Kinetics of amide formation through carbodiimide/N-hydroxybenzotriazole (HOBt) couplings

Article abstract of DOI:10.1021/jo701558y

(Chemical Equation Presented) The kinetics of formation of amide, 4, from the corresponding carboxylic acid by reaction with the isopropyl ester of methionine (MIPE), mediated by carbodiimide EDCI, 1, and HOBt, 2, have been studied in 1-methyl-2-pyrrolidi

Full text of DOI:10.1021/jo701558y

Kinetics of Amide Formation through Carbodiimide/
N-Hydroxybenzotriazole (HOBt) Couplings
Lai C. Chan* and Brian G. Cox
AstraZeneca PR&D, Silk Road Industrial Park, Charter Way, Macclesfield, Cheshire, SK10 2NA, U.K.
Lai.Chan@astrazeneca.com
ReceiVed July 18, 2007
The kinetics of formation of amide, 4, from the corresponding carboxylic acid by reaction with the isopropyl
ester of methionine (MIPE), mediated by carbodiimide EDCI, 1, and HOBt, 2, have been studied in
1-methyl-2-pyrrolidinone (NMP) using reaction calorimetry. The reaction rates have been found to be
independent of the concentration of HOBt, showing that the rate-determining step is the reaction between
the carboxylic acid and EDCI to give the corresponding O-acylisourea. The pH dependence of the observed
rate constants for O-acylisourea formation is consistent with a second-order reaction between doubly
protonated EDCI (EDCIH22+, 6) and the carboxylate group. The observed rate constants fall sharply at
2+
high pH, as the fraction of EDCI as EDCIH2 continues to fall strongly, whereas the carboxylic acid
group is already fully ionized. The rate constant, kP, for reaction between the carboxylate group of acid,
2+
3, and EDCIH2 has a value of kP ) 4.1 × 104 M-1 s-1 at 20 °C, some 105 times higher than similar
rate constants measured in water. The subsequent catalytic cycle, involving reaction of O-acylisourea
with HOBt to give HOBt ester, which then reacts with the amine to give the amide with regeneration of
HOBt, determines the product distribution. In the case of the amino acid, 3, reaction of the O-acylisourea
with MIPE to give amide, 4, is increasingly favored at higher pH values over that with the less basic
internal aromatic amine of 3 to give the diamide 5.
Introduction
The reaction is, therefore, catalytic in HOBt but stoichiometric
in EDCI, which is converted to the corresponding urea (eq 2).
The overall stoichiometry of the reaction therefore involves
reaction of the acid, amine, and EDCI to produce the amide
and the EDCI-derived urea.
Little is known about the kinetics of the reactions contained
within the scheme, apart from that between EDCI and various
carboxylic acids, which has been studied in aqueous solution.4
In particular, we wished to investigate the role of HOBt in
determining the overall rates of the product formation. In the
present paper, we report the results of a study of formation of
the amide, isopropyl N-{5-amino-2-[2-(4-fluorophenyl)ethyl]-
benzoyl}-L-methioninate, 4 (Scheme 2).
The formation of amide bonds by condensation of amines
with carboxylic acids in the presence of carbodiimides, such as
1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDCI, 1),
continues to be one of the most common synthetic procedures.1
To optimize these couplings, additives such as N-hydroxyben-
zotriazole (HOBt, 2) are widely used for the generation of active
esters capable of efficient acylation of amino groups, especially
in the case of amino acids and peptides.2,3
There are several areas of interest: the reaction rates, the
reaction order with respect to EDCI and HOBt, the influence
of added base, and, in the former case, the reaction selectivity
A typical reaction scheme for the coupling of acid RCO2H
with amine R′NH2 to form the amide, RCONHR′, is shown in
Scheme 1.
(2) Bodanszky, M. Peptide Chemistry: A Practical Textbook; Springer:
New York, 1988.
(3) Wang, S. S.; Tam, B. S.; Wang, B. S.; Merrifield, B. Int. J. Pept.
Protein Res. 1981, 19, 459.
(4) Williams, A.; Ibrahim, I. T. J. Am. Chem. Soc. 1981, 103, 7090.
(1) Shelkov, K.; Nahmany, N.; Melman, A. Org. Biomol. Chem. 2004,
2, 397.
10.1021/jo701558y CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/12/2007
J. Org. Chem. 2007, 72, 8863-8869
8863

Chan and Cox
formation is strongly exothermic (∆H ) -135 kJ mol-1), and
this proved to be particularly convenient for a calorimetric
investigation. The work was carried out in an Omnical Ultralow
Super CRC calorimeter, using an external circulating bath
(Presto bath with Baysilone KT3 oil) to maintain the reaction
temperature at 19.6 °C. In a typical reaction procedure, acid
(1.45 mmol) in 1.5 mL of NMP (3.5 rel vol) was added to an
NMP solution (1.5 mL, 3.5 rel vol) of amine hydrochloride (1.1
molar equiv), HOBt‚H2O (0.4 molar equiv), EDCI‚HCl (1.1
molar equiv), water (0.16 mL, 0.24 rel vol), and base (3.2 molar
equiv) at 19.6 °C. The power output was recorded every 10 s,
and a sample was taken for HPLC analysis when the power
output had returned to baseline level. The progressive and total
heat output was calculated using baseline-to-baseline extrapola-
tion in MS Excel.
SCHEME 1. Typical Reaction for Coupling Acid with
Amine in the Presence of EDCI/HOBt
A typical heat-output profile is shown in Figure 1. In all cases,
there is an apparent initial lag in the rate of heat generation,
due to the finite response time of the calorimeter. It was shown
in separate experiments that the measured heat output for the
“instantaneous” reaction between HCl and NaOH required
approximately 6 min to reach 99% completion, and for all
measurements, data accumulated over the first 6-10 min were
neglected for the purposes of kinetic analysis. The total heat
output of the reaction is unaffected by the instrument response
time. The concentrations of reactants at any time, t, during the
reactions were determined from the known initial concentrations
combined with the fraction of reaction (equivalent to the fraction
of the total heat output) that had occurred at time t, and these
were then used to determine the second-order rate constants.
SCHEME 2. Formation of Amide 4
Influence of HOBt Concentration. Reactions of amino acid,
3, with MIPE‚HCl (1.1 mol), N-methylmorpholine (NMM, 3.2
mol), and EDCI‚HCl (1.1 mol) were measured at various levels
of HOBt: 1.0 molar equiv, 0.4 molar equiv, 0.1 molar equiv,
and 0 mol. The results are shown in Figure 2 (1.0 molar equiv
of HOBt data has been omitted for clarity, but coincides with
that for 0.4 and 0.1 molar equiv).
It is immediately clear from the data presented that the
reaction rates and heat outputs are identical for HOBt levels
between 0.1 and 1.0 molar equiv. For reactions with no added
HOBt, the rate is very similar, but the total heat output is
considerably lower. When all data sets are replotted as a
percentage of total heat output against time (Figure 3), it can
be seen that the reaction profiles are identical, independent of
the level (or indeed the presence) of HOBt.
HPLC analysis of the reaction mixtures showed, however,
that whereas, in the presence of HOBt, high yields of the desired
amide were produced (>80%), the yield of the amide was only
14% in the absence of HOBt, and high levels of amino acid 3
and its decomposition products were obtained.
with respect to product formation. Thus, the amine R′NH2 in
eq 3 may be either isopropyl L-methioninate (MIPE), which
would give rise to the desired amide, isopropyl N-{5-amino-2-
[2-(4-fluorophenyl)ethyl]benzoyl}-L-methioninate, 4, or the
amino function of a second molecule of the starting amino acid,
5-amino-2-[2-(4-fluorophenyl)ethyl]benzoic acid, 3, leading to
an unwanted 2:1 adduct, isopropyl N-{5-({5-amino-2-[2-(4-
fluorophenyl)ethyl]benzoyl}amino)-2-(4-fluorophenyl)ethyl]ben-
zoyl}-L-methioninate, 5.
We conclude from these results that the kinetics of the
reactions are controlled by the reaction between the acid and
EDCI to give the O-acylisourea (rate-determining step), but that
the product distribution is controlled by the catalytic step
involving formation of the HOBt ester and its subsequent
reaction with the relevant amine.
Kinetics of the EDCI-Mediated Reactions of Amino Acid,
3. The reaction between EDCI and various nucleophiles,
including carboxylic acids, has been subject to a detailed study
in aqueous solution.4 It has been shown that the rate-determining
step for carboxylic acid addition is the reaction between the
carboxylate anion and doubly protonated EDCI (6), shown by
NMR studies in water and dimethylsulfoxide to comprise a mix-
Results and Discussion
Kinetic Measurements. Reaction rates were measured at
19.6 °C in 1-methyl-2-pyrrolidinone (NMP) as solvent. Amide
8864 J. Org. Chem., Vol. 72, No. 23, 2007

Kinetics of Amide Formation
FIGURE 1. Calorimetric trace for reaction of amino acid 3 at 20 °C with 0.4 mol HOBt and 3.2 molar equiv of N-methylmorpholine as base.
SCHEME 3
which EDCIH+ represents monoprotonated EDCI (protonated
on the alkylamino group).
It follows that the rate law for the reaction is given by eqs 8
and 9, where [EDCI]t and [RCO2H]t represent the total
FIGURE 2. Influence of HOBt concentration on reaction of amino
concentrations of EDCI and RCO2H in their various protonated
acid 3.
and tautomeric forms, and ke is the observed second-order rate
constant.
d[RCO ]
d[EDCI]
d[P]
) -
2 t ) -
t ) ke[RCO2H]t[EDCI]t (8)
dt
dt
dt
where
kp
ke )
(9)
[H+]
K
a(EDCIH2)
1 +
1 +
a(RCO2H)
[H+]
{
}{
}
K
In eq 9, the two terms in the denominator arise from the
-
fractions, respectively, of RCO2H as RCO2 and of EDCI as
2+
FIGURE 3. Time dependence of percentage heat output for reaction
of amino acid 3.
EDCIH2
.
It follows from eqs 8 and 9 that the rate constant, ke, should
show a bell-shaped variation with pH and reach a maximum
when the pH is halfway between the pKa values of EDCIH2
ture of an open chain and a cyclic form in rapid equilibrium.5,6
2+
and RCO2H, that is, at pH ) (pKa(RCO2H) + pKa(EDCIH2))/
2. For EDCI and acetic acid in water, pKa values of 3.1 and
4.7, respectively, this corresponds to a rate maximum at pH
3.9, with a rapid fall off in rate at either high (pH > 4.7) or
low (pH < 3.1) pH values because of increasingly reduced levels
(5) Yavari, I.; Roberts, J. D. J. Org. Chem. 1978, 43, 4689.
(6) Tenforde, T.; Fawwaz, R. A.; Freeman, N. K. J. Org. Chem. 1972,
37, 3372.
The kinetics may, therefore, be represented by Scheme 3, in
J. Org. Chem, Vol. 72, No. 23, 2007 8865

Chan and Cox
TABLE 1. Second-Order Rate Constants for Reaction of Amino
Acid 3 with EDCI in NMPa
[3]
(M)
[EDCI‚HCl]
HOBt
(molar equiv)
103ke
b
(M)
(M-1 s-1
)
0.291
0.291
0.145
0.291
0.291
0.320
0.320
0.160
0.320
0.320
1.0
0.4
0.4
0.1
0
4.30
4.50
4.63
4.45
4.50
a T ) 19.6 °C; N-methylmorpholine (3.2 molar equiv) as base; MIPE‚HCl
(1.1 molar equiv); water 3.2 vol %. b The ke values are (5%.
of EDCIH2 and RCO2-, respectively. In NMP, pKa values
2+
for carboxylic acids are much higher than those in water (e.g.,
pKa(benzoic) ∼ 11.5, in pure NMP,7 cf. 4.2 in water), but those
for protonated amines remain relatively unchanged (see below).
The presence of water (3.2 vol %) in the present system
(required to stabilize the HOBt) will reduce the pKa of amino
acid, 3, to a value somewhat below 11.5, but nevertheless eqs
8 and 9 predict that there should be a substantial pH window
in which the rate of product formation remains high.
Influence of pH. The rates of reaction and product distribu-
tions were measured in different buffer systems, prepared by
the addition of the following bases to the methioninate (MIPE)
hydrochloride: N,N′-dimethylaniline (DMeA) (pKa(H2O) )
5.12), N-methylmorpholine (NMM) (pKa(H2O) ) 7.41), tribu-
tylamine (Bu3N) (pKa(H2O) ) 9.75), N-methylpiperidine (NMeP)
(pKa(H2O) ) 10.08), triethylamine (Et3N) (pKa(H2O ) 10.62),
and diazobicyclo(5.4.0)undecane (DBU) (pKa(H2O) ) 13.28).8
Reactions were initially carried out with N-methylmorpholine
as base with several levels of HOBt and two different initial
concentrations of amino acid, 3. In all cases, reactions showed
excellent second-order kinetics in accordance with eq 8, and
the observed rate constants are recorded in Table 1. The constant
values confirm the independence of the reaction rate on the level
of HOBt.
Rate constants were then measured in the different base
solutions, with HOBt fixed at 0.4 molar equiv. The pKa values
of the various tertiary amines in NMP, required for calculation
of the solution pH values, have in most cases not been reported.
Extensive compilations in dipolar aprotic solvents, however,
show that pKa value of a tertiary amine is constant within (0.2
units in NMP, dimethylformamide (DMF), and dimethylsul-
foxide (DMSO)7,9-11 and so may be used interchangeably.
Where values are not reported in any of the solvents (NMM,
NMeP, DBU, and EDCI), values were estimated by subtracting
1.5 units from the aqueous values, this being the typical
reduction in pKa observed on transferring tertiary amines from
water to NMP, DMF, or DMSO. The pKa values for primary
amines in these solvents are closer to their aqueous values,
typically about 0.4 units lower than in water; thus the pKa for
protonated MIPE has been taken as 6.8. The pH values for the
various reactions were then calculated from the simple equi-
librium between MIPE hydrochloride and the added base.
Observed rate constants are listed in Table 2.
FIGURE 4. Amino acid 3: pH dependence of EDCI reaction rate.
TABLE 2. Second-Order Rate Constants for Reaction of Met Sat
Acid with EDCI in NMPa
molar equiv
of base
pKa
103ke
base
(NMP)b
pHb
(M-1 s-1
)
DMA
NMM
NMM
NMM
Bu3N
NMeP
Et3N
3.2
1.3
2.1
3.2
2.1
3.2
3.2
3.2
3.8
5.9
5.9
5.9
8.2
8.6
9.2
11.8
4.8
6.4
6.7
6.8
8.2
8.9
9.5
10.9
c
4.83
4.60
4.6
4.28
1.62
d
0.69
0.060
e
DBU
a T ) 19.6 °C; MIPE‚HCl (1.1 molar equiv); water 3.2 vol %. b See
text. c Time scale similar to NMM reactions, but with significant deviation
from second-order kinetics and high levels of diamide 5 and HOBt ester in
product. d Reaction terminated after 4 h at 65% completion. e Reaction
terminated after 3 h at 17% completion.
Analysis of the results in Table 2 according to eq 9 is shown
in Figure 4, in which the calculated (best-fit) line corresponds
to kP ) 4.07 × 104 M-1 s-1 and pKa (3) (NMP) ) 8.75. The kP
value is not sensitive to the estimated pKa values of the various
bases, provided that EDCI does not behave in a very different
manner to the other bases.
The rate constant, kP, is some 105 times that for carboxylic
acids in water,4 in keeping with the lower solvation of the
carboxylate anion in this medium. It is important to note,
however, that the maximum obserVed rate constant, ke, is very
similar to that for the reaction between acetate and EDCI in
water (ke ) 8 × 10-3 M-1 s-1 (25 °C); cf. ke(NMP) ) 4.7 ×
10-3 M-1 s-1 (20 °C)), despite the much higher intrinsic
reactivity of the benzoate in NMP. This highlights the impor-
tance of both the available concentrations of the reacting species
(population) and their intrinsic reactivity in determining the
overall reaction rates. Thus the lower reactivity of the carboxy-
late in water compared with NMP is compensated for by the
-
fact that relatively high levels of the reactive species (RCO2
and EDCIH22+) can coexist in water because of the similarity
of the pKa values for carboxylic acids and EDCIH22+. The pKa
value of acid, 3, in the present system, pKa ) 8.75, is
considerably higher than that in water (pKa ∼ 3.1) but
significantly lower than that, for example, of benzoic acid in
pure NMP (pKa ∼ 11.5). The lower value relative to that
expected in pure NMP is attributable to the presence of some
water (3.2 vol %), combined with relatively high levels of
RN3H+ in the reactions; it has been shown that carboxylate
anions are subject to strong stabilization by H-bond formation
(7) Izutsu, K. Dissociation Constants in Dipolar Aprotic SolVents;
Blackwell: Oxford, 1990.
(8) The pKa values in water are from the database ACD/pKa DB and
referenced therein.
(9) Kolthoff, I. M.; Chantooni, M. K. Anal. Chem. 1970, 42, 1622.
(10) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
(11) Kolthoff, M. K.; Chantooni, M. K.; Bhowmik, S. J. Am. Chem.
Soc. 1968, 90, 23.
8866 J. Org. Chem., Vol. 72, No. 23, 2007

Kinetics of Amide Formation
TABLE 3. Amide 4 and Diamide 5 Yieldsa
SCHEME 4. Synthetic Route to Amino Acid 3
molar equiv
of base
% methionine
free base
amide 4
% yield
diamide 5
% yield
base
DMA
NMM
NMM
NMM
Bu3N
NMeP
Et3N
3.2
1.3
2.1
3.2
2.1
3.2
3.2
3.2
1.0
29
6.6b
82
15.4
12.1
6.8
4.6
3.1
41.7
49.3
96.1
99.1
99.8
100
86.6
89.6
92.5
93.5
91.6c
(94)d
2.2
2.1
DBU
(0)d
a Solution yields (HPLC) based on acid consumed. b High level (42%)
HOBt ester. c Reaction terminated after 4 h at 65% completion. d Reaction
terminated after 3 h at 17% completion.
with suitable donors in NMP and related solvents, such as
dimethylformamide and dimethylsulfoxide.11,12
pH Dependence of Product Distribution. Although it is
clear from the results above that the overall kinetics of the amide
coupling reaction are determined by the rate of formation of
the O-acylisourea, the product distribution is controlled by the
subsequent formation of the HOBt ester and its reaction with
the amine (eqs 2 and 3, Scheme 1). In particular, at low pH
values, protonation of the methioninate amine group will reduce
the rate of the desired reaction relative to that of coupling with
the less basic amino group of acid, 3 (to give diamide 5), and
ultimately severely inhibit product formation. This is confirmed
by the yields of amide and diamide 5 in the various base
solutions given in Table 3.
Further work13 has shown that the conclusions from this work
are not unique to the current system. Thus reactions involving
a simple benzoic acid, rather than the present amino acid, again
show reaction rates independent of HOBt levels, high yields of
amide (>90%) in the presence of HOBt, but very poor
conversion to amide (<50%) in the absence of HOBt. As in
the present case, reaction rates fall off at high pH, but the
product yields do not show a dependence on pH because there
is no internal amino group to compete for the HOBt ester.
without further drying. The yield of nitrostilbene 7 was about
90%: 1H NMR (DMSO-d6, 400 MHz, 300 K) δ 8.56 (d, J ) 2.1
Hz, 1H), 8.39 (dd, J ) 8.8, 2.6 Hz, 1H), 8.14 (d, J ) 8.8 Hz, 1H),
7.84 (d, J ) 16.4 Hz, 1H), 7.72-7.66 (m, 2H), 7.47 (d, J ) 16.8
Hz, 1H), 7.31-7.24 (m, 2H), 3.94 (s, 3H); 13C NMR (DMSO-d6,
100 MHz, 300 K) δ 165.5, 162.4 (d, JFC ) 241.1 Hz), 145.7, 144.0,
134.0, 132.9 (d, J FCCCC ) 3.0 Hz), 129.3 (d, J FCCC ) 8.3 Hz, 2C),
129.2, 128.0, 126.4, 125.3, 124.4 (d, JFCCCCC ) 2.2 Hz), 115.9 (d,
J FCC ) 21.8 Hz, 2C), 52.8; IR νmax (KBr disc) 1728, 1585, 1505,
1331, 1227 cm-1
.
Preparation of Methyl 5-amino-2-[2-(4-fluorophenyl)ethyl]-
benzoate] (8). Methanol (216 kg) was added to a mixture of a
nitrostilbene (7) (60 kg of methanol-wet solid containing an
estimated 39 kg at 100% strength, 1.00 molar equiv) and 10%
palladium on carbon-wet paste (0.78 kg, 0.24 mol %). The mixture
was stirred and heated at 35 °C under a hydrogen atmosphere (3.0
bar gauge) for 2 h. When the reaction was complete, the mixture
was heated to 50 °C and passed through a filter aid to remove the
catalyst; the filter cake was washed with methanol (26 kg). The
total filtrate was diluted with water (56.7 kg), cooled to -5 °C
with stirring, and maintained at -5 °C for 30 min. The crystalline
product was collected by filtration, washed with a mixture of
methanol (23 kg) and water (28.7 kg), and dried at ambient
temperature and pressure. The yield of aminoester 8 (mean of four
batches) was 31.8 kg (88%): 1H NMR (DMSO-d6, 400 MHz, 300
K) δ 7.24-7.16 (m, 2H), 7.11-7.04 (m, 3H), 6.95 (d, J ) 8.4 Hz,
1H), 6.69 (dd, J ) 8.3, 2.6 Hz, 1H), 5.16 (br s, 2H), 3.80 (s, 3H),
2.99-2.92 (m, 2H), 2.76-2.68 (m, 2H); MS (ES) m/z 274.1 (M +
H)+.
Preparation of 5-Amino-2-[2-(4-fluorophenyl)ethyl]benzoic
acid (3). A mixture of aminoester 8 (73.3 kg, 1.00 molar equiv),
methanol (220 L), and 47% w/w sodium hydroxide solution (34.5
kg, 1.50 molar equiv) was stirred and heated at 60 °C for 6.5 h.
The solution was cooled to room temperature and adjusted to pH
5 using 1 M aqueous hydrochloric acid (402 L, about 1.50 molar
equiv). The precipitated solid was collected by filtration, washed
with water (2 × 147 L), and dried in vacuo at 40 °C. The yield of
acid 3 (mean of three batches) was 65.0 kg (93%): 1H NMR
(DMSO-d6, 400 MHz, 300 K) δ 7.25-7.17 (m, 2H), 7.11-7.02
(m, 3H), 6.90 (d, J ) 8.2 Hz, 1H), 6.64 (dd, J ) 8.1, 2.6 Hz, 1H),
3.33 (br s, 1H), 3.01-2.93 (m, 2H), 2.77-2.69 (m, 2H); 13C NMR
(DMSO-d6, 100 MHz, 300 K) δ 169.2, 160.5 (d, JFC ) 241.1 Hz),
Experimental Section
5-Amino-2-[2-(4-fluorophenyl)ethyl]benzoic acid (3) was pre-
pared from 2-chloro-5-nitrobenzoic acid, via methyl 2[(E)-2-(4-
fluorophenyl)vinyl]-5-nitrobenzoate (7) and methyl 5-amino-2-[2-
(4-fluorophenyl)ethyl]benzoate (8), according to Scheme 4.
Preparation of Methyl 2[(E)-2-(4-fluorophenyl)vinyl]-5-ni-
trobenzoate (7). A mixture of methyl 5-nitro-2-chlorobenzoate
(31.0 kg, 1.00 molar equiv), sodium carbonate (16.0 kg, 1.05 molar
equiv), tetra-n-butylammonium bromide (4.65 kg, 0.10 molar
equiv), 4-fluorostyrene (22.0 kg, 1.25 molar equiv), and N,N-
dimethylacetamide (110 kg) was stirred at ambient temperature
under a nitrogen atmosphere. Palladium chloride (1.075 kg, 0.040
molar equiv) and triethyl phosphite (1.025 kg, 0.043 molar equiv)
were added, and the mixture was heated at 90 °C for 4 h. The hot
mixture was diluted with toluene (82.5 kg) and filtered to remove
catalyst and other insoluble material; the filter cake was washed
with hot toluene (34.5 kg). The total filtrate was concentrated by
distillation under reduced pressure until all of the toluene had been
removed. The residue was diluted with methanol (159 kg) and
cooled and stirred at -5 °C for 30 min. The crystalline product
was collected by filtration and washed with cold methanol (49 kg).
The methanol-wet solid (60 kg) was used directly in the next stage
146.6, 138.2 (d, JFCCCC ) 3.0 Hz), 131.4, 130.4, 129.9 (d, JFCCC
)
(12) Bordwell, F. G.; Branca, J. C.; Hughes, D. L.; Olmstead, W. N. J.
Org. Chem. 1980, 45, 3305.
(13) Chan, L. C. Unpublished results.
7.9 Hz, 2C), 129.3, 117.2, 115.5, 114.8 (d, JFCC ) 21.1 Hz, 2C),
37.0, 35.5; MS (ES) m/z 260.1 (M + H)+, 301 (MH + CH3CN)+.
J. Org. Chem, Vol. 72, No. 23, 2007 8867

Chan and Cox
SCHEME 5. Synthetic Route to Dimer 5
Preparation of Isopropyl N-{5-Amino-2-[2-(4-fluorophenyl)-
ethyl]benzoyl}-L-methioninate (4). A mixture of L-methionine
isopropyl ester hydrochloride (135.3 kg, 1.10 molar equiv), 1-(3-
dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (113.9 kg,
1.10 molar equiv), and N-methylpyrrolidin-2-one (NMP) (420 L)
was stirred at about 20 °C. N-Methylmorpholine (27.3 kg, 0.50
molar equiv) was added to the mixture followed by a solution of
1-hydroxybenzotriazole (29.2 kg, 0.40 molar equiv) in N-methyl-
morpholine (87.6 kg, 1.60 molar equiv) and water (29.1 kg) and a
wash of NMP (35 L). A solution of the amino acid 3 (140.0 kg,
1.00 molar equiv) in NMP (350 L) was added over 2 h, keeping
the temperature at about 20 °C, and this was followed by a wash
of NMP (35 L). The mixture was maintained at 20 °C for a further
8 h to complete the reaction. Water (420 L) was added, and after
stirring for 1 h at 20 °C, the product began to crystallize. The
mixture was warmed to 40 °C, and more water (980 L) was added
over 2 h. After stirring the mixture at 40 °C for 1 h, the solid was
collected by filtration, washed with water (2 × 420 L), and dried
in vacuo at 40 °C. The yield of amine 4 (mean of four batches)
was 208.7 kg (89%): 1H NMR (DMSO-d6, 400 MHz, 300 K) δ
8.56 (d, J ) 7.7 Hz, 1H), 7.23-7.16 (m, 2H), 7.08-7.01 (m, 2H),
6.86 (d, J ) 7.9 Hz, 1H), 6.60 (d, J ) 2.1 Hz, 1H), 6.52 (dd, J )
8.1, 2.4 Hz, 1H), 5.06 (br s, 2H), 4.97 (septet, J ) 6.3 Hz, 1H),
2.84-2.66 (m, 4H), 2.64-2.51 (m, 2H), 2.04 (s, 3H), 1.99 (q, J )
6.4 Hz, 2H), 1.20 (d, J ) 6.3 Hz, 3H), 1.17 (d, J ) 6.2 Hz, 3H);
13C NMR (DMSO-d6, 125 MHz, 300 K) δ 171.4, 170.2, 160.5 (d,
JFC ) 240.7 Hz), 146.4, 138.2 (d, JFCCCC ) 2.8 Hz), 136.7, 130.2,
130.0 (d, JFCCC ) 8.9 Hz, 2C), 125.9, 114.8, 114.7 (d, JFCC ) 20.7
Hz, 2C), 112.9, 67.9, 51.4, 36.8, 34.4, 30.0, 29.8, 21.5, 21.4, 14.5;
MS (ES) m/z 433.2 (M + H)+.
Preparation of 5-[(tert-Butoxycarbonyl)amino]-2-[2-(4-fluo-
rophenyl)ethylbenzoic acid (9). To a mixture of methyl 5-amino-
2-[2-(4-fluorophenyl)ethyl]benzoate (8) (11 g, 1.0 molar equiv) and
di-tert-butyldicarbonate (11.40 g, 1.30 molar equiv) in dichlo-
romethane (70 mL) was added zinc perchlorate hexahydrate (0.9
g, 0.06 molar equiv), washed in with 10 mL of dichloromethane.
The mixture was stirred at ambient temperature overnight and then
heated to 32 °C for 4 h to complete the reaction. The mixture was
cooled to ambient temperature, and water (80 mL) was added,
followed by sodium hydroxide (0.75 mL, 50% w/w, 0.35 molar
equiv) so pH 12 was achieved. The mixture was stirred for 10 min,
and the lower aqueous phase was separated and discarded.
The organic solution was then concentrated under reduced vacuo
on a rotary evaporator to give an oil (19 g). The oil was redissolved
in methanol (100 mL), and water (80 mL) was added, followed by
sodium hydroxide (10 mL, 50% w/w, 4.7 molar equiv). The mixture
was heated to 75 °C and was held at 75 °C for 1 h. The mixture
was then cooled to ambient temperature and left to stir overnight.
Water (80 mL) was added and the mixture cooled to 12 °C, before
acidification with concentrated hydrochloric acid (15 mL, 4.33
molar equiv) to pH 1. The mixture was stirred for 10 min, then
filtered, and the solid washed with water (80 mL). The product
was dried in vacuo at 40 °C. The yield of benzoic acid 9 was 13.94
g (93%): 1H NMR (DMSO-d6, 400 MHz, 300 K) δ 9.41 (s, 1H),
8.00 (d, J ) 2.0 Hz, 1H), 7.47 (dd, J ) 8.3, 2.4 Hz, 1H), 7.27-
7.17 (m, 2H), 7.14 (d, J ) 8.3 Hz, 1H), 7.11-7.03 (m, 2H), 3.37
(br s, 1H), 3.13-3.03 (m, 2H), 2.82-2.71 (m, 2H), 1.47 (s, 9H);
13C NMR ((DMSO-d6, 100 MHz, 300 K) δ 168.7, 160.6 (d, JFC
)
240.7 Hz), 152.8, 137.9 (d, JFCCCC ) 3.0 Hz), 137.6, 135.9, 131.2,
130.4, 130.0 (d, JFCCC ) 8.3 Hz, 2C), 121.3, 119.8, 114.8 (d, JFCC
) 21.1 Hz, 2C), 79.1, 36.6, 35.4, 28.1 (3C); MS (ES) m/z 382
(MH + Na)+.
Preparation of Isopropyl N-{5-({5-Amino-2-[2-(4-fluorophenyl)-
ethyl]benzoyl}amino)-2-[2-(4-fluorophenyl)ethyl]benzoyl}-L-me-
thioninate hydrochloride (5). N-Boc amino ester 8 was hydro-
lyzed with base to N-Boc amino acid 9, which was then coupled
with methioninate 4 in the presence of EDCI‚HCl and HOBt-
in N-methylpyrrolidinone to give N-Boc dimer 10. The tert-
butoxy group was then removed under acid conditions to give
hydrochloride salt of dimer 5. The overall reaction is shown in
Scheme 5.
Preparation of Isopropyl N-{5({5-[(tert-Butoxycarbonyl)-
amino]-2-[2-(4-fluorophenyl)ethyl]benzoyl}amino)-2-[2-(4-fluo-
rophenyl)ethyl]benzoyl}-L-methioninate (10). N-Boc amino acid
9 (11.63 g, 1.0 molar equiv) was added in portions to a mixture of
methioninate 4 (15.54 g, 98.4% w/w, 1.10 molar equiv), EDCI‚
HCl (6.72 g, 1.08 molar equiv), and HOBt (20% w/w solution;
10.50 mL, 0.48 molar equiv) in N-methylpyrrolidin-2-one (42 mL),
8868 J. Org. Chem., Vol. 72, No. 23, 2007

Kinetics of Amide Formation
keeping the reaction temperature at 20 °C. The mixture was stirred
at ambient temperature overnight. Water (100 mL) and ethyl acetate
(100 mL) were added, and the mixture was stirred for 10 min. The
mixture was allowed to settle, and the lower aqueous phase was
separated off and discarded. The organic phase was washed with
more water (2 × 100 mL) and then concentrated under reduced
vacuo on a rotary evaporator to give a solid. The solid was then
recrystallized from methyl-tert-butyl ether (100 mL). The product
(22.6 g, 90%) yield was isolated: 1H NMR (DMSO-d6, 400 MHz,
300 K) δ 10.43 (s, 1H), 9.46 (s, 1H), 8.78 (d, J ) 7.5 Hz, 1H),
7.83-7.78 (m, 1H), 7.72 (dd, J ) 8.3, 1.9 Hz, 1H), 7.67-7.61 (m,
1H), 7.39 (dd, J ) 8.3, 2.0 Hz, 1H), 7.29-7.12 (m, 6H), 7.12-
6.99 (m, 4H), 4.93 (septet, J ) 6.3 Hz, 1H), 4.58-4.88 (m, 1H),
3.00-2.76 (m, 8H), 2.68-2.53 (m, 2H), 2.11-1.94 (m, 5H), 1.48
(s, 9H), 1.21 (d, J ) 6.3 Hz, 3H), 1.18 (d, J ) 6.3 Hz, 3H); 13C
NMR (DMSO-d6, 100 MHz, 300 K) δ 171.3, 169.5 (2C), 167.9,
160.5 (d, JFC ) 241.0 Hz, 2C), 152.8, 137.8 (d, JFCCCC ) 3.0 Hz),
137.7 (d, JFCCCC ) 3.0 Hz), 137.4, 137.1, 136.9 (d, JFCC ) 21.9
Hz, 2C), 134.1, 132.3, 130.1, 130.0 (d, JFCCC ) 7.9 Hz, 2C), 129.9
(d, JFCCC ) 7.9 Hz, 2), 120.4, 119.2, 118.7, 116.7, 114.9 (d, JFCC
) 21.1 Hz, 2C), 114.8 (d, JFCC ) 21.1 Hz, 2C), 79.2, 68.1, 51.5,
36.3 (2C), 34.5, 34.4, 30.1, 29.8, 28.1, 21.5, 21.4, 14.5; MS (ES)
m/z 774 (M + H)+, 718 (minus tert-butyl).
Preparation of Isopropyl N-{5-({5-Amino-2-[2-(4-fluorophenyl)-
ethyl]benzoyl}amino)-2-[2-(4-fluorophenyl)ethyl]benzoyl}-L-me-
thioninate hydrochloride (11). Boc dimer 10 (21.50 g, 1.0 molar
equiv) in toluene (60 mL) was heated to 40 °C. To this solution
was added HCl in isopropanol (5 M, 16 mL, 2.87 molar equiv).
The solution was stirred at 40 °C for 1.5 h and then cooled to
ambient temperature. An aqueous solution of potassium bicarbonate
(9 g, 3.3 molar equiv) in water (80 mL) was added to the mixture.
The organic phase was separated off and dried over anhydrous
magnesium sulfate. The solution was filtered and concentrated under
reduced vacuo on the rotary evaporator to give an oil (18 g).
The oil was redissolved in diethyl ether (100 mL), and HCl in
isopropanol (5 M, 5.50 mL, 0.98 molar equiv) was added. The
mixture was stirred at ambient temperature overnight. The solid
was filtered off and dried in vacuo at 40 °C. The yield of
hydrochloride salt of dimer 5 was 14.1 g (74%): 1H NMR (DMSO-
d6, 400 MHz, 300 K) δ 10.63 (s, 1H), 8.79 (d, J ) 7.7 Hz, 1H),
7.80 (d, J ) 2.3 Hz, 1H), 7.71 (dd, J ) 8.4, 2.0 Hz, 1H), 7.41-
7.35 (m, 1H), 7.34-7.29 (m, 1H), 7.27-7.14 (m, 5H), 7.11-7.00
(m, 4H), 4.93 (septet, J ) 6.3 Hz, 1H), 4.57-4.49 (m, 1H), 4.57-
4.49 (m, 1H), 3.50 (br s, 2H), 3.04-2.74 (m, 8H), 2.67-2.52 (m,
2H), 2.07-1.95 (m, 5H), 1.21 (d, J ) 6.3 Hz, 3H), 1.18 (d, J )
6.2 Hz, 3H); 13C NMR (DMSO-d6, 100 MHz, 300 K) δ 171.3,
169.5, 166.9, 160.7 (d, JFC ) 241.1 Hz, 2C), 138.0, 137.8 (d, JFCCCC
) 3.0 Hz), 137.3 (d, JFCCCC ) 3.0 Hz), 136.9, 136.7, 134.4 (2C),
131.2, 130.1 (2C), 130.5 (d, JFCCC ) 7.9 Hz, 2C), 130.0 (d, JFCCC
) 7.9 Hz, 2C), 123.4, 120.9, 120.5, 118.7, 114.9 (d, JFCC ) 21.1
Hz, 2C), 114.8 (d, JFCC ) 21.1 Hz, 2C), 68.1, 51.5, 36.2, 35.9,
34.5, 30.0, 29.8, 21.5, 21.4, 14.5.
HPLC Method
column
Genesis, C-18,
15 cm × 3.0 mm
i.d., 3 µm
40 °C
temperature
wavelength
flow
sample injection volume
run time
265 nm
1.0 mL/min
10 µL
16 min
post time
2.0 min
Gradient
1%
trifluoroacetic
acid
time
(min)
%
water
%
CH3CN
0
12
14
16
60
20
20
60
30
70
70
30
10
10
10
10
Sample Preparation:
Take 50 µL of reaction mixture into a 5 mL volumetric flask.
Dilute to volume with methanol.
rt
component
HOBt
amino acid 3
amide 4
(min)
1.00
1.48
4.7
HOBt ester
diamide 5
6.0
8.5
Acknowledgment. We thank Dr G.E. Robinson, Dr. S.
Abbas, and Dr. Ian C. Jones for discussions and for help with
analytical and synthetic work.
Supporting Information Available: 1H, 13C NMR and LCMS
spectra of compounds 7, 8, 3, 4, 9, 10, and 11. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO701558Y
J. Org. Chem, Vol. 72, No. 23, 2007 8869