Reactions of nitroxides, part XI: O-Aryl phenylselenophosphonates bearing a nitroxyl moiety

Article abstract of DOI:10.1002/hc.20667

O-Aryl phenylselenophosphonates bearing a nitroxyl moiety were synthesized in the presence of pyridine in diethyl ether from 4-hydroxy-2,2,6,6- tetramethylpiperidine-1-oxyl, and either phenyldichlorophosphine and elemental selenium in 9-35% yield or phenyldichlorophosphine selenide in 36-62% yield. O,O-Diaryl phenylselenophosphonates and O,O-bis(2,2,6,6-tetramethyl-1-oxyl-4- piperidyl)phenylselenophosphonates were obtained as by-products. O,O-Diaryl phenylselenophosphonates were obtained from phenyldichlorophosphine selenide and 2 mol of phenols in 55-86% yield. Antifungal activity of some synthesized selenophosphonates was found.

Full text of DOI:10.1002/hc.20667

Heteroatom Chemistry
Volume 22, Number 2, 2011
Reactions of Nitroxides, Part XI: O-Aryl
Phenylselenophosphonates Bearing a Nitroxyl
Moiety
Bogumiła Huras, Jerzy Zakrzewski, and Maria Krawczyk
Institute of Industrial Organic Chemistry, Annopol 6, 03–236 Warsaw, Poland
Received 20 September 2010; revised 25 November 2010
teine (HSeCH2CH(NH2)COOH) as a part of an ac-
ABSTRACT: O-Aryl phenylselenophosphonates
tive site [7,10]. The important relation of selenium-
bearing a nitroxyl moiety were synthesized in the
containing compounds to cancer is discussed [3,5–
presence of pyridine in diethyl ether from 4-hydroxy-
7,11].
2,2,6,6-tetramethylpiperidine-1-oxyl,
and
either
The role of selenium in agriculture has been re-
cently reviewed [10]. This role is controversial. The
hyperaccumulation of selenium in plants can cause
selenosis in animals [2]. The toxicity of selenophos-
phates to mice was also investigated [12]. In spite
of that, attempts to apply selenium-containing com-
pounds as insecticides: beta-selenolactams [13] and
selenophosphates [12] were described. Recently, we
have investigated antifungal properties of the se-
lenoureas bearing a nitroxyl moiety [14]. Because
there are only a few examples of nitroxyl radicals
containing a selenium atom [15,16] or P=Se moiety
[17,18], we are encouraged to continue the inves-
tigation of the antifungal properties of the nitroxyl
radicals with a P=Se function. Herein, we present
the synthesis and antifungal evaluation of phenylse-
lenophosphonates bearing a nitroxyl moiety.
phenyldichlorophosphine and elemental selenium in
9–35% yield or phenyldichlorophosphine selenide
in 36–62% yield. O,O-Diaryl phenylselenophos-
phonates and O,O-bis(2,2,6,6-tetramethyl-1-oxyl-4-
piperidyl)phenylselenophosphonates were obtained as
by-products. O,O-Diaryl phenylselenophosphonates
were obtained from phenyldichlorophosphine selenide
and 2 mol of phenols in 55–86% yield. Antifungal
activity of some synthesized selenophosphonates
ꢀ
C
was found.
2011 Wiley Periodicals, Inc. Heteroatom
Chem 22:137–147, 2011; View this article online at
wileyonlinelibrary.com. DOI 10.1002/hc.20667
INTRODUCTION
Selenium plays an important role in living organ-
isms (for some recent reviews, see [2–10]). This ele-
ment is found in plants mostly as selenomethionine
(CH3SeCH2CH2CH(NH2)COOH) [10]. In animals, all
essential functions of selenium have been associ-
ated with selenoproteins, which contain selenocys-
RESULTS AND DISCUSSION
Synthesis of phenylselenophosphonates bearing a
nitroxyl moiety is shown in Scheme 1.
Phenyldichlorophosphine
selenide
[1b,
PhP( Se)Cl2] was obtained by direct seleniza-
tion of phenyldichlorophosphine (1a) at 175–180◦C
for 1.5 h in 59.7% yield (double distillation) [19,20].
As selenization of trivalent phosphorus is a typ-
ical method of building the P Se bond, phospho-
rylation of 2,2,6,6-tetramethyl-4-piperidinol-1-oxyl
Correspondence to: Jerzy Zakrzewski; e-mail: zakrzewski@
ipo.waw.pl.
For Part X, see [1].
Contract grant sponsor: Polish Ministry of Science and Higher
Education.
Contract grant number: 429/E-142/S/2009-2.
2011 Wiley Periodicals, Inc.
c
ꢀ
137

138 Huras, Zakrzewski, and Krawczyk
SCHEME 1 Synthesis of phenylselenophosphonates bearing a nitroxyl moiety.
(TEMPOL, 2) and phenols 3a–f was achieved with
phenyldichlorophosphine (1a) and black selenium
in diethyl ether in the presence of pyridine (method
A). Alternatively, TEMPOL (2) and phenols 3a–f
were phosphorylated with phenyldichlorophosphine
selenide (1b) (method B). The O-aryl phenylse-
lenophosphonates bearing a nitroxyl moiety 5a–f,
diaryl esters 6a–f, and dinitroxyl diradical 7 were
the products in both cases (Scheme 1 and Table 1).
As shown in Table 1, when phenyldichlorophos-
phine selenide (1b) was used as a starting compound
(method B), the target nitroxides 5a–f were obtained
in higher yields.
In the reaction of 1a, Se (or 1b instead), phe-
nols 3a–f, and TEMPOL (2), diaryl esters 6a–f were
formed as minor products (see above). To obtain
larger amounts of 6a–f and to evaluate their bio-
logical properties, they were prepared via the direct
reaction of phenyldichlorophosphine selenide (1b)
with 2 mol of phenols 3a–f (Scheme 2 and Table 2).
Monochlorides 8c and 8e were isolated as side
products. 8e was unstable, and the confirmation
of its structure was not possible. The presence of
monochlorides 8a and 8b was proved from 1H NMR
spectra.
It is worthy to note that the attempts to react
both monochloride 4 with an equivalent amount of
phenol 3a to get 5a and monochloride 8c with an
equivalent of 4-methoxyphenol 3c to get the diester
6c, were unsuccessful (Scheme 3). Unreacted start-
ing 4 or 8c remained unchanged (TLC).
Phenylselenophosphonic 2,2,6,6-tetramethyl-1-
oxyl-4-oxypiperidyl chloride (4) was obtained from
phenyldichlorophosphine selenide (1b) and TEM-
POL (2) in the presence of pyridine in diethyl ether
in 35.4% yield (mp 42–52◦C).
TABLE 1 Products 5a–f/6a–f/7 in the Reaction of a Phenol + TEMPOL (2) + PhPCl2 (1a) + Se (Method A) versus the
Reaction of a Phenol + TEMPOL (2) + PhP( Se)Cl2 (1b) (Method B)
5a–f Method A
(%)/meth B (%)
6a–f Method A
(%)/method B (%)
7a Method A
(%)/method B (%)
Phenol 3a–f
a
b
c
d
e
f
C6H5OH
35.5/51.2
9.1/35.9
8.6/19.7
—/7.4
33.4/14.4
34.1/8.0
34.1/24.6
11.6/—
17.7/28.1
58.7/23.1
4-O2NC6H4OH
4-CH3OC6H4OH
4-CF3C6H4OH
4-CH3C6H4OH
2,4-di-Cl2C6H3OH
10.6/56.8b
16.8/48.4
22.4/61.8
33.5/59.5
2.2/9.2
—/38.1c
30.0/13.2
—/20.6
amp: 85–89◦C (110–116◦C [17]).
bmp: 71–75◦C.
cmp: 41–46◦C.
Heteroatom Chemistry DOI 10.1002/hc

Reactions of Nitroxides, Part XI: O-Aryl Phenylselenophosphonates Bearing a Nitroxyl Moiety 139
Ar
3,6,8
a
b
c
d
e
f
C6H5
Ar
Cl
Ar
4-O2NC6H4
4-H3COC6H4
4-F3CC6H4
4-H3CC6H4
2,4-Cl2C6H3
O
P
O
P
Se
Cl
P
Se
py
Et O
Se
2
O
+
+ 2 Ar-OH
Cl
Ar
1b
3a–f
6a–f
8a–c, e
1
8a,b: identified by H NMR,
8c: isolated,
8e: isolated, lack of spectral data
SCHEME 2 Synthesis of diaryl phenylselenophosphonates 6a–f.
The reaction of TEMPOL (2), and either 1a and
13C NMR, 31P NMR, and 77Se NMR spectra were
recorded (Tables 3 and 4).
selenium (method A) or 1b (method B), was repeated
with alcohols (cyclohexanol, cyclopentanol, benzyl
alcohol, n-hexanol, tert-butanol, and isopropanol).
Analogous phenylselenophosphonates 5 and appro-
priate diaryl esters 6 were obtained; however, the
products were strongly contaminated with phenylse-
lenophosphonic monochlorides 8. The isolation of
pure phenylselenophosphonates 5 and the diaryl es-
ters 6 by flash chromatography was unsuccessful,
so the activity of aliphatic nitroxide selenophospho-
nates and diesters was not tested.
Mass spectra (EI) of nitroxyl esters 5a–f show
characteristic fragmentation ions M-153. In most
cases, the abundances of the corresponding M-
153 peaks are strong (5a: 299 (100), 5b: 344 (20),
5c: 329 (84), 5d: 367 (52), 5e: 313 (100), 5f: 367
(8)). More importantly, there are no other fragmen-
tation peaks observed between M and M-153. In
Fig. 1, the exemplary mass spectrum of 5e is pre-
sented. Because the M-153 signal shows character-
istic selenium pattern, the origin of a M-153 peak
should be the elimination of a nitroxyl fragment.
In Scheme 4, the elimination of the nitroxyl radical
M = 154 is presented. The observed [M-153]+ frag-
ment has likely a structure presented in Scheme 4.
The proposed structure of [M-153]+ fragment
was confirmed by HR EI MS analysis (for 5e):
C13H14O2P80Se: calcd: 312.98966, found: 312.98838.
The stability of all synthesized compounds is
limited in spite of storage in a refrigerator. Evolution
of red selenium is observed sooner or later. Some of
the compounds are crystalline; however, their melt-
ing points are not sharp. The observed melting point
of 7 (85–89◦C) is essentially different from the re-
ported literature value (110–116◦C [17]).
The structures of the synthesized compounds
were confirmed by spectroscopic methods. The MS,
HR MS, and IR spectra were performed for the com-
pounds bearing a nitroxyl moiety. For the diamag-
netic derivatives, the MS, HR MS, IR, 1H NMR,
•
O
N
•
O
N
O
P
Cl
P
Se
3a, py, Et
O
2, py, Et
O
2
Se
2
O
P
O
Cl
Se
TABLE
2 Diaryl Phenylselenophoshonates 6a–f from
5a
1b
4
Cl
Phenyldichlorophosphine Selenide (1b) and 2 mol of Phe-
noles 3a–f (Method B)
OCH3
Phenol 3a–f
6a–f (%)
8 (%)
OCH3
a
b
c
d
e
f
C6H5OH
61.8
56.6a
54.5
55.3
59.5
86.3d
–
–
O
Se
O
4-O2NC6H4OH
4-CH3OC6H4OH
4-CF3C6H4OH
4-CH3C6H4OH
2,4-di-Cl2C6H3OH
3c, py, Et
O
2
P
Se
O
46.3b
–
P
Cl
22.1c
–
8c
6c
OCH3
amp: 50–75◦C; decay with selenium evolution.
bmp: 45–48◦C.
cDecomposition with selenium evolution, no spectroscopic data.
dmp: 62–71◦C.
SCHEME 3 No products in the reaction of the monochlo-
rides 4 and 8c with phenols 3a and 3c, respectively.
Heteroatom Chemistry DOI 10.1002/hc

140 Huras, Zakrzewski, and Krawczyk
TABLE 3 MS, HR MS, and IR Data for the Investigated Compounds
Compound
MS (m/z, int%)
HR MS
IR (ν, cm−1 b
)
1b
EI: 260 (13), 258 (23, M), 256 (9), 223 (7), 178
(31), 159 (17), 157 (20), 143 (100), 107 (37),
77 (42), 51 (30), 50 (14); ESI: a) 251 (100, [M
+ H]+), 249 (20), 157 (20)
ESIa for [M + Na]+
C8H11O2P80SeNa calcd.:
272.9560, found:
272.9555
Film: 1438, 1096, 745, 712,
683, 619, 589, 517
4
EI: 397 (1), 396 (2), 395 (2), 394 (4, M), 393 (1), EI: for M
KBr: 1465, 1439, 1363,
1244, 1179, 1108, 979,
884, 826, 745, 722, 680,
590, 556, 507
392 (2), 243 (8), 242 (3), 241 (14), 240 (6),
239 (7), 238 (5), 237 (4), 225 (2), 223 (2), 221
(3), 205 (3), 203 (1.5), 187 (4), 185 (2), 155
(28), 154 (92), 140 (15), 139 (14), 124 (100),
109 (67), 98 (20)
C15H22NO2P80SeCl
calcd.: 394.02419, found:
394.02321.
5a
5b
EI: 453 (4), 452 (8, M), 450 (4), 299 (100), 297
(48), 217 (21), 205 (27), 203 (17), 154 (87),
141 (8), 140 (13), 139 (12), 124 (52), 109
(37), 95 (13), 94 (29), 69 (14), 56 (11), 41 (24)
EI: for M C21H27NO3P80Se
calcd.: 452.08938, found:
452.08781
Film: 1592, 1488, 1197,
1012, 919, 770, 690, 596,
539
EI: 499 (1), 498 (2), 497 (4, M), 496 (1), 495 (2), EI: for M C21H26N2O5P80Se Film: 1590, 1524, 1489,
344 (20), 342 (11), 326 (4), 262 (9), 205 (15),
203 (10), 154 (100), 140 (10), 139 (10), 124
(69), 109 (38), 69 (18), 56 (19), 55 (32), 41
(27)
EI: 484 (3), 483 (6), 482 (9, M), 480 (6), 331
(19), 330 (14), 329 (84), 328 (10), 327 (56),
326 (21), 325 (18), 311 (19), 247 (12), 231
(7), 205 (15), 189 (11), 188 (9), 187 (7), 186
(6), 185 (4), 154 (100), 140 (27), 125 (14),
124 (76), 123 (43), 109 (48), 100 (6), 98 (9),
95 (11), 83 (8), 82 (6), 81 (13), 77 (12), 74
(12), 69 (20), 67 (15), 58 (14), 56 (15), 55
(39), 41 (42)
calcd.: 497.07446, found:
497.07564.
1346, 1216, 1115, 1010,
902, 742, 681, 626, 540
5c
EI: for M C22H29NO4P80Se
calcd.: 482.09994, found:
482.09854.
Film: 1503, 1250, 1193,
1116, 1011, 905, 832,
725, 692, 590
5d
5e
EI: 520 (8, M), 518 (4), 367 (52), 365 (28), 349
(6), 347 (4), 271 (5), 269 (7), 207 (7), 205
(23), 203 (12), 154 (100), 140 (13), 124 (46),
109 (14), 56 (4), 55 (6), 41 (9)
EI: 467 (4), 466 (12, M), 464 (6), 463 (3), 462
(2), 315 (18), 314 (15), 313 (100), 311 (48),
310 (18), 309 (20), 231 (20), 205 (22), 172
(6), 154 (92), 140 (23), 124 (59), 109 (39),
108 (24), 107 (10), 91 (15)
EI: for M
Film: 1611, 1511, 1439,
1325, 1214, 1168, 1125,
1067, 1011, 995, 904
C22H26F3NO3P80Se
calcd.: 520.07676, found:
520.07785
EI: for M C22H29NO3P80Se
calcd.: 466.10503, found:
466.10317
Film: 3337, 1615, 1595,
1505, 1460, 1438, 1364,
1198, 1115, 1011, 990,
909, 821, 750, 723, 690,
590, 557, 508
5f
EI: 522 (2), 520 (3, M), 518 (2), 396 (2), 394 (3), EI: for M
Film: 1476, 988, 590, 558,
509
392 (2), 369 (13), 367 (8), 365 (8), 287 (4),
285 (6), 241 (11), 205 (7), 154 (95), 140 (40),
124 (100), 109 (47)
C21H25Cl2NO3P80Se35
calcd: 520.01143, found:
520.01396
6a
6b
EI: 376 (8), 375 (8), 374 (35, M), 372 (18), 371
(7), 299 (3), 297 (3), 281 (12), 279 (6), 217
(100), 203 (22), 201 (14), 77 (29)
EI: for M C18H15O2P80Se
calcd: 373.99749, found:
373.99832
Film: 1590, 1487, 1185,
917, 771, 688, 595, 528
ESI: 465 (8.5, [M + H]+), 463 (1), 413 (3), 262
(4), 242 (100)
ESI: for [M + H]+
C18H14N2O6P80Se calcd.
464.9755, found:
Film: 1615, 1590, 1523,
1488, 1346, 1200, 1160,
1112, 903, 859, 744, 685,
638
Film: 1501, 1250, 1178,
1116, 1032, 904, 831,
800, 725, 691, 570, 520
464.9769
6c
6d
ESI: 537 (1, contaminant), 535 (3, contaminant), ESI: for [M + H]+
533 (4, contaminant), 531 (1, contaminant),
435 (50, [M + H]+), 433 (10), 327 (75), 325
(15), 311 (100), 309 (21), 247 (30)
EI: 512 (9), 511 (10), 510 (44, M), 508 (23), 507
(8). 506 (8), 491 (5), 489 (3), 349 (34), 347
(17), 285 (100), 271 (28), 269 (26), 206 (6),
188 (9), 187 (7), 145 (8), 107 (8), 77 (8)
C20H20O4P80Se calcd.
435.0264, found:
435.0235
EI: for M C20H13F6O2P80Se
calcd.: 509.97226, found:
509.97147
KBr: 1611, 1510, 1440,
1420, 1325, 1215, 1200,
1170, 1117, 1066, 1020,
910, 845, 722, 626, 590
(Continued)
Heteroatom Chemistry DOI 10.1002/hc

Reactions of Nitroxides, Part XI: O-Aryl Phenylselenophosphonates Bearing a Nitroxyl Moiety 141
TABLE 3 (Continued)
Compound
MS (m/z, int%)
HR MS
IR (ν, cm−1 b
)
6e
6f
7
ESI: 425 (100, [M + Na]+), 423 (25)
ESI: for [M + Na]+
C20H19O2P80SeNa
calcd.: 425.0186, found:
425.0187
Film 1602, 1503, 1438,
1186, 1160, 1114, 1018,
906, 821, 750, 725, 691,
566
KBr: 1472, 1252, 1217,
1120, 1099, 1056, 901,
822, 798, 735, 689, 650,
565, 540
ESI: 537 (30, M + Na + 4), 535 (M + Na + 2,
100, M + 2), 533 (98, [M + Na]+), 531 (20, M
+ Na – 2)
ESI: for [M + Na]+
C18H11Cl4O2P80SeNa
calcd: 532.8314, found:
532.8301
EI: 530 (3, M), 528 (2), 374 (19), 319 (3), 310
(47), 309 (55), 235 (71), 223 (51), 222 (10),
217 (100), 203 (15), 201 (8), 170 (30),154
(97), 141 (14), 140 (19), 124 (12), 109 (9), 94
(79), 78 (50), 77 (79), 74 (24), 68 (4), 65 (11),
58 (14), 56 (14), 55 (18), 41 (19)
EI: 348 (46), 346 (100, M), 344 (49), 343 (17),
342 (18), 311 (3), 265 (7), 231 (10), 225 (9),
223 (19), 221 (10), 189 (22), 187 (74), 185
(37), 184 (15), 183 (14), 161 (18), 159 (59),
145 (5), 143 (16), 123 (96), 109 (16), 107
(24), 95 (30), 77 (28); ESI a): 365 (75, [M +
Na]+), 363 (20), 347 (20), 343 (100, [M +
H]+), 341 (20), 311 (15)
EI: for M C24H39N2O4P80Se KBr: 965, 566, 547
calcd: 530.18127, found:
530.18330
8c
ESIa: for [M + Na]+
C14H15O3P80SeNa:
calcd.: 364.9822, found:
364.9731
KBr: 1500, 1440, 1252,
1178, 1104, 1026, 903,
828, 760, 720, 693, 577
aBecause of the performing the MS ESI spectra of 1b and 8c in methanol solution, in fact the spectrum of PhP(Se)(OCH3)2 (M = 250) is
recorded, instead the spectrum of PhP(Se)Cl2 (1b, M = 258), and the spectrum of PhP(Se)(OC6H4OCH3)(OCH3) (M = 342) is recorded,
instead the spectrum of PhP(Se)(CH3OC6H4O)Cl (8c, M = 346).
bFrequency of the P Se vibrations: 580–590 [21], 504–535, and 565–577 cm−1 [22].
84–86 ppm, δSe = −36 to −40 ppm, 6: δP = 92–95
The chemical shift of P atom (relative to H3PO4)
and especially of a selenium atom (relative to
(CH3)2Se) in 1b, 6, 8 depends on a number of chlo-
rine and oxygen atoms connected to the phosphorus
atom: 1b: δP = 59.41 ppm, δSe = 148.6 ppm, 8: δP =
ppm, δSe = −242 to −245 ppm.
The observed coupling constants of P Se bond
(910–940 Hz) are in good agreement with the liter-
ature data: 852–1064 Hz [23] reported for O-acyl
FIGURE 1 EI MS of 5e.
Heteroatom Chemistry DOI 10.1002/hc

142 Huras, Zakrzewski, and Krawczyk
TABLE 4 1H, 13C, 31P, and 77Se NMR Data for the Investigated Compounds
1H NMR (500 MHz,
CDCl3)
13C NMR (125 MHz,
CDCl3)
31P NMRa
(202.5 MHz)
77Se NMRa
(95.4 MHz)
Compound
1b
7.50–7.61 (m, 2H, C6H5),
7.62–7.67 (m, 1H),
8.12–8.21 (dm, 2H, JPH
19.2 Hz)
7.15–7.27 (m, 5H), 7.29–7.46
(m, 5H), 7.54–7.62 (m, 2H,
P-C6H5), 7.62–7.68 (m, 1H,
P-C6H5), 8.17–8.26 (m, 2H,
P-C6H5)
128.69 (d, JCP = 17.1 Hz),
130.34 (d, JCP = 14.7 Hz),
133.83 (d, JCP = 3.9 Hz),
138.88 (d, JCP = 100.1 Hz)
121.77 (d, JCP = 4.7 Hz, 4 ×
CHar /o-O/), 125.14 (d, J
= 1.9 Hz, 2 × CHar /p-OC/)P,
128.34 (d, JCP = 15.4 Hz),
129.23 (d, JCP = 1.5 Hz, 4
× CHar /m-O/), 131.38 (d,
JCP = 12.8 Hz, 2 × CHar
59.41 (s, JPSe = 935 148.6 (dd, JPSe =
Hz)
935 Hz, J =
=
4.4 Hz)
6a
92.60 (s, JPSe = 914 –244.17 (d, JPSe
Hz) 914 Hz)
=
/m-P/), 132.87 (d, JCP
=
3.1 Hz, CHar /p-P/), 134.20
(d, JCP = 134.9 Hz, C-P),
150.69 (d, JCP = 8.7 Hz, 2
× COP)
8ab
–
121.66 (d, JCP = 5.4 Hz),
126.08 (d, JCP = 2.5 Hz),
128.47 (d, JCP = 17.6 Hz),
129.53 (d, JCP = 2 Hz),
130.58 (d, JCP = 13.6 Hz),
133.33 (d, JCP = 3.5 Hz),
136.29 (d, JCP = 123.0 Hz),
150.05 (d, JCP = 11.2 Hz)
122.49 (d, JCP = 4.9 Hz, 4 ×
CHar /m-NO2/), 125.40 (d,
JCP = 1.1 Hz, 4 × CHar
83.99 (s, JPSe = 932 –37.07 (d, JPSe
Hz) 932 Hz)
=
6b
7.26–7.30 (m, 3H), 7.36 (s,
2H), 7.58–7.65 (m, 2H,
P-C6H5), 7.66–7.74 (m, 1H,
P-C6H5), 8.12–8.20 (m, 2H,
P-C6H5), 8.21–8.25 (m, 3H)
94.14 (s, JPSe = 933 –237.18 (d, JPSe
Hz) 933 Hz)
=
/o-NO2/), 128.89 (d, JCP
=
15.7 Hz, 2 × CHar /o-P/),
131.59 (d, JCP = 13.3 Hz,
2 × CHar/m-P/), 132.54 (d,
JCP = 133.6 Hz/C-P/),
134.03 (d, JCP = 3.3 Hz,
CHar/p-P/), 145.23 (d, JCP
= 1.9 Hz, 2 × C-NO2),
154.99 (d, JCP = 7.9 Hz, 2
× COP)
8bb
6c
–
122.60 (d, JCP = 5.8 Hz),
128.73 (d, JCP = 16.7 Hz),
130.72 (d, JCP = 14.0 Hz),
133.89 (d, JCP = 3.65 Hz),
154.57 (d, JCP = 11.3 Hz)
55.46 (s, 2 × OCH3), 114.35
(d, JCP = 1.6 Hz, 4 × CHar
84.13 (s, JPSe = 940 –36.07 (d, JPSe
Hz) 942 Hz)
=
3.77 (s, 6H, 2 × OCH3),
94.47 (s, JPSe = 908 –249.65 (d, JPSe
Hz) 908 Hz)
=
6.81–6.85 (m, 4H, OC6H4O),
7.01–7.06 (m, 4H, OC6H4O),
7.52–7.57 (m, 2H, C6H5-P),
7.59–7.64 (m, 1H, C6H5-P),
/o-OCH3/), 122.65 (d,JCP
4.4 Hz, 4 × CHar
=
/m-OCH3/), 128.33 (d, JCP
= 15.1 Hz, 2 × CHar /o-P/),
131.48 (d, JCP = 12.6 Hz, 2
× CHar /m-P/), 132.84 (d,
JCP = 3.1 Hz, CHar /p-P/),
134.28 (d, JCP = 134.2 Hz,
/C-P/), 144.24 (d, JCP = 8.8
Hz, 2 × COP), 156.95 (d,
JCP = 2.0 Hz, 2 × COCH3)
8.11–8.17 (dm, 2H, JPH
14.9 Hz, C6H5-P).
=
(Continued)
Heteroatom Chemistry DOI 10.1002/hc

Reactions of Nitroxides, Part XI: O-Aryl Phenylselenophosphonates Bearing a Nitroxyl Moiety 143
TABLE 4 (Continued)
1H NMR (500 MHz,
13C NMR (125 MHz,
CDCl3)
31P NMRa
(202.5 MHz)
77Se NMRa
(95.4 MHz)
Compound
CDCl3)
8cc
3.78 (s, 3H, OCH3), 6.87–6.91
(m, 2H, OC6H4O), 7.21–7.25
(m, 2H, OC6H4O), 7.52–7.57
(m, 2H, C6H5-P), 7.59–7.64
(m, 1H, C6H5-P), 8.12–8.19
(dm, 2H, JPH = 16.7 Hz,
C6H5-P)
55.53 (s, OCH3), 114.52 (d, JCP
= 2.1 Hz, 2 × CHar /o-OCH3/),
122.60 (d, JCP = 5.2 Hz, 2 ×
CHar /m-OCH3/), 128.49 (d,
JCP = 16.4 Hz, 2 × CHar
85.51 (s, JPSe
930 Hz)
=
–39.98 (d, JPSe
930 Hz)
=
/o-P/), 130.68 (d, JCP = 13.6
Hz, 2 × CHar /m-P/), 133.35
(d, JCP = 3.5 Hz, CHar/p-P/),
136.31 (d, JCP = 122.6
Hz,/C-P/), 143.51 (d, JCP
=
11.4 Hz, 2 × COP), 157.55 (d,
JCP = 2.5 Hz, 2 × COCH3)
6d
7.20–7.24 (m, 4 H), 7.54–7.62
(m, 6 H), 7.68 (pseudo tq, 1H,
J = 7.5 Hz, J = 1.5 Hz),
122.26 (d, JCP = 4.8 Hz, CHar
93.64 (s, JPSe
926 Hz)
=
–242.51 (d, JPSe
= 926 Hz)
/m-CF3/), 123.82 (q, JCF
=
271.8 Hz, CF3), 126.99 (qd,
JCF = 3.9 Hz, JCP = 1.5 Hz,
CHar /o-CF3/), 127.96 (qd, JCF
= 32.7 Hz, JCP = 1.9 Hz, Car
8.11–8.18 (dm, 2H, JPH
15.5 Hz)
=
/C–CF3/), 128.76 (d, JCP
=
15.7 Hz, CHar/o-P/), 131.63
(d, JCP = 13.1 Hz, CHar/m-P/),
133.32 (d, JCP = 134.2 Hz,
Car/C–P/), 133.66 (d, JCP
=
3.4 Hz, CHar/p-P/), 153.02
(dq, JCP = 6.3 Hz, JCF = 1.4
Hz, Car/C–O/)
6e
2.29 (s, 6H, 2 × CH3), 6.95–7.00 20.79 (s, 2 × CH3), 121.55 (d,
92.67 (s, JPSe
910 Hz)
=
=
–247.26 (d, JPSe
= 910 Hz)
(m, 4H, OC6H4CH3),
JCP = 4.9 Hz, 4 ×
7.05–7.10 (m, 4H,
CHar/m-CH3/), 128.36 (d, J
= 15.1 Hz, 2 × CHar/o-P/), CP
129.92 (d, JCP = 1.9 Hz, 4 ×
CHar/o-CH3/), 131.48 (d, J
= 12.7 Hz, 2 × CHar/m-P/)C, P
132.84 (d, JCP = 3.4 Hz,
OC6H4CH3), 7.49–7.54 (m,
2H, C6H5-P), 7.55–7.60 (m,
1H, C6H5-P), 8.08–8.16 (dm,
2H, JPH = 15.5 Hz, C6H5-P).
CHar/p-P/) 134.43 (d, JCP
=
134.43 Hz,/C-P/), 134.99 (d,
JCP = 2.4 Hz, /C-CH3/),
148.56 (d, JCP = 8.7 Hz, 2 ×
COP)
6f
7.17 (dd, 2H, J = 8.8 Hz, J = 2.5 124.04 (d, JCP = 3.9 Hz, 2 ×
94.76 (s, JPSe
931 Hz)
–244.19 (d, JPSe
= 931 Hz)
Hz, POC6H3Cl2), 7.22 (dd,
2H, J = 8.8 Hz, JPH = 2.0 Hz,
POC6H3Cl2), 7.42 (d, 2H, J =
2.5 Hz, POC6H3Cl2),
7.54–7.60 (m, 2H, C6H5-P),
7.63–7.68 (m, 1H, C6H5-P),
CHar /C6H3Cl2/), 127.64 (d,
JCP = 1.9 Hz, 2 × CHar
/C6H3Cl2/), 127.88 (d, JCP
5.3 Hz, 2 × Car /C6H3Cl2/),
128.55 (d, JCP = 15.6 Hz, 2 ×
=
CHar /o-P/), 130.38 (d, JCP
=
8.19–8.26 (dm, 2H, JPH
15.5 Hz, C6H5-P)
=
2.0 Hz, 2 × CHar /C6H3Cl2/),
131.24 (d, JCP = 2.4 Hz, 2 ×
Car /C6H3Cl2/), 131.74 (d, JCP
= 13.2 Hz, 2 × CHar /m-P/),
133.37 (d, JCP = 136.7 Hz,
/C-P/), 133.57 (d, JCP = 3.4
Hz, CHar /p-P/), 145.71 (d,
JCP = 8.3 Hz, 2 × COP)
aP Se coupling constants are calculated from the 77Se NMR spectrum, as well as compared with the satellite bands observed in 31P NMR
spectrum.
bot isolated; observed in 13C NMR spectrum of diaryl esters.
csolated.
Heteroatom Chemistry DOI 10.1002/hc

144 Huras, Zakrzewski, and Krawczyk
inhibition of two other species: P. cactorum (5b) or
B. cinerea (6b).
+
•
Ar
O
O
P
Ar
Se
O
P
⎯
Moreover, a diester 6d with a trifluoromethyl
group shows a moderate activity against R. solani.
The compounds, 6a and 6c, reduced growth of pow-
dery mildew on wheat (B. graminis), whereas they
were devoid of any antifungal activity against the
rest of the pathogens. The compound 6e was inac-
tive in a preliminary test toward all the tested fun-
gal strains. Three compounds 5b, 6b, and 5e have
better antifungal activity against R. solani than the
commercial fungicide chlorothalonil. The most ac-
tive compound 5e displayed better activity against
B. cinerea and was comparable against P. cacto-
rum with the chlorothalonil, but all of the com-
pounds were inferior to the commercial fungicide
carboxine.
HSe
N
+
•
[154]
OH
O
•
+ H
+
[M-153]
+
M
N
•
O
SCHEME 4 M-153 fragment observed in mass spectra of
5a–f.
selenophosphates (R1R2P( Se)OCOR3). For com-
parison, the coupling constant of the P Se bond for
(CH3O)Ph2P Se and (CH3O)2PhP Se are reported
as 788 and 881 Hz, respectively [24].
The plant protection activity of the synthesized
phenylselenophosphonic aryl nitroxyl esters 5a–f
and phenylselenophosphonic diaryl esters 6a–f
was evaluated. All the synthesized compounds
(5a–f, 6a–f) were inactive as herbicides and in-
secticides, but some of them showed antifungal
activity. The inhibition effect of selenium deriva-
tives on phytopathogenic fungi was studied by a
method previously described [25] using five species:
Botrytis cinerea, Fusarium culmorum, Phytoph-
thora cactorum, Rhizoctonia solani, and Blumeria
graminis. Three fungicides, chlorothalonil (2,4,5,6-
CONCLUSIONS
In conclusion, we proposed a novel and efficient
method to synthesize O-aryl phenylselenophos-
phonates bearing
a nitroxyl moiety 5a–f us-
ing phenyldichlorophosphine selenide (1b) as a
key intermediate. The application of 1b results
in much better yields than the method where
phenyldichlorophosphine (1a) and black selenium
are used. Some selenophenylphosphonates bearing
a nitroxyl moiety 5a–f and diaryl selenophenylphos-
phonates 6a–f showed a weak to good antifungal
activity in vitro against B. cinerea, F. culmorum, P.
cactorum, and R. solani and in vivo against B. grami-
nis. Monoester with the 4-CH3 group and mono- and
diester with the 4-NO2 group were particularly active
in inhibition of R. solani mycelium growth.
tetrachloroisophthalonitrile,
carboxin (5,6-dihydro-2-methyl-1,4-oxathiine-3-
carboxanilide, CAS: 5234-68-4), and metcona-
zole (5-[(4-chlorophenyl)methyl]-2,2-dimethyl-1-
(1H-1,2,4-triazol-1-ylmethyl)cyclopentanol, CAS:
CAS:
1897-45-6)),
125116-23-6), were used as the reference com-
pounds. The results of the evaluation are shown in
Table 5.
The antifungal activity of some obtained deriva-
tives was observed. The investigated compounds
generally showed a weak to good broad-spectrum
activity against several plant diseases. It was found
that the monoester with a 4-CH3 group (5e) showed
the highest level of protection against R. solani and
against B. cinerea, and a moderate inhibition effect
against P. cactorum, at the concentration of 200
mg/L. Probably, introduction of the methyl groups
in the compound increases the lipophilicity and im-
proves absorption into a cell membrane. The acti-
vating character of a methyl group is known for the
mode of action of some herbicides and fungicides.
Lipophilic herbicides more readily diffuse across the
membrane than hydrophilic ones do [26]. Two sele-
nium derivatives, monoester and diester bearing a
nitro group (5b, 6b), exhibited 100% reduction of R.
solani mycelium growth, and a remarkable growth
EXPERIMENTAL
General
2,2,6,6-Tetramethyl-4-piperidinol-1-oxyl (2) was
synthesized by oxidation of 2,2,6,6-tetramethyl-4-
piperidinol with 30% hydrogen peroxide (76.5%,
mp 71–73◦C) according to methods described in
[27–29]. Phenyldichlorophosphine and phenols 3a–f
was purchased from Aldrich and used without fur-
ther purification. All experiments were performed
in a three-necked flask of 25 mL capacity protected
from humidity (a tube with calcium chloride) and
air (dry argon), equipped with a magnetic bar, a
thermometer, a dropping funnel, a reflux condenser,
and an ice-salt bath (with exception of 1b synthesis
where a silicon oil bath was used). TLC monitoring
and column chromatography were done on Merck
silica gel Alurolle 5562, Alufolien 5554, and Merck
Heteroatom Chemistry DOI 10.1002/hc

Reactions of Nitroxides, Part XI: O-Aryl Phenylselenophosphonates Bearing a Nitroxyl Moiety 145
TABLE 5 Antifungal Activity of 5a–f, 6a–f, and 7 (R• = Nitroxyl Moiety). Reduction of the Colonies Growth (%)
B. cinerea,
200 mg/L
in vitro
F. culmorum,
200 mg/L
in vitro
P. cactorum,
200 mg/L
in vitro
R. solani,
200 mg/L
in vitro
B. graminis,
1000 mg/L
in vivo
#
Compound
5a
6a
5b
6b
5c
6c
5d
6d
5e
6e
5f
(Ph)(OC6H5)(OR•) PSe
(Ph)(OC6H5)2PSe
0
0
37
66
4
0
0
0
95
0
21
25
25
76
100
–
0
0
17
14
0
0
0
0
41
0
0
0
14
0
54
10
13
5
24
0
61
0
0
0
4
61
–
9
2
91
100
0
34
84
0
32
0
51
0
26
0
0
0
0
21
–
(Ph)(OR•) (OC6H4NO2)PSe
(Ph)(OC6H4NO2)2PSe
(Ph)(OR•) (OC6H4OCH3)PSe
(Ph)(OC6H4OCH3)2PSe
(Ph)(OR•) (OC6H4CF3)PSe
(Ph)(OC6H4CF3)2PSe
(Ph)(OR•) (OC6H4CH3)PSe
(Ph)(OC6H4CH3)2PSe
(Ph)(OR•) (OC6H3Cl2)PSe
(Ph)(OC6H3Cl2)2PSe
(Ph)(OR•)2 PSe
0
28
62
100
0
21
25
9
85
100
–
6f
7
10
38
80
–
Chlorothalonil
Carboxin
Metconazole
–
100
–
1.09385.1000 (0.040–0.063 mm, 230–400 mesh),
respectively. The following abbreviations for mobile
phases were used throughout the text: HxA4, HxA9
= hexane: ethyl acetate 4:1, 9:1, respectively, BA4,
BA9 = benzene: ethyl acetate 4:1, 9:1, respectively.
The progress of the reactions was monitored by
means of TLC (silica gel plates (Merck 5562,
5554), mobile phase: HxA4, HxA9). Visualization of
TLC plates: UV 254, and/or iodine vapors, and/or
spraying with 1% ethanolic PdCl2. MS (EI, 70
eV, m/z, int. (%)) data were recorded using an
AMD 604 and Agilent Technologies 5975 B mass
spectrometers. HR MS (EI) data were recorded
using an AMD 604 mass spectrometer. MS and HR
MS (ESI, positive ions, CH3OH as a solvent) were
recorded using Micromass LCT apparatus. IR (ν,
cm−1) data were recorded using a FT/IR Jasco 420
spectrophotometer. NMR data were collected using
Varian UNITYplus 200, Varian UNITYplus 500 (1H,
13C, 31P), and Bruker DRX 500 Advance [1H, 13C, 31P,
77Se (referenced to (CH3)2Se)].
natively, filtration through Cellite bed followed by
washing with diethyl ether may be used. The fil-
trate was subjected to distillation followed by redis-
tillation under reduced pressure (diffusion pump)
to give phenyldichlorophosphine selenide (1b, 7.697
g, 59.7%, bp 68–74◦C/0.01 mmHg (60◦C/0.01 mmHg
[19], 79◦C/0.04 mmHg [20]).
Phenylselenophosphonic
2,2,6,6-Tetramethyl-1-oxyl-4-oxypiperidyl
Chloride (4)
Phenyldichlorophosphine selenide (1b, 2.5 mmol,
0.645 g, 0.4 mL) in dry ether (10 mL) was cooled
down to 0–5◦C. The solution of 2,2,6,6-tetramethyl-
4-piperidinol-1-oxyl (2, 2.5 mmol, 0.430 g) and pyri-
dine (2.5 mmol, 0.189 g, 0.2 mL) in ether (5 mL) was
added dropwise. The reaction mixture was stirred for
2 h at room temperature. After the reaction had been
terminated, the precipitate of pyridinium hydrochlo-
ride was filtered off. The filtrate was concentrated
to dryness. The residue (1.078 g) was subjected to
column chromatography (mobile phase: benzene) to
give 4 (0.349 g, 35.4%, mp 42–52◦C).
The methods for assessing antifungal activity in-
volving both in vitro and in vivo tests were identical
to those previously published [25].
Phenyldichlorophosphine Selenide (1b)
Phenylselenophosphonates 5a–f from
Phenyldichlorophosphine (1a) (Method A)
Phenyldichlorophosphine (1a, 8.95 g, 0.05 mol,
6.8 mL) and black selenium (4.75 g, 0.06 mol) were
stirred for 1.5 h at 175–180◦C. After the reaction
had been terminated, the unreacted selenium was
filtered off under reduced pressure through a pad of
white cellulose wool and a cotton wool plug. Alter-
Phenyldichlorophosphine (1a, 2.5 mmol, 0.339 g,
0.34 mL) in dry ether (10 mL) was cooled down
to 0–5◦C. A solution of the appropriate phenol (3a–
f, 2.5 mmol) and pyridine (2.5 mmol, 0.189 g,
0.2 mL) in ether (3 mL) was added dropwise into
Heteroatom Chemistry DOI 10.1002/hc

146 Huras, Zakrzewski, and Krawczyk
the reaction mixture. After 5 min, the solution of
2,2,6,6-tetramethyl-4-piperidinol-1-oxyl (2, 0.430 g,
2.5 mmol) and pyridine (2.5 mmol, 0.189 g, 0.2 mL)
in ether (5 mL) was added dropwise. The mixture
was stirred for 15 min, and black selenium (0.5 g,
6.3 mmol) was added. After approximately 2 h, the
second portion of selenium (0.5 g, 6.3 mmol) was
added. The reaction mixture was stirred for 20 h at
room temperature. After the reaction had been ter-
minated, the suspension was filtered through silica
bed of 1-cm height. The filtrate was concentrated to
dryness. The residue was subjected to column chro-
matography (mobile phase: HxA4, HxA9, BA4, BA9)
to give 5a–f, 6a–f, and 7.
(0.310 g, 2.5 mmol) and pyridine (2.5 mmol, 0.189
g, 0.2 mL) in ether (5 mL) was added dropwise. The
reaction mixture was stirred for 5.5 h at room tem-
perature. After the reaction had been terminated,
the precipitate of pyridinium hydrochloride was fil-
tered off. The filtrate was concentrated to dryness.
The residue (0.589 g) was subjected to column chro-
matography (mobile phase: HxA9) to give 8c (0.2 g,
46.3%, mp 45–48◦C) and 6c (0.295 g, 54.5%).
REFERENCES
[1] Zakrzewski, J.; Krawczyk, M. Bioorg Med Chem Lett
2011, 21, 514–516.
[2] Quinn, C. F.; Galeas, M. L.; Freeman, J. L.; Pilon-
Smits, E. A. H. Integr Environ Assess Manag 2007,
3(3), 460–462.
[3] Vinceti, M.; Maraldi, T.; Bergomi, M.; Malagoli, C.
Rev Environ Health 2009, 24(3), 231–248.
[4] Whanger, P. D. J Am Coll Nutr 2002, 21(3), 223–232.
[5] Whanger, P. D. Br J Nutr 2004, 91, 11–28.
[6] Birringer, M.; Pilawa, S.; Flohe´, L. Nat Prod Rep
2002, 19, 693–718.
[7] Jackson-Rosario, S. E.; Self, W. T. Metallomics 2010,
2, 112–116.
[8] Roy, G.; Sarma, B. K.; Phadnis, P. P.; Mugesh, G. J
Chem Sci 2005, 117(4), 287–303.
[9] Comasseto, J. V.; Gariani, R. A. Tetrahedron 2009,
65, 8447–8459.
[10] Sager, M. Pure Appl Chem 2006, 78(1), 111–133.
[11] Madhunapantula, S. V.; Desai, D.; Sharma, A.; Huh,
S. J.; Amin, S.; Robertson, G. P. Mol Cancer Ther
2008, 7(5), 1297–1308.
[12] Nuretdinov, I. A.; Neklesova, I. D.; Kudrina, M. A.;
Iraidova, M. A.; Buina, N. A. Izv Akad Nauk SSSR,
Ser Khim 1971; 20, 1266 (in Russian); Bull Acad Sci
USSR, Div Chem Sci 1971, 20, 1170 (in English).
[13] Kimura, M.; Ishihara, H.; Kato, S. Arch Pharm Res
2007, 30(8), 938–944.
Selenophenylphosphonates 5a–f from Phenyl-
dichlorophosphine Selenide (1b) (Method B)
Phenyldichlorophosphine selenide (1b, 2.5 mmol,
0.645 g, 0.4 mL) in dry ether (10 mL) was cooled
down to 0–5◦C. Appropriate solution of phenol (3a-
f, 2.5 mmol) and pyridine (2.5 mmol, 0.189 g,
0.2 mL) in ether (3 mL) was added dropwise into
the reaction mixture. After 5 min, the solution of
2,2,6,6-tetramethyl-4-piperidinol-1-oxyl (2, 0.430 g,
2.5 mmol) and pyridine (2.5 mmol, 0.189 g, 0.2 mL)
in ether (5 mL) was added dropwise. The reaction
mixture was stirred for 2 h at room temperature.
After the reaction had been terminated, the pyri-
dinium hydrochloride was filtered off. The filtrate
was concentrated to dryness. The residue was sub-
jected to column chromatography (mobile phase:
HxA4, HxA9, BA4, BA9) to give 5a–f, 6a–f and 7.
Diaryl selenophenylphosphonates 6a–f from Ph-
enyldichlorophosphine Selenide (1b) (Method B)
[14] Zakrzewski, J.; Krawczyk, M. Heteroat Chem 2008,
19(6), 549–556.
[15] Ka´lai, T.; Balog, M.; Jeko¨, J.; Hideg, K. Synthesis
1999(6), 973–980.
[16] Ka´lai, T.; Mugesh, G.; Roy, G.; Sies, H.; an Berente,
Z.; Hideg, K. Org Biomol Chem 2005, 3, 3564–3569.
[17] Konieczny, M.; Sosnovsky, G. Z Naturforsch B 1978,
33, 1040–1046.
[18] Sosnovsky, G.; Konieczny, M. Synthesis 1978(8),
583–585.
[19] Roesky, H. W.; Kloker, W. Z Naturforsch B 1973, 28,
697–706.
[20] Bayandina, E. V.; Sadkova, D. N.; Osankina, E. I.;
Nuretdinov, I. A. Zh Obsch Khim 1989, 59(9), 1999–
2003.
[21] Turkevich, V. V.; Viblyi, I. F. Zh Prikl Spektroskop
1966, 4(1), 77–79; J Appl Spectr 1966, 4(1), 64–65.
[22] Piffath, R. J., Edgewood Chemical Biological Center,
U.S. Army Research, Development and Engineering
Command, ECBC-TR-392, May 2007, pp 469–470.
[23] Rachon´, J.; Cholewin´ski, G.; Witt, D. Chem Commun
2005, 2692–2694.
Phenyldichlorophosphine selenide (1b, 2.5 mmol,
0.645 g, 0.4 mL) in dry ether (10 mL) was cooled
down to 0–5◦C. Appropriate solution of phenol (3a–
f, 5 mmol) and pyridine (5 mmol, 0.378 g, 0.4 mL) in
ether (5 mL) was added dropwise into the reaction
mixture. The reaction mixture was stirred for 2 h at
room temperature. After the reaction had been ter-
minated, the pyridinium hydrochloride was filtered
off. The filtrate was concentrated to dryness. The
residue was subjected to column chromatography
(mobile phase: HxA4, HxA9, BA4, BA9) to give 6a–f.
Phenylselenophosphonic (4-methoxyphenoxy)
chloride (8c)
Phenyldichlorophosphine selenide (1b, 1.25 mmol,
0.322 g, 0.2 mL) in dry ether (10 mL) was cooled
down to 0–5◦C. The solution of 4-methoxyphenol
[24] Krawczyk, E.; Skowron´ska, A.; Michalski, J. J Chem
Soc, Dalton Trans 2002(23) 4471–4478.
Heteroatom Chemistry DOI 10.1002/hc

Reactions of Nitroxides, Part XI: O-Aryl Phenylselenophosphonates Bearing a Nitroxyl Moiety 147
[25] Zakrzewski, J.; Krawczyk, M. Heteroat Chem 2006,
[27] Brie`re, R.; Lemaire, H.; Rassat, A. Bull Soc Chim Fr
1965, 3273–3283.
17(5), 393–401.
[26] Monaco, T. J.; Weller, S. C.; Ashton, F. M. Weed Sci-
ence: Principles and Practices; Wiley-Blackwell: New
York, 2002; p.107.
[28] Sosnovsky, G.; Konieczny, M. Z Naturforsch B 1976,
31, 1376–1378.
[29] Zakrzewski, J. Monatsh Chem 1990, 121, 803–808.
Heteroatom Chemistry DOI 10.1002/hc