Kinetics and mechanism of base-catalysed degradations of substituted aryl-N-hydroxycarbamates, their N-methyl and N-phenyl analogues

Article abstract of DOI:10.1039/b310454k

The kinetics and mechanism of the degradation reactions of substituted phenyl N-hydroxycarbamates and their N-methyl and N-phenyl analogues have been studied at pseudo-first-order reaction conditions in aqueous buffers and sodium hydroxide solutions at 20°C and 60°C and at I = 1 mol·1 ^-1. The dependence of log k_obs on pH for phenyl N-hydroxycarbamates at pH < 9 and pH > 13 is linear with the unit slope; at pH 10-12 log k_obs is pH independent. The Bronsted coefficient β_lg is about -1 (pH 7-13) and -1.53 (pH > 13) indicating that the degradation reaction of phenyl N-hydroxycarbamates follows an ElcB mechanism giving the corresponding phenol/phenolate and HO-N=C=O. The latter species undergoes further decomposition to give carbonate, nitrogen and ammonia as final products. In contrast to the phenyl N-hydroxycarbamates the N-methyl derivatives at pH 7-9 undergo degradation to the corresponding phenol/phenolate, carbonate and methylamine via a concerted mechanism (β_lg is about - 0.75). The only exception is 4-nitrophenyl N-hydroxy-N-methylcarbamate in which the predominant break down pathway proceeds via the Smiles rearrangement to give sodium N-methyl-(4-nitrophenoxy)carbamate. At pH > 9 the reaction of N-hydroxy-N-methylcarbamates is kinetically complex: the dependence of absorbance on time is not exponential and it proceeds as a consecutive two-step reaction. N-Hydroxy-N-phenylcarbamate under the same conditions undergoes degradation to phenol, carbonate, aniline and azoxybenzene.

Full text of DOI:10.1039/b310454k

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Kinetics and mechanism of base-catalysed degradations of
substituted aryl-N-hydroxycarbamates, their N-methyl and
N-phenyl analogues
ˇ
Petr Beier, Jaromír Mindl, Vojeslav Steˇrba and Jirˇí Hanusek
Department of Organic Chemistry, Faculty of Chemical Technology, University of Pardubice,
532 10, Czech Republic
Received 29th August 2003, Accepted 1st December 2003
First published as an Advance Article on the web 19th January 2004
The kinetics and mechanism of the degradation reactions of substituted phenyl N-hydroxycarbamates and their
N-methyl and N-phenyl analogues have been studied at pseudo-first-order reaction conditions in aqueous buffers
and sodium hydroxide solutions at 20 ЊC and 60 ЊC and at I = 1 molؒlϪ1. The dependence of log kobs on pH for phenyl
N-hydroxycarbamates at pH < 9 and pH > 13 is linear with the unit slope; at pH 10–12 log kobs is pH independent.
The Bronsted coefficient βlg is about Ϫ1 (pH 7–13) and Ϫ1.53 (pH > 13) indicating that the degradation reaction of
phenyl N-hydroxycarbamates follows an E1cB mechanism giving the corresponding phenol/phenolate and HO–N᎐
᎐
C᎐O. The latter species undergoes further decomposition to give carbonate, nitrogen and ammonia as final products.
᎐
In contrast to the phenyl N-hydroxycarbamates the N-methyl derivatives at pH 7–9 undergo degradation to the
corresponding phenol/phenolate, carbonate and methylamine via a concerted mechanism (βlg is about Ϫ0.75). The
only exception is 4-nitrophenyl N-hydroxy-N-methylcarbamate in which the predominant break down pathway
proceeds via the Smiles rearrangement to give sodium N-methyl-(4-nitrophenoxy)carbamate. At pH > 9 the reaction
of N-hydroxy-N-methylcarbamates is kinetically complex: the dependence of absorbance on time is not exponential
and it proceeds as a consecutive two-step reaction. N-Hydroxy-N-phenylcarbamate under the same conditions
undergoes degradation to phenol, carbonate, aniline and azoxybenzene.
separated ether phase was dried, the solvent was distilled off,
and the evaporation residue was recrystallised from a suitable
solvent.
Introduction
Certain aryl N-hydroxycarbamic acids display biological activ-
ity1 as fungicides, acaricides and ovicides. The presence of an
ionisable hydroxyl group near the reactive carbonyl group in
these substrates can induce intramolecular reactions, out of
which only the Smiles rearrangement has been studied so
Phenyl-N-hydroxycarbamate (1a)
(51%), Mp 106 Ϫ106.5 ЊC (n-heptane–ether), (lit.4 105–107 ЊC).
IR (nujol), νmax/cmϪ1: 3305s, 1287s (NH), 3128wb (OH), 1690s
far. 4-Nitrophenyl N-hydroxycarbamate undergoes the Smiles
rearrangement under base-catalysed conditions.2 Phenyl
(C᎐O), 1600w, 1492m, 689m (C C ), 1207s, 1170m, 1028w
᎐
Ar Ar
(C–O), 795m (CH). NMR δH(360 MHz; CDCl3; Me4Si): 6.40
(1H, br s, OH), 7.14 (2H, d, J 7.8, H-2,6), 7.23 (1H, t, J 7.2,
H-4), 7.38 (2H, t, J = 8.0, H-3,5), 7.50 (1H, br s, NH). δC(90.6
MHz; CDCl3; Me4Si): 121.7 (C-2,6), 125.3 (C-4), 129.5 (C-3,5),
N-hydroxycarbamate, on the other hand does not undergo this
rearrangement, but its hydrolysis has been shown to proceed at
two orders of magnitude faster than that of phenyl N-methoxy-
carbamate.3 The aim of this work was to carry out a detailed
study of the mechanism of the base catalysed degradation of
N-hydroxycarbamates (1–3). The structures investigated are
shown in Fig. 1.
150.9 (C-1), 155.7 (C᎐O). UV λ
(MeOH)/nm, (ε/m2 molϪ1):
᎐
max
204 (893), 260 (29).
4-Methylphenyl-N-hydroxycarbamate (1b)
(36%), Mp 99–100.5 ЊC (n-heptane), lit.5 99 ЊC. IR (nujol),
νmax/cmϪ1: 3262sb, 1285sh, 1272s (NH), 3325sb (OH), 1700vs
(C᎐O), 1506s (C C ), 1378w, 827m (CH), 1208s, 1173w,
᎐
Ar Ar
1020m (C–O). NMR δH(360 MHz; CDCl3; Me4Si): 1.70 (1H, br
s, NH), 2.33 (3H, s, CH3), 7.00 (2H, d, J 8.8, H-2,6), 7.16 (2H, d,
J 9.0, H-3,5), 7.55 (1H, br s, OH). δC(90.6 MHz; CDCl3; Me4Si):
21.3 (CH3), 121.2 (C-2,6), 130.2 (C-3,5), 135.9 (C-4), 148.3
(MeOH)/nm, (ε/m2 molϪ1): 202
Fig. 1
(C-1), 157.7 (C᎐O). UV λ
᎐
max
(898), 266 (52), 272 (48).
Experimental
3-Chlorophenyl-N-hydroxycarbamate (1c)
General preparation of aryl N-hydroxycarbamates and their
N-methyl and N-phenyl analogues
(42%), Mp 99.5–101 ЊC (n-heptane), IR (nujol), νmax/cmϪ1
:
3318s, 1285s (NH), 3168sb (OH), 1746vs, 1717vs (C᎐O),
᎐
A solution of substituted phenyl chloroformate (10 mmol) in
ether (10 ml) was added within 30 min drop by drop with stir-
ring and cooling at 5–10 ЊC into a suspension of substituted
hydroxylamine hydrochloride (10 mmol), potassium carbonate
(11 mmol) in ether (10 ml) and water (1 ml). Then the reaction
mixture was stirred at room temperature for another 5 h. The
1598m, 1593m, 675s (CArCAr), 1224s, 1167w (C–O), 1071w
(CAr–Cl), 868m, 793m (CH). NMR δH(360 MHz; CDCl3;
Me4Si): 7.06 (1H, d, J 9.1, H-2), 7.18–7.23 (2H, m, H-4,6), 7.28–
7.37 (1H, d, J 9.0, H-3). δC(90.6 MHz; CDCl3; Me4Si):120.7
(C-2), 121.8 (C-6), 125.3 (C-4), 129.6 (C-3), 133.4 (C-5), 151.6
(C-1), 155.7 (C-7). Elemental analysis: found C, 44.76; H, 3.16;
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 5 6 2 – 5 6 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
562

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N, 7.57. Calc. for C7H6NO3Cl: C, 44.82; H, 3.22; N, 7.45%. UV
Me4Si): 38.4 (NCH3), 120.0 (C-2), 122.3 (C-6), 126.2 (C-4),
λmax (MeOH)/nm, (ε/m2 molϪ1): 204 (1417), 266 (39), 273 (34).
130.3 (C-3), 134.8 (C-5), 151.5 (C-1), 155.6 (C᎐O). Elemental
᎐
analysis: found C, 47.82; H, 4.05; N, 6.92. Calc. for C8H8-
ClNO3: C, 47.66; H, 4.00; N, 6.95%. UV λmax (MeOH)/nm,
(ε/m2 molϪ1): 203 (1603), 266 (39).
4-Chlorophenyl-N-hydroxycarbamate (1d)
(45%), Mp 140–142 ЊC (n-heptane), lit.5 127 ЊC. IR (nujol),
νmax/cmϪ1: 3320vs, 1285m (NH), 1745vs (C᎐O), 1480s, 1492m
᎐
4-Chlorophenyl-N-hydroxy-N-methylcarbamate (2d)
(CArCAr), 1220s, 1171w (C–O), 1082s (CAr–Cl), 847s (CH).
NMR δH(360 MHz; CDCl3; Me4Si): 7.20 (2H, d, J = 7.0, H-2,6),
7.48 (2H, d, J 7.0, H-3,5), 9.19 (1H, br s, NH), 10.45 (1H, br s,
OH). δC(90.6 MHz; CDCl3; Me4Si): 123.6 (C-2,6), 129.3
(55%), Mp 121–123 ЊC (n-heptane), lit.10 114–115 ЊC. IR
(nujol), νmax/cmϪ1: 3208vsb, 3150sh (OH), 1728vs, 1961vs
(C᎐O), 1428m, 809s (CH), 1222s, 1173m, 1020m, (C–O), 1089m
᎐
(C-3,5), 129.4 (C-4), 149.6 (C-1), 155.2 (C᎐O). Elemental
(CAr–Cl). NMR δH(360 MHz; CDCl3; Me4Si): 3.32 (3H, s,
NCH3), 7.04 (2H, d, J 9.0, H-2,6), 7.31 (2H, d, J 9.0, H-3,5),
7.4–8.2 (1H, br s, OH). δC(90.6 MHz; CDCl3; Me4Si): 38.3
(NCH3), 123.0 (C-2,6), 129.7 (C-3,5), 131.4 (C-4), 149.5 (C-1),
᎐
analysis: found C, 44.82; H, 3.22; N, 7.45. Calc. for C7H6NO3-
Cl: C, 45.32; H, 2.93; N, 7.64%. UV λmax (MeOH)/nm, (ε/m2
molϪ1): 201 (883), 220 (1095), 269 (49), 275 (43).
155.8 (C᎐O). Elemental analysis: found C, 47.81; H, 4.11; N,
᎐
3-Nitrophenyl-N-hydroxycarbamate (1e)
6.84. Calc. for C8H8ClNO3: C, 47.66; H, 4.00; N, 6.95%. UV
λmax (MeOH)/nm, (ε/m2 molϪ1): 203 (1107), 220 (1245), 268 (47).
(22%), Mp 132–134 ЊC (xylene), lit.5 130.5 ЊC. IR (nujol),
νmax/cmϪ1: 3378w, 1278m (NH), 3280mb, 3120w (OH), 1749vs,
3-Nitrophenyl-N-hydroxy-N-methylcarbamate (2e)
1728vs, 1702s (C᎐O), 1616m, 1596m, 685w (C C ), 1520sh,
᎐
Ar Ar
1510s, 1358s (NO2), 1215s, 1160m, 1020m (C–O), 869m (CH).
NMR δH(360 MHz; CDCl3; Me4Si): 5.15 (1H, br s, NH), 7.58
(2H, m, H-2,3), 8.06 (1H, m, H-6), 8.12 (1H, td, J 8.0, H-4).
δC(90.6 MHz; CDCl3; Me4Si):116.6 (C-6), 120.1 (C-4), 128.5
(57%), Mp 150–151 ЊC (n-heptane, xylene). IR (nujol),
νmax/cmϪ1: 3260mb, 3122w (OH); 1732vs, 1708s (C᎐O), 1617m,
᎐
1598m, 689w (CArCAr), 1528vs, 1520vs, 1350vs (NO2), 1265m
(N–H), 1229s, 1162m, 1025m (C–O), 869m (CH). NMR δH(360
MHz; CDCl3; Me4Si): 3.40 (3H, s, NCH3), 8.05 (1H, t, J 2.1,
H-2), 8.15 (1H, m, H-4), 7.74 (1H, t, J 8.2, H-5), 7.68 (1H, m,
H-6). δC(90.6 MHz; CDCl3; Me4Si): 38.6 (NCH3), 116.9 (C-6),
120.3 (C-4), 128.6 (C-2), 130.7 (C-3), 148.2 (C-5), 151.3 (C-1),
(C-2), 130.7 (C-3), 148.3 (C-5), 151.1 (C-1), 154.5 (C᎐O). UV
᎐
λmax (MeOH)/nm, (ε/m2 molϪ1): 205 (1600), 259 (745).
4-Nitrophenyl-N-hydroxycarbamate (1f)
153.8 (C᎐O). Elemental analysis: found C, 45.15; H, 3.62; N,
᎐
(33%), Mp 137–139.5 ЊC (xylene), lit.6 122–123 ЊC. IR (nujol),
νmax/cmϪ1: 3400w, 3378w, 1280m (NH), 3280wb, 3180wb (OH),
13.11. Calc. for C8H8N2O5: C, 45.29; H, 3.80; N, 13.20%. UV
λmax (MeOH)/nm, (ε/m2 molϪ1): 208 (1789), 259 (794).
1730vs, 1728vs, 1705s (C᎐O), 1618m, 1598m (C C ), 1518vs,
᎐
Ar Ar
1510vs, 1360s (NO2), 1217s, 1170m (C–O). NMR δH(360 MHz;
DMSO-d6; Me4Si): 6.98 (2H, m, H-2), 8.16 (1H, m, H-3), 8.42
(1H, br s, OH), 8.88 (1H, br s, NH). δC(90.6 MHz; DMSO-d6;
4-Nitrophenyl-N-hydroxy-N-methylcarbamate (2f)
(76%), Mp 113–114 ЊC (xylene). IR (nujol), νmax/cmϪ1: 3250sb,
Me Si): 115.8 (C-2,6), 126.2 (C-3,5), 139.6 (C-4), 163.9 (C᎐O),
3125w (OH); 1735vs, 1708s (C᎐O); 1618m, 1598m (C C ),
᎐
᎐
4
Ar Ar
164.1 (C-1). Elemental analysis: found C, 42.43; H, 3.05; N,
14.14; Calc. for C7H6N2O5: C, 42.82; H, 2.76; N, 13.71%. UV
λmax (MeOH)/nm, (ε/m2 molϪ1): 202 (1107), 274 (855).
1528vs, 1521vs, 1351s (NO2), 1230vs, 1164m, 1027m (C–O),
749m (CH). NMR δH(360 MHz; CDCl3; Me4Si): 3.38 (3H, s,
NCH3), 7.31 (2H, d, J 9.0, H-2,6), 7.75 (2H, d, J 9.0, H-3,5).
δC(90.6 MHz; CDCl3; Me4Si): 38.6 (NCH3), 113.6 (C-2,6),
Phenyl-N-hydroxy-N-methylcarbamate (2a)
124.1 (C-3,5), 125.8 (C-4), 140.7 (C-1), 165.9 (C᎐O). Elemental
᎐
analysis: found C, 45.37; H, 3.65; N, 12.95. Calc. for C8H8N2O5:
C, 45.29; H, 3.80; N, 13.20%. UV λmax (MeOH)/nm, (ε/m2
molϪ1): 204 (1279), 286 (771).
(92%), Mp 60–61.5 ЊC (n-heptane), lit.7 58–60 ЊC. IR (nujol),
νmax/cmϪ1: 3128sb (OH), 1732vs, 1697vs (C᎐O), 1595m, 1488s,
᎐
691m (CArCAr), 1198s, 1170s, 1037m (C–O), 758s (CH). NMR
δH(360 MHz; CDCl3; Me4Si): 3.28 (3H, s, NCH3), 7.08 (2H, d,
J 8.0, (H-2,6), 7.17 (1H, t, J 8.0, H-4), 7.34 (2H, t, J 8.0, H-3,5),
8.15 (1H, br s, OH). δC(90.6 MHz; CDCl3; Me4Si): 38.4
(NCH3), 121.6 (C-2,6), 125.9 (C-4), 129.5 (C-3,5), 150.9 (C-1),
3,5-Dinitrophenyl-N-hydroxy-N-methylcarbamate (2g)
(11%), Mp 121–122.5 ЊC, (n-hexane–toluene). NMR δH(360
MHz; CDCl3; Me4Si): 3.47 3H, s, NCH3), 6.4–7.1 (1H, br s,
OH), 8.44 (2H, d, J 1.5, H-2,6), 8.96 (1H, t, J 1.6, H-4).
Elemental analysis: found C, 37.34; H, 2.84; N, 16.47. Calc. for
C8H7N3O7: C, 37.36; H, 2.75; N, 16.34%. UV λmax (MeOH)/nm,
(ε/m2 molϪ1): 205 (1119), 268 (752).
156.2 (C᎐O). UV λ
(MeOH)/nm, (ε/m2 molϪ1): 204 (1029),
᎐
max
260 (29).
4-Methylphenyl-N-hydroxy-N-methylcarbamate (2b)
(46%), Mp 64–65.5 ЊC (n-heptane–toluene), lit.8 35–36 ЊC IR
4-Chloro-3-nitro-N-hydroxy-N-methylcarbamate (2h)
(nujol), νmax/cmϪ1: 3300sb (OH), 1680vs (C᎐O), 1510m (C -
᎐
Ar
CAr), 1385vs, 810m (CH), 1190s, 1170w, 1020m (C–O). NMR
δH(360 MHz; CDCl3; Me4Si): 2.31 (3H, s, ArCH3), 3.27 (3H, s,
NCH3); 6.96 (2H, d, J 8.0 (H-2,6), 7.13 (2H, d, J 8.0, (H-3,5),
8.1 (1H, br s, OH). δC(90.6 MHz; CDCl3; Me4Si): 21.0 (ArCH3),
38.4 (NCH3), 121.3 (C-2,6), 130.1 (C-3,5), 135.6 (C-4), 148.5
(35%) Mp 159–161 ЊC (n-heptane–xylene). NMR δH(360 MHz;
CDCl3; Me4Si): 3.38 (3H, s, NCH3), 7.57 (1H, dd, J 8.8, J 2.7,
H-6), 7.84 (1H, d, J 8.8, H-5), 8.02 (1H, d, J 2.7, H-2).
Elemental analysis: found C, 39.14; H, 2.84; Cl, 14.47; N, 11.15.
Calc. for C8H7ClN2O5: C, 38.96; H, 2.86; Cl 14.38; N, 11.36%.
UV λmax (MeOH)/nm, (ε/m2 molϪ1): 261 (815).
(C-1); 156.5 (C᎐O). UV λ
(MeOH)/nm, (ε/m2 molϪ1): 203
᎐
max
(1150), 266 (52), 272 (47).
Phenyl-N-phenyl-N-hydroxycarbamate (3a)
3-Chlorophenyl-N-hydroxy-N-methylcarbamate (2c)
(53%), Mp 92.5–94 ЊC (n-heptane), IR (nujol), νmax/cmϪ1
(46%) Mp 127.5–128.5 ЊC (n-heptane–toluene), lit.5 mp 124 ЊC.
:
IR (nujol), νmax/cmϪ1: 3270sb, 3158w (OH), 1680vs (C᎐O),
᎐
3205sb (OH); 1738vs, 1698vs (C᎐O); 1595s, 678m (C C );
1595m, 1497m, 692m, 689m (CArCAr), 1198s, 1160m (C–O),
752s, 741s (CH). NMR δH(360 MHz; CDCl3; Me4Si): 7.18 (2H,
m, Ar), 7.19–7.24 (2H, m, Ar), 7.31–7.38 (4H, m, Ar), 7.52 (4H,
m, Ar), 7.75 (1H, br s, OH). δC(90.6 MHz; CDCl3; Me4Si):
121.6, 122.6, 126.2, 126.7, 129.0, 129.7, 140.3, 150.9, 153.8
᎐
Ar Ar
1214s, 1151s (C–O); 1071m (CAr–Cl); 875m, 782m (CH). NMR
δH(360 MHz; CDCl3; Me4Si): 3.33 (3H, s, NCH3), 7.02 (1H, t,
J8.0, H-2), 7.15 (1H, m,. H-6), 7.20 (1H, d, J 8.0, H-4), 7.28
(1H, m, H-3), 7.35–7.95 (1H, br s, OH). δC(90.6 MHz; CDCl3;
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 5 6 2 – 5 6 9
563

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(C᎐O). UV λ
(27).
(MeOH)/nm, (ε/m2 molϪ1): 205 (1610), 1437
50 min, the mixture was cooled to 25 ЊC and a solution of 10%
aqueous HCl was injected till pH 1–3. The gaseous products
which contained carbon dioxide were bubbled into a solution of
barium hydroxide to generate insoluble barium carbonate.
Alternatively, the liberated gaseous products were carried in a
stream of nitrogen through a tube packed with magnesium per-
chlorate (p.a.,0.8–1.5 mm, Merck) into a small column packed
with sodium hydroxide on asbestos (0.8–1.6 mm, Merck). The
carbon dioxide was determined from the mass increase of the
column. The mass increase from 1a was 97.5% of the theoreti-
cal value. The percentage of CO2 in a similarly processed mix-
ture of carbamates 2a and 3a was 85.7 and 96.0%, respectively.
᎐
max
Phenyl-N-methyl-N-methoxycarbamate
20
20
(61%) nD = 1.5072 lit.,3 nD = 1.5069. IR (nujol), νmax/cmϪ1
:
3070wb (CArH), 2944w, 2820w, 1441m, 1374s, 750s (CH),
1750vs, 1730vs (C᎐O), 1594m, 690m (C C ), 1212s, 1168m,
1030m (C–O). NMR δH(360 MHz; CDCl3; Me4Si): 3.17 (3H, s,
NCH3), 3.68 (3H, s, OCH3), 7.08–7.15 (3H, m, H-2,4,6), 7.29
(1H, t, J 11.0, H-3,5). δC(90.6 MHz; CDCl3; Me4Si): 35.2
(NCH3), 61.3 (OCH3), 121.3 (C-2,6), 125.3 (C-4), 129.0 (C-3,5),
᎐
Ar Ar
150.8 (C-1), 154.8 (C᎐O). UV λ
(MeOH)/nm, (ε/m2 molϪ1):
᎐
max
205 (1110), 259 (27).
Procedure C. A sample of 1a (or 1e, 2a and 3a respectively)
(10 mmol) was submitted to hydrolysis under the same condi-
tions as described in Procedure B. The cooled mixture was acid-
ified with conc. HCl, extracted into ether (2 × 25 ml) and the
extract was analysed by HPLC. The phenol concentration was
quantified relative to a solution. The recovery of phenol was
91.0, 96.9, and 93.3% for 1a, 2a, and 3a, respectively. Carb-
amate 1e was submitted to the same procedure in phosphate
buffer (pH 7.05) at 60 ЊC over 10 h. The yield of 3-nitrophenol
was 94.4%.
N-methyl-O-(4-nitrophenyl)hydroxylamine
Carbamate 2f (0.15g, 1.4 mmol) was dissolved at room temper-
ature in 0.1 M NaOH (30 ml). After 20 min the solution was
acidified by conc. HCl (pH ≈ 2) and after 10 min treated with
5% aqueous NaOH (pH 12). Then the mixture was extracted
into ether (3 ×15 ml), dried and evaporated. The residue (0.09g,
71%) was crystallised from toluene. Mp 127–129 ЊC. NMR
δH(360 MHz; CDCl3; Me4Si): 3.36 (3H, s, NCH3), 7.21 (2H, d,
J 9.0, H-3,5), 8.24 (2H, d, J 9.0, H-4,6). Elemental analysis:
found C, 49.74; H, 4.84; N, 16.41. Calc. for C7H8N2O3: C, 50.00;
H, 4.80; N, 16.66%. UV λmax (MeOH)/nm, (ε/m2 molϪ1): 204
(230), 228 (681), 311 (1099). Hydrochloride mp 114–117 ЊC
(decomp.).
Procedure D. Solutions of carbamates 1a, 2a, 3a (10 mmol) in
1 M NaOH (5 ml) were submitted to partial hydrolysis at room
temperature for 10 min, and then the pH was adjusted at 9 by
addition of acetic acid. The same pH value was also adjusted
in the hydrolysis products from 1a, 2aand 3a obtained by
Procedure C. Then the individual solutions were mixed with
acetic anhydride (10 mmol), stirred at room temperature for 10
min, and finally mixed with a 3% aqueous solution of ferric
chloride (1 ml), and the colour monitored. An analogous pro-
cedure was also applied to potassium N-hydroxycarbamate
freshly prepared according to Voigt.9 The colour of individual
mixtures was compared with solutions of ferric complexes of
acethydroxamic acids formed from the same molar amounts
of hydroxylamine hydrochloride, or N-methyl- or N-phenyl-
hydroxylamine hydrochloride and acetic anhydride in the
presence of phenol.
Methods
The chromatographic measurements were carried out on a
Varian HPLC, Model 5000 using Separon SGX C18 in 60%
aqueous methanol. 1H and 13C NMR spectra were recorded in
CDCl3 or DMSO-d6 on a Bruker AMX 360 apparatus at 296 K.
The chemical shifts are reported in ppm relative to TMS as an
internal standard, and the coupling constants J are given in Hz.
The IR spectra were recorded in Nujol or in a film on Perkin
Elmer 684 and Data Station 3600 spectrometers. Mass spectra
were recorded on Platform Micromass quadrupole mass
spectrometer equipped with ESI and APCI with a column
Separon SGX C18 in 60% aqueous methanol. The masses of
the gaseous mixtures were measured on GC-MS-QP5050 with
a Carboxene-1006 column of 30 cm length and 0.53 mm ID.
UV-VIS spectra were measured on a Hewlett-Packard 8453
Diode Array apparatus. The pH values were determined with
the use of an MV 870 instrument (VEB Pröcitronic) with a
combination of glass electrode and silver electrode calibrated
with standard aqueous buffers. The ionisation constants Ka of
carbamates 1–3 were determined by spectral methods or from
kinetic data or taken from the literature.2
Procedure E. The 13C NMR spectra of the reaction mixture
obtained by dissolving 1a (10 mmol) in 10% NaOD (3 ml) in
D2O were measured at 30 min intervals at 20 ЊC. The decrease
in the peak at 155.9 ppm (C᎐O) of the carbamate and the
᎐
increase of the new peak at 165.9 ppm was monitored. The
intensity of the latter peak increased after adding potassium
carbonate to the reaction mixture. A similar procedure was also
applied to study the course of decarboxylation of the hydroxy-
carbamate ion which is formed from 2a and 3a or potassium
N-hydroxycarbamate.
Identification of degradation products of N-hydroxycarbamates
1a–3a
Procedure F. Carbamate 3a (1 g) was dissolved in 1 M NaOH
(50 ml), and the mixture was stirred under nitrogen at room
temperature for three weeks. The solution was washed with
chloroform (3 × 30 ml) and the aqueous solution acidified with
hydrochloric acid to pH 1 and extracted into ether. Both
extracts were submitted to chromatographic analysis. The
chloroform extract was concentrated and the products
separated on silica gel using chloroform as the eluent. The
major product was azoxybenzene (0.21 g, 75%); mp 33–35 ЊC.
MS-HPLC (m/z = 198). The formation of this product was
observed as early as after 24 h. In the chloroform extract, anil-
ine was identified by GC-MS (m/z = 93) and phenol (0.345 g,
86%) was identified (GC-MS, m/z = 94) in the ether extract.
Procedure A. 1a (10 mmol) was suspended in water (5 ml) and
covered with a layer of n-heptane in a sealed vessel equipped
with an electromagnetic stirrer and a tube connected to
an eudiometer. The vessel was kept at 60 ЊC and 1 M NaOH
(10 ml) was injected via a septum into the suspension, and the
aqueous phase was stirred for 60 min. During this period, the
evolved gas was measured in the eudiometer. A sample of the
gas was analysed by MS to determine the presence of nitrogen
(m/z = 28). After mixing the aqueous phase with solid sodium
hydroxide and heating the reaction mixture, ammonia gas
evolved. The same procedure was adopted in the case of
hydroxylamine hydrochloride and the potassium N-hydroxy-
carbamate freshly prepared according to Voigt.9
Kinetic experiments
For the kinetic analyses of carbamate hydrolysis the following
buffers were used: phosphate, borax, carbonate, 2-amino-2-
(hydroxymethyl)propan-1,3-diol; sodium hydroxide solutions
Procedure B. A sample of 1a (10 mmol) was hydrolysed by
stirring in a solution of 1 M NaOH (30 ml) at 60 ЊC. After
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were used for pH above 12. The ionic strength was adjusted to
I = 1.0 by addition of potassium chloride. The pH values of
the resultant solution were measured by means of a combined
electrode GK 2401 B, using a PHM-93 (Radiometer-Copen-
hagen) pH-meter at 25 ЊC. The pH values at 60 ЊC were deter-
mined on the same pH-meter calibrated at the respective tem-
perature with the IUPAC standards with known pH-temper-
ature dependences.10 The ionic strength of 1 M NaOH and
stronger solutions was unadjusted, and their basicity was
taken from the literature.11 The carbamate degradation reaction
rates were followed spectrophotometrically, using a UV-VIS
spectrometer HP 8453 (Hewlett-Packard) and quartz cells at
constant temperature (thermostat). Very slow reactions (with
half-lives below 1 h) were followed with a UV-VIS M 40 appar-
atus (Carl Zeiss). Solutions of the carbamates in methanol
or dioxane were injected into the cell to a final concentration
of 10Ϫ4–10Ϫ5 M. Absorbance changes were followed at the
wavelength corresponding to the absorption maximum of the
liberated phenol/phenolate. These measurements were not per-
turbed by subsequent reactions, as the products of these reac-
tions do not absorb at the wavelengths used, and there were no
other reactions involving phenol/phenolate. The parameters A
and kobs were optimised by the method of non-linear regression
using Eqn. (1):
The formation of nitrogen from hydroxylamine is about eight
times (81% conversion) slower than the formation of phenolate
from carbamate 1a. The disproportionation of hydroxylamine
in aqueous basic media was studied earlier by Lunˇák and
14
ˇ
Veprˇek-Sisˇka. The authors claim that hydroxylamine is
decomposed in 1 M NaOH to ammonia and nitroxyl, which
produces nitrogen by reaction with the remaining non-reacted
hydroxylamine. At pH 7 they also found N2O.
The base-catalysed decomposition of carbamates 1a–e to
substituted phenols and N-hydroxycarbamate proceeds kinetic-
ally as a pseudo-first-order reaction. The dependences of log
kobs on pH or log [OHϪ] are presented in Fig. 2.
obst
A Ϫ A0 = (A∞ Ϫ A0) (1 Ϫ ek
)
(1)
where A, A0, and A∞ are absorbances at time t, at the beginning,
and at the end of the reaction, respectively.
Results and discussion
Substituted phenyl N-hydroxycarbamates 1a–f and 3a are stable
crystalline solids. Their N-methyl analogues 2a–2f are unstable
substances slowly decomposing at 25 ЊC. In contrast to the
substituted phenyl carbamates or N-methoxycarbamates, all
N-hydroxycarbamates 1a–3a are much more soluble in water as
a consequence of the hydroxyl group.
Fig. 2 Dependence of log kobs of degradation of substituted phenyl
N-hydroxycarbamates 1a(᭿), 1b(ꢀ), 1c(᭢), 1d(᭜), 1e(᭹) and 1f(ᮀ)
on pH or 14 ϩ log cOHϪ at 25 ЊC. The curves correspond to Eqn. (5).
The dependence shown can generally be expressed by kinetic
Eqn (2):15,16
Mechanism of base-catalysed degradation of substituted phenyl
N-hydroxycarbamates
(2)
Carbamates 1a–e are hydrolysed above pH 7 and generate the
corresponding phenol or phenolate and the N-hydroxycarb-
amate almost quantitatively (see Procedure C and Scheme 1).
An exception is the 4-nitro derivative 1f, which at pH 12–13
generates N-(4-nitrophenoxy)carbamate, which is formed as a
result of a rearrangement as described earlier.2,12
where a, b and c are coefficients of the rate equation. The first
linear dependence in Fig. 2 with slope 1 and the transition to
plateau with slope 0 are defined by the equation kobs = a[OHϪ]/
(1ϩb[OHϪ]). At low concentrations of hydroxyl ions, when
b[OHϪ] << 1, the value of kobs is directly proportional to the
concentration of hydroxyl ions, i.e. kobs = a[OHϪ]. At high con-
centrations of hydroxyl ions, when b[OHϪ] >> 1, it is kobs = kdЈ =
a/b, hence the reaction rate is independent of hydroxyl ion
1
concentration. In Table 1, the value of constant kOH is given
by experimentally determined coefficient a, and the value of
constant kdЈ is given by the ratio a/b.
The hydrolysis of carbamates can proceed either by an attack
of hydroxyl ion on the carbonyl group with concomitant cleav-
age of the C–OAr bond (concerted mechanism) or by cleavage
of a phenolate ion from the conjugate base of carbamate 1 in
the rate-limiting step (E1cB) (Scheme 2).
kc is the rate constant for the concerted reaction, kd is the rate
constant for the decomposition of the carbamate 1 anion. The
reaction mechanism can be determined on the basis of the
Brønsted dependence of log kOH1 or log kdЈ from Table 1 on the
change in pKaP of the leaving phenol (Eqns 3, 4).
Scheme 1
N-Hydroxycarbamate could not be detected because it is
rapidly decarboxylated to hydroxylamine and carbonate
(Scheme 1), in contrast to N-aryl carbamates, N-alkyl carb-
amates and N-alkoxycarbamates,3,13 which are relatively stable
in basic media. After complete conversion of carbamate 1a to
the phenolate ion, hydroxylamine could not be detected. The
formation of phenolate ion was accompanied by liberation of
nitrogen from the reaction mixture and (after addition of fur-
ther sodium hydroxide) by liberation of ammonia (Scheme 1).
In a separate experiment the decomposition of hydroxylamine
was monitored in 1 M aqueous NaOH at 60 ЊC, giving gaseous
nitrogen and ammonia. Also the potassium N-hydroxycarb-
amate gives the same mixture of products (see Procedure A).
(3)
(4)
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Table 1 Kinetic data of degradation of carbamates 1a–3f, 2a–h and 3a at 25 or 60 ЊC
(105ؒK1)a
(pKa)a
(104ؒkdЈ)b
(kOH
)
(103ؒkOH
)
(pKa )d
1
c
2
c
p
1a
5.25 0.8
4.90 0.9
6.20 0.7
5.62 0.9
6.46 0.6
7.59 1.2
—
—
—
—
9.28 0.14
9.31 0.06
9.21 0.07
9.25 0.08
9.19 0.06
∼9.1 0.1
9.29 0.02i
9.36 0.05i
9.16 0.02i
9.23 0.02i
9.04 0.04i
8.85 0.03i
—
0.73 0.04
0.52 0.03
6.12 0.33
3.15 0.15
32.9 1.77
∼850 50
—
—
—
—
—
—
—
—
—
5.13 0.33
2.95 0.15
38.1 2.65
14.1 0.62
230 20.4
∼4500
11.7 0.75
10.7 0.64
45.1 3.21
22.9 1.77
258 16.5
2187 110
4467 167
758 48.6
11.6 0.31
1.51 0.08
0.71 0.05
23.4 0.15
6.45 0.25
9.99
10.25
9.08
9.42
8.38
7.16
9.99
10.30
9.08
9.42
8.38
7.16
6.70g
7.75h
9.99
1b
1c
1d
1e
173 11.7
1fe
2af
2bf
2cf
2df
2ef
2f
∼45
—
—
—
—
—
—
—
—
—
—
71
9
2gf
2hf
3af
—
—
72 0.1
—
8.86 0.06i
a Ka = K1ؒKW; K1 is defined in Eqn. (4) and its value is determined from kinetic data. b [sϪ1]. c [dm3.molϪ1.sϪ1]; kOH2 is defined in Eqn.(6). d pKa of the
corresponding substituted phenol, taken from ref. 17. e Taken from ref. 2. f Measured at 60 ЊC. g Taken from ref. 18. h Taken from ref. 19. i Determined
by spectral methods.
Scheme 2
For an E1cB reaction of this type βlg is about Ϫ1 (refs. 20,
21). For a concerted mechanism, the βlg values vary from Ϫ0.4
to Ϫ0.6, exceptionally22 to Ϫ0.8. The calculated values are =
Ϫ0.99 and = Ϫ0.93. That means that the hydrolysis of carb-
amates 1 proceeds by the E1cB mechanism (Scheme 3).
In order to roughly estimate the extent to which the proton is
removed from N–H and O–H groups, we adopted the value of
the ρ constant of ionisation of substituted carbamates 1 to the
first degree (ρ = 0.13) and that of ionisation of substituted
phenyl N-methoxycarbamates, which possess only one acidic
proton N–H (ρ = 0.83).3 The large difference between the two
constants is due also to the fact that during the dissociation of
the N–H bond, the negative charge is nearer to the substituent.
We suppose that in carbamate 1 the O–H bond is predomin-
antly cleaved, KT is much higher than 1 and the value of kd is
substantially greater than kdЈ.
The value (β1g)k = Ϫ1.53 found from the dependence of
OH2
2
p
log kOH vs. pKa corresponds to decomposition of the dianion
also according to the E1cB mechanism (Scheme 4).
The values of rate constants kobs in the measured range of
hydroxyl ion concentration in Fig. 2 obey Eqn. (6):
(6)
Scheme 3
In this case, the constants kobs are defined by Eqn (5):
At a high concentration of hydroxyl ions K1[OHϪ] ӷ 1, and
Eqn. (6) is simplified to Eqn. (7):
(5)
kobs = kdЈ ϩ kd2K2[OHϪ]
where kd K2 = kOH2 (Table 1).
(7)
2
where kOH1 = kdЈؒK1 and K1 = KNH ϩ KOH
.
The high negative value (Ϫ1.53) is caused by the fact that the
2
The equilibrium constants K1 and rate constants kdЈ are
experimentally determined constants and were calculated from
Eqn. (5) by the non-linear regression method. KNH, KOH and kd
cannot be experimentally determined.
substituent effect on (β1g)k comes from the effects on both kd
OH2
and K2 in a similar way as that on (β1g)k , but the effect on K2
OH1
is much higher, because predominantly the N–H bond is
broken. As the dependence of kobs vs. log [OHϪ] was linear in
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Scheme 4
the whole range measured (with the slope equal to 1), it was not
2
possible to determine the values of K2 and kd from experi-
mental values.
Mechanism of base-catalysed degradation of phenyl N-hydroxy-
N-methylcarbamates and of phenyl N-hydroxy-N-phenyl-
carbamates
The degradation products of substituted phenyl N-methyl-
carbamates 2a–2e, 2g, 2h in aqueous bases are the corre-
sponding phenols, methylamine, and carbonate. The qualitative
test with Fe3ϩ did not prove the presence of N-methylhydroxyl-
amine, even in the initial phase of the hydrolysis. In a separ-
ate experiment, N-methylhydroxylammonium chloride itself
gave methylamine at the conditions of hydrolysis. Kjellin23
reported that hydrolysis of N-methylhydroxylamine in aqueous
sodium hydroxide produces ammonia, methylamine, and
formate.
After base-catalysed degradation of carbamate 3a, phenol,
carbonate, azoxybenzene and aniline were detected in the
reaction mixture (Scheme 5).
Fig. 3 Dependence of log kobs of hydrolysis of substituted phenyl
N-hydroxy-N-phenylcarbamates 2a(᭿), 2b(ꢀ), 2c(᭢), 2d(᭜), 2e(᭹),
2g(ᮀ) 2h(᭺) and phenyl N-hydroxy-N-phenylcarbamate 3a(᭞) on pH
at 60 ЊC.
1
The Brønsted coefficient βlg calculated from the log kOH
values for the carbamates 2a–e was Ϫ0.75. This corresponds to
hydrolysis by a concerted mechanism (Scheme 6).
Scheme 6
In the case of addition–elimination mechanisms the addition
of hydroxide ions to generate a tetrahedral intermediate is gen-
erally the rate limiting step because OHϪ is a poor leaving group
compared with Ar–OϪ. Accordingly the Brønsted coefficient βlg
for this reaction varies26 from 0.1 to 0.3.
Scheme 5
It was also found that phenylhydroxylamine disproportion-
ated in aqueous sodium hydroxide solution to aniline and
azoxybenzene (Scheme 5). The formation of these products
from phenylhydroxylamine under similar conditions was
described by Bamberger and Brady.24 Boyland and Nery25
heated benzyl N-hydroxy-N-phenylcarbamate in sodium
hydroxide solution and identified nitrosobenzene and phenyl-
hydroxylamine. The pH dependence of log kobs of carbamates
2a–h at pH <8.5 is linear with the slope 1 (Fig. 3).
In this pH range, carbamates 2 are present in their neutral
1
form, hence the reaction rate v = kc.[2][OHϪ], and kOH = kc is
the rate of formation of substituted phenol and N-hydroxy-N-
methylcarbamate at 25 ЊC (except for 2f ). The 4-nitro derivative
2f provides a mixture of N-hydroxy-N-methyl carbamate and
4-nitrophenol besides N-methyl-N-(4-nitrophenoxy)-carbamate
(Scheme 7) in a ratio of 2 : 98. The major product of hydrolysis
of 2f after decarboxylation (during acidification) is N-methyl-
O-(4-nitrophenyl)hydroxylamine similar to the 1f derivative.12
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567

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Scheme 7
1
The value of the kOH constant for the rearrangement of
carbamate 2f at 25 ЊC is almost one order of magnitude
1
higher than the kOH for the degradation of the 3-nitro deriv-
1
ative 2e at 60 ЊC, but comparable with kOH for derivative 1f,
which predominantly reacts by a E1cB mechanism (Fig. 4).
Fig. 5 Dependences of rate constants log ks (•) and log kf (᭺) on the
Hammett σ constants for base-catalysed degradation of carbamates
2a–e and 2g at pH 9.5 at 60 ЊC.
N–H bond splitting is higher than that of O–H bond splitting).
The lower ρ constant of carbamates 1 is obviously due to the
strong hydrogen bond between a water molecule and N–H
group of the neutral carbamate 1, which stabilises the neutral
substrate, whereas in its conjugate O-base the analogous bond
is almost negligible. Electron-withdrawing substituents X in 1
stabilize the hydrogen bonding to the N–H bond, and ionis-
ation of O–H in the molecule H–ON(H ؒ ؒ ؒ OH2)COOC6H4X
is diminished. In carbamates 2 H–ON(CH3)COOC6H4X the
analogous ionisation is not affected by hydrogen bonding, and
thus the ρ value is higher than that of carbamates 1. A similar
large effect on ionisation was observed when hydrogen was
replaced by the methyl group in N-acetylthioureas:30 CH3-
Fig. 4 The pH dependence of log kobs of degradation and rearrange-
ment of carbamates 1f(ᮀ) and 2f(᭹) at 25 ЊC.
At pH 8.5–9, the slope value of dependence of log kobs vs. pH
for carbamates 2 decreases, because their dissociation is increas-
ingly important. At pH ≈ 10, the pH dependence of log kobs
should be linear and independent of pH, because the concen-
tration of substrate is inversely proportional to the concen-
tration of OHϪ ions, hence the product [2]ؒ[OHϪ] is constant.
Measuring the degradation rate of compounds 2a–e and 2g in
carbonate buffers at pH 9.5, it was found that the absorbance–
time dependence is no longer exponential, as two consecutive
reactions are taking place. Their rates were determined from
Eqn. (8)27,28
CONHC(᎐S)NHCH , pK = 16.6 and CH CON(CH )C(᎐S)-
᎐
᎐
3
a
3
3
NHCH3, pKa = 14.5.
Fig. 6 gives the pH dependence of log kobs for 1a and its
ft
st
At Ϫ A∞ = M ؒ eϪk ϩ N ؒ eϪk
(8)
where kf and ks are rate constants of the faster and slower steps,
respectively.
The dependences of the rate constants log ks and log kf on the
Hammett substituent constants σ are presented in Fig. 5.
The values of the reaction constants, ρf = 0.65 0.17 and
ρs = 1.09 0.14, show a large scattering, which is due to the
small difference between the two rate constants, and hence a
relatively large error in their determination.29 So far we have no
explanation for this degradation course.
At pH > 9 the degradation is complex and we could not
determine the values pKa of carbamates 2 from the reaction
kinetics. However, these values were determined by spectral
method at 25 ЊC and were used for comparison with carbamates
1. Although carbamates 2 only have one acidic proton, which is
bound to oxygen, the ρ constant is about 0.32. This value is
almost 3 times as large as that of carbamates 1, where partial
splitting of the N–H bond also takes place (the ρ constant of
Fig. 6 Dependence of rate of base-catalysed degradation on pH
values for carbamates 1a(᭿), 2a(ᮀ), phenyl N-methoxycarbamate
4a(᭺) and phenyl N-methoxy-N-methylcarbamate 5a(᭹) at 60 ЊC.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 5 6 2 – 5 6 9
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6 R. L. Blakeley, J. A. Hinds, E. C. Webb and B. Zerner, Biochemistry,
1969, 8, 1991.
O-methyl derivative (4a), and for 2a and its O-methyl derivative
(5a). Carbamates 2a and 5a (ref. 3) are hydrolysed at almost
identical rates at pH < 10 at 60 ЊC. On the other hand, carb-
amate 1a is hydrolysed almost two orders of magnitude faster
than 4a in spite of the fact that carbamate 1a undergoes
predominant O–H bond splitting, which gives a non-reactive
conjugate base. Both carbamates (1a and 4a) react by a E1cB
mechanism. In the base formed by dissociation of the N–H
bond in carbamate 1a, the hydrogen bond between the adjacent
OH and water is very weak. The value βlg = Ϫ0.92 suggests a
transition state in which the splitting of the C–O bond has
advanced to a large extent. The structure of the transition state
thus resembles that of a complex formed from phenolate ion
7 P. Gröbner and E. Müller, J. Med. Chim. Ther., 1974, 9, 341.
8 J. A. Baskakow, T. V. Gorskaya, P. V. Tibanov, G. C. Schvinblerman
and A. F. Basilev, Zh. Org. Khim., 1967, 3, 112.
9 C. W. Voigt, G. Glattow and H. Hlawatschek, Z. Anorg. Allg. Chem.,
1978, 439, 227.
10 R. C. Weast, A. J. Meluin and W. H. Beyer, in Handbook of
Chemistry and Physics, 64th edition, D-150, CRC Press. Inc.,
Boca Raton, 1984.
11 G. Yagil, J. Phys. Chem., 1967, 71, 1034.
12 R. L. Blakeley and B. Zerner, Chem. Ind. (London), 1973, 133.
13 S. L. Johnson and D. L. Morrison, J. Am. Chem. Soc., 1972, 94,
1323.
ˇ
14 S. Lunˇák and J. Veprˇek-Sisˇka, Collect. Czech. Chem. Commun.,
1974, 39, 391.
15 J. Birk, J. Chem. Educ., 1976, 53, 704.
16 H. F. Gilbert, J. Chem. Educ., 1977, 54, 492.
17 F. G. Bordwell and G. D. Cooper, J. Am. Chem. Soc., 1952, 74, 1058.
18 J. Butler and D. Martin, J. Chem. Soc., 1939, 1366.
19 M. J. Colthurst, M. Nanni and A. Williams, J. Chem. Soc.,
Perkin Trans. 2, 1996, 2285.
and the acid HO–N᎐C᎐O, which forms a strong hydrogen bond
᎐ ᎐
with a water molecule. In this way, water can act as a base
catalyst facilitating the degradation of carbamate 1a.
Acknowledgements
20 A. Williams, J. Chem. Soc., Perkin Trans. 2, 1973, 1244.
21 A. F. Hegarty and L. Frost, J. Chem. Soc., Perkin Trans. 2, 1973,
1719.
22 B. S. Saif, A. K. Luthra and A. Williams, J. Am. Chem. Soc., 1989,
111, 2647.
This work was supported by the Ministry of Education, Youth
and Sports of the Czech Republic (Project No. CI MSM 253
100 001).
23 C. Kjellin, Svensk. Kem. Tids., 1921, 33, 213 (Chem. Abstr., 1922,
12209).
24 E. Bamberger and F. Brady, Ber. Dtsch. Chem. Ges., 1900, 33, 271.
25 E. Boyland and R. Nery, J. Chem. Soc. C, 1966, 346.
26 E. A. Castro, Chem. Rev., 1999, 99, 3505.
27 W. G. Jackson, J. M. Harrowfield and P. D. Vowles, Int. J. Chem.
Kinet., 1977, 9, 535.
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