Synthesis of carbamates from carbon dioxide promoted by organostannanes and alkoxysilanes

Article abstract of DOI:10.1002/aoc.3733

A cooperative methoxy transfer between orthosilicate esters and organotin oxides was developed for the synthesis of various N-alkyl and N-aryl carbamates from carbon dioxide in up to 97% isolated yield. The reaction is highly selective and N-alkylated amines are not observed. Density functional theory calculations of the reaction were performed and, together with NMR observations, a plausible mechanism featuring the catalytic regeneration of dialkyltin dialkoxide is proposed.

Full text of DOI:10.1002/aoc.3733

Received: 27 October 2016
DOI 10.1002/aoc.3733
Revised: 5 December 2016
Accepted: 13 December 2016
F U L L PA P E R
Synthesis of carbamates from carbon dioxide promoted by
organostannanes and alkoxysilanes
Nicolas Germain1 | Marko Hermsen1,2 | Thomas Schaub1,2
| Oliver Trapp1,3
1 Catalysis Research Laboratory (CaRLa), Im
A cooperative methoxy transfer between orthosilicate esters and organotin oxides
was developed for the synthesis of various N‐alkyl and N‐aryl carbamates from car-
bon dioxide in up to 97% isolated yield. The reaction is highly selective and N‐
alkylated amines are not observed. Density functional theory calculations of the
reaction were performed and, together with NMR observations, a plausible mecha-
nism featuring the catalytic regeneration of dialkyltin dialkoxide is proposed.
Neuenheimer Feld 584, 69120 Heidelberg,
Germany
2 BASF SE, Carl‐Bosch‐Str. 38, 67056
Ludwigshafen, Germany
3 Department of Chemistry, Ludwigs‐Maximilians‐
Universität München, Butenandstr. 5‐13, 81377
Munich, Germany
Correspondence
KEYWORDS
Thomas Schaub, BASF SE, Carl‐Bosch‐Str. 38,
67056 Ludwigshafen, Germany.
Email: thomas.schaub@basf.com
carbamates, catalysis, CO2, computational investigations, silicates, tin
Oliver Trapp, Department of Chemistry, Ludwigs‐
Maximilians‐Universität München, Butenandstr.
5‐13, 81377 München, Germany.
Email: oliver.trapp@cup.uni‐muenchen.de
1
|
INTRODUCTION
Sakakura and co‐workers presented a dialkyltin oxide‐
catalysed route to carbamates by using CO2 and 2,2‐
Carbon dioxide (CO2) is a readily available feedstock and
a valuable C1 unit for organic synthesis.[1] The carboxyla-
tion reaction by catalytic insertion into electron‐rich metal–
heteroatom bonds offers a sustainable alternative to routes
using carbon monoxide (CO) or strong oxidants.[2] Recently,
some studies reported the use of CO2 in organic carbonate
and carbamate synthesis.[3] While the first is well docu-
mented, new systems are still required to achieve high‐yield-
ing and selective syntheses of carbamates, precursors of
isocyanates and polycarbamates.[4] Conventionally, carba-
mates are prepared at the laboratory scale from amines and
chloroformates.[5] Chloroformates are toxic, not compatible
with many organic functions and HCl is produced in the
course of the reaction. Chloroformates are obtained from
phosgene. Conversely, several phosgene‐free synthetic routes
towards carbamates have been reported, but there are only a
few reports of the carbonylation of amines.[6] One method
uses dimethyl carbonate (DMC), but this technique has two
major drawbacks: (a) DMC is an excellent alkylating agent
forming methylamine as by‐product and (b) the preparation
of DMC by oxidative carbonylation requires a prior step using
methanol, CO and O2.[7]
dimethoxypropane to remove water with good yields and
selectivities avoiding N‐alkylation, but this approach could
only be applied to aliphatic amines.[8]
In recent papers from Andrioletti and co‐workers[9] on
aliphatic amines and Kleij and co‐workers[10] on aromatic
amines, cyclic carbonates were used which can form under
remarkably mild conditions the corresponding urethanes by
ring opening of the cyclic carbonate. This approach gives
high yields and selectivities (no N‐alkylation), but is limited
to the synthesis of hydroxyurethanes and not simple
arylalkylurethanes, which are potentially easier to cleave to
afford the corresponding isocyanates.
Therefore, a strategy was devised to directly use
organotin derivatives, aromatic amines and CO2 to obtain
arylalkylurethanes without N‐alkylation or the use of
chloroformates.
In our approach we use dialkoxydialkyltin compounds as
potent reagents to formally transfer an alkoxy moiety to
carbamic acid.[8,11] Although the mechanism is as yet
unclear, it seems plausible that dialkoxydialkyltin first inserts
CO2 followed by a nucleophilic attack of the amine onto the
tin carbonate.[11]
Appl Organometal Chem 2017;e3733.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3733

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GERMAIN ET AL.
from SiO2 and alcohols was disclosed.[14] Herein, the
carbamation of anilines and aliphatic amines is described
using CO2, orthosilicates and sub‐stoichiometric to catalytic
amounts of organotin oxide.
The proposed concept was tested with aniline and various
commercially available organostannanes and alkoxysilanes
(Table 1).[15] Using 20 mol% of Bu2SnO, the reaction with
methoxytrimethylsilane (entry 1, 3%) yielded only a low
amount of 2a. Increasing the stoichiometry of
methoxy equivalent did not improve the conversion signifi-
cantly (entry 2, 10%). Dimethoxydimethylsilane and
dimethoxydibutylsilane also afforded low yields (entries 3
and 4, respectively 66 and 35%). The conversion could be
improved by using trimethoxymethylsilane (entry 5, 40%)
but the best result was obtained with tetramethyl orthosilicate
(entry 6, 81%). The NMR yield of the crude mixture was
92%. Tetramethyl orthosilicate can potentially deliver up to
four equivalents of methoxy moiety but it is reasonable to
assume that only two methoxy can be transferred (vide infra).
Among all the silicates tested, tetraalkyl orthosilicates are the
only ones to form silicon oxides which can be easily filtered
after hydrolysis. This is in contrast to stable and soluble
oligo‐ or polysiloxanes formed after the reactions using
alkylmethoxysilanes.
Three dialkyltin or diaryltin oxides are commercially
available. While diphenyltin oxide gave a poor yield (Table 1,
entry 7, 22%), the conversion was improved by using
dialkyltin oxide. In addition to the above‐mentioned Bu2SnO
(entry 6), the reaction using Me2SnO yielded 55% of isolated
MPC 2a (entry 8). The superior activity of Bu2SnO can be
explained by the facts that the catalytically active tin species
must be electron rich and Me2SnO as a three‐dimensional
oligomer is more stable than the dibutyl counterpart.
Despite that tin oxides are stable, minimal loadings of
catalyst are desirable. It was thus attempted to decrease the
amount of the tin catalyst starting from 1 mol% (Table 1,
entry 9). As expected, the reaction was slower and 40% of
2a was isolated after 24 h. When the reaction was run for
62 h, the yield increased to 53% (entry 10). However, increas-
ing the loading of Si(OMe)4 to 5 and 10 mol% improved the
isolated yields to 64 and 75% for only 5 h of reaction time
(entries 11 and 12), respectively.
SCHEME 1 Synthesis of carbamates using catalytic amount of tin oxide,
CO2 and amines
An industrially relevant process for the preparation of N‐
phenylmethylcarbamate (MPC), toluenedicarbamate and
4,4′‐methylenediphenylurethane was achieved using this
method.[11] However, the scope has not been extended to
other substrates and stoichiometric amounts of dialkyltin
dimethoxide are required. Organotin oxides can also be used
in sub‐stoichiometric amounts in combination with
orthoformates and an excess of alcohols (Scheme 1a).[8]
However, a rather restricted scope of aliphatic amines and
only a moderate selectivity were obtained. The reaction of a
relatively inert and inexpensive diorganotin oxide with
another source of methoxy equivalents was therefore investi-
gated here. The cooperative transfer of methoxy
equivalents from main group metals to tin oxides has been
demonstrated for the acid‐catalysed synthesis of
organometallohydroxystannanes (Scheme 1b).[12] Here,
Bu2SnO is possibly activated by the carboxylic acid to
weaken and break the polymeric structure.
2
| RESULTS AND DISCUSSION
2.1 | Concept and alkoxysilane screening
In our work, an access to carbamates from CO2 using
tetraalkyl orthosilicate as a methoxy equivalent source and
sub‐stoichiometric to catalytic amount of dialkyltin oxide
was developed (Scheme 1c). The challenge is the transfer of
two methoxy moieties to generate catalytic amounts of
dialkyltin dialkoxide. Alkoxysilanes are versatile reagents
industrially used for coatings, construction, zeolite prepara-
tion, semiconductors and silicon polymers (polydimethylsi-
loxane, polydimethoxysiloxane).[13] Orthosilicates are
obtained by reacting tetrachlorosilanes with alcohols. None-
theless, to compete with halogen‐ and thus phosgene‐free car-
bamate synthesis, this work is supported by recent reports,
wherein a novel approach to generate alkoxysilane directly
Ultimately, longer reaction time with 20 mol% loading
afforded the desired reaction product in 97% isolated yield
(Table 1, entry 13). Importantly, no by‐product such as
methylaniline or diphenylurea could be detected in the crude
mixtures isolated after longer reaction time. This is again in
sharp contrast to the approach using dimethyl carbonate.[16]
Dichloromethane, pentane and tetrahydrofuran gave slightly
lower yields, while toluene gave the best results.
2.2 | Substrate scope
With the optimized reaction conditions in hand, we studied
the scope of the reaction (Table 2). The amount of tin oxide

GERMAIN ET AL.
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TABLE 1 Optimizations of carbamation of anilinea
Entry
R
x (mol%)
[Si]
Yield (%)b
1
Bu
Bu
Bu
Bu
Bu
Bu
Ph
20
20
20
20
20
20
20
20
1
Me3Si(OMe)
Me3Si(OMe)
Me2Si(OMe)2
Bu2Si(OMe)2
MeSi(OMe)3
Si(OMe)4
3
10c
66
2
3
4
35
5
40
81d
6
7
Si(OMe)4
22
8
Me
Bu
Bu
Bu
Bu
Bu
Si(OMe)4
45
9
Si(OMe)4
40e
53f
64
10
11
12
13
1
Si(OMe)4
5
Si(OMe)4
10
20
Si(OMe)4
75
97g
Si(OMe)4
aConditions: in a 50 ml stainless steel autoclave, 1a (2 mmol), [Si] (4 mmol), R2SnO in toluene (10 ml), CO2 (70 bar) at 135 °C for 5 h.
bIsolated yield.
c8 equiv. of Me3Si(OMe) were used.
d92% NMR yield.
eReaction time was 24 h.
fReaction time was 62 h.
gReaction time was 15 h.
was kept at 20 mol% in order to easily compare the reaction
rate between substrates. The reaction was tolerant to substitu-
tion of the aromatic ring even if the conversion strongly
depends on the electronic properties of the amines. In com-
parison to the synthesis of MPC 2a, electron‐donating groups
such as methoxy and methyl were beneficial for the conver-
sion. Products 2b and 2c were isolated in 91 and 85% yield,
respectively. Following the same trend, the methylcarbamate
of 1‐naphthylamine 2d was obtained in 82% yield. Halogen‐
substituted anilines also afforded the corresponding
carbamates 2e–g in moderate to good yields (58–86%). Here,
electronic effects may play a significant role since 4‐
iodoaniline (1 g) reacted slower than 4‐chloroanoline and
even slower than 4‐fluoroaniline. The conversion was
improved by increasing the reaction time (58% after 5 h ver-
sus 75% after 13 h).
Carbamate 2 h was isolated in 61% yield whereas the
strong electron‐withdrawing nitro group dramatically reduced
the reaction rate (19% after 5 h, 30% after 10 h). If the reac-
tion with 3‐chloroaniline to produce 2j can be compared to
4‐chloroaniline (77 and 76%, respectively), an o‐substituent
affected the reactivity (2 k, 45% after 13 h). The transfer of
the ethoxy group from inexpensive Si(OEt)4 was also
examined. Ethylcarbamate 2 l was successfully isolated in
68% yield.
required longer reactions times. After 24 h, benzylamine
1 m was converted to 2 m in 77% isolated yield. Excellent
results were obtained with cyclohexylamine 1n (97%) but
lower yields with butylamine 1o (51%, volatile colourless
oil). The lower reactivity of aliphatic amines can be explained
by a competing mechanism. Electron‐rich and uncongested
aliphatic counterparts are able to bind to tin first thus lower-
ing their ability to react with the tin carbonate. This hypothe-
sis was supported experimentally since the best result was
obtained with secondary amine 1n.
The scope of substrates showed a correlation between
the yield and the electronic properties of the aniline deriva-
tives. This suggests a step where anilines act as nucleophiles
without a prior coordination to tin. To test this hypothesis,
the conversion against the Hammett parameters was plotted
(Figure 1). Beforehand, the reaction profile for the conver-
sion of 1b and 1f was monitored. The comparison of initial
rates corroborated that the fastest reaction is observed for
electron‐rich amines.[15,17] Seven competing reactions
including p‐ and m‐substituted anilines with 2a were con-
ducted for Hammett studies. 1,3,5‐Trimethoxybenzene was
used as internal standard to control that the conversion
stayed below 10% after 15 min. Excellent correlation
(ρ = −1.02) and accuracy (R2 = 0.9855) were found. This
confirms the observations in Table 2 and corroborates the
intermolecular nucleophilic attack of amine to a tin carbon-
ate species.
Notwithstanding the superior reactivity of electron‐rich
anilines, carbamation of electron‐enriched aliphatic amines

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GERMAIN ET AL.
TABLE 2 Scope of carbamation of anilines and alkylated aminesa
aConditions: in a 50 ml stainless steel autoclave, amine (2 mmol), Bu2SnO (0.4 mmol), tetramethyl orthosilicate (4 mmol), CO2 (70 bar), toluene (10 ml) at 135 °C for 5 h.
Isolated yields.
bReaction time was 13 h.
cReaction time was 10 h.
dReaction time was 24 h.
by insertion leading to 3. The NMR spectra indicated that 3
has an oligomeric structure (Scheme 2), which would also
fit with the penta‐coordinated tin species detected by NMR.
As we did not observe dialkyl carbonate formation, we
assume that the tin atoms can also be linked in this first step
in the formation of 5 by carbonate units and not by methyl
carbonate leading to an oligomeric structure. In the later step
of the reaction when 8 is formed (see below), a dimeric
distannoxane linked by methyl carbonate units as suggested
by Ballivet‐Tkatchenko et al.[18] is more likely. Unfortu-
nately, we were never able get any crystalline material for
FIGURE 1 Hammett correlation
an X‐ray structure determination of 3. The tin carbonate olig-
omer 3 can only react with alcohols or acids to afford
organotin methylcarbamate or organotin hydroxyl‐based
2.3 | Mechanistic studies
complexes.[18] The interaction of 3 with orthosilicates is
Compared to the previous work on methoxy transfer between
organosilicate and tin, two methoxy moieties are exchanged.
Moreover, no carboxylic acid is required as additive: CO2
activates the Sn─O bond in the presence of the orthosilicate
assumed to proceed through σ‐bond metathesis which causes
an entropically favoured opening towards 4. To reach 5, a
methoxy fragment is transferred in an intramolecular fashion
via a six‐membered ring intermediate as depicted in

GERMAIN ET AL.
5 of 10
2.4 | Computational investigations
Taking these experimental observations into consideration, a
tentative mechanism is proposed (Scheme 3). The thermody-
namic values were computed for each step, considering
Bu2SnO and polydimethoxysiloxane as oligomers;[15] the
monomers are not present under these conditions. The calcu-
lations were mostly limited to tetra‐coordinated species
although empty d‐orbitals of silicon and tin allow for higher
coordinated species. The formation of oligomers with penta‐
coordinated siloxanes and stannanes reduces the free
enthalpy for some of the steps. (Pseudo‐)penta‐coordinated
species could also be involved in transition states to decrease
the activation barrier of some steps. As proposed above, the
reaction starts with the transfer of an alkoxy moiety from
the orthosilicate to activate the poorly reactive dialkyltin or
diphenyltin oxides to form silicon–tin species 5. The assis-
tance of CO2 most likely reduces the activation barrier for
this step, and the relative stability of 5 accounts for a
favourable transformation. The reaction proceeds via the
transfer of another methoxy group from silicon onto tin,
yielding dibutyltin dimethoxide and polydimethoxysiloxane
(step B, ΔG = 5.1 kJ mol−1).
SCHEME 2 Rationale for the formation of 5 by activation of polymeric
Bu2SnO
Scheme 2. The driving force of this last transformation is the
release of CO2 and polymeric Bu2SnO which reacts again in
a new cycle.
Stoichiometric NMR studies were conducted with and
without CO2 pressure with various amounts of silicates and
tin oxide to detect some of the species suggested above.[15,18]
An equimolar mixture of Bu2SnO and tetramethyl
orthosilicate was heated at 135 °C for 5 h in the presence of
CO2 (70 bar). The observations we made were in accordance
with those reported by Ballivet‐Tkatchenko et al.,[18] who
performed experiments for the investigation on dialkyl car-
bonate formation in methanol with silicates and CO2 using
dibutlytin oxide as catalyst. Two signals for carbonyl function
were observed in the 13C NMR spectrum at 162.8 and
162.6 ppm indicating the possible presence of 4 and
dimethylmethoxytin methylcarbonate as a result of step A
and B (see below). When the ratio of tin and silicon is the
same as that of Table 2, the excess of orthosilicate allows
one to conclude that two organosilicates coexist during the
reaction (29Si; δ = −78.7, −85.9 ppm). The starting material,
tetramethyl orthosilicate, was identified but the second chem-
ical shift can be attributed to either 4 or 5. Two‐dimensional
experiments indicated the presence of Sn─OMe bonds which
was concluded from the correlation between methoxy groups
and the butyl chain. Moreover, even if the quantity of
dimethyltin dimethoxide was too low to be detected, two res-
onance forms are visible in the area of penta‐coordinated tin
derivatives suggesting the formation of tin carbonates – fur-
ther confirmed by 13C NMR (156.2 ppm). Ultimately, the
title reaction was monitored using NMR. With or without
Although the formation of 6 was expected to entropically
favour the formation of 7, the stability of 5 is shown by a high
enthalpic contribution in step A yielded in a slightly
disfavoured step. This could be compensated by an exergonic
CO2 insertion into
a
Sn─OMe bond.
A
Gibbs
energy of −3.7 kJ mol−1 was found for B. Experimentally,
the formation of 8 is more stabilized due to a favourable
dimerization in non‐coordinative solvents. The tin carbonate
8 then undergoes a nucleophilic attack of aniline to
produce methylphenyl carbamate and the reactive
hydroxymethoxydibutyl tin species 9. The carbamation step
1
aniline, the same signals were observed in H NMR, 13C
NMR, 29Si NMR and 119Sn NMR spectra. Overall, qualita-
tive NMR measurements shed light on the activation of
Bu2SnO in the course of the carbamation. Additionally, a
new silicon signal was detected as well as those of tin carbon-
ate derivatives.
SCHEME 3 Proposed mechanism for the carbamation of aniline. Energies
are given in italics (kJ mol−1
)

6 of 10
GERMAIN ET AL.
C is the most endergonic step in the process but the formation
of the unstable hydroxystannane intermediate 9 (ΔG = 21.3 kJ
mol−1) is then compensated by D and F. Indeed, 9 can then
either regenerate Bu2SnO through loss of methanol as in step
D (ΔG = −52.3 kJ mol−1), forming a very stable tetramer in
which Sn is penta‐coordinate with 7, or directly react with
orthosilicate to form Si/Sn species 5 (step F, ΔG = −19.7 kJ
mol−1). The Si–Sn species 5 can also be obtained through
step E, although breakup of the penta‐coordinate tetramer to
release Bu2SnO is naturally endergonic.
AVANCE III 500 and Bruker AVANCE III 600 spectrome-
ters at the Organisch‐Chemisches Institut der Universität
Heidelberg. Chemical shifts δ are given in ppm referenced
to solvent (1H, 13C, 29Si, 119Sn) and NMR abbreviations used
are s (singlet), d (doublet), t (triplet), br (broad). Coupling
constants are reported in hertz (Hz). NMR spectra were mea-
sured in toluene‐d8 (2.08 ppm for 1H, PhCH3; and 20.43 ppm
for 13C, PhCH3), dichloromethane‐d2 (5.32 ppm for 1H,
CH2Cl2; and 53.84 ppm for 13C, CH2Cl2) or CDCl3
1
(7.26 ppm for H, CHCl3; and 77.16 ppm for 13C, CHCl3).
In any case, both overall steps are exergonic, closing the
catalytic cycle featuring a cooperative Si–Sn species as the
key intermediate for methoxylation of the dialkyltin oxide.
Judged by the free enthalpies of the intermediates involved,
this is assumed to be a viable pathway that is supported
through coordination of various sized oligomers, stabilizing
the above species, but not participating at the reactive site
of the intermediates themselves.
Gas chromatography data were collected using an Agilent
6890 N modular GC base equipped with a split‐mode capil-
lary injection system and flame ionization detector using a
standard HP‐5 capillary column (30 m × 0.32 mm × 0.25 mm;
He flow: 2.0 ml min−1; programme: 50 °C (1 min), 20 °C
min−1 to 250 °C (10 min)). HPLC data were collected with
an HPLC Chiralpak IB using a hexane–isopropanol mixture.
Mass spectrometry was performed with an MS‐Labor,
Organisch‐Chemisches Institut der Universität Heidelberg.
Elemental analyses were performed at the Mikroanalytisches
Laboratorium der Chemischen Institute der Universität
Heidelberg.
3
| CONCLUSIONS
Carbamate synthesis from aromatic and aliphatic amines
using CO2 was successfully achieved in isolated yields of
up to 97%. The reaction is promoted by a cooperative
methoxy transfer from tetraalkyl orthosilicates to dialkyltin
oxides in which the role of CO2 was evaluated. An exhaustive
transfer of two methoxy moieties was achieved to generate
dialkyltin dialkoxides which insert CO2 and promote the car-
bamate formation. Hammett correlation, NMR studies and
the calculation of thermodynamic parameters of the reaction
converged to a tentative catalytic cycle.
4.2 | Computational details
Geometries were optimized at the B3LYP/def2‐TZVPP
level of theory.[19–21] Final electronic energies were com-
puted at the B3LYP‐D3(BJ)/def2‐TZVPP level using
dispersion correction[22] with Becke–Johnson damping.[23]
Electronic structure calculations were carried out with
TURBOMOLE,[24] employing the resolution‐of‐identity
approximation[25] and the corresponding auxiliary basis
sets.[26,27] Zero‐point vibrational energies and thermochemi-
cal corrections were computed in the gas phase within the
usual harmonic‐oscillator, rigid‐rotor approximation at the
B3LYP/def2‐TZVPP level for T = 408.15 K and p = 1 bar
with TURBOMOLE's freeh program. Symmetry numbers
for the quasi‐classical rotational partition sum were chosen
based on the symmetry of each molecule. All positive vibra-
tional frequencies were included in the calculation of the
vibrational partition function. Free enthalpies of solvation
for each compound at infinite dilution in toluene were
obtained at the BP86/def‐TZVP level through COSMO‐
RS[28] (conductor‐like screening model for realistic solvents)
using COSMOtherm[29] (Version C3.0, Release 1501). The
thermodynamic reference point for each species was a molar
fraction of χ = 0.1, except for gaseous CO2, where the
reference point was shifted to p = 70 bar. The conformational
partition function was set to one, so that only the energeti-
cally lowest conformer was considered for each species. For
larger molecules, a broad conformational search was con-
ducted considering possible rotations around single bonds
and the best conformer at the BP86/def‐TZVP/COSMO level
(with ε = ∞) was chosen.
4
| EXPERIMENTAL
4.1 | General remarks
All experiments were carried out under an atmosphere of dry
argon using standard Schlenk techniques or in an MBraun
glovebox. Solvents were dried over appropriate drying agents,
distilled under an atmosphere of argon, purchased dry from
Aldrich, or dispensed from an MBraun SPS‐800 solvent
system. Solvents were stored over either sodium ingots or
molecular sieves (3 Å). Solvents were degassed via three
freeze–pump–thaw cycles, argon bubbling or vacuum transfer.
Dimethyltin(IV) oxide, dibutyltin(IV) oxide, diphenyltin(IV)
oxide, tetramethyl orthosilicate, tetraethyl orthosilicate,
trimethylmethoxysilicate, dimethyldimethoxysilicate and
methyltrimethoxysilicate were purchased from Aldrich,
ABCR or TCI and used without any further purification. Ani-
line was distilled and stored in a glovebox and protected from
light. Methanol was distilled over potassium carbonate and
stored in a glovebox under molecular sieves. NMR spectra
were recorded using a Bruker 200 at CaRLa or Bruker
AVANCE III 300, Bruker AVANCE III 400, Bruker

GERMAIN ET AL.
7 of 10
4.3 | General procedure for synthesis of carbamates
4.4.3 | N‐(4‐Methylphenyl)methylcarbamate (2c)
In a glovebox, a tall Premex stainless autoclave was
charged with amine (1 equiv., 2 mmol), tetramethyl
orthosilicate (2 equiv., 4 mmol), dibutyltin oxide (0.2 equiv.,
0.4 mmol) and toluene (10 ml). The autoclave was sealed and
pressurized outside the glovebox with CO2 (50 bar). After
stirring for 10 min, the pressure dropped to 40 bar. The auto-
clave was heated to 135 °C (70 bar). After 5 h, the autoclave
was depressurized and opened to air. The crude mixture was
concentrated to dryness and dissolved in ethyl acetate. Tin
and silicon residues were filtered off. The crude mixture
was then washed with 1 M HCl to remove the unreacted
amine, sodium bicarbonate and brine, and concentrated for
analysis. If some dialkyltin residues remained, another filtra-
tion over silica gel with dichloromethane was performed.
The general procedure was followed using p‐toluidine (1
equiv., 214.3 mg, 2 mmol). The desired product was obtained
pure as a white solid (330.4 mg, 85% yield). 1H NMR
(600 MHz, CD2Cl2, δ, ppm): 7.27 (d, J3 = 16.4 Hz, Har,
2H), 7.13 (d, J3 = 16.4 Hz, Har, 2H), 6.70 (bs, NH, 1H),
3.74 (s, NHCO2CH3, 3H), 2.30 (s, CH3, 3H). 13C NMR
(133 MHz, CD2Cl2, δ, ppm): 154.0, 136.3, 129.2, 128.6,
120.1, 52.5. Elemental analysis: calcd (%) for C8H8ClNO2
(185.0): C 65.44, H 6.71, N 8.48; found (%): C 65.66, H
6.75, N 8.33. MS (EI): m/z (%): calcd 165.0790; found
165.0782.
4.4 | Synthesis of mixed aliphatic–aromatic carbamates
4.4.4
| N‐Naphthylmethylcarbamate (2d)
4.4.1
| N‐Phenylmethylcarbamate (2a)
Thegeneralprocedurewasfollowedusinganiline (1equiv.,
186.2 mg, 2 mmol). The desired product was obtained pure
as a white solid (300 mg; 98% yield). H NMR (600 MHz,
The general procedure was followed using 1‐naphthyl-
amine (1 equiv., 286.1 mg, 2 mmol). The desired product
1
1
was obtained pure as a white solid (324 mg, 81% yield). H
CD2Cl2, δ, ppm): 7.39 (d, J3 = 7.6 Hz; Har, 2H), 7.33 (t,
J3 = 7.6 Hz, Har, 2H), 7.08 (dt, J3 = 7.6; 1.2 Hz, Har,
1H), 6.70 (bs, NH, 1H), 3.75 (s, NHCO2CH3, 3H). 13C
NMR (133 MHz, CD2Cl2, δ, ppm): 154.4, 138.6, 129.4
(2C), 123.7 (2C), 119.1, 52.6. Elemental analysis: calcd
(%) for C6H9NO2 (151.1): C 63.56, H 6.00, N 9.27; found
(%): C 63.55, H 5.92, N 9.10. MS (EI): m/z (%): calcd
151.0633; found 151.0626.
NMR (600 MHz, CD2Cl2, δ, ppm): 7.92–7.88 (m, Har, 2H),
7.83 (d, J3 = 7.2 Hz, Har, 1H), 7.71 (d, J3 = 8.4 Hz, Har,
1H), 7.56–7.46 (m, Har, 3H), 7.04 (bs, NH, 1H), 3.81 (s,
NHCO2CH3, 3H). 13C NMR (133 MHz, CD2Cl2, δ, ppm):
155.4, 134.6, 133.2, 129.0, 126.6, 126.5, 126.1, 125.5,
121.1, 52.8. Elemental analysis: calcd (%) for C12H11NO2
(201.1): C 71.63, H 5.51, N 6.96; found (%): C 71.60, H
5.65, N 7.01. MS (EI): m/z (%) calcd 201.0790; found
201.0783.
4.4.2
| N‐(4‐Methoxyphenyl)methylcarbamate (2b)
4.4.5
| N‐(4‐Fluorophenyl)methylcarbamate (2e)
The general procedure was followed using 1‐naphthyl-
amine (1 equiv., 246.3 mg, 2 mmol). The desired product
was obtained pure as a white solid (256 mg, 71% yield).
11H NMR (600 MHz, CD2Cl2, δ, ppm): 7.27 (d,
J3 = 16.4 Hz, Har, 2H), 7.13 (m, J3 = 16.4 Hz, Har, 2H),
6.70 (bs, NH, 1H), 3.74 (s, OCH3, 3H), 2.30 (s, CH3, 3H).
13C NMR (133 MHz, CD2Cl2, δ, ppm): 154.5, 135.9,
133.4, 129.8 (3C), 119.2, 52.7, 20.8. Elemental analysis:
calcd (%) for C9H11NO3 (181.1): C 59.66, H 6.12, N 7.73;
found (%): C 59.90, H 6.15, N 7.73. MS (EI): m/z (%): calcd
181.0739; found 181.0736.
The general procedure was followed using 4‐fluoroamine
(1 equiv., 222.2 mg, 2 mmol). The desired product was
obtained pure as a white solid (204 mg, 60% yield). 1H
NMR (600 MHz, CD2Cl2, δ, ppm): 7.38–7.33 (m, Har, 2H),
7.05–6.97 (m, Har, 2H), 6.76 (bs, NH, 1H), 6.48 (bs, NH,
1H), 3.74 (s, NHCO2CH3, 3H). 13C NMR (133 MHz,
CD2Cl2, δ, ppm): 160.0 (q, J4 = 212.8 Hz), 154.5, 134.6,
115.9 (2C), 115.8 (2C), 52.7. 19F NMR (282 MHz, CD2Cl2,
δ, ppm): −120.5. Elemental analysis: calcd (%) for
C8H8FNO2 (169.1): C 56.80, H 4.77, N 8.28; found (%): C

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GERMAIN ET AL.
54.94, H 4.79, N 7.27. MS (EI): m/z (%) calcd 169.0539;
4.4.9 | N‐4‐Nitrophenylmethylcarbamate (2i)
found 169.0539.
4.4.6
| N‐(4‐Chlorophenyl)methylcarbamate (2f)
The general procedure was followed using 4‐nitroaniline
(1 equiv., 276.3 mg, 2 mmol) for 13 h. The desired product
1
was obtained pure as a white solid (118 mg, 30% yield). H
The general procedure was followed using 4‐
chloroaniline (1 equiv., 255.1 mg, 2 mmol). The desired
product was obtained pure as a white solid (280 mg, 76%
yield). 1H NMR (600 MHz, CDCl3, δ, ppm): 7.34 (s,
J3 = 11.8 Hz, Har, 2H), 6.92 (dt, J3 = 12; 4.4 Hz, Har, 2H),
6.66 (bs, NH, 1H), 3.83 (s, NHCO2CH3, 3H), 3.80 (s,
OCH3, 3H). 13C NMR (133 MHz, CDCl3, δ, ppm): 154.0,
136.3, 129.2, 128.6, 120.1, 52.5. Elemental analysis: calcd
(%) for C8H8ClNO2 (185.0): C 51.77, H 4.34, N 7.55; found
(%): C 52.16, H 4.55, N 7.48. MS (EI): m/z (%) calcd
185.0244; found 185.0233.
NMR (600 MHz, CD2Cl2, δ, ppm): 8.20 (dt, J3 = 19.8;
6.0 Hz, Har, 2H), 7.60 (dt, J3 = 19.8; 6.0 Hz, Har, 2H),
7.16 (bs, NH, 1H), 3.80 (s, NHCO2CH3, 3H). 13C NMR
(133 MHz, CD2Cl2, δ, ppm): 153.7, 144.6, 143.4, 125.5,
118.1, 53.2. MS (EI): m/z (%) calcd 196.0484; found
194.0479.
4.4.10
| N‐(3‐Chlorophenyl)methylcarbamate (2j)
4.4.7
| N‐(4‐Iodophenyl)methylcarbamate (2g)
The general procedure was followed using 3‐chloroamine
(1 equiv., 255.0 mg, 2 mmol). The desired product was
obtained pure as a white solid (284 mg, 77% yield). 1H
NMR (600 MHz, CD2Cl2, δ, ppm): 7.53 (s, Har, 1H), 7.24–
7.23 (m, Har, 2H), 7.06–7.03 (m, Har, 1H), 6.87 (bs, NH,
1H), 3.76 (s, NHCO2CH3, 3H). 13C NMR (133 MHz,
CD2Cl2, δ, ppm): 154.2, 139.9, 135.0, 130.5, 131.0, 123.7,
118.9, 117.1, 52.8. Elemental analysis: calcd (%) for
C8H8ClNO2 (185.0): C 51.77, H 4.34, N 7.55; found (%):
C 52.07, H 4.43, N 7.58. MS (EI): m/z (%) calcd 185.0244;
found 185.0255.
The general procedure was followed using 4‐iodoaniline
(1 equiv., 438.0 mg, 2 mmol) during 10 h. The desired prod-
uct was obtained pure as a white solid (418 mg, 75% yield).
1H NMR (600 MHz, CD2Cl2, δ, ppm): 7.62 (dt, J3 = 14.4;
4.2 Hz, Har, 2H), 6.92 (d, J3 = 13.2 Hz, Har, 2H), 6.76 (bs,
NH, 1H), 3.75 (s, OCH3, 3H). 13C NMR (133 MHz, CD2Cl2,
δ, ppm): 154.2, 138.5, 138.3, 120.9, 52.8. Elemental analysis:
calcd (%) for C8H8ClNO2 (185.0): C 34.68, H 2.91, N 5.06;
found (%): C 34.71, H 2.90, N 5.09. MS (EI): m/z (%) calcd
185.0244; found 185.0233.
4.4.11
| N‐(2‐Bromophenyl)methylcarbamate (2k)
4.4.8
| N‐(4‐(Trifluoromethyl)phenyl)methylcarbamate (2h)
The general procedure was followed using 2‐
bromoaniline (1 equiv., 350.0 mg, 2 mmol). The desired
product was obtained in an inseparable mixture with starting
material (207 mg, 45% yield). 1H NMR (600 MHz, CD2Cl2,
δ, ppm): 8.13 (dd, J3 = 16.4, 3.0 Hz, Har, 1H), 7.55 (dd,
J3 = 16.4, 3.0 Hz, Har, 1H), 7.35 (dt, J3 = 15.6, 3.0 Hz,
Har, 2H), 7.15 (bs, NH, 1H), 6.98 (dt, J3 = 15.6, 3.0 Hz,
Har, 2H), 3.79 (s, NHCO2CH3, 3H). 13C NMR (133 MHz,
CD2Cl2, δ, ppm): 154.0, 136.4, 132.7, 128.8, 124.7, 120.7,
113.1, 52.9. Elemental analysis: calcd (%) for C8H8BrNO2
(185.0): C 41.77, H 3.51, N 6.09; found (%): C 42.07, H
3.48, N 6.07. MS (EI): m/z (%) calcd 228.9733; found
228.9768.
The general procedure was followed using 4‐
(trifluoromethyl)aniline (1 equiv., 322.3 mg, 2 mmol). The
desired product was obtained pure as a white solid
(360 mg, 82% yield). 1H NMR (600 MHz, CD2Cl2, δ,
ppm): 7.58 (m, Har, 4H), 6.96 (bs, NH, 1H), 3.78 (s,
NHCO2CH3, 3H). Dirty NMR. 13C NMR (133 MHz,
CD2Cl2, δ, ppm): 154.1, 141.9, 126.8 (q, J4 = 4 Hz), 125.8
(q, J4 = 32 Hz), 118.5, 114.5, 52.9. 19F NMR (282 MHz,
CD2Cl2, δ, ppm): −62.3. Elemental analysis: calcd (%) for
C9H8F3NO2 (219.1): C 49.32, H 3.68, N 6.39; found (%):
C 49.69, H 3.69, N 6.38. MS (EI): m/z (%) calcd 219.0502;
found 219.0504.

GERMAIN ET AL.
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4.4.12
|
N‐Phenylethylcarbamate (2l)
4.5.3 | N‐Butylmethylcarbamate (2o)
The general procedure was followed using tetraethyl
orthosilicate (2equiv., 833.2mg, 4mmol)and aniline (1equiv.,
186.2 mg, 2 mmol). The desired product was obtained pure
The general procedure was followed using butylamine
(1 equiv., 146.3 mg, 2 mmol). The desired product was
obtained pure as a colourless oil (135 mg, 51% yield). H
1
1
as a white solid (112 mg, 68% yield). H NMR (600 MHz,
NMR (600 MHz, CD2Cl2, δ, ppm): 5.12 (bs, NH, 1H),
3.60 (s, OCH3, 3H), 3.14 (dd, J3 = 13.2; 6.4 Hz,
CH2NHCO2CH3, 2H), 1.48–1.41 (m, CH2CH2NHCO2CH3,
2H), 1.36–1.29 (m, CH2CH2CH2NHCO2CH3, 2H), 0.92 (t,
J3 = 7.2 Hz, CH3, 3H). 13C NMR (133 MHz, CD2Cl2, δ,
ppm): 157.6, 52.0, 41.2, 32.6, 20.3, 13.9. Elemental analy-
sis: calcd (%) for C6H13NO2 (131.1): C 54.94, H 9.99, N
10.68; found (%): C 55.15, H 9.74, N 10.55. MS (EI): m/
z (%) calcd 131.0946; found 131.0941.
CD2Cl2, δ, ppm): 7.43–7.07 (m, Har, 2H), 7.5–7.30 (m, 2H),
7.10–7.06 (m, 1H), 6.83 (bs, NH, 1H), 4.26 (t, J3 = 10.8 Hz,
HEt, 2H), 1.34 (q, J3 = 10.8 Hz, HEt, 3H). 13C NMR
(133 MHz, CD2Cl2, δ, ppm): 153.5, 138.3, 128.9, 123.1,
118.6, 61.1, 14.3. Elemental analysis: calcd (%) for
C9H11NO2 (165.2): C 65.44, H 6.71, N 8.48; found (%): C
65.10, H 6.63, N 8.63. MS (EI): m/z (%): calcd 151.0633;
found 151.0626.
ACKNOWLEDGMENTS
4.5 | Synthesis of aliphatic carbamates
CaRLa (Catalysis Research Laboratory) is co‐financed by the
Ruprechts‐Karls‐University Heidelberg (Heidelberg Univer-
sity) and BASF SE. The authors are grateful to Prof. Shannon
Stahl for helpful discussions. The authors thank Dr Jürgen
Graf and the NMR service of the Organisch‐Chemisches
Institute, Universität Heidelberg for discussions and NMR
measurements.
4.5.1
| N‐Benzylmethylcarbamate (2m)
Following the general procedure, benzylamine (1 equiv.,
214.3 mg, 2 mmol) was used to afford the product as a white
solid (271 mg, 82% yield). H NMR (600 MHz, CD2Cl2, δ,
ppm): 7.34–7.27 (m, Har, 5H), 5.05 (bs, NH, 1H), 4.38 (d,
J3 = 6 Hz, CHbenz, 2H), 3.70 (s, CH3, 3H). 13C NMR
(133 MHz, CD2Cl2, δ, ppm): 157.1, 138.5, 128.7 (2C),
128.6 (2C), 127.5, 52.5, 45.1. Elemental analysis: calcd (%)
for C9H11NO2 (165.0): C 65.44, H 6.71, N 8.48; found (%):
C 65.68, H 6.67, N 8.45. MS (EI): m/z (%) calcd 165.0790;
found 165.0786.
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How to cite this article: Germain N, Hermsen M,
Schaub T, Trapp O. Synthesis of carbamates from car-
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