Trapping of carbamic acid species with (trimethylsilyl)diazomethane

Article abstract of DOI:10.1016/j.tet.2005.09.116

Methoxycarbonylation of a variety of amines into the corresponding methyl carbamates was accomplished by allowing them to react with (trimethylsilyl) diazomethane TMSCHN₂ under bubbling of CO₂. The reaction was performed at room temperature for a period of ca. 2 h in benzene-MeOH (4/1 v/v), which was the solvent of choice. In this mixed solvent, undesirable bicarbonate is formed in equilibrium along with carbamate anion. Owing to the irreversibility in the esterification step by TMSCHN₂, however, the yield of methyl carbamate can reach very high.

Full text of DOI:10.1016/j.tet.2005.09.116

Tetrahedron 62 (2006) 226–235
Trapping of carbamic acid species with
(trimethylsilyl)diazomethane
*
Yoshikatsu Ito and Hiromi Ushitora
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Kyoto 615-8510, Japan
Received 18 August 2005; accepted 21 September 2005
Available online 20 October 2005
Abstract—Methoxycarbonylation of a variety of amines into the corresponding methyl carbamates was accomplished by allowing them to
react with (trimethylsilyl)diazomethane TMSCHN2 under bubbling of CO2. The reaction was performed at room temperature for a period of
ca. 2 h in benzene–MeOH (4/1 v/v), which was the solvent of choice. In this mixed solvent, undesirable bicarbonate is formed in equilibrium
along with carbamate anion. Owing to the irreversibility in the esterification step by TMSCHN2, however, the yield of methyl carbamate can
reach very high.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
benzene or CHCl3 (apolar, aprotic solvent), or in 2-PrOH or
MeOH (dipolar, amphiprotic solvent), ammonium carba-
mates rather than carbamic acids are formed, although the
ammonium bicarbonates/carbonates are competitively
formed in hydrous MeOH. Not only solvent polarity and
protic character but also its protophilicity was found to be a
crucial factor. The selective generation of the undissociated
carbamic acids in preference to the ammonium carbamates
in protophilic, dipolar, aprotic solvent (DMSO, DMF,
pyridine, or dioxane) was ascribed to the larger pKa values3
for the carbamic acids than for the amines 1a–c in this class
of solvent, that is, the equilibrium reaction 1 is shifted far to
the left.
We and others have recently reported studies on the solvent
dependence of the carbamic acid formation from u-(1-
naphthyl)alkylamines 1a–c and carbon dioxide.1,2 On the
basis of NMR and IR analyses in situ after bubbling of CO2
through solutions of these amines in DMSO, DMF or
pyridine, it is concluded that they are completely converted
to the corresponding carbamic acids in the protophilic,
highly dipolar, aprotic solvents (see Scheme 1).1 In dioxane
(protophilic, dipolar, aprotic solvent), the carbamic acid and
a small amount of the ammonium carbamate are formed. By
contrast, in MeCN (protophobic, dipolar, aprotic solvent), in
Scheme 1.
Keywords: Amine; Carbon dioxide; Carbamic acid; (Trimethylsilyl)diazomethane; Methyl carbamate.
*
Corresponding author. Tel./fax: C81 75 383 2718; e-mail: yito@sbchem.kyoto-u.ac.jp
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.09.116

Y. Ito, H. Ushitora / Tetrahedron 62 (2006) 226–235
227
solvents were added at room temperature 2–3equiv of
TMSCHN2 (an Aldrich 2.0 M solution in hexanes was used)
with stirring under bubbling of CO2. In less than half an
hour, a yellow color of TMSCHN2 disappeared and the
corresponding methyl carbamates (MC’s) resulted with
excellent yields (83–100%) when benzene–MeOH (4/1) or
dioxane–MeOH (4/1) was used (Table 1).‡ However, the
yields were much lower in the solvent like pyridine–MeOH
(4/1 or 9/1), DMF–MeOH (4/1) or MeOH (22–61%). In the
case of pyridine–MeOH, formation of a large amount of
brown tar greatly impaired the reaction. When pure MeOH
was used, TMSCHN2 decomposed more rapidly as
compared with the benzene–MeOH system. This is
probably due to some side reactions. In DMF–MeOH,
formamides 6 and 7 were obtained as additional products
from 1a and 2a, respectively. From these results, benzene–
MeOH (4/1) was employed as the solvent of choice for
further experiments.
(1)
The ammonium carbamates often precipitated out from the
solution and hence they could be separated. In order to trap
unstable carbamic acids as the stable esters, we reacted 3-(1-
naphthyl)propylamine (1a) in 4:1 v/v benzene/MeOH with
(trimethylsilyl)diazomethane TMSCHN2 under bubbling of
CO2.1a The corresponding methyl carbamate (1a-MC) was
successfully obtained in essentially quantitative yield
(Eq. 2). TMSCHN2 is a well-known reagent for methylation
of carboxylic acids.4 In this paper, application of this
N-methoxycarbonylation reaction to other amines such as
1–5 is investigated. Lately, amine has been utilized to fix
carbon dioxide through various types of chemical reaction.5
An example is the formation of carbamate ester (urethane)
by using alkyl halide,6 alcohol,7 carbonate ester,8 alkene,9
alkyne,9 or epoxide9 as the alkylating agent, but TMSCHN2
had not been employed to this aim before our previous
report.1a
Solutions of amines 1–5 in 4:1 benzene–MeOH were treated
with a TMSCHN2 solution under bubbling of CO2, as
described above. As summarized in Table 2, primary alkyl
amines 1a–c, 2a–d, 3a, 3b and secondary alkylamine 4
afforded their methyl carbamates (MC’s) in excellent to
good yields (76–100%). MC was obtained even from
dopamine (2e), an important neurotransmitter in brain,10 in
(2)
2. Results and discussion
a modest yield (17%) by using dopamine$3,5-dinitro-
benzoic acid salt (2e$dnba)11 as the starting amine. This
methoxycarbonylation reaction is interesting, because 2e
itself is easily susceptible to oxidative polymerization to
generate melanin, a structurally unelucidated black pig-
ment.11,12 Similar treatment of commercially available
dopamine hydrochloride 2e$HCl with TMSCHN2 gave a
very low yield of the crude MC (6%). MC was also obtained
from DL-noradrenaline (2f) in 12% yield. One reason for the
low yields of MC from catecholamines 2e and 2f is that their
reactions had to be carried out in 4:1 pyridine/MeOH, since
2.1. Methoxycarbonylation with TMSCHN2
First, we looked for a suitable solvent for the N-methoxy-
carbonylation reaction. Since, according to Shioiri and
co-workers,4 methanol is indispensable for methylation of
acids with TMSCHN2, we carried out the reaction in
benzene, dioxane, pyridine or DMF, each containing 20 (or
10) volume % of MeOH, or in pure MeOH.† Thus, to
solutions of amines 1a, 2a, 2c, 3a, and 5a in one of these
† The possibility that the Me group comes from methanol in the
methylation reaction was excluded by Shioiri by using methanol–13C.4
‡ The use of 1 equiv of TMSCHN2 instead of 2 equiv decreased the yield of
2a-MC from 97% (Table 1) to 78% in benzene–MeOH (4/1).

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Y. Ito, H. Ushitora / Tetrahedron 62 (2006) 226–235
Table 1. Solvent effect on methoxycarbonylation of several amines with (trimethylsilyl)diazomethane under atmospheric pressure of CO2 at room temperature
Amine
Solvent (v/v)
Product yield (%)a
Others
Recovered amine (%)b
Methyl carbamate
1a
1a
1a
1a
1a
1a
2a
2a
2c
2c
3a
3a
5a
5a
Benzene–MeOH (4/1)
Dioxane–MeOH (4/1)
Pyridine–MeOH (9/1)
Pyridine–MeOH (4/1)
DMF–MeOH (4/1)
MeOH
Benzene–MeOH (4/1)
DMF–MeOH (4/1)
Benzene–MeOH (4/1)
MeOH
99#
98#
42#
40
Trace
0
51
51
0
55
0
0
c
c
22#
38#
97#
25
6, 52
7, 7
100#
29
0
45
10
Trace
2
Benzene–MeOH (4/1)
Pyridine–MeOH (4/1)
Benzene–MeOH (4/1)
Pyridine–MeOH (4/1)
83#
61
c
c
98 (97#)
32
50
a Isolation yield, unless marked with #, where it is the NMR yield. Values are based on the initial amount of amine.
b Estimated by NMR.
c A large amount of brown tar was formed.
2e$dnba, 2e$HCl and 2f are insoluble in 4:1 benzene/
MeOH.
(5a-MC) in an excellent yield (98%). The MC yields from
electron-rich p-anisidine (2j) and N,N-dimethyl-p-phenyl-
enediamine (2k) (27 and 19%, respectively) were better
than aniline (2g) (8%).
Other significant points found from inspection of Table 2
are that methoxycarbonylation reactions of arylamines 1d,
2g, 2h, 2j, 2k, and 5b are difficult (0–27% yields), although
indoline (5a), which is an N-alkyl-substituted arylamine
whose nitrogen atom is expected to be sterically less
hindered than that of 5b or 2h, gave its methyl carbamate
Alcohols react with CO2 to form alkylcarbonic acids in the
presence of tertiary amine bases under high-pressure or in
the presence of 1,8-diazabicyclo-[5.4.0]-undec-7-ene
(DBU).13 Under our reaction conditions, however,
Table 2. Methoxycarbonylation of various amines with (trimethylsilyl)diazomethane under atmospheric pressure of CO2 at room temperature
Amine
Product yield (%)a
Others
Recovered amine (%)b
Methyl carbamate
Primary alkylamine: RNH2/RNH–CO2Me
1a
1b
1c
2a
2b
2c
2d
2ec
2fg
3a
3b
99#
95
Trace
0
0
0
7
0
95
97#
88
100#
82
0
e,f
d
d
17
12
15
10
83#
76
14h
Secondary alkylamine: RR0NH/RR0N–CO2Me
4
78
3
Arylamine: ArRNH/ArRN–CO2Me
1d
2g
2h
2j
0
8
0
27
1e, 13
2i, 7
82
e,i
O75i
52
2k
5a
5b
19
98 (97#)
24 (40#)
66
2
57
The solvent employed is benzene–MeOH (4/1 v/v) except for 2e and 2f.
a Isolation yield, unless marked with #, where it is the NMR yield. Values are based on the initial amount of amine.
b Estimated by NMR.
c Dopamine$3,5-dinitrobenzoic acid salt (2e$dnba)11 dissolved in 4:1 pyridine/MeOH was reacted (2e$dnba is insoluble in 4:1 benzene/MeOH). The reaction
of dopamine hydrochloride (2e$HCl) under the same conditions gave a lower yield of MC (6%).
d A large amount of brown tar was formed.
e Not estimated.
f
Polymerized to melanin during the workup.
g The reaction was carried out in 4:1 pyridine/MeOH, since DL-noradrenaline (2f) is insoluble in 4:1 benzene/MeOH.
h Isolated.
i
The recovered amine was partially lost on evaporation of the solvent.

Y. Ito, H. Ushitora / Tetrahedron 62 (2006) 226–235
229
Figure 1. The 1H and 13C NMR spectra of 1b in 4:1 C6D6/CD3OD before and after CO2 bubbling.
Figure 2. The HMBC spectrum of 1b in 4:1 C6D6/CD3OD after CO2 bubbling.

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Y. Ito, H. Ushitora / Tetrahedron 62 (2006) 226–235
Figure 3. The IR spectra of 1b in 4:1 benzene/MeOH before and after CO2 bubbling.
methylcarbonic acid CH3OCOK2 was probably not formed
from the following reasons. Dimethyl carbonate, which is a
possible product from the reaction of methylcarbonic
acid with TMSCHN2, was undetectable (NMR) in the
reaction mixtures. Furthermore, the signals corresponding
to CD3OCOK2 were none by the in situ 13C NMR
experiments in C6D6–CD3OD or in CD3OD (Section 2.2
and Ref. 1a).
between ammonium carbamate and ammonium bicarbonate
is estimated as 54:46. The carbamyl carbon appeared at d
163.5 and the bicarbonate carbon at d 160.9 (Fig. 1B, right).
In the HMBC spectrum (Fig. 2), there is an expected cross
peak between the a-methylene proton and the carbamyl
carbon, demonstrating the N– Cbond formation between the
amine nitrogen and the CO2 carbon. The IR spectrum
(Fig. 3, bottom) showed a broad band at about 1580 cmK1
,
which is assignable to the NHCOOK group of the carbamate
anion.14 The bands at 1640 and 1312 cmK1 should be
attributed to bicarbonate.1a For comparison, the pertinent
spectra in DMSO and DMF are displayed in Figures 4–6. As
reported previously,1 1b was quantitatively converted to the
carbamic acid (1-Naph)CH2CH2NHCO2H in DMSO, DMF
or pyridine (e.g., as can be seen from the NMR spectra in
Figures 4 and 5).
2.2. Carbamate anion formation in 4:1 benzene/MeOH
When we previously studied the solvent dependence of
carbamic acid formation,1 we did not investigate the
benzene–MeOH mixed solvent. Therefore, by selecting
2-(1-naphthyl)ethylamine (1b) as a typical example, the 1H
and 13C NMR, HMBC, and IR spectra in 4:1 benzene–
MeOH before and after bubbling CO2 were measured in situ
(Figs. 1–3). It can be seen from these figures that a mixture
of ammonium carbamate and bicarbonate was predomi-
2.3. Mechanistic consideration
1
nantly formed in this mixed solvent. From the H NMR
spectrum after CO2 bubbling (Fig. 1A, right), the molar ratio
Interconversion reactions among amine, carbamic acid,
ammonium carbamate, and ammonium bicarbonate (formed

Y. Ito, H. Ushitora / Tetrahedron 62 (2006) 226–235
231
if H2O is present as a contaminant in MeOH; see Scheme 1)
3. Conclusion
are reversible processes,5,15 whereas the formation of
methyl carbamate (MC) is irreversible (Eqs. 3–6). It is
assumed that TMSCHN2 can esterify either undissociated or
dissociated carbamic acid (Eq. 6). As aforementioned, upon
CO2 bubbling through the solution of 1b in 4:1 benzene/
MeOH, formation of both ammonium carbamate and
bicarbonate (molar ratio 54:46) but no carbamic acid was
observed (Figs. 1–3). Of course, there may exist a low
concentration of the carbamic acid below the detection limit
of the NMR and IR analysis. Apart from the ambiguity
about the true species involved in the esterification,
however, production of the methyl carbamate 1b-MC in
high yield (95%, Table 2) from the ammonium carbamate–
bicarbonate mixture (carbamate content 54%) in 4:1
benzene/MeOH is feasible owing to the irreversibility of
the reaction 6.
Methoxycarbonylation of a variety of amines into the
corresponding methyl carbamates was accomplished by
allowing them to react with (trimethylsilyl)diazomethane
TMSCHN2 under bubbling of CO2 at room temperature.
Benzene–MeOH (4/1 v/v) was the solvent of choice. Thus,
primary alkyl amines 1a–c, 2a–d, 3a, 3b and secondary
alkylamine 4 afforded their methyl carbamates in excellent
to good yields (76–100%). On the other hand, methoxy-
carbonylation of arylamines (1d, 2g, 2h, 2j, 2k and 5b) was
difficult (0–27% yield), although indoline (5a), whose
nitrogen atom is sterically less hindered, gave its methyl
carbamate in an excellent yield (98%). Even oxidatively
labile dopamine (2e) and DL-noradrenaline (2f) were
successfully methoxycarbonylated, albeit a low yield (17
and 12%, respectively). By selecting 2-(1-naphthyl)ethyl-
1
amine (1b) as a typical example, the H and 13C NMR,
RR0NH CCO2 #RR0NCO2H
(3)
(4)
(5)
HMBC, and IR spectra in 4:1 benzene–MeOH were
measured before and after CO2 bubbling. These spectra
have revealed that about an equimolar amount of the
ammonium carbamate and the ammonium bicarbonate is
formed in equilibrium in this mixed solvent. However, since
the methyl esterification step by TMSCHN2 is irreversible,
the conversion of amine to methyl carbamate can be nearly
quantitative.
RR0NCO2H CRR0NH#RR0NHC2 CRR0NCOK2
RR0NH CCO2 CH2O#RR0NHC2 CHCOK3
4. Experimental
4.1. General
(6)
1H and 13C NMR spectra were measured on a JEOL
EX-270J, AL-300, or JUM-A400 spectrometer. Measure-
ments of 2D NMR were carried out with JEOL JUM-A400.
Mass and IR spectra were recorded on JEOL JMS-HX 110A
and SHIMADZU FTIR-8400 spectrometers, respectively.
In the preceding paper1 we have mentioned that the in situ
yield of carbamic acid in CO2-saturated DMSO-d6 is
generally much lower for arylamines than for alkylamines.
More complete data supporting this statement are shown in
Table 3. Many alkylamines underwent nearly complete
conversion (w100%) into their respective carbamic acids.
Among arylamines, those having an electron-donating
group (2j and 2k) and cyclic ones (5a and 5b) exhibit
relatively high carbamic acid yields (7–24%). It is
interesting to note that the carbamic acid yields in DMSO-
d6 for alkyl- and arylamines (Table 3) are roughly correlated
with their methoxycarbonylation yields (Table 2). On the
assumption that the structural effect by different amines on
the formation of either carbamic acid or carbamate anion is
similar, it seems that TMSCHN2 is a good reagent for
trapping unstable carbamic acid species (carbamic acid or
carbamate anion).
The in situ measurements of 1H and 13C NMR, HMBC, and
IR spectra in benzene–MeOH (4/1 v/v) before and after
bubbling CO2 were carried out as described previously.1a
4.2. Methoxycarbonylation
4.2.1. A typical procedure: isolation of methyl N-[3-(1-
naphthyl)propyl]carbamate (1a-MC). Through 5 mL of
a
4:1 benzene/MeOH solution containing 73 mg
(0.393 mmol) of amine 1a was bubbled CO2 gas for
15 min. Into this solution was added 0.5 mL (1.0 mmol) of
(trimethylsilyl)diazomethane TMSCHN2 (Aldrich 2.0 M
solution in hexanes) in one portion at room temperature
with both stirring and CO2 bubbling. The yellow color of
TMSCHN2 disappeared in 20 min. The mixture was stirred
for an additional 2 h under CO2 bubbling. The resultant
colorless solution was rotary-evaporated to afford 93 mg of
a colorless viscous oil, which was almost pure 1a-MC
containing only a trace of 1a on the basis of the NMR
analysis. Further purification was carried out by preparative
TLC on silica gel (CHCl3/MeOH 20:1 v/v) to give 93 mg
(97% yield) of 1a-MC as a colorless viscous oil.
Table 3. Carbamic acid formation from amines in DMSO-d6 through
bubbling of CO2 at room temperature
Alkylamine
Carbamic acid
yield (%)
Arylamine
Carbamic acid
yield (%)
1a–c
2a–d, 2f
2e$dnba
3a
w100
w100
12 (31a)
w100
82
1d, 1e, 2h
2g
2j
2k
5a
5b
0
1
11
24
22
7
4
The conversion or the carbamic acid yield was estimated in situ by 1H NMR
analysis. Some data are from Ref. 1a.
4.2.2. Isolation of methyl N-[2-(3,4-dihydroxyphenyl)-
ethyl]carbamate (2e-MC). Through 12.5 mL of a 4:1
a In pyridine-d5.

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Y. Ito, H. Ushitora / Tetrahedron 62 (2006) 226–235
pyridine/MeOH solution containing 150 mg (0.411 mmol)
of the salt 2e$dnba11 was bubbled CO2 gas for 15 min. Into
this solution was added 1.1 mL (2.2 mmol) of TMSCHN2 in
one portion at room temperature with both stirring and CO2
bubbling. The mixture was stirred at room temperature for
2.5 h under CO2 bubbling. Then the solvent was rotary-
evaporated to leave 452 mg of a dark brown residue, which
was dissolved in 10 mL of MeOH and was left overnight.
An insoluble black solid (probably melanin) appeared and
this was removed by filtration. The filtrate was subjected to
repeated preparative TLC on silica gel (CHCl3/MeOH 10:1
v/v) to afford 14 mg (17% yield) of 2e-MC as a pale brown
semisolid. Methyl 3,5-dinitrobenzoate was undetectable in
the reaction mixture (NMR and TLC).
JZ7.1 Hz); 13C NMR (75.5 MHz, CDCl3) d 157.05,
134.77, 133.90, 131.87, 128.79, 127.31, 126.77, 126.14,
125.66, 125.45, 123.55, 52.04, 41.65, 33.37; IR (neat) 3337
(m), 1705 (s), 1530 (s), 1254 (s), 777 (s) cmK1; MS (EIC)
m/z 229 (MC, 31), 154 (83), 141 (100), 115 (58), 88 (52);
HRMS (EIC) calcd for C14H15NO2 229.1103, found
229.1105.
4.2.3.4. Methyl N-[(1-naphthyl)methyl]carbamate
(1c-MC). Obtained as a pale yellow solid, mp 84–88 8C
1
(lit.16 mp 66–69 8C); H NMR (300 MHz, CDCl3) d 8.01
(1H, d, JZ7.8 Hz), 7.86 (1H, d,¶ JZ8.1 Hz), 7.79 (1H, dd,
JZ6.5, 2.8 Hz), 7.56–7.47 (2H, m), 7.42–7.37 (2H, m), 5.00
(1H, br s), 4.81 (2H, d, JZ5.4 Hz), 3.69 (3H, s);8b 13C NMR
(75.5 MHz, CDCl3) d 156.82, 133.82, 133.62, 131.23,
128.77, 128.49, 126.52, 126.13, 125.91, 125.36, 123.29,
52.24, 43.19; IR (neat) 3298 (m), 1695 (s), 1550 (m), 1296
(m), 1259 (m), 799 (m) cmK1; MS (EIC) m/z 215 (MC,
100), 200 (72), 156 (61), 141 (77), 129 (85); 8b HRMS (EIC)
calcd for C13H13NO2 215.0946, found 215.0944.
4.2.3. Spectral data for isolated products. Other amines
were reacted in a manner similar to that described at Section
4.2.1. The products were separated by preparative TLC on
silica gel (CHCl3–MeOH) and were characterized by 1H and
13C NMR, IR, MS, and HRMS measurements. Further
purification of the isolated products by recrystallization or
distillation was usually not done. Spectral data for known
compounds were well in agreement with the reported data.
4.2.3.5. Methyl N-(2-phenylethyl)carbamate (2a-MC).
Obtained as a colorless oil; H NMR (270 MHz, CDCl3) d
1
7.38–7.21 (5H, m), 4.75 (1H, br s), 3.70 (3H, s), 3.48 (2H,
quar, JZ6.5 Hz), 2.85 (2H, t, JZ6.9 Hz);6c 13C NMR
(67.7 MHz, CDCl3) d 156.84, 138.64, 128.68, 128.53,
126.41, 52.07, 42.22, 36.19;6c IR (neat) 3335 (m), 1705
(s), 1532 (s), 1258 (s), 700 (m) cmK1; MS (EIC) m/z 179
(MC, 32), 104 (60), 91 (59), 88 (100); HRMS (EIC) calcd
for C10H13NO2 179.0946, found 179.0944.
4.2.3.1. Methyl N-[3-(1-naphthyl)propyl]carbamate
(1a-MC). Obtained as a colorless thick oil; 1H NMR
(400 MHz, CDCl3) d 8.00 (1H, d, JZ8.2 Hz), 7.85 (1H, dd,
JZ8, 1.7 Hz), 7.71 (1H, d, JZ8.1 Hz), 7.53–7.44 (2H, m),
7.38 (1H, t, JZ7.6 Hz), 7.31 (1H, d, JZ7 Hz), 4.74 (1H, br
s), 3.67 (3H, s), 3.28 (2H, quar, JZ6.5 Hz), 3.10 (2H, t, JZ
7.6 Hz), 1.96 (2H, quin, JZ7.6 Hz); 13C NMR (100 MHz,
CDCl3) d 157.13, 137.44, 133.91, 131.72, 128.84, 126.83,
125.97, 125.88, 125.53, 125.52, 123.58, 52.06, 40.95, 30.88,
30.11; IR (neat) 3336 (m), 1705 (br s), 1538 (br s), 1258 (s),
778 (s) cmK1; MS (FABC) m/z 244 (MHC, 100), 243 (MC,
58), 212 (13), 168 (21); HRMS (FABC) calcd for
C15H17NO2 243.1259, found 243.1259.
4.2.3.6. N-(2-Phenylethyl)formamide (7). Obtained as a
colorless oil; 1H NMR (270 MHz, CDCl3) d 8.07 (0.85H, s),
7.85 (0.15H, d, JZ11.9 Hz), 7.28–7.09 (5H, m), 5.7 and
5.56 (1H, two br s), 3.52 (1.7H, quar, JZ6.7 Hz), 3.42
(0.3H, quar, JZ6.7 Hz), 2.78 (2H, t, JZ6.9 Hz);17 13C
NMR (67.7 MHz, CDCl3) d 164.33,§ 161.07, 138.31,
137.41,§ 128.78,§ 128.75,§ 128.65 (two peaks), 126.86,§
126.60, 43.20,§ 39.25, 37.73,§ 35.50;17 IR (neat) 3283 (m),
4.2.3.2. N-[3-(1-Naphthyl)propyl]formamide (6).
Obtained as a colorless thick oil; 1H NMR (300 MHz,
CDCl3) d 8.15 (0.84H, s), 8.05 (0.16H, d, JZ12.3 Hz),
8.01–7.95 (1H, m), 7.87–7.83 (1H, m), 7.76–7.70 (1H, m),
7.54–7.28 (4H, m), 5.8 and 5.59 (ca. 0.15 and 0.85H, two br
s), 3.41 (1.66H, quar, JZ6.8 Hz), 3.28 (0.34H, quar, JZ
6.7 Hz), 3.12 (2H, t, JZ7.8 Hz), 1.99 (2H, quin, JZ7.4 Hz);
13C NMR (67.7 MHz, CDCl3) d 164.55,§ 161.09, 137.06,
136.49,§ 133.84,§ 133.78, 131.54, 131.47,§ 129.42,§
128.84,§ 128.77, 127.03,§ 126.83, 126.00,§ 125.94, 125.85,
125.57,§ 125.47, 125.44, 125.40,§ 123.41, 123.28,§ 41.41,§
38.07, 31.92,§ 30.40, 30.31, 29.73§; IR (neat) 3287 (m),
1666 (s), 1537 (m), 1386 (m), 779 (s) cmK1; MS (EIC) m/z
213 (MC, 89), 168 (100), 153 (71), 141 (95), 115 (48);
HRMS (EIC) calcd for C14H15NO 213.1154, found
213.1155.
1667 (s), 1534 (m), 1384 (m), 1239 (m), 699 (m) cmK1 17
;
MS (EIC) m/z 149 (MC, 13), 104 (100), 91 (44); HRMS
(EIC) calcd for C9H11NO 149.0841, found 149.0842.
4.2.3.7. Methyl N-[2-(4-hydroxyphenyl)ethyl]carba-
mate or N-(methoxycarbonyl)tyramine (2b-MC).
Obtained as a colorless thick oil; 1H NMR (270 MHz,
CDCl3) d 6.99 (2H, d,¶ JZ8.5 Hz), 6.76 (2H, d,¶ JZ
8.5 Hz), 4.80 and 4.69 (1H, two br s), 3.64 (3H, s), 3.37 (2H,
quar, JZ6.7 Hz), 2.69 (2H, t, JZ7.0 Hz); 13C NMR
(67.7 MHz, CDCl3) d 157.23, 154.64, 130.03, 129.70,
115.45, 52.27, 42.47, 35.21; IR (neat) 3335 (s), 1700 (s),
1515 (s), 1260 (s), 827 (m) cmK1; MS (EIC) m/z 195 (MC,
14), 120 (100), 107 (95); HRMS (EIC) calcd for C10H13NO3
195.0895, found 195.0896.
4.2.3.3. Methyl N-[2-(1-naphthyl)ethyl]carbamate
(1b-MC). Obtained as a pale yellow thick oil; H NMR
(300 MHz, CDCl3) d 8.08 (1H, d, JZ7.8 Hz), 7.85 (1H, d,
JZ7.5 Hz), 7.79 (1H, d, JZ7.8 Hz), 7.55–7.45 (2H, m),
7.39 (1H, t, JZ7.7 Hz), 7.31 (1H, d, JZ6.6 Hz), 4.78 (1H,
br s), 3.67 (3H, s), 3.54 (2H, quar, JZ6.6 Hz), 3.28 (2H, t,
4.2.3.8. Methyl N-[2-(3,4-dimethoxyphenyl)ethyl]car-
bamate (2c-MC). Obtained as a colorless thick oil; 1H
NMR (270 MHz, CDCl3) d 6.81 (1H, d, JZ7.9 Hz), 6.74–
6.70 (2H, m), 4.69 (1H, br s), 3.87 (3H, s), 3.86 (3H, s), 3.66
(3H, s), 3.42 (2H, quar, JZ6.6 Hz), 2.76 (2H, t, JZ
6.9 Hz);18 13C NMR (75.5 MHz, CDCl3) d 156.95, 148.99,
1
§ The peaks probably belonging to the minor isomer.
¶ Each peak is finely split (JZ1–2 Hz).

Y. Ito, H. Ushitora / Tetrahedron 62 (2006) 226–235
233
147.66, 131.21, 120.64, 111.87, 111.32, 55.89, 55.84, 52.03,
42.28, 35.69;18 IR (neat) 3365 (m), 1710 (s), 1516 (s), 1260
(96), 122 (85); HRMS (EIC) calcd for C9H11NO3 181.0739,
found 181.0740.
(s), 1236 (s), 1142 (s), 1028 (s) cmK1 18 MS (FABC) m/z
;
240 (MHC, 87), 239 (MC, 100), 165 (59); HRMS (FABC)
calcd for C12H17NO4 239.1158, found 239.1164.
4.2.3.14. Methyl N-[4-(dimethylamino)phenyl]carba-
mate (2k-MC). Obtained as a dark plates (from benzene–
hexane), mp 87–99 8C (dec); 1H NMR (300 MHz, CDCl3) d
7.20 (2H, br d, JZ7.8 Hz), 6.68 (2H, d,¶ JZ8 Hz), 6.52
(1H, br s), 3.73 (3H, s), 2.88 (6H, s); 13C NMR (75.5 MHz,
CDCl3) d 154.76, 147.71, 127.67, 121.00, 113.28, 52.11,
40.97; IR (KBr) 3331 (m), 1700 (s), 1536 (s), 1323 (m),
1244 (s), 1074 (m), 811 (m) cmK1; MS (EIC) m/z 194 (MC,
100), 162 (89), 135 (66); HRMS (EIC) calcd for
C10H14N2O2 194.1055, found 194.1055.
4.2.3.9. Methyl N-(2-hydroxy-2-phenylethyl)carba-
mate (2d-MC). Obtained as a colorless solid, mp 85–
1
89 8C (lit.8a mp 88–90 8C); H NMR (300 MHz, CDCl3) d
7.29–7.19 (5H, m), 5.11 (1H, br s), 4.74 (1H, dd, JZ8,
3 Hz), 3.60 (3H, s), 3.44 (1H, br d, JZ14 Hz), 3.21 (1H, br
dd, JZ14, 8 Hz), 2.56 (1H, br s);8a 13C NMR (75.5 MHz,
CDCl3) d 157.81, 141.52, 128.54, 127.93, 125.84, 73.58,
52.33, 48.53;8a IR (nujol) 3374 (m), 3272 (m), 1689 (s),
1551 (m), 1285 (m), 1277 (m), 704 (m) cm ;
K1 8a MS (EIC)
4.2.3.15. Methyl N-[2-(3-indolyl)ethyl]carbamate or
N-(methoxycarbonyl)tryptamine (3a-MC). Obtained as a
m/z 195 (MC, 11), 177 (20), 120 (29), 107 (58), 89 (100);
HRMS (EIC) calcd for C10H13NO3 195.0895, found
195.0895.
1
colorless semisolid; H NMR (270 MHz, CDCl3) d 8.11
(1H, br s), 7.62 (1H, d, JZ7.6 Hz), 7.38 (1H, d,¶ JZ7.9 Hz),
7.22 (1H, t,¶ JZ7.5 Hz), 7.14 (1H, t,¶ JZ7.5 Hz), 7.04 (1H,
s¶), 4.78 (1H, br s), 3.67 (3H, s), 3.53 (2H, quar, JZ6.8 Hz),
2.98 (2H, t, JZ6.8 Hz);21 13C NMR (67.7 MHz, CDCl3) d
156.89, 136.21, 127.08, 122.03, 121.91, 119.32, 118.57,
112.74, 111.08, 51.97, 41.20, 25.75; IR (neat) 3412 (s),
3334 (s), 1700 (s), 1530 (m), 1454 (m), 1260 (m), 744
(m) cmK1; MS (FABC) m/z 219 (MHC, 73), 218 (MC,
100), 144 (69), 130 (72);21 HRMS (FABC) calcd for
C12H14N2O2 218.1055, found 218.1055.
4.2.3.10. Methyl N-[2-(3,4-dihydroxyphenyl)ethyl]-
carbamate or N-(methoxycarbonyl)dopamine (2e-MC).
Obtained as a pale brown semisolid; H NMR (400 MHz,
1
CD3OD) d 6.66 (1H, d, JZ8.1 Hz), 6.62 (1H, d, JZ2.0 Hz),
6.50 (1H, dd, JZ8.0, 2.0 Hz), 3.60 (3H, s), 3.23 (2H, t, JZ
7.5 Hz), 2.60 (2H, t, JZ7.5 Hz); 13C NMR (100 MHz,
CD3OD) d 159.54, 146.23, 144.74, 132.04, 121.02, 116.85,
116.33, 52.36, 43.76, 36.57; IR (neat) 3335 (s), 1700 (s),
1525 (s), 1283 (s) cmK1; MS (EIC) m/z 211 (MC, 21), 136
(100), 123 (92); HRMS (EIC) calcd for C10H13NO4
211.0845, found 211.0842.
4.2.3.16. Na-Methoxycarbonyl-L-tryptophan methyl
ester (3b-MC). Obtained as a colorless semisolid; 1H NMR
(270 MHz, CD3OD) d 7.56 (1H, d, JZ7.9 Hz), 7.38 (1H, d,
JZ7.9 Hz), 7.14 (1H, t, JZ7.1 Hz), 7.11 (1H, s), 7.05 (1H,
t, JZ7.0 Hz), 4.54 (1H, t, JZ6.7 Hz), 3.69 (3H, s), 3.64
(3H, s), 3.32 (1H, dd, JZ14.6, 6 Hz), 3.18 (1H, dd, JZ14.6,
7.7 Hz);22 13C NMR (67.7 MHz, CD3OD) d 174.28, 158.87,
137.90, 128.53, 124.33, 122.33, 119.71, 118.96, 112.22,
110.60, 56.44, 52.63, 28.71;22 IR (neat) 3400 (m), 1733 (s),
1700 (s), 1454 (m), 1356 (m), 1221 (m), 745 (m) cmK1; MS
(EIC) m/z 276 (MC, 26), 130 (100); HRMS (EIC) calcd for
C14H16N2O4 276.1110, found 276.1116.
4.2.3.11. Methyl N-[2-hydroxy-2-(3,4-dihydroxy-
phenyl)ethyl]carbamate or N-(methoxycarbonyl)nor-
adrenaline (2f-MC). Obtained as a pale brown semisolid;
1H NMR (400 MHz, CD3OD) d 6.80 (1H, d, JZ1.8 Hz),
6.72 (1H, d, JZ8.1 Hz), 6.66 (1H, dd, JZ8.1, 1.8 Hz), 4.55
(1H, dd, JZ7.9, 4.9 Hz), 3.61 (3H, s), 3.29–3.17 (2H, m);
13C NMR (100 MHz, CD3OD) d 159.70, 146.25, 145.89,
135.39, 118.73, 116.12, 114.39, 73.71, 52.49, 49.47; IR
(KBr) 3335 (s), 1700 (s), 1535 (m), 1290 (s) cmK1; MS
(EIC) m/z 227 (MC, 9), 209 (82), 177 (23), 152 (21), 139
(100), 93 (45); HRMS (EIC) calcd for C10H13NO5
227.0794, found 227.0793.
4.2.3.17. Methyl N,N-dibenzylcarbamate (4-MC).
Obtained as a colorless oil; H NMR (270 MHz, CDCl3) d
1
7.33–7.15 (10H, m), 4.43 (2H, s), 4.38 (2H, s), 3.79 (3H,
s);23 13C NMR (67.7 MHz, CDCl3) d 157.29, 137.26,
128.50, 128.03, 127.31, 52.98, 49.43, 48.90; IR (neat) 1700
(s), 1471 (m), 1454 (m), 1407 (m), 1238 (s), 1119 (m), 699
4.2.3.12. Methyl N-phenylcarbamate (2g-MC).
Obtained as a colorless thick oil; 1H NMR (270 MHz,
CDCl3) d 7.33–7.19 (4H, m), 7.00 (1H, t,¶ JZ7.2 Hz), 6.54
(1H, br s), 3.71 (3H, s);19 13C NMR (67.7 MHz, CDCl3) d
153.91, 137.70, 128.98, 123.41, 118.62, 52.37;19 IR (neat)
3318 (m), 1713 (s), 1602 (m), 1544 (s), 1448 (s), 1235 (s),
(m) cmK1 23 MS (EIC) m/z 255 (MC, 21), 164 (93), 121
;
(25), 91 (100); HRMS (EIC) calcd for C16H17NO2
255.1259, found 255.1257.
1070 (m), 755 (m), 691 (m) cmK1 19 MS (EIC) m/z 151
;
(MC, 100), 119 (47), 106 (60); HRMS (EIC) calcd for
C8H9NO2 151.0633, found 151.0636.
4.2.3.18. 1-(Methoxycarbonyl)indoline (5a-MC).
Obtained as a pale yellow oil; 1H NMR (300 MHz,
CDCl3) d 7.9–7.15 (1H, br), 7.07–7.00 (2H, m), 6.81 (1H,
t, JZ7.5 Hz), 3.86 (2H, t, JZ8.3 Hz), 3.69 (3H, s), 2.97
(2H, t, JZ8.7 Hz);24 13C NMR (75.5 MHz, CDCl3) d
153.68, 142.47, 130.82, 127.41, 124.65, 122.48, 114.66,
52.45, 47.36, 27.46; IR (neat) 1710 (s), 1488 (s), 1444 (s),
1395 (s), 1337 (m), 1317 (m), 1137 (m), 1058 (m), 752
4.2.3.13. Methyl N-(4-methoxyphenyl)carbamate
(2j-MC). Obtained as a pale brown semisolid; H NMR
1
(300 MHz, CDCl3) d 7.25 (2H, d, JZ8.2 Hz), 6.82 (2H, d,
JZ8.2 Hz), 6.58 (1H, br s), 3.76 (3H, s), 3.74 (3H, s);20 13C
NMR (75.5 MHz, CDCl3) d 156.04, 154.49, 130.87, 120.83,
114.23, 55.47, 52.25; 20 IR (neat) 3319 (m), 1710 (s), 1602
(m), 1514 (s), 1298 (m), 1231 (s), 1180 (m), 1073 (m), 1034
(m) cmK1 24 MS (EIC) m/z 177 (MC, 100), 162 (19), 132
;
(30), 118 (48), 91 (38); HRMS (EIC) calcd for C10H11NO2
177.0790, found 177.0787.
(m), 829 (m) cmK1 20 MS (EIC) m/z 181 (MC, 100), 149
;

234
Y. Ito, H. Ushitora / Tetrahedron 62 (2006) 226–235
2820–2830. (g) Aresta, M.; Quaranta, E. J. Chem. Soc., Dalton
4.2.3.19. 1-Methoxycarbonyl-2-methylindoline (5b-
MC). Obtained as a pale brown thick oil; 1H NMR
(300 MHz, CDCl3) d 7.6 (1H, very br), 7.07–7.00 (2H,
m), 6.82 (1H, t, JZ7.3 Hz), 4.41 (1H, br quin, JZ7 Hz),
3.70 (3H, s), 3.22 (1H, dd, JZ15.9, 9.6 Hz), 2.49 (1H, dd,
JZ15.9, 2.4 Hz), 1.15 (3H, d, JZ6.3 Hz); 13C NMR
(75.5 MHz, CDCl3) d 153.62, 130.03, 127.42, 125.01,
122.67, 115.32, 55.33, 52.40, 35.83, 21.06; IR (neat) 1708
(s), 1485 (s), 1441 (s), 1390 (s), 1285 (m), 1060 (m), 751
(m) cmK1; MS (EIC) m/z 191 (MC, 83), 176 (100), 132
(40), 117 (76); HRMS (EIC) calcd for C11H13NO2
191.0946, found 191.0942.
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Acknowledgements
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We thank Mr. Koji Masuda and Mr. Haruo Fujita for help to
take a part of spectral data.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.09.
116. The 1H and 13C NMR spectra of 1b in DMF-d7 before
and after CO2 bubbling (Fig. 4). The HMBC spectrum of 1b
in DMSO-d6 after CO2 bubbling (Fig. 5). The IR spectra of
1b in DMSO before and after CO2 bubbling (Fig. 6).
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