Stereoselective synthesis of trans β-lactams via palladium/N- heterocyclic carbene-catalyzed carbonylative [2+2] cycloaddition

Article abstract of DOI:10.1016/j.tetlet.2012.01.073

The carbonylative [2+2] cycloaddition of benzyl chlorides and allyl derivatives with imines and CO for synthesis of β-lactam is effectively catalyzed by palladium/N-heterocyclic carbene complex. The desired β-lactam could be obtained in good to excellent yields (61-96%) with excellent regioselectivities (trans/cis > 95:5) and chiral lactams could be obtained with moderate diastereoselectivities. The KIE experimental studies have revealed that the C-H cleavage is most likely to be the rate-limiting step for the carbonylative cycloaddition.

Full text of DOI:10.1016/j.tetlet.2012.01.073

Tetrahedron Letters 53 (2012) 1613–1616
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stereoselective synthesis of trans b-lactams via palladium/N-heterocyclic
carbene-catalyzed carbonylative [2+2] cycloaddition
⇑
⇑
Pan Xie, Bo Qian, Hanmin Huang , Chungu Xia
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The carbonylative [2+2] cycloaddition of benzyl chlorides and allyl derivatives with imines and CO for
synthesis of b-lactam is effectively catalyzed by palladium/N-heterocyclic carbene complex. The desired
b-lactam could be obtained in good to excellent yields (61–96%) with excellent regioselectivities (trans/
cis > 95:5) and chiral lactams could be obtained with moderate diastereoselectivities. The KIE experimen-
tal studies have revealed that the C–H cleavage is most likely to be the rate-limiting step for the carbony-
lative cycloaddition.
Received 6 December 2011
Revised 12 January 2012
Accepted 18 January 2012
Available online 28 January 2012
Keywords:
trans-b-Lactam
Ó 2012 Elsevier Ltd. All rights reserved.
N-heterocyclic carbene
Palladium
Carbonylative [2+2] Cycloaddition
1. Introduction
OH
H
H
H
N
H
H
N
H
N
H
S
R1
N
S
R
N
The b-lactam is an important four-membered heterocycle that
exists extensively in numerous natural products and biologically
active compounds (Fig. 1).1 Therefore, intense effort has been
devoted to the synthesis of this four-membered ring system.
Among the many types of synthesis methods for b-lactams docu-
mented, the Staudinger reaction,2 consisting of a [2+2] cycloaddi-
tion of ketene to imine, has provided a rapid and straightforward
access to the b-lactam skeleton.3 Due to the importance of ketene
intermediate in this reaction, many strategies to generate this live
species have been developed. Nowadays, the conventional meth-
ods for the generation of ketenes are the Wolff rearrangement
and the dehydrohalogenation of acid chlorides with tertiary
amines.3
On the other hand, in the carbonylation reaction of allyl phos-
phate, allyl and benzyl halides catalyzed by palladium, a ketene or
an equivalent could also be generated under some appropriate basic
conditions.4 Based on this concept, a large number of b-lactams
have been synthesized through the carbonylation of allyl diethyl
phosphate, allyl halides and imines.5 However, to the best of our
knowledge, only two examples of carbonylative cycloaddition of
benzyl halides and imines have been achieved using Pd(PPh3)Cl2/
NEt3 and Pd(OAc)2/PPh3/NEt3 as catalytic systems, respectively.
Comparatively lower reactivities [usually a longer reaction time
(60–90 h) was needed to achieve good yields and stereoselectivi-
R2
COOH
Cephalosporin
OMe
O
O
O
O
O
COOH
COOH
GV 104326
Penicillin
Figure 1. Pharmaceutical and biologically active b-lactams.
ties] were observed in these two catalytic systems.6 Therefore,
further development of efficient catalyst systems for obtaining sole
trans- or cis-isomer of b-lactams with high yields in the above
carbonylative cycloaddition reaction would be highly desired.
Thanks to the strong
r-donor and weak p-acceptor properties,
N-heterocyclic carbene (NHC) has proved to be an alternative
ligand to phosphorus for many catalytic organic reactions in the
last decade.7 Among the many metal/NHC complexes, palladium
carbene complexes have successfully been employed as catalysts
for catalyzing some of the carbonylation reactions.8 However, to
the best of our knowledge, no example of catalytic carbonylative
cycloaddition of benzyl halides using N-heterocyclic carbenes as
ligands has been reported to date. Considering the priorities of
the N-heterocyclic carbene ligands and the challenge of the
carbonylative [2+2] cycloaddition reaction, we envisioned that
both yields and stereoselectivities of the above carbonylative
[2+2] cycloaddition might be improved with NHC as ligand. Herein,
we describe an efficient palladium/NHC catalyst system for the
highly stereoselective carbonylative [2+2] cycloaddition of benzyl
halides and imines in the presence of CO, which provides an
efficient protocol for the synthesis of trans b-lactams.9
⇑
Corresponding authors. Tel.: +86 931 4968326; fax: +86 931 4968129.
E-mail addresses: hmhuang@licp.cas.cn (H. Huang), cgxia@licp.cas.cn (C. Xia).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.073

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P. Xie et al. / Tetrahedron Letters 53 (2012) 1613–1616
Table 2
2. Results and discussion
Substrate scope of benzyl derivativesa
To examine the catalytic activities of carbene–palladium
complexes, benzyl chloride (1a) and imine (2a) derived from benz-
aldehyde were chosen as model substrates, and the reaction was
performed under 30 atm of CO pressure at 100 °C for 24 h. To our de-
light, the desired cycloaddition reaction proceeded smoothly in the
presence of carbene–palladium complex (Bmim)2PdCl2 (Table 1,
entry 1). Screening of the synthesized carbene–palladium com-
plexes revealed that [(Bmim)PdI2]2 proved to be the most efficient
catalyst, while other complexes gave lower yields (Table 1, entries
1–4; see Supplementary data for details). With [(Bmim)PdI2]2 as
the catalyst, some other reaction parameters were then investi-
gated. Investigation of temperature revealed that the yield was tem-
perature-dependent. Running the reaction at 80 °C instead of 100 °C
only afforded a trace amount of the desired product (Table 1, entry
5). When the CO pressure was reduced from 30 atm to 20 atm, the
yield decreased from 96% to 62% (Table 1, entry 6). Solvent effect
was also examined. The reaction was found to proceed more effi-
ciently in polar solvents than in non-polar ones, and CH3CN was
found to be the best choice (Table 1, entries 1, 7 and 8). Finally,
we examined the effect of base on this reaction. Inorganic bases
were found to be completely inefficient (Table 1, entries 9 and
10). Organic base was found to be crucial to the success of the reac-
tion. The use of a stronger base such as DABCO or TMEDA gave no
desired product (Table 1, entries 11 and 12), but the addition of a
weaker base such as Et3N or n-Pr3N could produce the desired prod-
uct 3aa in moderate yields (Table 1, entries 13 and 14). Therefore it
was concluded that i-Pr2NEt was the best base for this reaction.
H
H
N
[(Bmim)PdI2]2
Ph
O
Ph
Bn
i-Pr2NEt
+
Ph
X
Ph
N
Ph
CO (30 atm)
100°C 24 h
CH3CN
1
2a
3aa
Entry
X
Yieldb,c (%)
1
2
3
4
5
6
Cl
Br
OAc
OCO2Me
SO2Ph
96
94
Trace
Trace
Trace
89
OP(O)(OEt)2
a
Conditions:
1 (1.5 mmol), 2a (0.5 mmol), [(Bmim)PdI2]2 (1 mol %), i-Pr2NEt
(0. 75 mmol), CO (30 atm), CH3CN (2 mL), 100 °C, 24 h.
b
Isolated yield.
trans/cis Ratio: >95:5, determined by 1H NMR of the crude product.
c
With the optimized reaction conditions in hand, we subse-
quently examined the scope of the benzyl derivatives in the
carbonylative [2+2] cycloaddition (Table 2). To our delight,
although acetate, carbonate, and sulfonate could not give the de-
sired product, benzyl halides and phosphate could undergo the
cycloaddition reaction and furnish the desired b-lactam in good
yields and with excellent regioselectivities.
Motivated by this result, we next explored the scope of aldimines
in the carbonylative [2+2] cycloaddition reaction (Table 3). A variety
of aldimines could tolerate and gave the corresponding trans
lactams in high yields. Imines bearing an electron-withdrawing
group, such as 2b and 2c, gave trans b-lactams 3ab and 3ac in 89%
and 84% yields, respectively (Table 3, entries 2 and 3). Substrates
with electron-donating groups also smoothly transformed to their
corresponding b-lactams in good yields (Table 3, entries 4–7). To
Table 1
Optimization of reaction conditionsa
H
H
N
Ph
O
Ph
Bn
Cat./base
+
Ph
Cl
Ph
N
Ph
CO, solvent
100ºC
expand the
employed as a substrate, and the corresponding product 3ah was
generated in an 87% yield (Table 3, entry 8). The ,b-unsaturated
p-conjugated system, naphthyl aldimine 2h was
1a
2a
3aa
a
N
I
N
N
N
imine 2i could also be applied to the present carbonylative cycload-
dition with benzyl chloride and gave the corresponding lactam 3ai
in a 61% yield (Table 3, entry 9). Besides the imines derived from
benzyl amine, the imines derived from aniline and n-propylamine
could also undergo the carbonylation cycloaddition reaction and
gave the corresponding products in 82% and 71% yields, respectively
(Table 3, entries 10 and 11). It is notable that the imine derived from
glycine methyl ester could also transform to the corresponding
product 3al in a good yield and with an excellent regioselectivity
(Table 3, entry 12).10
Furthermore, a range of substituted benzyl chlorides was also
investigated and the results are shown in Table 4. Substitution of
electron-withdrawing (–Cl, –Br) at the 4-position in benzyl chlo-
rides 1b–1c furnished the corresponding products in good yields
(Table 4, entries 1 and 2). An increase in the yield was observed
when an electron-donating group at the 4-position on benzene-
ring of benzyl chlorides such as 1d, provided the product 3da in
a 91% yield (Table 4, entry 3). A slight steric hindrance effect on
the reactivity was observed, which was demonstrated by the reac-
tivities of ortho-methoxy and para-methoxy substituted benzyl
chlorides 1e and 1f. (Table 4, entry 4 vs 5). Besides the substituted
benzyl chlorides, 1-(chloromethyl)naphthalene 1g could also
provide the product 3ga in a 90% yield (Table 4, entry 6).
Pd
Pd
X Pd X
I
I
I
N
N
N
N
X
Cl (Bmim)2PdCl2
[(Bmim)PdI2]2
Br (Bmim)2PdBr2
I
(Bmim)2PdI2
Entry
Cat.
Solvent
Base
Yieldb (%)
1c
2c
3c
4
(Bmim)2PdCl2
(Bmim)2PdBr2
(Bmim)2PdI2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMF
Toluene
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
i-Pr2NEt
NaHCO3
NaOH
29
32
91
96
Trace
62
12
Trace
—
—
—
—
68
70
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
[(Bmim)PdI2]2
5d
6e
7
8
9
10
11
12
13
14
DABCO
TMEDA
Et3N
n-Pr3N
a
Conditions: 1a (1.35 mmol), 2a (0.45 mmol), base (0.675 mmol), catalyst
(1 mol %), CO (30 atm), solvent (2 mL).
To further expand the scope of our current method, we also
tested the allyl derivatives (Scheme 1). When allyl carbonate was
served as the substrate, only an 11% yield was obtained. To our
delight, allyl bromide and phosphate underwent the cycloaddition
b
Isolated yield.
Catalyst: 2 mol %.
80 °C.
c
d
e
20 atm of CO pressure.

P. Xie et al. / Tetrahedron Letters 53 (2012) 1613–1616
1615
Table 3
[(Bmim)PdI2]2
i-Pr2NEt
H
H
N
H
H
N
Ar
Ph
Ar
Ph
Substrate scope of aldiminesa
+
Ar
Cl +
Ph
N
Ph
CO (30 atm)
Ph
Ph
O
O
[(Bmim)PdI2]2
i-Pr2NEt
H
H
N
100ºC 24 h
CH3CN
Ph
O
Ar
1
2l
6
R1
+
Ph
Cl
Ar
N
CO (30 atm)
100ºC 24 h
CH3CN
R1
6bl 85% yield 70:30 d.r.
6cl 83% yield 72:28 d.r.
1b
1c
Ar = 4-ClC6H4
Ar = 4-BrC6H4
1a
2
3
Scheme 2. Carbonylation of substituted benzyl chlorides with chiral imine.
Entry
2
Ar
R1
Productc
Yieldb (%)
1
2
3
2a
2b
2c
2d
2e
2f
2g
2h
2i
C6H5
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
96
89
84
84
70
87
70
87
61
82
71
74
4-ClC6H4
4-BrC6H4
4-MeC6H4
H
D
H
N
Ph
O
Ph
Bn
4
5d
6
2-MeOC6H4
CH2Cl
CD2Cl
[(Bmim)PdI2]2
i-Pr2NEt
2,3-MeO2C6H3
4-Me2NC6H4
1-Naphthyl
(E)-PhCH@CH
4-MeC6H4
C6H5
N
7
8
9
10
11
12
+
+
CO (30 atm)
100ºC MeCN
2 h, 21% yield
H
Ph
O
Ph
Bn
Bn
Ph
2j
2k
2l
3aj
3ak
3al
kH/kD = 4.9
N
n-Pr
CH2COOMe
C6H5
Scheme 3. Kinetic isotope effect experiment.
a
Conditions: 1a (1.35 mmol), 2 (0.45 mmol), [(Bmim)PdI2]2 (1 mol %), i-Pr2NEt
(0.675 mmol), CH3CN (2 mL), CO (30 atm), 100 °C, 24 h.
b
Isolated yield.
c
trans/cis Ratio: >95:5, determined by 1H NMR of the crude product.
[(Bmim)PdI2]2
d
36 h.
A
Ar
Ar'
R
CO
ArCH2Cl
N
(Bmim)Pd(CO)n
Table 4
Substrate scope of benzyl chloridesa
O
3
1
B
R
N
[(Bmim)PdI2]2
i-Pr2NEt
H
H
N
Ar
Ph
Bn
2
Ar'
Ar
Cl
+
Ph
N
Ph
CO (30 atm)
100°C 24 h
CH3CN
O
C
O
CH2Ar
2
1
a
3
Pd(Bmim)(CO)n
Bmim Pd CO
Ar
H
E
Cl
Productc
Yieldb (%)
C
Entry
1
Ar
1
2
3
4
5
6
1b
1c
1d
1e
1f
4-ClC6H4
4-BrC6H4
4-MeC6H4
2-MeOC6H4
4-MeOC6H4
1-Naphthyl
3ba
3ca
3da
3ea
3fa
87
76
91
84
89
90
i-Pr2NEt•HCl
CO
O
CCH2Ar
Bmim Pd CO
1g
3ga
i-Pr2NEt
Cl
D
a
Conditions: 1 (1.35 mmol), 2a (0.45 mmol), [(Bmim)PdI2]2 (1 mol %), i-Pr2NEt
(0.675 mmol), CH3CN (2 mL), CO (30 atm), 100 °C, 24 h.
Scheme 4. Mechanism of the carbonylative cycloaddition reaction.
b
Isolated yield.
c
trans/cis Ratio: >95:5, determined by 1H NMR of the crude product.
smoothly and gave the corresponding lactam 5da in an excellent
yield.
Considering the importance of chiral b-lactams, the cycloaddi-
tion reaction of chiral imines aiming at obtaining enantiomerically
pure b-lactams was also attempted. Using (R)-benzylidene-N-phen-
ylethylamine as the chiral imine substrate, the carbonylative cyclo-
addition reactions were carried out by using the substituted benzyl
chlorides 1b and 1c, the desired products 6bl and 6cl were obtained
in good yields under the optimized conditions. However, the
carbonylation reaction only gave the corresponding lactams in
70:30 and 72:28 diastereoselectivities, respectively (Scheme 2).5d,6b
To obtain some mechanistic insights, the kinetic isotope effects
(KIE) of benzyl chloride were studied. As shown in Scheme 3, the
kH/kD was found to be 4.9 (see Supplementary data). This observed
kinetic isotope effect indicates that C–H cleavage is most likely to
be the rate-limiting step of the carbonylative cycloaddition.11
On the basis of these experimental results and previous
work,5,6,13 a plausible reaction pathway is outlined in Scheme 4.
Initially, CO-complexed Pd(0) species B was generated in the pres-
ence of CO and base,12 which may serve as an effective catalytic
species to react with benzyl chloride 1 to give rise to the interme-
[(Bmim)PdI2]2
i-Pr2NEt
H
H
Ph
Bn
+
X
Ph
Ph
N
N
CO (30 atm)
100ºC 24 h
CH3CN
O
5aa
4
2a
4a X = Br
4b
4c
84% yield, trans/ cis ratio: >95:5
X = OP(O)(OEt)2 89% yield, trans/ cis ratio: >95:5
X = OCO2Me
11% yield, trans/ cis ratio: >95:5
[(Bmim)PdI2]2
Ph
i-Pr2NEt
H
H
N
Ph
Bn
+
Ph
Cl
Ph
Ph
N
CO (30 atm)
100°C 24 h
CH3CN
O
5da
4d
2a
92% yield, trans/ cis ratio: >95:5
Scheme 1. Carbonylation of the allyl derivatives.
as well as produced the desired product 5aa in good yields and
with excellent regioselectivities. The carbonylative [2+2] cycload-
dition of (E)-phenyl substituted allyl chloride 4d proceeded

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P. Xie et al. / Tetrahedron Letters 53 (2012) 1613–1616
diate C through oxidative addition.13a,14 The intermediate D would
be generated through the CO insertion under the CO atmosphere,
which was then transformed to E in the presence of base. The
[2+2] cycloaddition of E and imine 2 would afford the trans product
3 and the palladium (0) species B was regenerated to complete the
catalytic cycle.5a,13a
Heterocyclic Chemistry, Gordon, W. G., John, A. J., Eds.; Elsevier: Oxford, 2010;
Vol. 22, Chapter 4, pp 85–107.
2. Staudinger, H. Liebigs Ann. Chem. 1907, 356, 51–123.
3. (a) Tidwell, T. T. Ketenes; Wiley: New York, 1995. pp 77–100; (b) Wolff, L.
Liebigs Ann. Chem. 1902, 325, 129–195; (c) Kirmse, W. Eur. J. Org. Chem. 2002,
2193–2256; (d) Tidwell, T. T. Eur. J. Org. Chem. 2006, 563–576; (e) Seki, H.;
Georg, G. I. J. Am. Chem. Soc. 2010, 132, 15512–15513; (f) Litovitz, A. E.;
Keresztes, I.; Carpenter, B. K. J. Am. Chem. Soc. 2008, 130, 12085–12094.
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2008; (b) Liégault, B.; Renaud, J.; Bruneau, C. Chem. Soc. Rev. 2008, 37, 290.
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3. Conclusions
In summary, we have developed an efficient catalyst system for
the carbonylative [2+2] cycloaddition of benzyl chlorides or allyl
derivatives with imines. With 1 mol % [(Bmim)PdI2]2 as the cata-
lyst, the cycloaddition reaction can proceed smoothly to afford
the desired b-lactams with up to a 96% yield as well as with an
excellent stereoselectivity (the trans/cis ratio up to >95:5). This
efficient catalyst gives a supplement to the prior reported catalytic
system with palladium–phosphine complexes as catalysts. Further
studies on the asymmetric carbonylative cycloaddition are ongoing
in our group.
6. (a) Cho, C. H.; Jiang, L. H.; Shim, S. C. Synth. Commun. 1999, 29, 2695–2703; (b)
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Acknowledgment
This work was supported financially by the Chinese Academy of
Sciences and National Natural Science Foundation of China
(20802085).
9. For the assignment of cis- or trans-stereochemistry by coupling constants, see
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Supplementary data
10. Ketoimines have been applied in this reaction, but no desired [2+2]
cycloaddition products were observed.
11. Grate, J. W.; Frye, G. C. In Mechanism and Theory in Organic Chemistry; Harper
Collins: New York, 1996.
Supplementary data (characterization data, copies of the NMR
spectra) associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.tetlet.2012.01.073.
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