Catalyst- and steric-controlled alkenylation via chemoselective C-H activation and C-Br activation in Heck reaction of methyl 1-(2-bromoaryl)-3-(2- furyl/thienyl)-5-oxopyrrolidine-2-carboxylates and diethyl 1-(2-bromoaryl)-3-(2- furyl/thienyl)-5-oxopyrrolidine-2,2-dicarboxylate derivatives

Article abstract of DOI:10.1016/j.tetlet.2010.05.005

Pd(II)-catalyzed alkenylation of methyl 1-(2-bromoaryl)-3-(2-furyl/thienyl) -5-oxopyrrolidine-2-carboxylate derivatives 1(a-d) resulted in the formation of 3(a-d) exclusively via C-H activation in the heteroaryl moiety. Similar observations were observed for the corresponding diester analogues 4(a-d) to form 5(a-d). Normal Heck reaction, however, was observed in the case of 1(a-f) to furnish 2(a-f) when the reaction was carried out with Pd(0) catalyst generated in situ. Pd(0)-catalyzed vinylation of 4(a-f) via C-Br oxidation, however, failed due to steric reason.

Full text of DOI:10.1016/j.tetlet.2010.05.005

Tetrahedron Letters 51 (2010) 3580–3582
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Catalyst- and steric-controlled alkenylation via chemoselective C–H
activation and C–Br activation in Heck reaction of methyl
1-(2-bromoaryl)-3-(2-furyl/thienyl)-5-oxopyrrolidine-2-carboxylates
and diethyl 1-(2-bromoaryl)-3-(2-furyl/thienyl)-5-oxopyrrolidine-2,
2-dicarboxylate derivatives
Prasanta Patra a, Jayanta K. Ray b, Gandhi K. Kar a,
*
a Department of Chemistry and Biochemistry, Presidency College, 86/1 College Street, Kolkata 700 073, India
b Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Pd(II)-catalyzed alkenylation of methyl 1-(2-bromoaryl)-3-(2-furyl/thienyl)-5-oxopyrrolidine-2-carbox-
ylate derivatives 1(a–d) resulted in the formation of 3(a–d) exclusively via C–H activation in the hetero-
aryl moiety. Similar observations were observed for the corresponding diester analogues 4(a–d) to form
5(a–d). Normal Heck reaction, however, was observed in the case of 1(a–f) to furnish 2(a–f) when the
reaction was carried out with Pd(0) catalyst generated in situ. Pd(0)-catalyzed vinylation of 4(a–f) via
C–Br oxidation, however, failed due to steric reason.
Received 2 January 2010
Revised 1 May 2010
Accepted 6 May 2010
Available online 9 May 2010
Keywords:
Heck reaction
Ó 2010 Elsevier Ltd. All rights reserved.
Electrophilic palladation
C–H activation
Chemoselective alkenylation
c
-Lactam
The synthetic studies toward novel
c
-lactam antibacterial
PPh3)-catalyzed alkenylation of the c-lactam monocarboxylic ester
agents as non-b-lactam mimics of b-lactam antibiotics1 is a grow-
derivatives 1(a–f) to furnish 2(a–f) in moderate yields. Attempted
vinylation of the corresponding diester derivatives 4(a–f), under
identical condition, however, met with failure. Herein, we report
our results.
ing field since four decades and it has been further stimulated by
the isolation of many naturally occurring
c
-lactam derivatives with
-lactam
potential biological activities.2 Synthetic monocyclic
c
derivatives like 1-aryl-3-heteroaryl-5-oxopyrrolidine-2-carboxylic
acids have been found to display significant antibacterial activi-
ties.3 The activity in turn depends on the nature of 1- and 3-aryl
Thus when methyl 1-(2-bromophenyl)-3-(2-furyl)-5-oxopyr-
rolidine-2-carboxylate (1a) was subjected to Heck reaction with
methyl acrylate in the presence of Pd(OAc)2 (5 mol %), PPh3, NaO-
Ac, Bu4NCl (cat.) at 110–120 °C, in DMF, under argon atmosphere
afforded compound 2a in 35% yield. Similar results were obtained
in the case of 1b to produce 2b as the only isolable product. When
the 2-furyl group was replaced with a 2-thienyl group (1c and 1d)
or a phenyl group (1e and 2f), the vinylation occurred in the N-aryl
part only to furnish 2(c–d) (35–36%) and 2(e–f) (36–40%) via acti-
vation of C–Br bond (Scheme 1). The compounds have been charac-
terized by usual spectroscopic and analytical data.5
moiety and the C2–COOH group present in the
c-lactam frame-
work. In our ongoing project in search of novel antibacterial agents
with enhanced antibacterial activity, we were studying the Pd-cat-
alyzed Heck reaction4 of
c-lactam derivatives 1(a–f) and 4(a–f),
respectively, with methyl acrylate. We have observed some inter-
esting catalyst-dependent chemoselective C–H oxidation in the
heteroaryl part over the expected C–Br bond in the N-aryl part of
c-lactam dicarboxylic ester derivatives 4(a–d) (Scheme 2) and
the corresponding monoester derivatives 1(a–f) (Scheme 1). When
Pd(OAc)2 was used as catalyst, vinylation took place by replace-
ment of C5–H in the heteroaryl moiety. However, normal results
were obtained in the Pd(0) (generated in situ from Pd(OAc)2 and
Under identical conditions, attempted vinylation of diethyl 1-
(2-bromoaryl)-3-(2-furyl/thienyl)-5-oxopyrrolidine-2,2-dicarbox-
ylate derivatives 4(a–f) afforded no product (6) with N-aryl–vinyl
bond formation. Instead heteroaryl–vinyl bond formation via C–H
oxidation was noticed in the reaction of 4(a–d) with methyl acry-
late and 5(a–d) formed in 35–55% yield (Scheme 2). Thus when
compound 4a or 4b was subjected to Heck reaction with methyl
* Corresponding author. Tel.: +91 33 22413893.
E-mail address: gandhikar41@hotmail.com (G.K. Kar).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.005

P. Patra et al. / Tetrahedron Letters 51 (2010) 3580–3582
3581
Br
R
CO2Me
CO2Me
N
X
Br
N
3a. 57%
3b. 52%
3c. 52%
R
ii
CO2Me
O
3(a-d)
3d.
54%
X
i
CO2Me
CO2Me
O
1(a-f)
2a. 35%
2b. 35%
2c. 36%
2d. 35%
R
a.
c.
b.
d.
X = O, R = H;
X = S, R = H;
e. X = -CH=CH-, R = H; f. X = -CH=CH-, R = Me.
X = O, R = Me
X = S, R = Me
N
X
2e.
2f.
36%
40%
O
2(a-f)
Scheme 1. Reagents and conditions: (i) methyl acrylate, Pd(OAc)2 (5 mol %), PPh3, n-Bu4NCl (cat.), NaOAc, DMF, 110–120 °C, 8–10 h under argon atmosphere; (ii) methyl
acrylate Pd(OAc)2 (5 mol %), Cu(OAC)2, DMF 90–100 °C, 8–9 h.
CO2Me
R
EtO 2C
CO2Et
Br
R
N
EtO2C
X
i
CO2Et
6
O
N
O
X
Br
R
i or ii
EtO2C
CO2Et
CO2Me
4(a-f)
N
X
a. X = O, R = H;
c.
b. X = O, R = Me
d.
X = S, R = H;
X = S, R = Me
O
5a. 36% (59%) 5b. 55% (59%)
5c. 5d.
e. X = -CH=CH-, R = H, f. X = -CH=CH-, R = Me
5
35% (53%)
5e. 0% 5f. 0%
42% (57%)
Scheme 2. Reagents and conditions: (i) methyl acrylate, Pd(OAc)2 (5 mol %), PPh3, n-Bu4NCl (cat.), NaOAc, DMF, 110–120 °C, 8–12 h under argon atmosphere (yields 0–55%);
(ii) methyl acrylate, Pd(OAc)2 (5 mol %), Cu(OAC)2, DMF 90–100 °C, 8–9 h (yields 53–57%) (within parenthesis).
acrylate with Pd(OAc)2 (cat.), PPh3, NaOAc, Bu4NCl (cat.), DMF,
110–120 °C, under argon atmosphere compound 5a or 5b5 was
produced in 36% and 55% yields, respectively, with exclusive viny-
lation by activation of C5–H in 2-furyl ring. When the 2-furyl group
was replaced with a 2-thienyl group (4c and 4d), under similar
reaction conditions, vinylation occurred (in 35% and 42% yields,
respectively) in the C5-position of the thiophene ring also but no
alkenylation in N-aryl ring was observed. In the case of 3-phenyl
derivative (4e or 4f), however, there was no reaction at all and
the starting material was recovered unchanged showing the inert
nature of the N-(2-bromoaryl) group toward Pd(0)-catalyzed Heck
reaction.
(Schemes 1 and 2). Under this condition compound 1(a–d) pro-
duced only 3(a–d) by heteroaryl–vinyl bond formation but no for-
mation of compounds like 2(a–d) was noticed. For example, when
a mixture of 1d and methyl acrylate in DMF was heated with pal-
ladium acetate (ꢀ5 mol %), Cu(OAc)2 (2–3 equiv), and NaOAc at
90–100 °C (no argon atmosphere was used) compound 3d was
formed in 54% yield as indicated from spectral data.5
Here the heteroaryl moiety first undergoes electrophilic palla-
dation with Pd(II) species which is followed by addition–elimina-
tion6a with methyl acrylate to show vinylation in heteroaryl
moiety. The trans stereochemistry around the alkene functionality
was established from the coupling constant of the vinylic protons
(J = 15–16 Hz). In the case of vinylation of 4(a–d) with Pd–ace-
tate/Cu(OAc)2 much better yields (53–59%) were obtained. Thus
reaction of diethyl 1-(2-bromoaryl)-3-(2-furyl)-5-oxopyrrolidine-
2,2-dicarboxylate 4a with Pd(OAc)2 (cat.), NaOAc, and Cu(OAc)2
in DMF at 90–100 °C produced 5a in 59% yield. Other compounds
4(b–d) also behaved in a similar fashion to yield 5(b–d) (53–
59%). Pd-catalyzed C2–H oxidation of furan in the reaction with ac-
rylic esters though known is not common.6 But prior to this report,
we have not come across any C–H oxidation in aryl/heteroaryl ring
despite the presence of an aryl bromide moiety in the molecule.
The reason for this may be the steric effect due to two CO2Et
groups at C2 of the
c-lactam ring which restricts the oxidative
addition of Pd(0) to C–Br bond. Vinylation in the heteroaryl moiety
possibly takes place by electrophilic palladation6a at C-5 position
with Pd(OAc)2 present in the reaction medium, followed by alkeny-
lation with methyl acrylate. This is further proved by the fact that
when the reaction was carried out using only Pd(PPh3)4 as catalyst
instead of palladium acetate and triphenyl phosphine mixture
there was no reaction at all. However, when the c-lactam mono-
carboxylic ester derivatives 1(a–f) or the diester derivatives 4(a–
d) were allowed to react with Pd(OAc)2 (used in stoichiometric ra-
tio or in catalytic amount in combination with Cu(OAc)2 which
converts Pd(0) back to Pd(II) species) in the absence of PPh3 and in-
ert atmosphere, they afforded 3(a–d) or 5(a–d), respectively
Required c-lactam diester derivatives 4(a–f) were prepared, in
55–65% yields, by intermolecular Michael addition followed by
intramolecular amidification reaction7 of 2-bromoarylamino mal-
onate and b-aryl/heteroaryl-a,b-unsaturated acid chlorides in the

3582
P. Patra et al. / Tetrahedron Letters 51 (2010) 3580–3582
Br
Br
R
Br
H
NH2
CO2Et
N
EtO2C
CO2Et
ii
i
+
2
BrCH(CO2Et)2
CO2Et
N
O
R
X
R
7a.
7b.
R = H
R = Me
4(a-f)
R
Br
iii, iv
CO2Me
a. X = O, R = H;
c. X = S, R = H;
b. X = O, R = Me
d. X = S, R = Me
1(a-f)
N
X
e.
f.
X = -CH=CH-, R = H, X = -CH=CH-, R = Me
O
Scheme 3. Reagents and conditions: (i) 100–110 °C, 24 h; (ii) b-aryl/heteroaryl-
(iv) CH2N2, Et2O, 10–15 °C.
a,b-unsaturated acid chloride, Et3N, benzene, reflux, 8–10 h; (iii) KOH, EtOH, H2O, reflux, 4 h;
5. Physical and spectral data of representative compounds:
presence of Et3N/benzene under reflux (Scheme 3). Hydrolysis
(KOH, EtOH–H2O, and reflux) and in situ decarboxylation afforded
the trans acid (70–78% yield) (very minor amount cis product
formed was removed during recrystallization) which was esterified
with diazomethane to obtain the c-lactam monoester derivatives
1(a–f) in very good to excellent yields. The compounds have been
Compound 1b: Colorless solid, mp 124–125 °C (EtOH); 1H NMR (200 MHz, CDCl3)
d: 2.32 (s, 3H), 2.83 (dd, 1H, J = 4.8 and 17.0 Hz), 3.06 (dd, 1H, J = 8.8 and
17.0 Hz), 3.70 (s, 3H), 3.79–3.87 (m, 1H), 4.83 (d, 1H, J = 3.6 Hz), 6.29 (d, 1H,
J = 3.2 Hz), 6.35 (dd, 1H, J = 1.8 and 3.7 Hz), 7.14 (dd, 1H, J = 1.2 and 8.0 Hz), 7.29
(d, 1H, J = 8.0 Hz), 7.42–7.43 (m, 2H) ppm; 13C NMR (75 MHz, CDCl3) d: 21.34,
34.79, 36.37, 53.06, 66.61, 107.19, 110.90, 122.20, 129.59, 131.29, 133.21,
134.35, 140.93, 142.82, 153.49, 171.46, 173.92 ppm; IR (KBr)
mmax: 1703.8,
1732.7 cmÀ1
.
characterized by usual spectral and analytical data. The trans
geometry of the C4- and C5-substituents in the c-lactam ring was
Compound 2b: Colorless solid, mp 118–120 °C (Et2O/À15 °C); 1H NMR (200 MHz,
CDCl3) d: 2.36 (s, 3H), 2.80 (dd, 1H, J = 4.2 and 17.2 Hz), 3.15 (dd, 1H, J = 9.2 and
17.2 Hz), 3.70 (s, 3H), 3.74–3.84 (m, 1H), 3.77 (s, 3H), 4.59 (d, 1H, J = 3.2 Hz), 6.30
(d, 1H, J = 3.4 Hz), 6.38 (dd, 1H, J = 1.8 and 3.2 Hz), 6.39 (d, 1H, J = 15.8 Hz), 7.13
(d, 1H, J = 8.2 Hz), 7.21 (dd, 1H, J = 1.8 and 8.0), 7.46 (br s, 1H), 7.49 (br d, 1H,
assigned from the coupling constant values of C4–H and C5–H
(J = 3.2–4.7 Hz), respectively, as well as by analogy.3,8
Thus this Letter describes some Pd(II)-catalyzed chemoselective
C–H oxidation in heteroaryl moiety in vinylation of the c-lactam
diester derivatives 1(a–d) or 4(a–d) with methyl acrylate even in
the presence of aryl bromide functionality and Pd(0)-catalyzed
J = 1.2 Hz), 7.75 (d, 1H, J = 15.8 Hz ppm; IR (KBr) mmax: 1704.0–1734.6 (br
strong), 1753.9 cmÀ1; HRMS (ESI, 70 eV): m/z = 406.1318 (M++Na) [calculated
mass for C21H21NO6Na: 406.1267 (M++Na)].
Compound 3d: Colorless solid, mp 117–119 °C (Et2O/À15 °C); 1H NMR (500 MHz,
CDCl3) d: 2.26 (s, 3H), 2.75 (dd, 1H, J = 6.0 and 17.0 Hz), 3.11 (dd, 1H, J = 9.5 and
17.0 Hz), 3.63 (s, 3H), 3.72 (s, 3H), 3.93 (m, 1H), 4.69 (d, 1H, J = 4.5 Hz), 6.12 (d,
1H, J = 16.0 Hz), 6.93 (d, 1H, J = 4.0 Hz), 7.07 (d, 1H, J = 4.0 Hz), 7.08 (br d, 1H,
Jꢀ8.0 Hz), 7.21 (d, 1H, J = 8.0 Hz), 7.36 (br s, 1H), 7.65 (d, 1H, J = 16.0 Hz) ppm;
13C NMR (125 MHz, CDCl3) d: 20.99, 37.92, 38.13, 51.87, 52.88, 68.91, 116.99,
122.09, 128.28, 129.41, 131.09, 131.35, 132.85, 134.15, 137.10, 139.07, 140.87,
C–Br oxidation in vinylation of c-lactam diester derivatives 1(a–
f). To the best of our knowledge such types of observations are un-
known till date. Though the yields are not very high in some cases
there are wide scopes to study the reaction by variation of Pd-cat-
alysts, reagents, conditions, and/or solvents etc to improve the
yield which will be our future course of work in this connection.
147.39, 167.25, 170.78, 173.02 ppm IR (KBr)
mmax: 1618.0, 1714.4 (br strong),
1749.1 cmÀ1
.
Compound 4a: Colorless white, mp 156–157 °C (EtOH); 1H NMR (200 MHz,
CDCl3) d: 0.82 (t, 3H, J = 7.2 Hz), 0.99 (t, 3H, J = 7.2 Hz), 2.85 (dd, 1H, J = 8.8 and
16.8 Hz), 3.13 (dd, 1H, J = 12.2 and 16.8 Hz), 3.78 (m, 1H), 3.93–4.12 (m, 3H),
4.74 (dd, 1H, J = 8.8 and 12.2 Hz), 6.31–6.37 (m, 2H), 7.19–7.36 (m, 3H), 7.54–
7.61 (m, 2H) ppm; 13C NMR (75 MHz, CDCl3) d: 13.09, 13.54, 32.89, 39.55, 62.37,
62.68, 76.96, 109.66, 110.67, 125.38, 128.40, 130.01, 130.69, 132.65, 136.41,
Acknowledgments
Financial help from DST (Project no. SR/S-1/OC-10), New Delhi,
is gratefully acknowledged. Thanks to CSIR, New Delhi, for provid-
ing a NET fellowship to one of the authors (P.P.).
142.40, 149.17, 164.97, 167.61, 172.92 ppm; IR (KBr)
mmax: 1718.3, 1733.7,
1753.0 cmÀ1
.
Compound 5a: Colorless white, mp 118–120 °C (Et2O/À15 °C); 1H NMR
(200 MHz, CDCl3) d: 0.85 (t, 3H, J = 7.2 Hz), 0.98 (t, 3H, J = 7.2 Hz), 2.89 (dd,
1H, J = 8.8 and 16.6 Hz), 3.18 (dd, 1H, J = 12.2 and 16.6 Hz), 3.74–3.82 (m, 1H),
3.79 (s, 3H), 3.96–4.14 (m, 3H), 4.76 ((dd, 1H, J = 8.8 and 12.2 Hz), 6.27 (d, 1H,
J = 15.6 Hz), 6.50 (d, 1H, J = 3.4 Hz), 6.57 (d, 1H, J = 3.4 Hz), 7.22–7.40 (m, 2H),
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.05.005.
7.37 (d, 1H, J = 15.5), 7.58–7.64 (m, 2H) ppm; IR (KBr) mmax: 1638.2, 1702.8,
1730.8, 1752.0 cmÀ1 HRMS (ESI, 70 eV): m/z = 534.4321 (M++H), 536.4301
;
(M++2+H) [calculated mass for C24H25NO8Br: 534.0764 (M++H) and 536.0743
(M++2+H)].
References and notes
Compound 5b: Colorless white, mp 132–133 °C (Et2O/À15 °C); 1H NMR
(500 MHz, CDCl3) d: 0.94 (t, 3H, J = 7.2 Hz), 1.01 (t, 3H, J = 7.2 Hz), 2.37 (s, 3H),
2.92 (dd, 1H, J = 8.6 and 16.7 Hz), 3.19 (dd, 1H, J = 12.3 and 16.7 Hz), 3.82 (m,
1H), 3.83 (s, 3H), 4.04–4.08 (m, 1H), 4.08–4.12 (m, 2H), 4.79 ((dd, 1H, J = 8.8 and
12.2 Hz), 6.30 (d, 1H, J = 15.7 Hz), 6.53 (d, 1H, J = 3.2 Hz), 6.60 (d, 1H, J = 3.1 Hz),
7.17 (br d, 1H, J = 8.2 Hz), 7.41 (d, 1H, J = 15.7), 7.47 (br s, 1H), 7.49 (d, 1H,
J = 8.1 Hz) ppm; 13C NMR (125 MHz, CDCl3) d: 13.20, 13.56, 20.84, 32.77, 39.83,
51.69, 58.34, 62.57, 62.70, 112.70, 115.58, 115.85, 124.95, 129.26, 130.27,
130.62, 133.11, 133.41, 140.63, 150.63, 151.93, 164.87, 167.30, 167.41,
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172.67 ppm; IR (KBr)
mmax: 1636.3, 1710.0–1730.0 (br, strong), 1735.0–1752.0
(br, strong) cmÀ1
;
HRMS (ESI, 70 eV): m/z = 570.0859(M++Na) and 572.0838 (M++2+Na) [calculated
mass for C25H26NO8BrNa: 570.0739 (M++Na) and 572.0719 (M++2+Na)].
(Supplementary physical and spectral data enclosed for other compounds.)
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