798
N. Prusinowska and J. Gawronski
Figure 1. Isolated yields of the addition products of TMSNCS to aziridines 2a–e
and oxirane 2f at 20ꢁC. Solvent: THF, &; DCM, &; and PhMe, &.
reactive than its ‘‘activated’’ counterparts 2b–e. However, basicity of the
nitrogen atom in 2a may help to activate 1 for the addition reaction, thus
providing compensation for intrinsic lack of activation of electrophile 2a.
Building on the decreased reaction rate of N-tosyl aziridine 2e with 1
at ꢀ50ꢁC, we attempted asymmetric catalysis of the reaction with
N-benzylcinchoninium fluoride in THF at ꢀ78ꢁC. After 24 h, the
addition product 3e was isolated with 44% yield, in a racemic form.
Likewise, catalysis with cinchonine in DCM solution at ꢀ30ꢁC afforded
racemic addition product 3e in 36% yield.
Figure 2. Isolated yields of the addition products of TMSNCS to aziridines 2a–e
and oxirane 2f at ꢀ50ꢁC. Solvent: THF, &; DCM, &; and PhMe, &.

Addition of Trimethylsilyl Isothiocyanate
2799
In summary we have shown that TMSNCS (1) is a ‘‘hot’’ silylated
nucleophile, allowing uncatalyzed addition to aziridines and cyclohexene
oxide even at low temperatures with nonpolar solvents. This is in contrast
to a report already mentioned[19] and also in contrast to the reactivity of
other silylated nucleophiles (e.g., TMSCN, TMSN3), for which uncata-
lyzed additions are not common,[23] especially with less reactive electro-
philes such as aziridines and oxiranes. In the reported spontaneous
addition reactions of silylated nucleophiles, the role of the activator
usually is played by a polar solvent, such as dimethylformamide
(DMF),[24] acetonitrile (MeCN),[25] or dimethyl sulfoxide (DMSO).[26]
Our results indicate that asymmetric catalysis of addition of 1 to aziri-
dines and oxiranes remains a challenging and yet unreached goal.
EXPERIMENTAL
NMR spectra were recorded at ambient temperature on a Varian XL300
instrument and are reported in parts per million (ppm) with respect to
(CH3)4Si as a reference. Fourier transform (FT)–IR spectra were
measured using a Bruker IFS 113v instrument. Mass spectra (MS) were
recorded with an AMD 402 mass spectrometer. Melting points are
uncorrected.
General Procedure for Ring-Opening Reactions
TMSNCS (1.4 equiv.) was added to a stirred solution of aziridine
(0.25 mmol) or oxirane (0.5 mmol) in 3 ml of dry solvent at room
temperature or at ꢀ50ꢁC. The solution was stirred for 24 h, and then
the reaction mixture was quenched with water and extracted with
DCM. The organic fraction was dried over MgSO4 and evaporated,
and the product was purified by column chromatography on silica gel
(eluent: DCM and its mixtures with AcOEt or MeOH).
Supplementary Data
N-Benzyl-2-thiocyanatocyclohexanamine 3a
1
White solid, mp 172–176ꢁC; H NMR (CDCl3) d 1.28–1.32 (m, 4H),
1.77–1.85 (m, 2H), 2.0 (m, 2H), 3.15 (ddd, J ¼ 10.9, 10.7, 3.3 Hz, 1H),
3.3 (ddd, J ¼ 10.7, 9.6, 3.3 Hz, 1H), 4.4 (d, J ¼ 15.7 Hz, 1H), 4.8
(d, J ¼ 15.9 Hz, 1H), 7.2–7.6 (m, 5H); 13C NMR (CDCl3) d 23.8, 25.8,

2800
N. Prusinowska and J. Gawronski
28.7, 30.2, 46.5, 50.6, 68.2, 126.9, 127.5, 128.4, 137.6, 165.6; EI MS 246
(Mþ), 186, 155, 106, 91, 61; IR (KBr) 3379, 3302, 2150, 1645, 1542,
1080 cmꢀ1
.
N-(2-Thiocyanatocyclohexyl)benzamide 3b
1
White solid, mp 156–158ꢁC; H NMR (CDCl3) d 1.37–1.54 (m, 4H),
1.7–1.9 (m, 4H), 2.2 (m, 2H), 2.45 (m, 2H), 3.4 (ddd, J ¼ 11.8, 10.98,
4.12 Hz, 1H), 4.1 (ddd, J ¼ 10.98, 6.8, 4.4 Hz, 1H), 6.1 (d, J ¼ 8.2, 1H),
7.43–7.56 (m, 3H), 7.77–7.81 (m, 2H); 13C NMR (CDCl3) d 24.6, 25.8,
33.6, 33.8, 53.1, 53.6, 112, 127, 128.6, 131.8, 134, 163.4; EI MS 260
(Mþ) 202, 160, 105, 77; IR (KBr) 3306, 2155, 1635, 1549 cmꢀ1
.
4-Nitro-N-(2-thiocyanatocyclohexyl)benzamide 3c
1
White solid, mp 157–158ꢁC; H NMR (acetone-d6) d 1.43–1.7 (m, 3H),
1.78–1.92 (m, 3H), 2.26–2.4 (m, 2H), 3.5 (ddd, J ¼ 11.8, 11.5, 3.8 Hz,
1H), 4.1 (ddd, J ¼ 10.9, 9.0, 3.8 Hz, 1H), 8.08–8.16 (m, 2H), 8.2 (d,
J ¼ 9.1, 1H), 8.3 (m, 2H); 13C NMR (acetone-d6) d 25.3, 26.6, 34.1,
34.6, 53.5, 53.8, 111.5, 124.2, 129.3, 141, 150.3, 165.2; EI MS 305
(Mþ), 278, 246, 205, 167, 150, 139, 104, 76; IR (KBr) 3388, 2161, 1670,
1345, 850 cmꢀ1
.
3,5-Dinitro-N-(2-thiocyanatocyclohexyl)benzamide 3d
1
White solid, mp 183–188ꢁC; H NMR (acetone-d6) d 1.44–1.67 (m, 3H),
1.8–1.9 (m, 3H), 2.12–2.38 (m, 2H), 3.5 (ddd, J ¼ 12.8, 11.8, 4.1 Hz, 1H),
4.15 (ddd, J ¼ 9.3, 7.1, 4.7 Hz, 1H), 8.65 (s, 1H), 9.1 (s, 3H); 13C NMR
(acetone-d6) d 25.3, 26.6, 34.5, 53.3, 54.2, 54.9, 111.4, 121.6, 128.1,
138.2, 149.4, 162.8; EI MS 350 (Mþ), 291, 250, 195, 139; IR (KBr)
3261, 2183, 1670, 694 cmꢀ1
.
4-Methyl-N-(2-thiocyanatocyclohexyl)benzenesulfonamide 3e[8]
1
White solid, mp 108–109ꢁC; H NMR (CDCl3) d 1.26–1.32 (m, 3H),
1.58–1.77 (m, 3H), 1.9 (m, 1H), 2.29–2.33 (m, 1H), 2.44 (s, 3H), 3.0
(ddd, J ¼ 10.9, 10.4, 4.12 Hz, 1H), 3.1 (ddd, J ¼ 9.1, 8.8, 4.4 Hz, 1H),
5.38–5.41 (d, J ¼ 8.5 Hz, 1H), 7.32–7.35 (d, J ¼ 8.5 Hz, 2H), 7.79–7.82
(d, J ¼ 8.2 Hz, 2H); EI MS 310 (Mþ), 252, 210, 155, 91; IR (KBr) 3235,
2153, 1320, 1156.

Addition of Trimethylsilyl Isothiocyanate
2801
2-Thiocyanatocyclohexanol 3f
See Refs. 11 and 19.
N-(2-Isothiocyanatocyclohexyl)benzamide 4b
1
White solid; H NMR (CDCl3) d 1.25–1.52 (m, 4H), 1.66–1.83 (m, 2H),
2.13–2.2 (m, 2H), 3.7 (ddd, J ¼ 10.2, 4.12, 3.9 Hz, 1H), 4.1 (ddd, J ¼ 9.7,
9.6, 5.1 Hz, 1H), 6.1 (d, J ¼ 7.6 Hz, 1H), 7.42–7.55 (m, 3H), 7.6–7.8 (m,
2H); EI MS 260 (Mþ); IR (KBr) 3306, 2080, 1635, 1549 cmꢀ1
.
2-Nitro-N-(2-isothiocyanatocyclohexyl)benzamide 4c
White solid; d 1H NMR (acetone-d6) d 1.43–1.7 (m, 3H), 1.78–1.92
(m, 3H), 2.26–2.4 (m, 2H), 3.72 (ddd, J ¼ 9.1, 5.6, 3.6 Hz, 1H), 3.98
(ddd, J ¼ 8.5, 5.4, 3.9 Hz, 1H), 8.08–8.16 (m, 2H), 8.2 (d, J ¼ 9.1, 1H),
8.3 (m, 2H) (ddd); EI MS 305 (Mþ); IR (KBr) 3388, 2100, 1670, 1345,
850 cmꢀ1
.
3,5-Dinitro-N-(2-isothiocyanatocyclohexyl)benzamide 4d
White solid; d 1H NMR (acetone-d6) d 1.44–1.67 (m, 3H), 1.8–1.9
(m, 3H), 2.12–2.38 (m, 2H), 3.94 (ddd, J ¼ 10.2, 6.2, 3.98 Hz, 1H), 4.21
(ddd, J ¼ 8.8, 6.8, 1.99 Hz, 1H), 8.65 (s, 1H), 9.1 (s, 3H); EI MS 350
(Mþ); IR (KBr) 3261, 1670, 694 cmꢀ1
.
ACKNOWLEDGMENT
This work was supported by Grant PBZ-KBN-126=T09=10 from the
Ministry of Science and Higher Education.
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