Carbomethoxylating reactivity of methyl phenyl carbonate toward aromatic amines in the presence of group 3 metal (Sc, La) triflate catalysts

Article abstract of DOI:10.1016/j.jcat.2004.08.004

Methyl phenyl carbonate (MPC) has been investigated as a carbomethoxylating agent of aromatic amines in the presence of group 3 metal (Sc or La) triflate catalysts. Under mild conditions (363 K), both Sc(OTf)₃ and La(OTf)₃ (OTf=O₃SCF₃) can promote the carbamation of aniline and a few industrially relevant aromatic diamines, such as 4,4′-methylenedianiline (MDA) and 2,4-diaminotoluene (TDA), with MPC. Carbamate yield and selectivity are markedly affected by the experimental conditions (temperature, reaction medium, nature of the metal center). Sc(OTf)₃ is a more effective and selective carbamation catalyst than La(OTf)₃ . Ad hoc experiments have shown that, in the presence of the M(OTf)₃ (M=Sc, La) catalysts, MPC is not only more reactive but also a more selective carbomethoxylating agent than dimethyl carbonate (DMC).

Full text of DOI:10.1016/j.jcat.2004.08.004

Journal of Catalysis 228 (2004) 36–42
www.elsevier.com/locate/jcat
Carbomethoxylating reactivity of methyl phenyl carbonate toward
aromatic amines in the presence of group 3 metal (Sc, La) triflate catalysts
Monica Distaso a, Eugenio Quaranta a,b,∗
a
Dipartimento di Chimica, Università di Bari, Campus Universitario, 70126, Bari, Italy
ICCOM-CNR, Dipartimento di Chimica, Campus Universitario, 70126, Bari, Italy
b
Received 8 April 2004; revised 4 August 2004; accepted 6 August 2004
Available online 25 September 2004
Abstract
Methyl phenyl carbonate (MPC) has been investigated as a carbomethoxylating agent of aromatic amines in the presence of group 3 metal
(Sc or La) triflate catalysts. Under mild conditions (363 K), both Sc(OTf) and La(OTf) (OTf = O SCF ) can promote the carbamation
3
3
3
3
ꢀ
of aniline and a few industrially relevant aromatic diamines, such as 4,4 -methylenedianiline (MDA) and 2,4-diaminotoluene (TDA), with
MPC. Carbamate yield and selectivity are markedly affected by the experimental conditions (temperature, reaction medium, nature of the
metal center). Sc(OTf) is a more effective and selective carbamation catalyst than La(OTf) . Ad hoc experiments have shown that, in the
3
3
presence of the M(OTf) (M = Sc, La) catalysts, MPC is not only more reactive but also a more selective carbomethoxylating agent than
3
dimethyl carbonate (DMC).
2004 Elsevier Inc. All rights reserved.
Keywords: Methyl phenyl carbonate; Dimethyl carbonate; Organic carbamates; Scandium triflate; Lanthanum triflate
1. Introduction
with carbonic acid diesters [2] merits particular attention
as a “green” route to organic carbamates, since innovative
phosgene-free methodologies for the industrial synthesis of
dimethyl carbonate (DMC)
The substitution of hazardous reagents with innocuous or
less noxious compounds in synthetic chemistry is a major
target for the modern chemical industry [1]. A very risky
compound is phosgene [2], which, despite its toxicity, is
still being used as a starting material for the synthesis of
a number of chemicals, such as, for instance, organic car-
bamates RRꢀNHC(O)ORꢀꢀ (R = alkyl, aryl; Rꢀ = H, alkyl,
aryl; Rꢀꢀ = alkyl, aryl) [3]. Carbamate esters are very use-
ful compounds which find wide application in several fields
(pharmacology, agriculture) and also play a key role as inter-
mediates in the chemical industry for the production of fine
and commodity chemicals [3,4].
Cat.
2MeOH + CO + 1 O2 → (MeO)2CO + H2O
(1)
2
and other organic carbonates
(MeO)2CO + PhOH C→at. MeOC(O)OPh + MeOH
(2)
have been implemented in the last few years [8]. DMC, es-
pecially, is widely being studied as a carbomethoxylating
substrate for the carbamation of both aliphatic and aromatic
amines [9–27]. This process requires a suitable catalyst. Re-
cently, we have demonstrated that d0 transition metal sys-
tems, such as Sc(OTf)3 or La(OTf)3 (OTf = O3SCF3) are
active catalysts for the selective (≈ 100%) synthesis of car-
bamate methyl esters from primary or secondary aliphatic
amines and DMC at ambient temperature (293 K) [25].
In this work the M(OTf)3 (M = Sc, La) triflate salts
have been investigated as carbamation catalysts of aromatic
Scheme 1 shows a few synthetic ways to carbamate es-
ters [2,5–7], potentially an alternative to the current methods
based on phosgene. Among them, the reaction of amines
*
Corresponding author. Fax: 0039 080 5442083.
E-mail address: quaranta@chimica.uniba.it (E. Quaranta).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.08.004
2004 Elsevier Inc. All rights reserved.

M. Distaso, E. Quaranta / Journal of Catalysis 228 (2004) 36–42
37
2. Experimental
2.1. General
Unless otherwise stated, all reactions and manipulations
were conducted under a dinitrogen atmosphere, by using
vacuum line techniques. All solvents were dried accord-
ing to literature methods [34] and stored under N2. DMC
(Fluka) was dried over 5A molecular sieves for 24 h, fil-
tered, distilled, and stored under N2. MPC was prepared
as described in the literature [35]. The amines were Fluka
or Aldrich products. PhNH2 was dried over KOH, distilled,
and stored under N2. M(OTf)3 (M = Sc, La) salts (Fluka,
Aldrich) were used as received and manipulated under an
inert gas atmosphere.
IR spectra were obtained with a Perkin–Elmer 883 spec-
trophotometer or with a Perkin–Elmer FTIR 1710 instru-
ment. NMR spectra (chemical shift in ppm vs TMS) were
run on a Varian XL-200 or a Bruker AM 500 instrument
and referenced to the solvent peak. GC-MS analyses were
carried out with a Shimadzu GC-17A linked to a Shimadzu
GCMS-QP5050 selective mass detector (capillary column:
Supelco MDN-5S, 30 m × 0.25 mm, 0.25 µm film thick-
ness). GC analyses were performed with a HP 5890 Se-
ries II gas chromatograph (capillary column: Heliflex AT-5,
30 m × 0.25 mm, 0.25 µm film thickness). HPLC analyses
were carried out with a Perkin–Elmer Series 4 LC liquid
chromatograph connected with a LC 290 UV/vis detector.
Scheme 1. Phosgene-free synthetic routes to carbamate esters.
amines. We have focused also on methyl phenyl carbonate
(MPC) as a potential carbomethoxylating substrate:
ArNH2 + MeOC(O)OPh → ArNHC(O)OMe + PhOH (3)
Growing attention, in fact, is currently devoted to the
study of alternative carbomethoxy or, more generally, car-
boalkoxylating substrates [28–32], able to react selectively,
under not drastic conditions.
The use of MPC in carbamation reactions of amines
has been scarcely studied in the past [29–32]. In a previ-
ous study, we have shown that organo-phosphorous Brøn-
sted acids X2P(O)OH (X = Ph, OPh, OBu) can selectively
catalyze the synthesis of methylcarbamates from MPC and
4,4ꢀ-methylenedianiline (1, MDA) or 2,4-diaminotoluene(2,
TDA) [32]. Also organic bases have been used to promote
the carbamation of aromatic amines,
2.2. Reaction of PhNH2 with MPC in the presence of
M(OTf)3 (M = Sc, La) salts
2.2.1. General procedure
Into a 10-mL tube, containing the catalyst M(OTf)3 (M =
Sc or La), the reactants (MPC and aniline), the solvent
(THF), and the internal standard (n-dodecane) were intro-
duced. The tube was closed with a screw cap equipped with
a silicon septum through which the reaction mixture could be
sampled using a GC syringe. The reaction mixture was then
allowed to react at the working temperature. At measured
intervals of time, heating was stopped, and the reaction mix-
ture was cooled to room temperature and analyzed by GC.
In a few experiments MPC was used both as solvent and
reactant. In this case, at measured intervals of time, heating
was stopped, and 5 µL of reaction solution was sampled, di-
luted with THF (0.1 mL), and, after addition of n-dodecane,
analyzed by GC.
,
with MPC [30,31]. To date, metal salts or complexes
have been poorly investigated as catalysts in this reaction
[Eq. (3)]. Herein, for the first time, we fully report on the ac-
tivity of a few metal systems, such as Sc(OTf)3 and La(OTf)3
[25,33] as catalysts in this process. We also compare the
carbomethoxylating properties of MPC and DMC toward
PhNH2. We show that, in the presence of the above catalysts,
MPC is not only more reactive, but also a more selective car-
bomethoxylating agent than DMC.
2.2.2. Isolation of (PhNH3)(OTf) and PhNHC(O)OMe
To the solution of Sc(OTf)3 (0.08850 g, 0.179 mmol) in
THF (1.3 mL), MPC and aniline were added. The reaction
solution was stirred at 363 K for 24 h, cooled to 293 K, and
evaporated in vacuo. The residue was extracted with diethyl
ether. A white solid, poorly soluble in ether, was recov-
ered and identified as (PhNH3)(OTf) (42.6 mg, 0.174 mmol).
Anal. Calcd. for C7H8SNO3F3: C, 34.57; H, 3.32; N, 5.76.

38
M. Distaso, E. Quaranta / Journal of Catalysis 228 (2004) 36–42
Table 1
Reaction of PhNH with DMC in the presence of M(OTf) (M = Sc, La) salts, under solvent-free conditions
2
3
a
b
Entry
PhNH
DMC
M(OTf)
T
Time
GC yield vs PhNH (%)
2
3
2
(mmol)
(mmol)
(K)
(h)
M = Sc
M = La
PhNHCO Me
2
Ph(Me)NH
c
PhNMe
c
2
1
2
3
4
5.48
59.35
9.03
5.46
–
–
363
363
363
368
96
24
26
24
–
8
5
3
0.823
2.192
1.096
6.4
3.0
–
–
–
5.6
25
19
20
10
4
5
11.87
a
(mol M/mol PhNH )%.
2
b
c
Under the working conditions, the formation of other by-products, such as diphenylurea and Ph(Me)NCO(O)Me, was not observed.
The total yield of N-methylation products was less than 2%.
Table 2
a
MPC vs DMC reactivity toward PhNH in the presence of M(OTf) (M = Sc, La) salts (363 K)
2
3
PhNH
MPC
DMC
M(OTf) (mmol)
Time
GC yield vs PhNH (%)
2
2
3
(mmol)
(mmol)
(mmol)
(h)
M = Sc
M = La
PhNHCO Me
2
Ph(Me)NH
PhNMe
2
1.096
1.096
1.096
1.096
1.096
1.109
1.109
–
1.109
–
–
–
0.073
0.073
0.072
–
–
2
24
27
25
26
38
72
18
25
7
3
5
20
5
Traces
< 1
2
< 1
5
–
–
1.128
–
1.128
0.064
0.064
–
26
a
THF (0.5 mL) was the solvent, in all the runs.
Found: C, 35.08; H, 3.65; N, 5.97. IR (Nujol, cm−1): 3180,
1636, 1621, 1587, 1538, 1498, 1284 (vs, br), 1238 (vs, br),
1197, 1174, 1159, 1141, 1108, 1094, 1044 (s), 1026, 973,
764, 744, 688, 650, 618, 588, 575.
2.4. Reaction of PhNH2 with DMC in the presence of
M(OTf)3 (M = Sc, La) salts
Into a 10-mL tube, containing the catalyst M(OTf)3 and
anhydrous DMC, aniline and THF (if used) were introduced.
After the tube was sealed, the reaction mixture was allowed
to react at the working temperature for a variable time (see
Tables 1 and 2) and, after cooling to ambient temperature,
analyzed by GC (internal standard: n-dodecane).
The washing solutions, collected together, were extracted
with water, NaOH 0.6 M (4 × 25 mL), and again with water.
The organic phase was dried over MgSO4, filtered, and con-
centrated in vacuo. Upon addition of n-hexane and cooling
to 273 K, a white solid separated and was isolated by filtra-
tion and identified as PhNHC(O)OMe (60%). Anal. Calcd.
for C8H9NO2: C, 63.57; H, 6.00; N, 9.26. Found: C, 63.93;
H, 6.04; N, 9.33. The IR, MS, 1H and 13C NMR spectra of
the isolated product fully agree with those of an authentic
sample of PhNHC(O)OMe prepared by another route [13].
No attempt was made to recover minor amounts of product
from the mother solution.
2.5. Reaction of MPC (or DMC) with PhCH2NH2 in the
absence of any catalyst
To a MPC (0.100 mL, 0.822 mmol) solution in THF
(0.5 mL) an excess of PhCH2NH2 (0.250 mL, 2.291 mmol;
amine/MPC = 2.8 (mol/mol)) was added. The reaction
mixture was allowed to react at 293 K and periodically
monitored by GC-MS. The GC-MS analysis of the re-
action solution showed that the conversion of MPC into
PhCH2NHC(O)OMe was quantitative within 24 h.
A smooth reaction was observed also when an equimo-
lar amine/carbonate ratio was used (PhCH2NH2, 0.090 mL,
0.825 mmol; MPC, 0.100 mL, 0.822 mmol; THF, 0.5 mL).
Under the latter conditions (293 K), the conversion of the
reactants was found to be complete within 55 h.
In both cases, no evidence of the formation of N-meth-
ylation by-products was found by GC-MS.
Similar experiments carried out with DMC afforded, after
comparable times, PhCH2NC(O)OMe in much lower yield
(13 and 8%, respectively).
2.3. Reaction of aromatic diamines (MDA, TDA) with MPC
in the presence of M(OTf)3 (M = Sc, La) salts
Into a 10-mL tube containing the catalyst M(OTf)3, THF,
MPC, and the diamine (MDA or TDA) were introduced. Af-
ter sealing the tube, the reaction mixture was heated to 363 K
for about 24 h and, after cooling to 293 K, diluted with THF
(2.5 mL) and analyzed by HPLC (see Table 4).
For the reaction with MDA, the conditions are as fol-
lows: internal standard, mesitylene; column, Supelcosil LC-
DP, 5 µm, 250 × 4.6 mm; mobile phase, CH3CN/H2O
(35/65 v/v); flow, 1.5 mL/min.
For the reaction with TDA, the conditions are as follows:
internal standard, benzene; column, Supelcosil LC8, 5 µm,
250 × 4.6 mm; mobile phase, CH3CN/H2O (30/70 v/v);
flow, 1.5 mL/min.

M. Distaso, E. Quaranta / Journal of Catalysis 228 (2004) 36–42
39
3. Results
3.1. Carbomethoxylation of aniline in the presence of
M(OTf)3 (M = Sc, La) salts: MPC vs DMC
carbomethoxylating behavior
In this study aniline was chosen as model compound for
aromatic amines. Under the experimental conditions (293 K)
employed for the aliphatic ones [25], less reactive PhNH2
did no react with DMC in the presence of the M(OTf)3
(Sc, La) salts, even after very long reaction times (4 days).
At 363 K, both Sc(OTf)3 and La (OTf)3 (3–6% (mol/mol)
vs PhNH2) can promote the formation of PhNHC(O)OMe
from aniline and DMC (see Table 1), but slowly, with low
yield (< 10% after 24 h) and selectivity, because of the pre-
dominant formation of N-alkylation products (Ph(Me)NH,
PhNMe2).
Fig. 1. Carbomethoxylation of PhNH (0.100 mL, 1.096 mmol) with
2
MPC (0.135 mL, 1.109 mmol) in the presence of Sc(OTf) (0.03550 g,
3
0.073 mmol) or La(OTf) (0.03735 g, 0.064 mmol), at 363 K. Solvent: THF
3
(0.5 mL).
However, a drastic change of reactivity was observed
by suitably changing carbomethoxylating agent. In fact, at
363 K, in THF, the M(OTf)3 salts (M = Sc, La; 3–6%
(mol/mol) vs PhNH2) catalyzed the carbamation of PhNH2
with MPC to give PhNHC(O)OMe with higher yield and se-
lectivity. This is clearly emphasized in Table 2, where the
carbomethoxylating properties of MPC and DMC toward
PhNH2, in the presence of the M(OTf)3 (Sc, La) salts (under
otherwise analogous reaction conditions: solvent, catalyst
concentration, reactants concentration, etc.), are compared.
The data in Table 2 clearly demonstrate that MPC is, not
only more reactive, but also a more selective carbomethoxy-
lating agent than DMC.
Carbamate yield and selectivity depend on the catalyst
used and the experimental conditions. Table 3 summarizes
the results obtained by using Sc(OTf)3) (≈ 6% (mol/mol)
vs PhNH2). At 363 K, in THF (Entry 2, Table 3), the car-
bamation reaction was, by far, the favored process (selec-
tivity ≈ 92%). The formation of N-methylation by-products
(mainly Ph(Me)NH) was very modest, but tended to be more
significant at higher temperatures, as observed, for example,
at 393 K (Entry 3, Table 3).
with MPC, at 363 K, under otherwise analogous reaction
conditions. In the presence of the La salt, the carbamation
reaction occurred more slowly, with lower selectivity (82%,
after 30.4 h). In the case of Sc(OTf)3 more than 56% of ani-
line was selectively (93%) converted into carbamate in less
than 7.5 h. Afterward, the carbamation proceeded at a lower
rate, with a slower increase of the conversion of the amine,
until a plateau was reached corresponding to a yield close
to 70% after about 24 h. If the fresh reactants (both PhNH2
and MPC) are added to the reaction solution at the plateau
(Fig. 2), the reaction restarted with a noticeable formation
of more product (PhNHC(O)OMe). This proves that, at the
plateau, the catalyst is still active, although not so effective
as at the beginning.
This issue was studied in greater detail (see Section 2.2.2).
We have found that aniline can undergo protonation and
convert into the corresponding phenylammonium cation
PhNH3+. This species is not reactive toward MPC and can
be recovered at the end of the catalytic run as (PhNH3)OTf.
The formation of this salt may be explained according to
LnSc(OTf)3 + xPhOH + xPhNH2
→ LnSc(OPh)x(OTf)3 − x + x(PhNH3)OTf
(4)
La(OTf)3 also promotes aniline carbomethoxylation by
reaction with MPC, but it shows a lower catalytic activ-
ity than Sc(OTf)3. Fig. 1 shows the catalytic activities of
Sc(OTf)3 and La(OTf)3 in the carbomethoxylation of aniline
L = ligand
which suggests that the starting catalyst modifies during the
catalytic run. We have tried to isolate Sc-phenoxy derivatives
Table 3
Reaction of PhNH with MPC in the presence of Sc(OTf)
a
2
3
b
Entry
MPC
(mmol)
Sc(OTf)
(mmol)
T
Time
(h)
GC yield vs PhNH (%)
2
Solvent
(mL)
3
(K)
PhNHC(O)OMe
Ph(Me)NH
PhNMe
2
1
2
3
1.109
1.109
1.109
4.437
–
0.5
0.5
0.5
363
363
393
363
23
24
25
22
< 1
72
16
< 1
5
25
7
Traces
< 1
13
0.073
0.081
0.068
c
4
No solv.
52
12
a
PhNH : 0.100 mL (1.096 mmol) in all the runs.
2
Solvent: THF.
At the working temperature (393 K), the selectivity of the carbamation process was very low. The GC-MS analysis of the reaction mixture revealed the
b
c
formation of a few other by-products, whose identification and characterization are in progress.

40
M. Distaso, E. Quaranta / Journal of Catalysis 228 (2004) 36–42
3.2. Carbomethoxylation of aromatic diamines with MPC
in the presence of M(OTf)3 (M = Sc, La) salts
We have extended the study also to a few industrially rel-
evant aromatic diamines, such as MDA and TDA [32].
At 363 K, in THF as solvent, Sc(OTf)3 (6% (mol/mol)
vs diamine) effectively promotes the carbomethoxylation of
MDA (1 eq) with MPC (2 eq), which react to give monocar-
bamate 1a and dicarbamate 1b,
Fig. 2. Carbomethoxylation of PhNH with MPC in the presence of
2
Sc(OTf) at 363 K: effect of further addition of the fresh reactants (PhNH
3
2
and MPC) at the plateau. Experimental conditions: MPC (0.135 mL,
1.109 mmol); PhNH (0.100 mL, 1.096 mmol); Sc(OTf) (0.03550 g,
2
3
0.073 mmol); solvent THF (0.5 mL). After 24 h, 0.100 mL (1.096 mmol)
of PhNH and 0.135 mL (1.109 mmol) of MPC were added.
2
After 24 h, the overall carbamation yield was close to 80%
with a selectivity as high as 94% (Entry 2, Table 4). Un-
der these conditions the incidence of the N-methylation
reaction was very modest as (4-methylaminophenyl)-4-
aminophenylmethane (m/z: 212) and dimethylated MDA
derivatives (m/z: 226) were formed with an overall yield
not higher than 5%. N-Methyl monocarbamates (m/z: 270)
were found only in trace amounts. Under comparable con-
ditions, La(OTf)3 (Entry 3, Table 4) proved to be a less
effective and selective catalyst than Sc(OTf)3. After 26 h
at 363 K, the overall carbamate yield did not exceed 16%,
with the almost exclusive formation of monocarbamate 1a.
Also the selectivity of the carbamation process was not so
satisfactory (67%) as found for the Sc catalyst because of
the relevant formation of N-methylated amines.
Fig. 3. Reaction of PhNH (0.100 mL, 1.096 mmol) with MPC (0.540 mL,
2
4.437 mmol), used both as reagent and solvent, in the presence of La(OTf)
(0.03935 g, 0.067 mmol), at 363 K.
3
[36,37] from the reaction mixture. However, in no case such
species could be obtained in a pure form.
The carbomethoxylation reaction was investigated, at
363 K, under solvent-free conditions, employing an excess
of MPC (Entry 4 in Table 3 (Sc) and Fig. 3 (La)). Under
these conditions, a marked decrease of both carbamate yield
and selectivity was observed as a result of the increased inci-
dence of the N-methylation reaction. In both cases (Entry 4
in Table 3 (Sc) and Fig. 3 (La)), after prolonged reaction
times (> 20 h), the formation of PhNMe2 was found to pre-
vail significantly over that of Ph(Me)NH.
Under conditions analogous to those employed above for
MDA, TDA showed quite a different behavior. As a matter
of fact, in the presence of Sc(OTf)3, the reaction of TDA
with MPC, in THF, at 363 K, afforded monocarbamate 2a as
the major product (Entry 5, Table 4).
Table 4
Reactivity of MDA or TDA with MPC in the presence of M(OTf) (M = Sc, La) salts, at 363 K
3
a
b
Entry
mmol of
THF
Time
(h)
Carbamate yield (%)
(mL)
MDA
TDA
MPC
Sc(OTf)
La(OTf)
Mono-
c
Di-
c
3
3
1
2
3
4
5
0.154
1.083
0.951
–
–
–
–
0.308
2.136
1.889
0.664
2.136
–
–
–
0.5
2
2
1
2
24
24
26
6
0.072
–
–
50
15
<1
30
1
c
0.063
–
–
d
0.332
1.148
d
–
0.071
25
31
1
a
THF was the solvent.
HPLC yield vs diamine.
Not formed.
b
c
d
Monocarbamate 2a.

M. Distaso, E. Quaranta / Journal of Catalysis 228 (2004) 36–42
41
tractive, from the applicative point of view, the use of MPC
as carbomethoxylating agent in place of DMC, if they are
usefully exploited to set up selective, high yield, non-energy-
intensive, MPC-based carbamation processes.
The higher reactivity of MPC vs DMC as carbomethoxy-
lating agent can be related, at least partly, to the different
structures of the two substrates. This is clearly supported by
a few experiments carried out with aliphatic amines in the
absence of any added catalyst (see Section 2.5) and sug-
gests that electronic effects rather than steric factors con-
trol the addition of the nucleophile (amine) to the carbonyl
group [38]. The different chemical environments around
the carbonyl group of MPC and DMC, by influencing the
carbomethoxylating power of both substrates, can affect
also the selectivity of these species as carbomethoxylating
agents. In this regard, we note, also, that N-methylation, in-
volving amine attack to MPC or DMC methyl group, is an
irreversible process because the formed ROC(O)OH decom-
poses to ROH (R = Me or Ph) and CO2 [39]. Conversely,
amine attack to the carbonyl carbon atom of either MPC
or DMC may take place, in principle, reversibly [40], and,
in the case of MPC, subsequent conversion of the result-
ing tetrahedral intermediate into the carbamate product is
greatly facilitated by the presence of a leaving group, such
as phenoxide, better than methoxide. Nevertheless (see Ta-
ble 2), a crucial role may also be played by the catalyst
used, as the latter, by interacting with the substrate (DMC
or MPC), can modify the substrate reactivity and affect the
rates of the competitive processes (carbomethoxylation and
N-methylation).
The nature of the metal center markedly influences the
catalytic processes described above, as supported by the fact
that Sc(OTf)3 is a more effective and selective catalyst of
reaction (3) than the homologue La salt. An analogous or-
der of catalytic activity was previously found by us for the
carbomethoxylation of aliphatic amines with DMC at room
temperature [25]. The order observed in both cases matches
well the relative oxophilicities of Sc3+ and La3+ as recently
evaluated by tandem mass spectrometry [41], and can be re-
lated with the marked difference between the ionic radii of
the two metal cations (Sc3+, 0.89 Å; La3+, 1.17 Å) [42].
In fact, the higher the Lewis acidity of the metal center, the
more effective the activation of the substrate (MPC), which
will exhibit enhanced electrophilic reactivity at the carbonyl
group.
Regio-isomer 2aꢀ was formed only in very small amounts,
most probably because carbomethoxylation of the vicinal
amino group experiences steric constraints due to the prox-
imity of the methyl group. This fact can also explain why
dicarbamate 2b formed much more slowly (1% yield, after
25 h) than 2a. Nevertheless, the carbomethoxylation reac-
tion was still the favored process, although selectivity toward
carbamation (78%) was not so high as in the case of the cor-
responding reaction with MDA (94%).
In the experiments discussed above for both diamines
(MDA or TDA) we found no experimental evidence for
the formation of mono- and diphenyl carbamate esters or
ureas. Accordingly, in the case of PhNH2, by-products such
as PhNHC(O)OPh or PhNHC(O)NHPh were found to form
only in trace amounts. These features find an explanation in
the different properties of phenoxy and methoxy groups as
leaving groups, and considering the fact that urea formation
usually requires more drastic conditions of temperature than
those used in this work [13].
The above processes can be catalytically promoted also
by nonmetal catalysts such as organo-phosphorous Brøn-
sted acids X2P(O)OH [32]. With both classes of catalysts
[the M(OTf)3 (M = Sc, La) salts and X2P(O)OH acids], the
carbamation of both diamines occurs under relatively mild
temperature conditions (363 K), by far less severe than those
required (ꢀ 453 K) for the carbomethoxylation of the same
systems with DMC in the presence of other catalysts [24].
The most efficient catalyst, among the organo-phosphorous
acids investigated, was shown to be diphenylphosphinic acid
Ph2P(O)OH. Despite the lower catalyst loadings used in
the present study (max 6% (mol/mol) of M(OTf)3 vs di-
amine), Sc(OTf)3 compares well, as carbomethoxylation
catalyst, with Ph2P(O)OH, which was used in higher cat-
alytic amounts, up to 20% (mol/mol) vs diamine, in order to
make up for its tendency to deactivate.
4. Discussion
In the presence of the same catalysts [M(OTf)3 (M = Sc,
La)] we effectively used for the carbamation of aliphatic
amines [25], aromatic amines react with DMC in a differ-
ent way from the aliphatic ones. Nevertheless, aniline, for
instance, can be converted into methyl carbamate ester with
good yield and selectivity, under not severe conditions, by
employing a different substrate such as MPC, which has
been shown to act as a carbomethoxylating agent, which is
not only more reactive but also more selective than DMC.
The latter features deserve attention as they can make at-
5. Conclusions
In the presence of M(OTf)3 (M = Sc, La) salts, MPC
can act as a carbomethoxylating agent of aromatic amines,
which is not only more reactive but also more selective than
DMC. In THF, under not severe conditions (363 K), both
scandium and lanthanum triflate promote the carbamation of
aromatic mono- and diamines by reaction with MPC, used
as carbomethoxylating agent in place of DMC. Yield and se-

42
M. Distaso, E. Quaranta / Journal of Catalysis 228 (2004) 36–42
lectivity of the carbamation process are markedly influenced
by the experimental conditions. Sc(OTf)3 is a more effective
and selective catalyst than the homologue La salt. Higher
temperatures (> 363 K), or the use of MPC as both solvent
and reactant, lower carbamate yield and selectivity, because
of the higher incidence of N-methylation processes.
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Financial support from Italian MIUR (PRIN 2003039774)
is gratefully acknowledged. M.D. thanks the MIUR (L488/
92) for a fellowship.
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