Reaction of Nitroxides with Sulfur Containing Compounds II [1]. Preparation of Nitroxides Bearing an Isothiocyanate Substituent in View of the Nitroxyl Group Reduction with Thiophosgene

Article abstract of DOI:10.1007/s00706-002-0572-x

The reaction of thiophosgene with 2,2,6,6-tetramethylpiperidine-1-oxyl (used as a model nitroxyl radical) was examined. 2,2,6,6-Tetramethylpiperidine and 2,2,6,6-tetramethyl-1-hydroxypiperidine were identified as products. The reaction is not competitive with the reaction of thiophosgene with an amino group. Thus, three nitroxides with an isothiocyanate group were synthesized from thiophosgene and the nitroxides containing the amino substituent.

Full text of DOI:10.1007/s00706-002-0572-x

Monatshefte f€ur Chemie 134, 843–850 (2003)
DOI 10.1007/s00706-002-0572-x
Reaction of Nitroxides with Sulfur Containing
Compounds II [1]. Preparation of Nitroxides
Bearing an Isothiocyanate Substituent
in View of the Nitroxyl Group Reduction
with Thiophosgene
ꢀ
Jerzy Zakrzewski , Jaroslaw Hupko, and Krzysztof Kryczka
Institute of Industrial Organic Chemistry, 6 Annopol, 03-236 Warsaw, Poland
Received July 22, 2002; accepted (revised) September 3, 2002
Published online March 13, 2003 # Springer-Verlag 2003
Summary. The reaction of thiophosgene with 2,2,6,6-tetramethylpiperidine-1-oxyl (used as a model
nitroxyl radical) was examined. 2,2,6,6-Tetramethylpiperidine and 2,2,6,6-tetramethyl-1-hydroxypiper-
idine were identified as products. The reaction is not competitive with the reaction of thiophosgene
with an amino group. Thus, three nitroxides with an isothiocyanate group were synthesized from
thiophosgene and the nitroxides containing the amino substituent.
Keywords. Thiophosgene; Isothiocyanates; Nitroxides.
Introduction
The reaction of thiophosgene (1) [2–4] (and references cited therein) with com-
pounds containing a primary amino group is the most important method for the
synthesis of isothiocyanates [5–8] (and references cited therein). This basic method
was applied for the synthesis of nitroxide radicals bearing an isothiocyanate group
[9, 10a], however 1 may be suspected to reduce a nitroxyl group, as it is the case
with a number of low valent sulfur containing compounds which are reported to be
responsible for a reduction of nitroxides. Especially, many references can be found
for compounds containing a sulfhydryl group (e.g. [11–16]) which are important in
the biochemical environment. Most often, the corresponding hydroxylamines are
reported as products. However, reductions of nitroxides to the corresponding
amines were also described [1] (and references cited therein). Thus, 1 (as a com-
pound susceptible to oxidation [17]) may also interfere with a nitroxyl group.
ꢀ
Corresponding author. E-mail: zakrzewski@ipo.waw.pl

844
J. Zakrzewski et al.
We here report the results of the reaction of 1 with a nitroxyl group and with
nitroxides containing a primary amino function, which may give the corresponding
isothiocyanates.
Results and Discussion
Reaction of Thiophosgene ð1Þ with 2,2,6,6-Tetramethylpiperidine-1-oxyl ð2Þ
In order to investigate the reaction of 1 with the nitroxyl group, 2 – a radical
without extra functional groups – was selected as model compound. The reaction
of 1 with 2 was examined in benzene and dichloromethane as solvents. An excess
of 1, benzene with triethylamine as hydrogen chloride acceptor, and (to simulate
conditions for isothiocyanate synthesis) two-phase systems: dichloromethane=
aqueous sodium bicarbonate solution and dichloromethane=aqueous sodium hy-
droxide solution at ambient temperature were tested.
The course of the reaction was monitored by means of TLC, GLC, and
GLC=MS. Strong darkening of the reaction mixtures and the complete decay of
the radical 2 were observed after 8–10 hours. When benzene and an excess of 1
were used as solvents a black precipitate formed. After workup, analysis by means
of GLC and GLC=MS revealed a strong peak for 2,2,6,6-tetramethylpiperidine (3)
and – unexpectedly – the radical 2 again (Scheme 1). So, we assumed that 2,2,6,6-
tetramethyl-1-hydroxypiperidine (4) is the true product of the reaction. Its re-oxi-
dation (simply by air during workup of the reaction mixture) affords back radical 2
[10b, 18a, 19].
The presence of 4 in the reaction mixture was directly confirmed by compar-
ison of the reaction mixtures with an authentic sample by means of TLC. The GLC
results revealed, that the higher the starting ratio of 1:2, the higher was the final
ratio 3:4 (determined as the radical 2), (Table 1, entries 1–6). Isolation of the
products led to a mixture of 3 and the secondary formed radical 2. The GLC
analysis of this mixture allowed to estimate directly the yield of the amine 3
(Table 1, entries 7–9). When the process was carried out either with NaOH=
H2O=CH2Cl2 as base or in NEt3=C6H6, the hydrolysis of 1 and the reaction of 1
with triethylamine, respectively, were observed, instead of the reaction of radical 2
with 1.
Isothiocyanates with a Nitroxyl Group
The six- and five-membered cyclic nitroxides bearing a primary amino group
were allowed to react with 1. Reactions with sodium hydroxide (A) [9, 10a],
NaHCO3=H2O=CH2Cl2 [20–22] (B), and NEt3=C6H6 [23] (C) were tested.
Scheme 1. a) identified as starting radical 2 after re-oxidation by air

Reaction of Nitroxides with Sulfur Containing Compounds
845
Table 1. Reaction of thiophosgene 1 with the radical 2
Entry
1:2
Solvent
3:2a
Yield of 3
[mol=mol]
(GLC)
[%]
1
2
3
4
5
6
7
8
9
1:2
1:1
2:1
1:2
1:1
2:1
1:1
13:1
1:1
benzene
benzene
benzene
CH2Cl2
0.38
0.53
3.68
0.46
4.12
17.2
CH2Cl2
CH2Cl2
benzene
1
20
37
23
NaHCO3=H2O=CH2Cl2
a
The radical 2 detected in the mixture of products after re-oxidation of 4
#
n
1
m
0
Method
Yield of 6=%
5, 6a
A
B
C
A
B
C
A
B
C
54
66
67
33
72
77
33
76
63
5, 6b
5, 6c
0
0
0
1
Scheme 2
When the reactions were carried out below ambient temperature and quenched
after several minutes, only the amino group reacted with 1, whereas the nitroxyl
group remained unchanged. If NaOH=H2O=CH2Cl2 was used as base, the results
were apparently worse – obviously due to hydrolysis of 1. The synthesized nitr-
oxides bearing an isothiocyanate group are presented in Scheme 2.
Conclusions
The synthesis of nitroxides bearing an isothiocyanate group can be efficiently
performed by reaction of 1 with the amino group, both with NaHCO3=
H2O=CH2Cl2 and NEt3=C6H6 systems rather than in NaOH=H2O=CH2Cl2.

846
J. Zakrzewski et al.
Comparison of the reactions let us conclude that the relative reactivity of 1 towards
reagents should be arranged in the order: 5a–c ꢁ NaOH>NEt3>2 ꢁ NaHCO3.
The reduction of the nitroxyl group does not take place under the given conditions.
Experimental Part
Methods
Melting points are uncorrected. TLC was done on silica gel with 254 nm fluorescent: Merck 5554,
5562. Visualization: UV 254 nm and I2 vapour. Mobil phase for the evaluation of the radical 2 decay:
benzene:ethyl acetate ¼ 9:1. Column chromatography was done on silica gel: <0.08 mm (Merck
7729), 60 G for TLC (Merck 7731), Serva for TLC (27185), Merck 1.09385.1000. Mobil phase:
benzene:ethyl acetate ¼ 95:5. GLC was recorded on a Varian 3300 gas chromatograph: DB1, tc ¼ 90ꢂC
(3min) ä (10ꢂC=min)ä 260ꢂC (5min), ti ¼ 260ꢂC, td ¼ 280ꢂC (FID). GLC=MS was recorded on an
AMD M-40 gas chromatograph with mass detector: HP5, tc ¼ 50ꢂC (3min) ä (5ꢂC=min)ä 220ꢂC,
ti ¼ 240ꢂC. MS and IR data were determined using an AMD M-40 (EI, 70eV) and FT IR Jasco 420
apparatus.
Starting Compounds
All commercially available chemicals were used as received. Thiophosgene (1) (98%) was man-
ufactured in the Institute of Industrial Organic Chemistry. The radical 2 [18b, 24–26], as well as
the samples of the amine 3 [27] and the hydroxylamine 4 [28] were obtained according to literature
methods. 4-Amino-2,2,6,6-tetramethylpiperidine was either obtained from triacetonamine by oxima-
tion with hydroxylamine hydrochloride (18%) [24, 29], followed by reduction of the corresponding
oxime with sodium in ethanol (80%) 18c, 30, 31], or purchased from Georganics (Slovakia) as well as
received as a free sample from Degussa-Huels. 2,2,6,6-Tetramethyl-4-piperidinone-1-oxyl was syn-
thesized by oxidation of triacetonamine with hydrogen peroxide and sodium carbonate (73%) [32, 33].
The starting aminonitroxides 5a–5c were obtained in multi-step preparations from 2,2,6,6-tetramethyl-
4-piperidinone (triacetonamine) according to reported methods with some modifications described
below.
4-Amino-2,2,6,6-tetramethylpiperidine-1-oxyl (5a) [18d, 30, 34]
4-Amino-2,2,6,6-tetramethylpiperidine (117g, 0.75 mol) dissolved in 340 cm3 of diethyl ether was
protected by acetylation with 236.6g of acetic anhydride (adding dropwise at 5–10ꢂC for 3.5 h) to
give 197 g of 4-acetamido-2,2,6,6-tetramethylpiperidine acetate (mp 188–195ꢂC). It was alkalized with
148.1g of potassium carbonate in 1.42 dm3 of H2O at 25ꢂC for 15 min, followed by oxidation with
284 cm3 of 30% H2O2 in the presence of 14.2g of sodium tungstate and 14.2 g of EDTANa2 for 73h to
give 150.8 g of 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl, which was directly hydrolyzed with
482.3g of KOH in 1.32 dm3 of H2O under reflux and vigorous mechanical stirring for 29h. Saturation
of the solution with K2CO3, thorough extraction with about 1.5 dm3 of diethyl ether, followed by
fractional vacuum distillation afforded 63.3 g of 5a (49%), bp 84–86ꢂC=0.5 mm Hg; IR (film);
ꢀꢀ ¼ 3360; 3280 cmꢃ1; MS is consistent with Refs. [35a, 36a].
2,2,5,5-Tetramethyl-3-carbamoylpyrrolidine-1-oxyl
Ring contraction of 2,2,6,6-tetramethyl-4-piperidinone-1-oxyl by Favorski rearrangement [37, 38]: A
solution of 44.1g of I2 (0.1736 mol) in 882 cm3 of benzene was added dropwise to a solution of 30.0g
of 2,2,6,6-tetramethyl-4-piperidinone-1-oxyl (0.1765 mol) and 29.7 g of KOH (0.53 mol) in 882cm3 of

Reaction of Nitroxides with Sulfur Containing Compounds
847
25% aq. NH3 at 25ꢂC for 8 h. After workup, followed by crystallization from ethyl acetate, 17.3 g of
2,2,5,5-tetramethyl-3-carbamoylpyrrolidine-1-oxyl were obtained (53%), mp 168–170.5ꢂC.
2,2,5,5-Tetramethyl-3-aminopyrrolidine-1-oxyl (5b)
Hofmann rearrangement of 2,2,5,5-tetramethyl-3-carbamoylpyrrolidine-1-oxyl with NaOBr
[18e, 39, 40].
2,2,5,5-Tetramethyl-3-carbamoylpyrrolidine-1-oxyl (9.3g, 0.05 mol) was added to a solution of
11.3g of Br2 (0.07 mol) and 12.1g of NaOH in 130 cm3 of H2O at 5ꢂC. The solution was stirred at 5ꢂC
for 2 h and at 70ꢂC for 2 h. After workup, 4.13g of 5b were obtained (52%), bp 65.5–66ꢂC=0.6 mm Hg
(Refs. [18e, 39] 75–80ꢂC=1 mm Hg); mp 44–46ꢂC (Ref. [41] 34–35ꢂC); IR (film): ꢀꢀ ¼ 3378, 3312,
2970, 2929, 2870, 1464, 1361, 1106, 860 cmꢃ1; MS is consistent with Refs. [35b, 36b].
2,2,5,5-Tetramethyl-3-(aminomethyl)pyrrolidine-1-oxyl (5c) [42, 43]
2,2,5,5-Tetramethyl-3-carbamoylpyrrolidine-1-oxyl (5.676 g, 30.7mmol) was reduced with 7.32g of
LiAlH4 (0.193mol) (added in three 2.44g portions in equidistant time intervals) in 800 cm3 of anhy-
drous diethyl ether under reflux and vigorous magnetical stirring for 28h. After workup, 5.3g of 5c
(yellow oil) were obtained. The crude amine 5c was purified by vacuum distillation (2.0g, 38%), bp
84–7ꢂC=0.8 mm Hg (Ref. [43] 55ꢂC=0.1 mm Hg); IR (film): ꢀꢀ ¼ 3370, 3303, 2971, 2929, 2867, 1463,
1362 cmꢃ1; MS: m=z (%) ¼ 183 (2), 173 (12), 172 (30), 171 (100, M), 157 (38), 142 (6), 127 (6), 126
(16), 112 (8), 110 (6), 109 (22), 98 (3), 96 (2), 95 (8), 84 (5), 81 (7), 74 (18), 70 (42), 69 (12), 68 (10),
67 (13), 58 (12), 56 (15), 55 (13), 53 (9).
Reaction of Thiophosgene (1) with 2,2,6,6-Tetramethylpiperidine-1-oxyl (2)
a) In Benzene (Table 1, entry 7)
Thiophosgene (1) (0.115 g, 1 mmol, 77 mm3) was added with a syringe to a solution of 0.1578 2 of
(1mmol) in 1 cm3 of benzene and the solution was allowed to stand at r.t. for 24h. The precipitate
(0.121 g) was filtered off, washed with acetone, and placed in a separatory funnel. A 20% aq. K2CO3
solution (1cm3) was added and the mixture was extracted with CH2Cl2. The organic layer was dried
with anh. MgSO4, filtered, and the solvent was evaporated. A mixture of 3 and 2 (0.065g, reddish oil)
was obtained. The yield of the amine 3, estimated according to GLC analysis, was 20%.
b) In an Excess of 1 (Table 1, entry 8)
The radical 2 (0.158g, 1 mmol) was added to an excess of 1 (1.51 g, 13.1mol, 1 cm3) and the solution
was allowed to stand at r.t. for 24h. The precipitate was filtered off (0.1686g), washed with acetone,
and placed in a separatory funnel. The precipitate was alkalized as above providing 0.060g of an oil
(37% of 3 – estimated as above).
c) In the NaHCO3=H2O=CH2Cl2 System (Table 1, entry 9)
The radical 2 (0.3897 g, 2.5 mmol) dissolved in 1 cm3 of CH2Cl2 was added to a mixture of 0.504g of
NaHCO3, 1.25 cm3 of H2O, 1 cm3 of CH2Cl2, and 0.302 g of 1 (2.624mmol, 201 mm3). The reaction
mixture was vigorously stirred for 9 h at r.t., diluted with CH2Cl2, and then washed with sat. NaHCO3
solution. The organic layer was dried with anh. MgSO4. After filtration and evaporation, a dark
crystalline mass (0.407g) was obtained. The solid was triturated with benzene, then with benzene:ethyl

848
J. Zakrzewski et al.
acetate 95:5, filtered, and placed in a separatory funnel. The precipitate was alkalized as above yielding
0.175 g of an oil (23% of 3 – estimated as above).
2,2,6,6-Tetramethyl-4-isothiocyanatopiperidine-1-oxyl (6a)
Procedure A with NaOH=H2O=CH2Cl2
Thiophosgene (1) (0.302g, 2.62mmol, 200 mm3) was added dropwise with a syringe to 0.438 g of 5a
(2.56 mmol) dissolved in 10 cm3 of a 5% NaOH solution at 5–7.5ꢂC for 10 min. An exotermic process
was observed and the resulting mixture was stirred for 10 min. The precipitate1 (0.613g) was removed
by filtration and dried under vacuum to give 0.362 g of the raw product. It was subjected to column
chromatography to yield 0.293 g of 6a (54%), mp 125.5–127.5ꢂC (Refs. [9, 10a] 127–128ꢂC, Ref. [44]
131–132ꢂC); IR (film): ꢀꢀ ¼ 2187; 2126 cmꢃ1; MS: m=z (%) ¼ 214 (4), 213 (14, M), 199 (4), 198 (3),
183 (5), 154 (12), 140 (10), 128 (12), 127 (9), 126 (12), 124 (11), 112 (17), 109 (7), 100 (11), 98 (6), 92
(6), 85 (4), 81 (6), 78 (8), 74 (7), 72 (7), 69 (100), 56 (11), 55 (12), 53 (7), 41 (45).
2,2,5,5-Tetramethyl-3-isothiocyanatopyrrolidine-1-oxyl (6b; C9H15N2OS)
Procedure C with NEt3=C6H6
Thiophosgene 1 (0.279 g, 2.43 mmol, 185 mm3) in 1 cm3 of benzene (dried azeotropically) was added
dropwise with a syringe to the solution of 0.363 g of 5b (2.31 mmol) and 0.68cm3 of triethylamine in
5 cm3 of benzene at 10–17ꢂC with vigorous stirring. The mixture was stirred at 20ꢂC for 5 min and the
precipitate of triethylamine hydrochloride was filtered off and thoroughly washed with benzene. The
benzene filtrate was washed with sat. NaHCO3 solution, the aq. phase was extracted with 2ꢄ 3 cm3 of
benzene, and the organic layers were dried with anh. MgSO4, filtered, and the solvent was evaporated.
The residue was subjected to column chromatography to give 0.356 g of 6b (77%); IR (film):
ꢀꢀ ¼ 2098 cmꢃ1; MS: m=z (%) ¼ 200 (1), 199 (11, M), 185 (2), 169 (2), 167 (3), 154 (4), 149 (5),
140 (2), 139 (3), 126 (15), 113 (60), 112 (19), 111 (14), 110 (7), 100 (24), 99 (9), 98 (15), 95 (12), 92
(6), 90 (4), 84 (13), 83 (8), 80 (18), 78 (5), 74 (14), 69 (48), 67 (16), 58 (20), 57 (14), 56 (86), 55 (100),
53 (22), 43 (32).
2,2,5,5-Tetramethyl-3-(isothiocyanatomethyl)pyrrolidine-1-oxyl (6c)
Procedure B with NaHCO3=H2O=CH2Cl2
Amine 5c (0.375 g, 2.19 mmol) dissolved in 0.5 cm3 of CH2Cl2 was added dropwise with vigorous
stirring to the mixture of 0.493 g of NaHCO3, 1.25 cm3 of H2O, 1 cm3 of CH2Cl2, and 0.297 g of 1
(2.58 mmol, 197 mm3) at 10–15ꢂC. The mixture was stirred at r.t. for 5 min, CH2Cl2 (5cm3) and 5 cm3
of H2O were added, and the aq. layer was extracted with 3 ꢄ 2 cm3 of CH2Cl2. The organic layer was
dried with anh. MgSO4, filtered, and the solvent was evaporated. The dark-yellow oil (0.463g) was
subjected to column chromatography to give 0.353g of 6c (76%), mp 33–6ꢂC (Ref. [44] 36–7ꢂC);
IR (film): ꢀꢀ ¼ 2080 cmꢃ1; MS: m=z (%) ¼ 214 (2), 213 (14, M), 199 (1), 183 (5), 140 (4), 109 (3), 100
(3), 81 (5), 74 (13), 69 (100), 56 (9), 55 (10), 41 (37).
Acknowledgement
The authors are indebted to Mrs. A. Kielczewska (MSc) and to Mrs. S. Garbaczewska (MSc) for
GLC=MS and GLC analyses.
1
In the case of 6b and 6c, synthesis according to procedure A gave no precipitate. Thus, the reaction
mixture was extracted with CH2Cl2

Reaction of Nitroxides with Sulfur Containing Compounds
849
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