Investigation of dialkyltin compounds as catalysts for the synthesis of dialkyl carbonates from alkyl carbamates

Article abstract of DOI:10.1016/S0022-328X(97)00787-0

New syntheses for dibutyldimethoxytin, dibutyldiisocyanatotin and 1,1,3,3-tetrabutyl-1,3-diisocyanatodistannoxane as well as the novel compounds dibutylisocyanatomethoxytin and 1,1,3,3-tetrabutyl-1-methoxy-3-isocyanatodistannoxane are described. These com

Full text of DOI:10.1016/S0022-328X(97)00787-0

Journal of Organometallic Chemistry 556 (1998) 41–54
Investigation of dialkyltin compounds as catalysts for the synthesis of
dialkyl carbonates from alkyl carbamates
Elena N. Suciu *, Barbara Kuhlmann, George A. Knudsen, Robert C. Michaelson
Exxon Chemical Company, Route 22 (East), Annandale, NJ 08801, USA
Received 8 August 1997; received in revised form 6 November 1997
Abstract
New syntheses for dibutyldimethoxytin, dibutyldiisocyanatotin and 1,1,3,3-tetrabutyl-1,3-diisocyanatodistannoxane as well as
the novel compounds dibutylisocyanatomethoxytin and 1,1,3,3-tetrabutyl-1-methoxy-3-isocyanatodistannoxane are described.
119
13
These compounds were characterized by elemental analyses as well as Sn, C, and proton NMR, FTIR, MS and HPLC. They
were found to have good catalytic activity for the synthesis of symmetrical dialkyl carbonates from alkyl carbamates and the
corresponding alcohols. Extensive NMR and IR data indicate that tin compounds containing both an alkoxy and an isocyanato
moiety bounded to tin are reaction intermediates in the synthesis of dialkyl carbonate and their formation is a key step in this
catalytic process. The mechanism of carbonate synthesis involves the interconversion of the various tin species present in the
reaction mixture to these reaction intermediates. Dibutyldimethoxytin, dibutylisocyanatomethoxytin and 1,1,3,3-tetrabutyl-1-
methoxy-3-isocyanatodistannoxane have similar activity and are more active (10–14%) than dibutyloxotin for the synthesis of
symmetrical dialkyl carbonates. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Dialkyltin; Dialkyl carbonates; Catalyst
1. Introduction
sized in a two step reaction from urea and an alcohol via
a carbamate intermediate [Eq. (1)] [8,9]
Recent efforts have been directed to the replacement
of toxic chemicals such as phosgene and dimethyl sulfate
with environmentally friendly reagents such as dimethyl
carbonate (DMC). DMC is a versatile chemical reagent
that can replace phosgene in the syntheses of pharmaceu-
ticals, agro chemicals, dyes and polymers [1]. It can also
function as an alkylating agent [1]. DMC has good solvent
power compared to ethyl acetate. It can also be used as
a fuel additive for the enhancement of the octane number
(
1)
The most active catalysts reported in the literature for
the conversion of a carbamate to a carbonate are
organotin derivatives such as dibutyldimethoxytin,
(
Scheme 1) [8,9]. Dibutyldimethoxytin is more effective
as a catalyst for the synthesis of carbonates than dibuty-
loxotin [8], however, dibutyldimethoxytin is also an order
of magnitude more expensive than dibutyloxotin. Hence,
an inexpensive synthetic route to the former material was
developed based on dibutyloxotin, which would also
allow catalyst regeneration to be accomplished in a
cost-effective manner should deactivation occur. An
effort was also made to synthesize other organotin
compounds which, potentially, could have higher cata-
lytic reactivity than those reported in literature.
[
2]. The standard method for preparing organic carbon-
ates involves the treatment of alcohols with phosgene.
New methods for the manufacture of DMC which do not
employ phosgene are based on the oxidative carbonyla-
tion of methanol with copper salts [1,3–6] or alkyl nitrates
and platinum or palladium salts [7] Alternatively,
dimethyl and higher carbonate homologs can be synthe-
*
Corresponding author. Tel.: +1 908 7302568; fax: +1 908
3
03198.
0
022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00787-0

4
2
E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
Scheme 1. Conversion of carbamate to carbonate using the organotin derivative dibutyldimethoxytin.
The synthesis of dibutyldiisocyanatotin, Ib, based on
can be readily synthesized from dibutyloxotin. Based
on the technical literature and our experimental results,
the scheme shown in Fig. 1 summarizes the various
equilibria which exist when dibutyloxotin is treated
with methanol and methylcarbamate.
dibutyldichlorotin has been published [10] Compounds
IId, e, and f have also been described earlier [11–13].
However, organotin compounds which combine both
the methoxy and isocyanato groups in the same
molecule as in dibutylisocyanatomethoxytin, Ia, and
1,1,3,3-tetrabutyl-1-methoxy-3-isocyanatodistannoxane,
IIa, have not been reported in the literature to date.
Dibutyloxotin is a convenient starting material for the
synthesis of dibutyldiisocyanato-tin, Ib, 1,1,3,3-tetra-
butyl-1,3-diisocyanatodistannoxane, IIb and 1,1,3,3-te-
trabutyl-1-methoxy-3-isocyanatodistannoxane, IIa.
Stannoxanes and distannoxanes of type I, II respec-
tively are known to exist as dimers bound via pentaco-
ordinated tin atoms. [11–15]b However, dimers of type
I exhibit a single peak in Sn-NMR corresponding to
two equivalent pentacoordinated tin atoms while the
distannoxanes dimers, of type II exhibit pairs of non-
equivalent pentacoordinated tin atoms (Table 1). The
two Sn(1) atoms in II form an internal four membered
ring structure via bridging by the oxygen linkage,
whereas both Sn(2) atoms coordinate via Y substituents
resulting in external four-membered rings. Thus, the
catalytic activity of these compounds may also be a
function of the state of aggregation of the catalyst
2. Results and discussion
2.1. Synthesis of dibutyldimethoxytin, I
One synthetic route for the preparation of
dibutyldimethoxytin
involves
the
reaction
of
dibutyldichlorotin with sodium methoxide [16]. The
crude dibutyldimethoxytin is subsequently purified by
vacuum distillation. The cost of dibutyldimethoxytin
prepared using the above process is high which
prompted our investigation of alternative synthetic
routes using relatively inexpensive starting materials.
Dibutyltin dialkoxides based on primary alcohols have
been synthesized via a two step reaction. The first step
involves condensation of dibutyloxotin with a primary
alcohol in refluxing benzene or toluene at 80–110°C, to
produce a 1,1,3,3-tetrabutyl-1,3-dimethoxydistannox-
ane with concurrent azeotropic removal of water. In the
second step the distannoxane undergoes disproportion-
ation at 180–220°C under reduced pressure to produce
dibutyldialkoxytin and dibutyloxotin [17] [Eq. (2)]:
1
19
(
Scheme 1).
The objective of this study was to evaluate the cata-
lytic properties of various dialkyltin compounds which
(2)

E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
43
Table 1
Spectra data
Owing to the low boiling point of methanol the above
procedure is not effective for the synthesis of either
dibutyldimethoxytin or the corresponding distannox-
ane. The reaction of dibutyloxotin with dimethyl car-
bonate (in the presence of traces of methanol) in
refluxing toluene produces 1,1,3,3-tetrabutyl-1,3-
dimethoxydistannoxane, IIg, exclusively, as described
by Davies [17]. We have discovered that
dibutyldimethoxytin can be synthesized by reacting
dibutyloxotin with methanol and dimethyl carbonate at
elevated temperatures and pressures [18] (Scheme 2).
Alternatively, one can use 1,1,3,3-tetrabutyl-1,3-

4
4
E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
Fig. 1. Interconversion of tin compounds.
dimethoxydistannoxane as a starting material using the
same process conditions.
This synthetic method is general for primary and
secondary alcohols [18]. It is interesting that the car-
bonates appear to be unique as water scavengers in this
reaction. Substitution of the carbonates with other po-
tential water scavengers such as the ortho ester of
n-butyric acid results only in the formation of distan-
noxane derivatives [18].
Based on these observations we assume that the
mechanism of reaction includes the following steps as
depicted in Scheme 2: addition of alcohol to dialkylox-
otin to give Id which is in equilibrium with IIh, fol-
lowed by nucleophilic attack of Sn–OH on the
carbonyl carbon of the carbonate to yield alkoxytin
carbonate, Ie, which undergoes nucleophilic displace-
ment on the tin atom by the alcohol present in the
system to yield I and carbon dioxide.
This reaction requires the addition of a catalytic
amount of alcohol since in the second step, the alcohol
necessary to continue the reaction is generated via the
decomposition of the carbonate. Carbonate esters of
dialkyl and trialkyl tin, eq. n-Bu Sn(OCO Me) [19],
2
2
2
n-Bu Sn(OCO Et) [17], have been reported in the litera-
3
2
ture. Thus, the reaction sequence shown in Scheme 2
appears to be consistent with our experimental results.
2
.2. Synthesis of dibutylisocyanatomethoxytin, Ia
The reaction of dibutyldimethoxytin, I, with methyl
carbamate in refluxing toluene (110°C) yields
dibutylisocyanatomethoxytin, Ia, as an oil, after re-
moval of the solvent. Recrystallization from hexane
affords a white powder, Ia, with m.p. 103–105°C. The
dimeric structure of Ia was assigned based on the
following physical data (Table 1).
Scheme 2. Mechanism for the production of I.

E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
45
−
1
NMR), the appearance of Sn–OH band at 3545 cm
(
IR) as well as the appearance of the distannoxane
119
dimer peaks in Sn-NMR at l −171, l −211 ppm
and broadening of the peak at l −180 ppm. Similar
chemistry has been reported for other tin derivatives
such as Ib [10].
2
.2.3. Reaction of dibutylisocyanatomethoxytin, Ia with
benzyl alcohol
The reaction of Ia with benzyl alcohol at 175°C and
atmospheric pressure yields benzyl carbamate, methyl
benzyl carbonate, dibenzyl carbonate, dibutyldiben-
zoxytin, ammonia, and methanol [Eq. (4)]. These prod-
ucts were detected by C-NMR, IR, GC and MS.
This experiment supports the hypothesis that
dibutylisocyanatoalkoxytin compounds are plausible re-
action intermediates in the formation of carbonates by
the reaction of dibutyldialkoxytin with carbamates and
alcohols.
−
1
The Me–O bands at 1080 and 1037 cm
and two
−
1
bands for NꢀCꢀO at 2206 and 2178 cm are indicative
of the covalent and coordinate bonds of these groups,
each of the lower frequencies corresponds to the respec-
tive coordinate bond as described for similar com-
pounds [20,21] (Table 1) The C-NMR spectrum
displays a single resonance for OMe (l 51.44) and
NꢀCꢀO (l 128.10) indicating an averaged environment.
A single resonance at l −180, ca. 20 ppm upfield of I,
13
1
3
1
19
is also observed using
Sn-NMR, which indicates
2
.3. Synthesis of dibutyldiisocyanatotin, Ib
pentacoordination of the tin atom [15]b. The facile
formation of Ia by the reaction of I with methyl
carbamate in the absence of a solvent (molar ratio 1:1)
was monitored by Sn- and C-NMR. A 30% conver-
sion of I to Ia was achieved at room temperature (r.t.).
The conversion increased to 60% within minutes when
heated to 65°C.
Dibutyldimethoxytin, I, and excess methyl carbamate
(
mole ratio 1:2) in refluxing toluene (110°C) yields a
1
19
13
mixture of compounds which include Ia and Ib as
119
indicated by
Sn-NMR. However, in the absence of
solvent at 175°C and atmospheric pressure, only
dibutyldiisocyanatotin, Ib, was obtained after removal
of MeOH and DMC (Fig. 1). The MS fragmentation
via CI/CH and CI/NH displayed, among others, the
2
.2.1. Reaction of dibutylisocyanatomethoxytin, Ia with
4
3
methanol (Fig. 1)
parent peaks M+H (317) and M+NH (336), respec-
4
The formation of Ia is a reversible reaction. With an
excess methanol at higher temperature (175°C) the pres-
ence of methyl carbamate and dibutyldimethoxytin, I
was detected.
tively.
Although compound Ib exhibits two bands for
−1
NꢀCꢀO at 2209 and 2178 cm , the observation of
119
only a single peak at a high field frequency
Sn l
−
71.6 (Table 1) indicates that the compound exists as
2
.2.2. Reaction of dibutylisocyanatomethoxytin, Ia with
water
The presence of traces of water in the system results
a monomer containing only tetra coordinated tin atoms
[15]b. This structure was also favored by Leung and
Herber based on M o¨ ssbauer studies [21]. It is interest-
ing that the corresponding dibutyldiisothiocyanatotin,
Ic, exhibits two NꢀCꢀS IR bands at 2064 and 2010
in the formation of distannoxanes [Eq. (3)]. This was
evidenced by a decrease in melting point of Ia (65–
8
13
−1
119
0°C), the loss of the methoxy group (IR and C-
cm
and a broad
Sn peak at l −128. This is
(
(
3)
4)

4
6
E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
indicative of a polymeric structure, comprised of penta-
coordinated tin nuclei and is consistent with earlier
M o¨ ssbauer studies [22]. The fact, that Ic exists as a
dimer while Ib is a monomer, is probably due to
electronic differences between the NꢀCꢀO and the
tetrabutyl-1,3-diisocyanatodistannoxane,
IIb,
and
1,1,3,3-tetrabutyl-1-hydroxy-3-isocyanatodistannoxane,
119
IIc. The structures are supported by Sn-NMR with
resonances of diisocyanato derivative IIb at l −148
and −169 (broad) and those of the hydroxyisocyanato
IIc, at l −162 and −212, broad. It is interesting that
1
19
NꢀCꢀS ligands [23]. The
Sn-NMR data for com-
119
pounds Ib and Ic are recorded here for the first time.
these
analogous isothiocyanato compounds IIf and IIe.
Table 1). The elemental analysis for the dimer IIb–IIc
Sn characteristics are similar to those of the
2.4. Synthesis of 1,1,3,3-tetrabutyl-1-methoxy-3-iso-
(
cyanatodistannoxane, IIa from IIg and methyl
carbamate
is identical to that reported by Mufti [10]. Mufti inter-
preted the structure as the dimer IIc–IIc by omitting
the nitrogen component. We propose that the com-
pound has the IIb–IIc structure (Fig. 1) which is sup-
1,1,3,3-Tetrabutyl-1,3-dimethoxydistannoxane, IIg,
was synthesized according to Davies [17]. The crude
product was treated with a stoichiometric amount of
methyl carbamate in toluene at 110°C to give 1,1,3,3-
13
ported by the spectral evidence below. The C-NMR
spectra exhibits NꢀCꢀO peaks at l 130.68 and 129.96
and the IR reveals the two characteristic strong NꢀCꢀO
tetrabutyl-1-methoxy-3-isocyanatodistannoxane,
IIa,
bands corresponding to the covalent and bridged bonds
(
Fig. 1). The product IIa is a white powder with m.p.
−1
at 2209 and 2190 cm
.
124–128°C. It exhibits spectral data similar to those
The reaction of dimers IIb–IIc with MeOH at 175°C
at the autogenic pressure of the system afforded methyl
carbamate, urea, and I. The formation of these com-
pounds were accompanied by the disappearance of the
NꢀCꢀO IR bands.
described for isothiocyanato IId [13] (Table 1). Two
peaks, characteristic of a distannoxane dimer, appear at
1
19
Sn l −166 and −206. Coordination of the
monomers occurs via the methoxy oxygen bond, not
the NꢀCꢀX (X=O, S) moiety. This structure is indi-
cated by O–Me IR bands at 1060 (covalent) and 1020
2
.5.1. E6aluation of dialkyltin catalysts for the
−
1
cm
(coordinated) but only a single NꢀCꢀO band in
synthesis of bis-(2-ethylhexyl)-carbonate
−1
the region 2204–2050 cm . Again, as for compound
The dialkyltin compounds described above were eval-
uated as catalysts for the synthesis of bis-(2-ethylhexyl)-
carbonate from methyl carbamate and 2-ethyl-1-hexyl
alcohol [Eq. (1)]. This reaction was selected because
activity data for dibutyldimethoxytin and dibutyloxotin
as catalysts have been published [8]. The use of a higher
molecular weight alcohol than methanol allows evalua-
tion of catalytic activity at elevated temperatures and
ambient pressure. The first step of the reaction as
indicated in Eq. (1) consists of the transesterification of
methyl carbamate with 2-ethyl-1-hexyl alcohol to give
the corresponding 2-ethyl-1-hexyl carbamate accompa-
nied by the formation of methanol which is removed by
distillation. Transesterification occurs during the heat-
ing period (20 min) required to raise the reaction tem-
perature from r.t. to 190°C. The formation of the
carbonate by alcoholysis of the amide group was moni-
tored at 190°C by the evolution of ammonia as well as
by an analysis of the reaction mixture using GC and
13
Ia, only one OMe peak was detected by C-NMR
which appears at l 50.64. The elemental analyses ob-
tained for this compound are consistent with the pro-
1
19
posed structure for IIa. The
Sn-NMR spectrum
reveals two small peaks at l −148 and −169 corre-
sponding to the diisocyanato IIb. This is paralleled in
1
3
C-NMR by the appearance of two NꢀCꢀO peaks at l
28.80 and 128.0 along with two minor peaks at l
37.70 and 126.16.
1
1
The reaction of IIg, with excess methyl carbamate at
10°C surprisingly gave the same result, with formation
1
of IIa and only a small amount of the diisocyanato IIb.
2.4.1. Reaction of IIa with benzyl alcohol
Compound IIa reacts with benzyl alcohol at r.t.
Heating to 175°C for 4 h, results in the nearly complete
disappearance of the NꢀCꢀO band, and the formation
of a mixture of carbamates and carbonates as depicted
in Eq. (4). This experiment supports the hypotheses
that isocyanato alkoxy tin compounds are plausible
reaction intermediates in the formation of carbonates.
13
C-NMR. The data are summarized in Table 2.
The isothiocyanatotin compounds IId and IIe, which
are transesterification catalysts [12,13] performed
poorly because of decomposition at 190°C. Although
the difference in activity between the other catalysts
was not large, conversion of 92% for I, and IIa vs. 80%,
2
.5. Attempt to synthesize 1,1,3,3-tetrabutyl-1,3-
diisocyanatodistannoxane, IIb from dibutyloxotin and
methyl carbamate
for IIg, and n-Bu SnO, these differences are real and
2
The reaction of dibutyloxotin with methyl carbamate
in toluene yields a white powder with m.p. 175–177°C
they are based on a number of replicate runs.
Dibutyldimethoxytin, I, has higher activity then dibuty-
loxotin, which agrees with previous literature data [8].
(
Fig. 1). The material consists of a dimer of 1,1,3,3-

E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
47
Table 2
2
.6. Catalytic acti6ity of I 6ersus IIg
Catalyst activity for the conversion of methyl carbamate to bis-(2-
ethylhexyl)-carbonate
In order to determine the difference in activity be-
tween I and IIg the reaction was repeated at a higher
catalyst concentration (25×) which enabled us to mon-
119
13
itor the tin catalyst species by
Sn-, C-NMR and
FTIR. The key observations follow.
2
.6.1. Increasing catalyst le6els has a relati6ely small
effect on reaction rate
The initial rates (0–1.5 h) for I and IIg at high level
catalyst (0.5 mole catalyst/mole methyl carbamate) are
about four times higher then those at low level catalyst
(
0.02 mole catalyst/mole methyl carbamate) (Fig. 2).
Thus a twenty-five fold increase in catalyst concentra-
tion produced a four-fold increase in rate. At both
catalyst levels I is more active than IIg.
At low catalyst concentration conversions of 80–90%
were achieved in 12 h for IIg and I respectively, while at
high catalyst concentration the same conversion (87%)
was achieved in 6 h for both catalysts (Tables 2 and 3).
2
.6.2. Spectroscopic data indicate that the Sn–OR
band of I undergoes a rapid con6ersion to Sn–NꢀCꢀO
with methyl carbamate at r.t. while IIg does not
Samples of the reaction mixture were taken after
methyl carbamate had dissolved (65°C). The NMR
spectra of these samples exhibit a broadening of the
119
Sn peaks corresponding to I and IIg indicative of the
exchange between Sn–OMe and C H OH to give Sn–
8
17
OC H and methanol. The most significant difference
8
17
between I and IIg is that I undergoes a rapid exchange
of the Sn–OR with methyl carbamate to form Sn–
NꢀCꢀO at low reaction temperature (65°C) while IIg
requires a longer time and, or a higher reaction temper-
ature. This exchange reaction was confirmed by the
appearance of an isocyanato band (FTIR) at 2211
−
1
cm . The experiment containing the isocyanato tin
species, n-Bu Sn(NꢀCꢀO)(OC H ), produced 8% con-
2
8
17
version to carbonate during the heating time to 190°C.
The experiment utilizing catalyst IIg gave 0% conver-
sion during the same heating time (Fig. 2, Table 3).
Hence the isocyanatotin has a higher catalytic activity
than IIg. The relative rate of conversion of distannox-
ane IIg to IIa is slow versus the relative rate of forma-
tion of Ia from I. (Fig. 1). The slower conversion of IIg
to an active Sn–N–CꢀO species is probably due to
steric effects. It is interesting that distannoxane IIa
which contains both methoxy and isocyanato moieties
exhibits the same catalytic activity as I (Table 2), which
supports the hypothesis that an isocyanatotin com-
pound is an important intermediate in this synthetic
route to carbonates. The relative rate of conversion of
IIa to Ia is also slow, however IIa already contains the
isocyanato moiety in the molecule and hence has high
catalytic activity.
*
% Conversion calculated based on ammonia released, selectivity in
all cases was 100%.
Reaction Conditions
Methyl carbamate
2
mmole
240
730
mole%
24.60
74.80
0.560
-Ethyl-1-hexyl alcohol
Catalyst
5meq
Temperature 190–195°C; Time 12 hr.

4
8
E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
Fig. 2. High levels of catalyst have a small effect on rate at 190°C.
1
3
2
.6.3. Reaction of I or IIg with methyl carbamate
2.6.3.3. Analysis of the reaction mixture by C-NMR
119
proceeed through a common intermediate
and correlation with Sn-NMR data. Samples taken at
different reaction times from the reaction mixture were
13
analyzed also by C-NMR. The transesterification re-
action was monitored by the disappearance of the peak
2
.6.3.1. Distribution of tin species. At 190°C, both ex-
119
periments exhibited identical Sn-NMR spectra indi-
cating that the heating period (20 min) provides
sufficient time for the initially dissimilar reaction mix-
tures to equilibrate. The tin compounds detected were
Bu Sn (OC H ) , l −80 as a monomer, main peak
at l 157.5 (NH COOMe), and the peak at l 50.34
2
(NH COOMe) as well as the appearance of the peak at
2
l 157.4 (NH COOC8H17) and of the peak at l 65.58
2
(NH COOC8H17). The reaction of the tin species with
2
the carbamate to give the isocyanato tin moiety Sn–
2
8
17 2
(
67%), Bu Sn (OC H )(NꢀCꢀO) l −180, (~16%) as
NꢀCꢀO was evidenced by the peak at l 127. The
2
8
17
well as unidentified species at l −50 (~16%) (Scheme
). Speculations about the identity of the unknown
compounds are discussed below.
formation
of
bis-(2-ethylhexyl)-carbonate,
O=
3
C(OC H ) was followed by the intensity of the car-
8 17 2
bonyl resonance at l 154.4, and the alkoxy group at l
8.48 (see carbonyl data in Table 3).
6
13
119
Parallel analyses by C- and Sn-NMR show that
2
.6.3.2. The concentrations of n-Bu Sn (OC H )
the concentration of Sn–NꢀCꢀO and the unidentified
tin species decreases with decreasing concentration of
methyl carbamate.
2
8
17
(NꢀCꢀO) and the unknown species are the same and
depend on the concentration of methyl carbamate in the
reaction mixture. At the start of the reaction when the
concentration of carbamate was high (GC analysis) the
2.6.3.4. Additional support for the formation of n-Bu Sn
2
ratio
was
n-Bu Sn(OC H ) :n-Bu Sn(OC H )(NꢀCꢀO)
(OC H )(NꢀCꢀO) species obtained from FTIR data.
2
8
17 2
2
8
17
8
17
4:1
as
was
the
ratio
of
n-
The reaction was also monitored by FTIR. Strong
−
1
Bu Sn(OC H ) :unknowns (l −50). After 3 h, both
bands appear at 2210 cm
corresponding to Sn–
2
8
17 2
ratios were 10:1, corresponding to a decrease in the
concentration of carbamate due to its conversion to
carbonate (\70%; Fig. 2, Table 3). These data suggest
that the unidentified tin species are very likely in equi-
librium with the dialkoxytin compound and the alkoxy
isocyanato tin species.
NꢀCꢀO as well as the bands at 1710, 1746 and 3358
−
1
cm
corresponding to the carbonyl in carbamate,
carbonate and the hydroxyl of the alcohol repectively.
The decrease in Sn–NꢀCꢀO and alcohol concentration
parallels the conversion of carbamate to carbonate as
evidenced by IR.

E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
49
Table 3
1
3C NMRa data for the conversion of methyl carbamateb to 2-ethylhexyl carbonate at a high level of catalyst
Catalyst type
Temp (°C) Time (h)
NH COOMe l
NH COOC H
157.4 peak area
l
OꢀC(OC H
)
17 2
l 154.4
Conversion of
NH COOC H
2
2
8
17
8
157.5 peak area
peak area
%
17
2
8
I
IIg
I
IIg
I
IIg
I
IIg
I
IIg
65
190
190
190
190
—
—
0
5.8
3.7
0
0
0
0
0
0
0
0
0
0
0
0.4
0
2.9
2.0
3.4
2.4
3.5
2.5
0
0
8
4.7
4.1
2.2
1.8
1.1
1.0
0.5
0.4
0
1
3
6
57
53
77
71
87
86
0
a
External reference Me Sn 0.0 ppm.
Conversion calculated based on fraction of total peak area NH COOC H /OꢀC(OOC H ) .
4
b
2
8
17
8
17 2
2
.6.3.5. Spectroscopic data are consistent with the struc-
plained by the relative rate of their conversion to tin
compounds containing both alkoxy and carbamato or
isocyanato moieties. Evidence for the formation of
ture of tin carbamato species of type n-Bu Sn(NH–
COOC H )(OC H ). A carbamato species is the most
2
8
17
8
17
119
likely precursor of an isocyanato compound since iso-
cyanic acid is not present in the reaction mixture. The
tin atom in dibutylcarbamatoalkoxytin is hindered by
the 2-ethyl-hexyl group which inhibits dimerization.
The resonance signal for the monomeric tin species is
expected to appear at higher field frequency (l\ −
Sn–NꢀCꢀO species was provided by
Sn-NMR (l
13
−1
-180), C-NMR (l 127), and IR (2207–2210 cm ).
These characteristics are in agreement with those ob-
tained for compound Ia which was independently
synthesized (Table 1).
The formation of a dialkyl carbonate from an alkyl
carbamate is postulated to include the following steps:
Step 1, the reaction of dibutyldimethoxytin, I, with
alkyl carbamate yielding dibutylalkoxycarbamatotin,
1, in equilibrium with dibutylalkoxyisocyanatotin, Ia.
Step 2, an intramolecular or intermolecular nucle-
ophilic attack of the alcohol on the carbonyl carbon
of the carbamatotin species, resulting in elimination
of the carbonate and formation of an aminotin inter-
mediate 2. Finally step 3, which involves the reaction
of the carbamate with 2, to regenerate 1 with con-
comitant elimination of ammonia (Scheme 4).
Support for an intramolecular transfer of an alkoxy
group from the tin atom to the carbonyl carbon with
concomitant formation of a dialkyl carbonate can be
obtained from the work of Shizuyoshi [26] who showed
that carbonates can also be synthesized by insertion of
a carbon oxysulfide into a tin alkoxy bond followed by
an intramolecular rearrangement [Eq. (5)].
1
19
100) [24]. The peaks which appear in Sn-NMR at l
−
50 are consistent with this assumption. Although a
dibutylalkoxy-carbamatotin of this type has not been
reported in literature alkoxy N-substituted carbama-
totins have been reported [19] n-Bu Sn–NHCOOBu
was claimed by Gerega [25] resulting from the reaction
of the corresponding isocyanate and butanol at 135°C
in a sealed tube. He and his coworkers reported that
the compound readily dissociates into starting materials
when distillation is attempted. The evidence which sup-
ports the presence of a carbamato compound as a
reaction product are the IR bands observed at 3330 and
3
−
1
−1
1
615 cm
for NH, and the band at 1720 cm
which
corresponds to the carbonyl frequency. However the
infrared spectrum is also consistent with a mixture of
Bu SnOBu and NH COOBu which are compounds
likely to have been formed at the reaction conditions
reported by Gerega. Our attempt to synthesize the tin
carbamato
3
2
compound
n-Bu Sn(NHCOOC H )
2
8
17
However, the formation of carbonates in our system
could also occur by the intermolecular attack of an
alcohol molecule present in the reaction medium with
the postulated tin carbamato species.
(
OC H ), from NH COOC H and n-Bu Sn(OC H )
8
17
2
8
17
2
8
17 2
failed to produce compelling evidence for the formation
of this compound.
3. Proposed reaction mechanism
4. Conclusions
The evidence presented indicates that the difference
in reactivity between various tin catalysts may be ex-
Tin catalysts which have the highest activity for the
synthesis of dialkyl carbonates by the reaction of alkyl

5
0
E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
Scheme 3. 119Sn-NMR spectra of compounds I and IIg at 190°C.
carbamates with alcohols are stannoxane and distan-
noxane which contain both methoxy and isocyanato
groups bound to tin.
Observed differences in the catalytic activity of
alkoxy stannoxanes and distannoxanes are correlated
with the relative rates of their conversion to the above
derivatives at reaction conditions.
The alkoxy isocyanato stannoxanes and distannox-
anes can be readily synthesized from alcohol, alkyl
carbamate, and dibutyloxotin.
5. Experimental details
The NMR spectra were recorded on a Varian XL-
300 and Jeol-400. GC analyses were performed on
HP-5880 FID instrument using a capillary column
megabore DB5 (L=30 m, Film thickness=5 mm, D=
0.53 mm). HPLC analyses were performed on a HP-
1040, high performance, liquid chromatograph, using a
Supelco LC-18 column D=25 cm/2.1 mm, with mobile
−
1
phase methanol (isocratic), Flow: 0.200 ml min . pro-
(5)

E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
51
Scheme 4. Proposed reaction mechanism for the formation of a dialkyl carbonate from an alkyl carbamate.
vided with Diode Array detector: 240 nm, BW 4 nm,
Ref 450 nm, BW 80 nm, peak width 0.054 min. Column
temperature=40°C. A Mattson Polaris was employed
for IR studies. Melting points were determined on an
Electrothermal melting point apparatus and are uncor-
rected. Elemental analyses were performed by Gal-
braith (Sycamore, Knoxville). Mass spectra were
performed on Finnigan 4500 or TSQB quadrupole in-
struments using direct insertion probe chemical ioniza-
tion mass spectrometric analyses with either ammonia
or i-butane as CI reagent gas for tin compounds or
GC/MS on b.p. columns for regular organic mixtures
analyzed by EI or CI techniques.
with nitrogen. The reactor was heated at 179°C at a
pressure of ~3890 kPa for 4 h while sparging with
−
1
nitrogen at a flow rate of 200 ml min . The reactor
was cooled and depressurized. The product (170 g)
collected under nitrogen as a clear colorless solution
was stripped of excess methanol and dimethyl carbon-
1
19
ate. Analysis: Sn l −160 (external ref Me Sn at 0.00
4
13
ppm) C l 13.80 (CH ), l 19.29 (CH –Sn), l 27.25
3
2
(CH –CH –CH –Sn), l 27.81(CH –CH –Sn), l 51.91
2
2
2
2
2
(OMe). (ref at 0.1 ppm TMS) HPLC/retention time 3.14
min.
5.2. Synthesis of dibutyldimethoxytin, I from
dibutyloxotin
5.1. Synthesis of dibutyldimethoxytin, I
Dibutyloxotin (50.5 g, 203 mmol), methanol (100.1 g
3
130 mmol) and dimethyl carbonate (39.40 g, 438 mmol)
®
Crude 1,1,3,3-tetrabutyl-1,3-dimethoxydistannoxane,
were charged to a 300 ml Hastelloy C autoclave and
sparged with nitrogen. The reactor pressure was set at ~
3500 kPa by means of a back pressure regulator. The
nitrogen which exited the reactor was passed through a
trap containing drierite and then through a trap contain-
ing a weighed amount of ascarite. The gas exiting the
IIg, (36.2 g, 66.5 mmol) was synthesized as described by
®
Davies [17], and charged to a 300 ml Hastelloy
autoclave. After evaporation of solvent, methanol
101.9 g, 3184 mmol) and dimethyl carbonate (42.0 g,
66 mmol) were charged to the reactor and sparged
C
(
4

5
2
E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
ascarite trap was sent to a bubbler containing mineral oil
for visual evidence of gas flow. The reactor was heated
at 177°C. After 2 h the vessel containing ascarite became
hot to the touch as a result of carbon dioxide absorption.
After 1 h the ascarite trap began to cool. The reactor was
maintained at 177°C for an additional 1.5 h and then
trum: l 159.58 (NH –COOMe), 51.59 (OMe) and l
2
13.35, l 20.35, l 27.22, l 27.66, as assigned for I (Bu Sn)
2
(ref at 128 ppm, benzene).
5
.4.2. Reaction of dibutylisocyanatomethoxytin, Ia with
water
Compound Ia, when exposed to air, underwent a
119
13
cooled and depressurized. Analysis:
Sn- and C-
NMR as assigned for I. The ascarite trap showed a gain
of 8.87 g (200 mmol CO2).
decrease in the melting point from 103°C to 65–80°C
and a decrease in the intensity of the OMe IR band.
119
Analysis: Sn-NMR: l −180 (br), l −171, l −211,
5.3. Synthesis of dibutylisocyanatomethoxytin, Ia in
13
(
ext ref Me Sn at 0.00 ppm), C-NMR: l 128.0
4
Refluxing Toluene
(
NꢀCꢀO) no peak at l 50.44 for OMe. (ref at 77.00
−
1
ppm chloroform) and IR (nujol): 3540 cm
3
(Sn–OH),
To a solution of dibutyldimethoxytin (7.40 g, 25.10
mmol), in toluene (100 ml), methyl carbamate (1.90 g,
−
1
−1
425 cm
(bonded OH), 2207, 2178 cm
(NꢀCꢀO).
25.30 mmol) was added and the solution was refluxed
under nitrogen to remove methanol, ca. 1 h. The solu-
tion was concentrated in vacuum to dryness to give 7 g,
5
.4.3. Reaction of dibutylisocyanatomethoxytin, Ia with
benzyl alcohol
91.00% yield of a pale yellow oily material which
A solution of dibutylisocyanatomethoxytin Ia (42 g,
crystallized on standing.
1
.37 mmol) in benzyl alcohol (2.00 g, 18.50 mmol) was
1
19
Analysis: Sn (C D ): l −180 (external ref Me Sn
®
6
6
4
heated under nitrogen at 175°C for 6 h in a Swagelock
autoclave [27]. Analysis: C-NMR: l 157.18 (NH2–
COOBz), l 156.30 OꢀC (OMe) (OBz), l 154.58 OꢀC
1
3
at 0.00 ppm). C, solv CDCl : l 13.86 (CH ) l 22.50
13
3
3
(
(
CH –Sn), l 26.93 (CH –CH –CH –Sn), l 27.59
2 2 2 2
CH –CH –Sn) l 51.44 (OMe), l 128.10 (NꢀCꢀO), (ref
−1
2
2
(
(
OBz) . (ref 77.20 ppm chloroform). IR: 1711 cm
CꢀO). GC/MS: EI: methyl benzyl carbonate (166),
2
1
at 77.0 ppm, CDCl ). H: l 0.8 (CH ), l 1.1 (CH –Sn),
3
3
2
l 1.2 (CH –CH –CH –Sn), l 1.5 (CH –CH –Sn), l
2
2
2
2
2
benzyl carbamate (151), dibenzyl carbonate (242).
3
.2 (OCH ); CH /OCH =2/1 (ref at 77.2 ppm, CDCl ).
3 3 3 3
−
1
−1
IR (nujol): 2206 cm , 2178 cm (vs), (NꢀCꢀO), 1036
cm (s) (OCH ), 683cm (m) (CH Sn), 619 cm (m)
−
1
−1
−1
5.5. Synthesis of dibutyldiisocyanatotin, Ib
3
2
−
1
(
Sn–O) and 503 cm
(m) (Sn–C).
Recrystallization from hexane gave a white solid with
m.p. 103–105°C. Elemental analysis: MW=306. Anal.
A solution of methyl carbamate (6.10 g, 81.33 mmol)
in xylene (62 g), was refluxed in a 200 cc flask equipped
with a Dean Stark apparatus, and condenser, at 145°C
to remove traces of water. After half of the xylene was
removed, the condenser was connected to a H SO (1N)
Calc. for C H N O Sn : C, 39.26, H 6.92, N 4.58, O
1
0
21
1
2
1
1
3
0.46, Sn 38.79%. Found: C 38.55, H 6.58, N 4.31, Sn
8.99%. MS: DIP/IC4. Mass/R : 308 (5) M+H; 276
int
2
4
(
100), 291 (40), 276 (80), 235 (5), 119 (100).
trap for ammonia detection. Neat dibutyldimethoxytin,
I, (11.70 g, 39.66 mmol), was added, and the solution
was heated (silicon bath) at 175°C for 0.5 h. After
cooling to r.t. the volatiles MeOH, dimethyl carbonate,
xylene, and small amounts of methyl carbamate col-
lected in the Dean Stark apparatus were analyzed by
GC.
5
.4. Preparation of dibutylisocyanatomethoxytin, Ia at
ambient temperature
A reaction mixture of dibutyldimethoxytin, I, (2.95 g,
1
0 mmol), and methyl carbamate (0.75 g, 10 mmol) was
119
analyzed immediately following mixing by Sn-NMR.
ext. ref Sn(Me)4 at 0.0 ppm). At r.t. the solution
exhibited two peaks corresponding to I and Ia at l
160 and l −180 respectively. The yield of Ia varied
with temperature as follows: at 25°C, 30%, at 65°C, 60%.
No ammonia was detected in the acid trap. After
concentration of the reaction mixture, and drying at
r.t., P=0.1 mm, 15 g of a semi solid white ppt were
obtained. The major component of the mixture was Ib.
(
−
1
19
Analysis: Sn-NMR (CDCl ): l −71.50 (external ref
3
13
13
The carbonyl group of Sn–NꢀCꢀO resonated at C l
1
Me Sn at 0.0 ppm), C-NMR: l 13.49 (CH ) l 22.56
4
3
28.0.
(CH –Sn), l 26.39 (CH –CH –CH –Sn), l 26.72
2 2 2 2
(
CH CH –Sn) l 128.30 (NꢀCꢀO), (ref at 77.13 ppm,
−1 −1
2
2
5.4.1. Reaction of dibutylisocyanatomethoxytin, Ia with
CDCl ). IR (nujol): 2209 cm , 2178 cm , NꢀCꢀO,
−1 −1
3
methanol
691 cm
(m) (CH Sn), 619 cm
(m) (SnC). MS
2
A solution prepared from Ia (0.280 g, 90 mmol) in
DIP/IC4: Mass/Rint %: 317 (20) M+H, 291(40), 276
(100), 249 (5), 233(5), 177(5), 119(100). MS+/CI–
NH /Q1: 336(100) M+NH , 308(50), 293(70), 266(5),
MeOH (1.40 g, 43.75 mmol) was heated in a 10 ml
®
Swagelok autoclave [27] at 175°C for 1 h. The solution,
3
4
1
3
analyzed by C-NMR, exhibited the following spec-
252(10), 235(5).

E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
53
5
.6. Synthesis of 1,1,3,3-tetrabutyl-1-methoxy-3-
carbonyl groups of the carbamates and carbonates.
13
isocyanatodistannoxane, IIa
Analysis: C-NMR: l 157.18 (NH COOBz), l 156.30
2
(
OꢀC(OMe) (OBz), l 154.58 OꢀC(OBz) . The peak
2
1,1,3,3 - Tetrabutyl - 1,3 - dimethoxydistannoxane IIg
corresponding to –OMe at l 50.74 nearly disappeared
(ref 77.00 ppm chloroform). IR: 2206 cm
1711 cm
−
1
was synthesized from dibutyloxotin (50 g, 200 mmol),
dimethyl carbonate (18.25 g, 203 mmol), methanol
(NꢀCꢀO),
−
1
(CꢀO), MS.
(
0.63 g, 19 mmol) and toluene (240 ml), as described by
Davies [17]. The reaction was performed in a 500 ml
flask, equipped with a magnetic stirrer, temperature
control and a condenser which was connected through
a cold trap to a wet test meter. At reflux dibutyloxotin
5
.7. Attempt to synthesize 1,1,3,3-tetrabutyl-1,3-
diisocyanatodistannoxane IIb from dibutyloxotin and
methyl carbamate
dissolved within 1 h. The evolution of CO was moni-
tored via the wet test meter; reflux was continued until
2
A solution of dibutyloxotin (7.46 g, 30 mmol) and
methyl carbamate (2.25 g, 30 mmol) in toluene (100 ml)
was refluxed for 5 h. under nitrogen. After concentra-
tion in vacuum to dryness, a semisolid white precipitate
gas evolution ceased, (100 mmol CO were generated),
2
119
total reaction time approx 3 h. Analyses: Sn-NMR,
solvent toluene: l −171, l −183, solvent CDCl : l
3
(
7.50 g, 89% yield) was obtained. A portion of the
−
169, l −176 (ext. ref Me Sn at 0.00 ppm) IR (neat):
4
material, dried on a porous plate under nitrogen and
washed with hexane, exhibited m.p. 175–177°C. Analy-
1
5
067, 1020, (OMe), HPLC: one peak retention time
.26 min. After replacing the condenser with a Dewar
119
sis: Sn-NMR, solv. C D : l −147, l −169, (IIb), l
6
6
column length 18 cm provided with a distillation head,
which was connected to a 250 ml flask, a hot solution
of methyl carbamate (7.50 g, 100 mmol) in toluene (20
ml) was added dropwise over a period of 0.5 h under
reflux. During this time a mixture of methanol,
dimethyl carbonate, and toluene was collected in the
receiving flask until the temperature at the distillation
head reached 110°C. The clear solution in the reaction
pot was concentrated at 50–60°C, ~39 kPa, followed by
evaporation at 1 mmHg. 50 g of IIa were obtained
−
162, l −212, (IIc) (external ref Me Sn at 0.0 ppm).
4
13
C-NMR, solv C D : l 129.00 and 130.68 (NꢀCꢀO),
IR (nujol): 3529 cm
6
6
−
1
−1
(w) (OH), 2209, 2190 cm
−
1
(
NꢀCꢀO covalent and bridged), 677 cm
(m) (CH2–
−
1
Sn), 616, 562 cm
(SnO and Sn–C). The material
analysis for a dimer consisting of IIb and IIc 1/1,
MW=1107. Anal. Calc. for C35 H73 N O Sn : C:
3
6
4
3
3
7.99, H: 6.59, N: 3.79, O: 8.67, Sn: 42.99%. Found: C:
8.22, H: 6.70, N: 3.57, O: 6.83, Sn: 44.90%.
(
91% yield) as a semisolid white material. The same
material was obtained in a separate experiment, when
an excess of methyl carbamate (200 mmol) was used as
the reagent. The excess of methyl carbamate was sepa-
rated by filtration from the toluene solution, followed
by vacuum drying.
6. Evaluation of dialkyltin catalysts—synthesis of
bis-(2-ethylhexyl)-carbonate
A mixture of 2-ethyl-1-hexanol (95 g, 73 mol),
methylcarbamate (18.20 g, 0.24 mol) and catalyst (5
meq) was heated to 190°C within 30–45 min. with
stirring under nitrogen. in a 250 ml flask provided with
magnetic stirring and temperature control. During this
period the methanol resulting from the transesterifica-
tion reaction was collected in a Dean Stark trap. The
vapors containing ammonia were passed through a
condenser to a sulfuric acid (1.6 N) trap. Following
collection of methanol, the Dean Stark trap was discon-
nected, and the reaction flask was connected directly to
the condenser; the temperature was maintained at 190–
195°C for 12 h. Samples were removed periodically
from the sulfuric acid trap and analyzed by potentio-
metric titration with a solution of NaOH (1N). Analy-
sis of the reaction mixture was performed by GC and
The product, crystallized on standing (m.p. 124–
1
19
128°C), Analysis:
Sn-NMR, solvent toluene: l −
1
−
66, l −206, (ratio 1:1) minor peaks at l −148 and l
165 (external ref. Me Sn at 0.0 ppm). C-NMR, solv
4
13
CDCl : l 13.53 (−CH ), l 22.21, 22.56 (CH Sn), l
3
3
2
2
6.75 27.01, (CH CH CH –Sn), l 27.17 l 27.35,
2 2 2
(
CH –CH –Sn), l 50.65 (OMe), l 128.0 (NꢀCꢀO),
2
2
small peaks, at l 126.15 and l 137.70. IR neat: 2204
cm
cm
−
−
1
1
−1
(NꢀCꢀO), 1060 and 1024 cm
(vs) (OMe), 695
−1
(CH Sn), 613–580 cm
(SnO and SnC). Analy-
2
sis: MW=1110. Anal. Calc. for C36 H78 N O Sn : C:
2
6
4
3
3
8.90, H: 7.03, N: 2.52, Sn: 42.90%. Found C:39.08,
8.94 H:7.30, 7.02 N: 2.17, 2.67 O:7.61, 7.22
Sn:44.46,40.75%. HPLC: retention time 5.28 min.
13
5.6.1. Reaction of IIa with benzyl alcohol
C-NMR. Selectivity for the carbonate in all cases was
A solution of IIa (1.10 g, 1 mmol) in benzyl alcohol
1.57 g, 14.5 mmol) was heated for 4 h at 175°C.
100%. The conversion of methyl carbamate and 2-ethyl-
1-hexanol to dicarbonate for different catalysts is pre-
sented in Table 2. In the case of the isothiocyanatotin
catalysts decomposition with deposit of a black sulfide
was observed at 190°C.
(
Evolution of ammonia, concomitant with a decrease in
−
−
1
1
the NꢀCꢀO band at 2210 cm
was paralleled by an
increase of a band at 1715 cm , corresponding to the

5
4
E.N. Suciu et al. / Journal of Organometallic Chemistry 556 (1998) 41–54
6.1. E6aluation of dialkyltin catalysts-effect of catalyst
References
concentration
[
[
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In parallel experiments compound I (7.67 g, 26
mmol), and compound IIg (7.65 g, 14.06 mmol/28 meq
Sn) were treated with 2-ethyl-1-hexanol (19.76 g, 152
mmol) and methyl carbamate (3.75 g 50 mmol) and
heated to 190°C (20–30 min) with stirring under nitro-
gen in a 250 ml flask equipped with a magnetic stirrer
and temperature control device. The methanol which
was formed as a reaction product was collected in a
Dean Stark apparatus. Following the collection of
methanol, the Dean Stark apparatus was disconnected
and the reaction mixture was heated for an additional 6
h. Samples were collected periodically and analyzed.
Analytical data for the initial sample obtained from the
4
904279, 1990.
[3] P. Giancarlo, G. Maurizio, Enichem, E.P. 460735, A2, 1991.
[4] A.K. Bhattacharya, Texaco, U.S. patent 4879266, 1989.
[
[
[
[
5] R.A. Sawicki, H. Chafetz, Texaco, U.S. patent 4900705, 1990.
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[
9] R.Y. Saleh, R.C. Michaelson, E.N. Suciu, B. Kuhlmann,
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[
[
[
10] A.S. Mufti, R.C. Poller, J. Chem. Soc. C (1965) 5055.
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Japan Chem. Let. (1984) 2109.
[15] (a) T. Yano, K. Nakashima, J. Otera, R. Okawara,
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patent 5545600, 1996.
1
19
reaction with I: Sn-NMR: l −159 (br), (external ref
1
3
Me Sn at 0.0 ppm). C-NMR: NH COOMe l 157.50,
4
2
[
l 50.34, NH COOC H l 65.74, C H –OH l 63.17,
2
8
17
8
−
17
1
Sn–OMe moiety l 49.0; IR 2206 cm
for the initial sample obtained with IIg: Sn-NMR: l
Analytical data
1
19
−
175.6, l −185.5 br (External Ref Me Sn 0.0 ppm).
4
[
[
1
3
C-NMR: NH –COOMe l 157.65, l 50.31, C H –
2
8
17
OH l 63.19. Sn–OMe moiety l 49.0. All data recorded
at r.t. The data are presented in Fig. 2 and Table 3.
Conversion was calculated based on integrated NMR
peak areas for the carbamate and the carbonate.
[
19] A.G. Davies, P.G. Harrison, W. Ramsay, R. Forster J. Chem.
Soc. C (1967) 1313.
[
20] J. Mendelsohn, M.M. Pommier, J. Valade, C.R. Acad. Sci.
Paris C (1966) 921.
[
[
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Acknowledgements
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We would like to express our appreciation for the
valuable suggestions contributed by Dr. Istv a´ n Hor-
v a´ th, who started this work and to Professor Alan R.
Katritzky for helpful discussions. We also thank Bom-
ing Liang and Debbie Sysyn for providing excellent
NMR spectra.
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.