Alkoxycarbonylation of alcohols and phenols by nitrosoformates

Article abstract of DOI:10.1135/cccc19961053

Unstable neutral radicals [ROCONHO^?] 2 and nitrosoformates 3 are formed by oxidation of N-hydroxycarbamates with lead dioxide. In the presence of alcohols or phenols and water they solvolyzed to mixtures of symmetrical 4 and asymmetrical 5 carbonates. The content of asymmetrical carbonates 5 increases with increasing reactivity of the nitrosoformates 3 formed, temperature, the content of water in the reaction mixture, and with decreasing reactivity of alcohol. The reactivities of individual alcohols have been evaluated with the help of competitive alcoholysis. The new method of alcohol or phenol alkoxylation has been verified experimentally by preparing six asymmetrical carbonates which were obtained in 34 to 47% yields.

Full text of DOI:10.1135/cccc19961053

Alkoxycarbonylation of Alcohols and Phenols
1053
ALKOXYCARBONYLATION OF ALCOHOLS AND PHENOLS
BY NITROSOFORMATES
a1
b
Jaromir MINDL , Ales HALAMAa2 and Zdenek CERNOSEK
a Department of Organic Chemistry, University of Pardubice, 532 10 Pardubice, Czech Republic;
e-mail: 1 mindl@hlb.upce.cz, 2 halama@hlb.upce.cz
b Department of General and Inorganic Chemistry, University of Pardubice, 532 10 Pardubice,
Czech Republic; e-mail: cernosek@hlb.upce.cz
Received March 18, 1996
Accepted April 16, 1996
Unstable neutral radicals [ROCONHO•] 2 and nitrosoformates 3 are formed by oxidation of N-hydroxy-
carbamates with lead dioxide. In the presence of alcohols or phenols and water they solvolyzed to
mixtures of symmetrical 4 and asymmetrical 5 carbonates. The content of asymmetrical carbonates 5
increases with increasing reactivity of the nitrosoformates 3 formed, temperature, the content of
water in the reaction mixture, and with decreasing reactivity of alcohol. The reactivities of individual
alcohols have been evaluated with the help of competitive alcoholysis. The new method of alcohol or
phenol alkoxylation has been verified experimentally by preparing six asymmetrical carbonates which
were obtained in 34 to 47% yields.
Key words: Lead dioxide, hydroxycarbamates oxidation; Carbonates, preparation by alkoxycarbonyla-
tion; Alcohols, alkoxycarbonylation; Phenols, alkoxycarbonylation.
The reaction of iodine or silver oxide with N-hydroxycarbamates (ROCONHOH) 1 in
aqueous alcohols produces1 mixtures of O- or N-alkoxycarbonyl derivatives of N-hydroxy-
carbamates [ROCONHOCOOR, (ROCO)2NOH)]. Later it was found that tetra-
alkylammonium periodates oxidize N-hydroxycarbamates 1 to unstable nitrosoformates 3.
These compounds were used in situ as dienophiles for Diels–Alder cyclization to give
esters of 3,6-dihydro-2H-1,2-oxazine-2-carboxylic acid2 or similar polycyclic com-
pounds3. As a matter of fact, we wanted to prepare 3-benzyloxycarbonyl-2-oxa-3-aza-
bicyclo[2.2.1]hept-5-ene by cycloaddition of cyclopentadiene with benzyl nitrosoformate
3h according to Kirby et al.3, however, by a modified procedure using the safer oxida-
tion agent. With respect to almost the same oxidation potential (+1.6 V, ref.4) and
accessibility of the compound, we chose lead dioxide. Analysis of products of our experi-
ment revealed cycloadduct but in a mixture with dibenzyl carbonate 4h (procedure A).
The presence of carbonate 4h among the products made us to study the acylation activity
of nitrosoformates and to verify the preparative value of this activity.
The reaction of lead dioxide with N-hydroxycarbamates is exothermic and produces
dinitrogen oxide and carbon monoxide (procedure B). The formation of gases was ob-
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1054
Mindl, Halama, Cernosek:
served even in the presence of alcohols or phenols in the reaction mixture. Oxidation
course of isopropyl N-hydroxycarbamate 1d was monitored by ESR analysis (procedure B).
The observed spectrum (Fig. 1) is characterized by three resolved asymmetrical bands
formed by superposition of twelve bands, which was confirmed by computer simula-
tion. The found values of hyperfine splitting (HFS) can be assigned to the interaction of
unpaired electrons of nitrogen atom with the nuclear spins of two nonequivalent hy-
drogen atoms in the structure of the radical. The obtained HFS constants: aN = 8.7
gauss, aN–H = 3.4 gauss, aC–H = 1.5 gauss. The bands were of the Gaussian type with the
half-band with ∆Hpp = 4.35 gauss with 30% ratio of Lorentian type. The mean value of
g tensor is g = 2.008. Gutch and Watters5 found a radical with similar values of the above
given constants in the oxidation of ethyl N-hydroxycarbamate with cerium(IV) sulfate
in aqueous sulfuric acid. The presence of two nonequivalent protons in the molecule
studied can be explained by the structure of cyclic electrically neutral radical 2d.
H C
3
O
O
H C
3
H
N
O
H
2d
Diisopropyl carbonate 4d was the final product of the oxidation of carbamate 1d in
dichloromethane. Carbamate 1d was oxidized to 4d in isopropanol, as well.
Oxidation of 1d in methanol produced a mixture of carbonate 4d and isopropyl
methyl carbonate 5ad. Under the given conditions it can be generalized that oxidation
of hydroxycarbonates 1 by lead dioxide in dichloromethane produces neutral radical 2
which subsequently forms nitrosoformate 3 (Eq. (A)).
–H+, –e
(PbO2)
–e, –H+
(PbO2)
R1OCONH–OH
R1OCONH–O•
R1OCON=O + H2O
(A)
(B)
1
2
3
–N2O, –CO2
(H2O)
–N2O
2 R1OCON=O
R1OCOOR1
ROCON=O + R1OH
3
4
3
1
1
1
1
1
R
R OCONHOH R OCON=O
R OCOOR
Me
Et
i-Pr
Ph
PhCH
1a
1b
1d
1g
1h
3a
3b
3d
3g
3h
4a
4b
4d
4g
4h
2
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Alkoxycarbonylation of Alcohols and Phenols
1055
The symmetrical carbonate 4 is formed by another subsequent reaction of nitrosofor-
mate 3 with the present water or alcohol containing the same alkyl group as formate 3
(Eq. (B)).
An analogous oxidation of nitrosoformate 3 in R2OH alcohol containing a different
alkyl group produces a mixture of symmetrical 4 and asymmetrical 5 carbonate (Eq. (C)).
–N2O, –CO2
R1OCON=O + R2OH
R1OCOOR1 + R1OCOOR2
(C)
(H2O)
3
4
5
2
R
1
R
Me
Et
Pr
i-Pr
Bu
s-Bu
Ph
Me
Et
i-Pr
Ph
–
5ab
–
5bd
5bg
5ac
5bc
5cd
5cg
5ad
5bd
–
5ae
5be
5de
5eg
5af
5bf
5df
5fg
5ag
5bg
5dg
–
5ab
5ad
5ag
5dg
dY/dH
x
FIG. 1
ESR spectrum of [i-PrOCONHO•] (2d), curve x
represents simulated spectrum of Gaussian type
3 340
3 380
3 420
H, gauss
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1056
Mindl, Halama, Cernosek:
As the ESR analysis of the reaction mixture confirmed the presence of the only radical
(type 2), we do not suppose a homolytic decomposition of intermediates 3 to carbonates 4
but a nucleophilic reaction with water formed in the previous oxidation. Analysis of the
products of oxidation of isopropyl N-hydroxycarbamate 1b with methanolic solution of
sodium azide (procedure C) did not confirm formation of any other products. The simi-
lar experiment indicating SN1 mechanism of solvolysis was carried out by Olivier and
Waters6 who studied autoacylation of phenylacetohydroxamic acid. Table I gives the
results of reaction mixture compositions according procedure D.
The ratio of symmetrical carbamate 5 to the mixture of carbonate 4 and 5 depends on
the type of groups (R1 and R2) in both reaction components and on temperature. For
evaluation of the effect of alkyl groups, we used the values of the steric constants Es as
the dominant parameter in the Taft two-parameter equation7. As it can be seen in Fig. 2 the
above mentioned ratio 5/(4 + 5) (%) in the mixtures increases with decreasing reactivity
of alcohol. On the other hand, for a given alcohol this ratio increases with increasing
reactivity of nitrosoformate 3.
The oxidation of methyl N-hydroxycarbamate 1a with lead dioxide in the presence of
alcohols was carried out at 40 and –10 °C. As it follows from Fig. 2 (curves 1 and 4)
the 5/(4 + 5) (%) ratio also increases with increasing reaction temperature.
TABLE I
The ratio of carbonates 4 and 5 in reaction mixture at 40 °C
R1OCONO
R1
+
R2OH
R1OCOOR1
+
R1OCOOR2
+ …
R2
4
%
5
%
R1
R2
Bu
4
%
5
%
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Me
Et
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4b
4b
92.1
70.0
71.5
55.2
67.9
51.5
65.0
14.6
52.3
66.3
90.2
49.7
–
Et
4b
4d
4b
4d
4d
4d
4d
4d
4d
4g
4g
4g
51.2
25.9
2.0
5be
5bd
5bf
5ad
5bd
5cd
5de
29.6
46.3
47.7
7.1
5ab
5ab
5ac
5ac
5ae
5ae
5ad
5ad
5ab
14.3
4.3a
37.2
9.7a
39.6
8.1a
77.7
14.3a
25.1
–
Et
i-Pr
s-Bu
Me
Et
Et
Et
Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Ph
84.2
66.3
62.5
51.9
76.5
45.6
34.2
41.7
49.0
Pr
9.4
Bu
Bu
i-Pr
i-Pr
Me
Et
Pr
12.0
14.1
–
Bu
i-Pr
s-Bu
Et
5df
5bg
5eg
21.3
16.8
7.5
Et
Ph
Bu
Et
Pr
5bc
26.2
Ph
Ph
–
a The reaction mixture was kept at –10 °C in a thermostat.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Alkoxycarbonylation of Alcohols and Phenols
1057
In order to establish the quantitative relationship between structure of the reacting
alcohols and their reactivity towards nitrosoformates 3, we carried out series of competitive
alcoholyses using equimolar mixtures of various alcohols R2OH and R3OH (procedure E).
Under the given starting conditions [R2OH] = [R3OH] >> [3] (e.g. 50 : 50 : 1), the ratio
of the corresponding reaction rates k2/k3 was evaluated from the found carbonate con-
centration ratios [R1OCOOR2]/[R1OCOOR3]. The ratios of carbonates 5 were deter-
mined by chromatography and used for calculating the relative reactivities related to
methanol, rRMe = kROH/kMeOH = [R1OCOOR]/[R1OCOOMe]. The values of rMR e were
determined either directly or by a combined way in the cases where the difference
between reactivities of the alcohols compared was too big (see Table II). In the reac-
tions of nitrosoformate 3 with alcohols, the relative reactivities rRMe of the individual
alcohols decrease in the order: MeOH > EtOH > PrOH > BuOH > i-PrOH > s-BuOH,
which corresponds to the Es values7. The evaluation of log rMR e values of reactions of
phenyl nitrosoformate 3g with various alcohols using the Taft two-parameter equation
gave both corresponding reaction constants (ρ, δ):
log rRMe = ρ*σ* + δES = 8.5 σ* + 0.8 Es; r = 0.999 .
Figure 3 shows the content of carbonates 5ab, 4b and 4a formed by the reaction of
ethyl nitrosoformate 3b with aqueous methanol. The occurrence of the dominant ethyl
75
3
2
0.8
1
Y, %
50
5/4+5
0.4
3
25
2
4
1
–1.2
FIG. 2
–0.8
–0.4
0
0
20
40
Es
c(H O), vol.%
2
FIG. 3
Relationship between 5/(4 + 5) ratio (amounts
of compound in %) in the carbonate mixture
and the Taft steric constants Es of alkyl groups
in reacting alcohol and nitrosoformate. Alcoho-
lyses of methyl nitrosoformate 3a at 40 °C (1)
and –10 °C (4), respectively, ethyl nitrosofor-
mate 3b at 40 °C (2), isopropyl formate 3d at
40 °C (3)
Dependence of yield (Y, %) of carbonates 4a
(1), 4b (2), and 5ab (3) formed in the oxida-
tion of ethyl N-hydroxycarbamate 1b in
aqueous methanol upon the amount of water
present
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1058
Mindl, Halama, Cernosek:
methyl carbonate 5ab decreases owing to the competitive kinetically controlled hydro-
lysis to ethanol. The alcohol reacts subsequently with another molecule forming diethyl
carbonate 4b. This compound was found in the reaction mixture without the addition of
water. Its content increases distinctly even after small addition of water into methanol
but, after continued addition of water, it decreases again. We did not observe the for-
mation of dimethyl carbonate 4a in the reaction mixture without the water added from
outside. The formation of 4a can correspond the presence of water in tetrahedral inter-
mediate of nitrosoformate 3b with methanol.
H
Me
H
H
H
H
O
O
O
O
H
H2O
MeOH
(H2O)
1b
-(N2O)
EtOH
4b
3b
3a
-CO2, -(N2O)
O
O
NO
Et
(D)
Et
MeOH
-(N2O)
-(N2O)
5ab
4a
TABLE II
Relative ratio of products 5 obtained in reaction mixture from nitrosoformates 3 and equimolar mix-
ture of alcohols R1OH and R2OH
k2
R1OCONO
R1OCONO
+
+
R2OH
R3OH
R1OCOOR2
R1OCOOR3
+
+
…
…
k3
2
3
2
3
R1
Et
R2
R3
k2/k3
log rRR
R1
Ph
R2
R3
k2/k3
log rRR
Pr
Me
Pr
0.095
0.464
–
–1.022
–0.334
–1.356a
–0.978
–2.000a
–0.748
Bu
Me
Bu
0.133
0.262
–
–0.876
–0.582
–1.458a
–0.331
–0.667
–2.125a
Bu
Bu
Pr
i-Pr
i-Pr
Et
Me
i-Pr
Me
Me
Me
Me
i-Pr
Me
0.105
–
0.466
0.215
–
i-Pr
Pr
s-Bu
s-Bu
Ph
0.179
2
2
3
3
a Calculated from: log rMe = log rRR + log rMe
.
R
R
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Alkoxycarbonylation of Alcohols and Phenols
1059
On the basis of our previous experiments concerning the study of conditions of
alkoxycarbonylation of alcohols, we chose methyl N-hydroxycarbamate 1a as the start-
ing reagent for the preparation of six asymmetrical carbonates 5ai–5an. As it follows
from Table III the yields of those compounds are relatively low (34–47%). Spectros-
copic characteristics of the new compounds are given in Table IV.
OCO CH
OCO CH
OCO CH
2 3
2
3
2
3
O
O
NHCOCH
NHCO C H
2 2 5
3
5ai
5aj
5ak
OCO CH
2
3
C H OCO CH
C H OCO CH
3
7
15
2
3
9
19
2
NHCO CH
2
3
5am
5an
5al
EXPERIMENTAL
Melting points were measured on a Koffler instrument and are not corrected. IR spectra were
measured on an IR 75 Spectrometer (Zeiss), 1H and 13C NMR spectra on a Bruker AMX-360 ap-
paratus in deuteriochloroform, ESR spectra on ESR 221 ZWG, GC was performed on a Carlo–Erba
chromatograph, model GV, and GC-MS on a Fison–Mega 5160 apparatus with the mass detector
QMD-100 using Carboxen 1000 60/80 mesh and Se52 – Chromosorb A 60/80 mesh columns. HPLC
was carried out on an instrument produced by Laboratorni pristroje, Prague using C18 columns and
50% aqueous methanol. The lead content was measured by the FSDDP method on a polarographic
analyzer EP-10.
Chemicals
Lead dioxide was prepared from lead tetraacetate according to Brauer8. 3-Benzyloxycarbonyl-2-oxa-
3-azabicyclo[2.2.1]hept-5-ene was prepared by reported procedure3. Methanol, ethanol, 1-propanol,
2-propanol, 1-butanol, (±)-2-butanol (commercial products, Lachema Brno) were pre-dried with cal-
cium oxide, refluxed with 2% sodium metal and distilled before use. 1-Heptanol and 1-nonanol, phenol,
4-hydroxycoumarine, and 3-acetylaminophenol were commercial products (Aldrich). Methyl and ethyl
3-hydroxyphenylcarbamates (products of CHZ-Synthesia Semtin) were crystallized from toluene (m.p.
93–94 °C and 92–93 °C, respectively).
N-Hydroxycarbamates 1 were prepared according to a general procedure2. Their physical data
were in accordance with those given in literature: 1a (ref.2), 1b (ref.9), 1g (ref.10), 1h (ref.2), isopropyl
1
N-hydroxycarbonate 1d: m.p. 26–27 °C. IR spectrum (film): ν (C=O) 1 750 cm–1. H NMR spectrum
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1060
Mindl, Halama, Cernosek:
(CDCl3): 1.25 d, 6 H, J = 7 (2 × CH3); 4.95–5.09 m, 1 H (CH); 7.25 s, 2 H (HO/NH). 13C NMR
spectrum (CDCl3): 21.8 (CH3), 70.2 (CH), 159.3 (COO).
Carbonates 4a–4h, 5ab–5fg as chromatographic standards were prepared by the pyridine
method11. Physical data of other individual esters prepared agree with those given in literature: 4a,
5ae, 5ad, 5be, 5bc, 5bd, 5cd (ref.12), 5ab (ref.11), 5ac (ref.13), 5ag, 5bg, 5cg, 5eg, 5dg (ref.14), 5fg
(ref.15), 5bf (ref.16), 4d (ref.17), 4b, (Aldrich), 4h (ref.18).
Synthesis of Carbonates 5ai–5an
To a solution of hydroxy compound (0.025 mol) in dichloromethane (15 ml) cooled to –10 °C,
freshly prepared lead dioxide (10 g) was added with intense stirring, and then methyl N-hydroxy-
carbamate (1a; 4.4 g) dissolved in dichloromethane (25 ml) was added during 30 min. The reaction
mixture was stirred until it reached room temperature and then for another half an hour until the
evolution of gases stopped. After treating with 5 ml water, the solid was separated by centrifuging.
The solution was shaken twice with 5 ml 5% sodium hydroxide, then with water, dried, and the pro-
duct was isolated by vacuum fraction or crystallization from methanol.
TABLE III
Analytical data for the carbonates ROCOOMe 5ai–5an
Calculated/Found
M.p., °C
Yield, %
Formula
M.w.
No.
R
% C
% H
5ai
5aj
2-oxo-1-benzopyran-4-yl
3-acetylaminophenyl
3-ethoxykarbonylaminophenyl
3-methoxycarbonylaminophenyl
heptyl
98–99 C11H8O5
60.00
59.78
3.66
3.68
45
220.1
115–116 C10H11NO4
57.41
57.20
5.30a
5.32
47
209.2
5ak
5al
86–87 C11H13NO5
42 239.2
55.23
55.37
5.48a
5.41
105–106 C10H11NO5
53.33
53.23
4.93a
5.08
45
225.2
b
5am
5an
C9H18O3
188.1
57.42
57.15
9.57
9.75
34
b
nonyl
C11H22O3
202.1
65.31
65.03
10.89
10.67
39
a
b
% N calculated/found: 5aj 6.70/6.95, 5ak 5.86/6.05, 5al 6.22/6.25. B.p. (°C/268 Pa), n2D0: 5am
69.0–71.0, 1.4155; 5an 85.0–88.0, 1.4242.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Alkoxycarbonylation of Alcohols and Phenols
1061
TABLE IV
Spectroscopic data of carbonates 5ai–5an
IR, cm–1
No.
NMR, ppm
ν(C=O)
5ai
5aj
5ak
5al
1 765
1 702
1H: 3.98 s, 3 H (CH3O); 6.63 s, 1 H (H-3); 7.29 m, 1 H (H-6); 7.31 m, 1 H
4
(H-8); 7.56 dt, 1 H, J = 7.9, J = 1.4 (H-7); 7.69 dd, 1 H, 3J = 7.9,
4J = 1.4 (H-5)
13C: 56.1 (CH3O), 103.2 (C-2), 114.7 (C-5a), 116.7 (C-8), 122 (C-5), 124.3
(C-6), 132.8 (C-7), 150.9 (OCOOCH3), 153.3 (C-8a), 158.1 (C-4),
161.2 (C-2)
1 776
1 681
1H: 2.05 s, 3 H (CH3CONH); 3.87 s, 3 H (CH3OCOO); 6.85 td 1 H,
4
3J = 8.0, J = 2.0 (H-4); 7.18 m, 1 H (H-6); 7.22 m, 1 H (H-5); 7.53 t,
1 H, 4J = 2.0 (H-2); 8.17 bb, 1 H (NH)
13C: 24.2 (CH3CONH), 55.4 (CH3OCOO), 112.8 (C-2), 116.3 (C-4), 117.2
(C-6), 129.5 (C-5), 139.2 (C-3), 151.1 (C-1), 154.4 (OCOOCH3), 168.9
(CH3CONH)
1H: 1.21 t, 3 H (CH3CH2O); 3.81 s, 3 H CH3OCOO); 4.14 q, 2 H, 3J = 7.1
(CH3CH2O); 6.79 m, 1 H (H-4); 7.10 m, 1 H (H-6); 7.17 t, 1 H,
3J = 7.9 (H-5); 7.41 bb, 1 H (H-2); 7.52 bb, 1 H (NH)
13C: 14.0 (CH3CH2O), 55.0 (CH3O), 60.8 (CH3CH2O), 111.1 (C-2), 115.0
(C-4), 115.8 (C-6), 129.1 (C-5), 139.3 (C-3), 151.1 (C-1), 153.4
(NHCOO), 154.0 (CH3OCOO)
1 743
1 620
1 730
1 608
1H: 3.73 s, 3 H (CH3OCONH); 3.87 s, 3 H (CH3OCOO); 6.83 m, 1 H
3
(H-4); 6.97 bb, 1 H (NH); 7.13 m, 1 H (H-6); 7.24 t, 1 H, J = 8.1
(H-5); 7.36 bb, 1 H (H-2)
13C: 52.3 (CH3OCONH), 55.36 (CH3OCOO), 111.5 (C-2), 115.6 (C-4),
115.9 (C-6), 129.6 (C-5), 139.1 (C-3), 151.4 (C-1), 153.8 (NHCOO),
154.1 (OCOO)
5am
5an
1 731
1 735
1H: 0.87 t, 3 H (H-7); 1.20–1.40 m, 8 H (H-8–H-3); 1.65 m, 2 H (H-2);
3.76 s, 3 H (CH3O); 4.12 t, 2 H (H-1)
13C: 13.9 (C-7), 22.5 (C-6), 25.6 (C-5), 28.6 (C-4), 28.8 (C-3), 31.6 (C-2),
54.5 (CH3O), 68.2 (C-1), 155.8 (CH3OCOO)
3
1H: 0.86 t, 3 H, J = 6.7 (H-9); 1.20–1.40 m, 12 H (H-3–H-8); 1.65 m, 2 H
3
(H-2); 3.76 s, 3 H (CH3O); 4.12 t, 2 H, J = 7.9 (H-1)
13C 13.9 (C-9), 22.5 (C-8), 25.6 (C-7), 28.6 (C-6), 29.1 (C-5 + C-4), 29.3
(C-3), 31.7 (C-2), 54.4 (CH3O), 68.1 (C-1), 155.8 (CH3OCOO)
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1062
Mindl, Halama, Cernosek:
Mass Balance Experiments
Procedure A. Benzyl N-hydroxycarbamate (1h; 0.5 g, 3 mmol) solution in dichloromethane (5 ml)
was added to a suspension of lead dioxide (0.8 g) in cyclopentadiene (0.2 g) and dichloromethane (3 ml)
at 0 °C with stirring for 20 min. The mixture was then kept at the room temperature for 30 min. The
solid portion was removed by centrifuging and the solution was analyzed by HPLC.
Procedure B. Lead dioxide (1.2 g, 5 mmol) was mixed with a solution of isopropyl N-hydroxy-
carbamate (1d; 0.6 g) in dichloromethane (10 ml). After 2 min of spontaneous reaction, the evolution
of gaseous oxides ceased, and the sample was transferred into a cell and its ESR spectrum was
measured. Another sample was mixed with methanol and after 10 min it was analyzed by GC-MS
method. The original mixture was also analyzed by chromatography after 10 min. The same experi-
ment was carried out in 1-propanol. The content of dissolved Pb2+ in the reaction mixture determined
by polarography was 0.87 g (4.2 mmol). Having repeated this experiment under nitrogen during 40 min,
we obtained 95 ml gaseous products. Gaseous phase: IR spectrum (νmax, cm–1): 2 349 (CO2), 2 224 (NN),
1 349 (CO2), 1 286 (NO), 667 (CO2). Gaseous products were compared with the authentic dinitrogen
oxide and carbon dioxide by means of GC.
Procedure C. Lead dioxide (1.2 g, 5 mmol) was added to a solution of isopropyl N-hydroxycarba-
mate (1d; 0.5 g, 5 mmol) in methanol (5 ml) and after 3 min a mixture of sodium azide (0.32 g, 5 mmol)
and methanol (5 ml) was added thereto. The reaction mixture was kept at 40 °C with stirring for
another 15 min and then analyzed by HPLC and GC.
Procedure D. In a flask equipped with a reflux condenser, a stirrer, and a gas burette, a solution
of N-hydroxycarbamate (0.025 mol) in alcohol (or aqueous alcohol) (0.25 mol) was added into stirred
suspension of lead dioxide (1.2 g, 5 mmol) in dichloromethane (5 ml) at 40 °C during 10 min. The
mixture was kept at this temperature until the evolution of gases stopped. The solid phase was separated
by centrifuging and the solution was analyzed by chromatography. A similar procedure was used for
the chromatographic comparison at –10 °C.
Procedure E. N-Hydroxycarbamate (2.5 mmol) was dissolved in a solution containing two chosen
alcohols (0.125 mol of each) and lead dioxide (0.6 g) was added thereto in one portion with stirring
at 40 °C. Ten minutes later, an inorganic portion was removed on centrifuge and the solution was
analyzed chromatographically.
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