Stereoselective intramolecular carbene C-H insertion catalyzed by rhodium(III) porphyrin complexes

Article abstract of DOI:10.1055/s-0032-1317322

Rhodium(III) porphyrin complexes [Rh(Por)Me] are catalytically active towards stereoselective intramolecular carbene C-H insertions of α-diazoacetamides to give cis-β-lactams or trans-γ-lactams in yields up to 99% with regioselectivities up to 100% and ci

Full text of DOI:10.1055/s-0032-1317322

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▌2753
cluster
Stereoselective Intramolecular Carbene C–H Insertion Catalyzed by
Rhodium(III) Porphyrin Complexes
Stereoselective Intramolecular Carbene C–H Insertion
Vanessa Kar-Yan Lo, Hung-Yat Thu, Yu-Man Chan, Tsz-Lung Lam, Wing-Yiu Yu,1 Chi-Ming Che*
Department of Chemistry, State Key Laboratory of Synthetic Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular
Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. of China
Fax +85229155176; E-mail: cmche@hku.hk
Received: 01.08.2012; Accepted after revision: 05.09.2012
EtO2C
R1
O
Abstract: Rhodium(III) porphyrin complexes [Rh(Por)Me] are cat-
alytically active towards stereoselective intramolecular carbene
C–H insertions of α-diazoacetamides to give cis-β-lactams or trans-
γ-lactams in yields up to 99% with regioselectivities up to 100% and
cis/trans ratios up to 83:17 for β-lactams.
O
O
N
R1
N
OEt
R2
R2
N2
yield up to 99%
cis/trans up to 83:17
[Rh(TTP)Me] (1 mol%)
toluene, reflux
Key words: rhodium porphyrin, carbene C–H insertion, stereo-
selectivity, intramolecular cyclization, lactams
R1
CO2Et
O
O
O
R1
N
OEt
N
R2
N2
Metalloporphyrin complexes are well documented as effi-
cient catalysts for atom-/group-transfer catalysis2 for
C–C,3 C–N,4 and C–O5 bond-formation reactions. The re-
activity of metalloporphyrin catalysts can be tuned by
modifying either the substituents on the porphyrin ligand
or the axial ligands.
R2
yield up to 97%
regioselectivity up to 100%
Scheme 1
Z
We have demonstrated that an axial ligand with a strong
trans effect facilitates carbene-transfer reactions by weak-
ening the trans metal–carbene carbon bond.6 Previously,
we have synthesized and characterized the biscarbene os-
mium porphyrin complex [Os(TPFPP)(CPh2)2].6b Its
X-ray crystal structure reveales a significantly long Os–C
(carbene) bond distance [1.910(4) Å] imposed by the
strong trans effect of the CPh2 group. This leads to its high
reactivity towards stoichiometric carbene transfer for
cyclopropanation. We have applied this concept of a
strong trans effect to the design of highly active chiral
rhodium(III) porphyrin [Rh(D4-Por)Me]7 and iridium(III)
porphyrin [Ir(D4-Por)Me]8 catalysts for the enantioselec-
tive carbene C–H bond-insertion reaction. We envision
that rhodium(III) porphyrin complexes could catalyze
other carbene-transfer reactions.
Y
X
Y
X
Me
Rh
X
Y
Y
Y
X
X
N
N
N
N
Z
Z
X
Y
X
X
Y
Y
Z
[Rh(TTP)Me] Z = Me, X = Y = H
[Rh(TDCPP)Me] Y = Z = H, X = Cl
[Rh(TMP)Me] X = Z = Me, Y = H
[Rh(TPPPP)Me] X = Z = Ph, Y = H
The transition-metal-catalyzed decomposition of α-diazo-
acetamides for intramolecular carbene C–H insertion to
give lactams has been extensively studied.9 With our re-
cent experiences in ruthenium-catalyzed intramolecular
carbene C–H insertions,10 herein we report the
[Rh(Por)Me]-catalyzed stereoselective intramolecular
carbene C–H insertion of α-diazoacetamides, giving cis-
β-lactams or trans-γ-lactams in yields up to 99% with re-
gioselectivities up to 100% and cis/trans ratios up to
83:17 for β-lactams (Scheme 1, Figure 1).
Figure 1 Rhodium porphyrin catalysts employed in this work
The rhodium(III) porphyrin complexes [Rh(Por)Me] were
synthesized according to the reported procedure.11 In
brief, heating [Rh(CO)2Cl]2 with the corresponding free
base porphyrin H2Por in refluxing toluene, followed by
subsequent reductive alkylation with NaBH4/MeI in etha-
nol afforded [Rh(Por)Me] complexes in 67–72% yields
(Scheme 2).
The catalytic activity of these rhodium(III) porphyrin
complexes towards intramolecular carbene C–H insertion
was examined.12 The results are depicted in Table 1.
SYNLETT 2012, 23, 2753–2757
Advanced online publication: 18.10.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1317322; Art ID: ST-2012-Y0608-C
© Georg Thieme Verlag Stuttgart · New York

2754
V. K.-Y. Lo et al.
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groups of the porphyrin ligand exert significant steric hin-
drance leading to a reduction in catalytic activity of the
rhodium catalyst towards this intramolecular carbene in-
sertion.
1) toluene, reflux
[Rh(Por)Me]
[Rh(CO)2Cl]2 + H2Por
2) NaBH4, MeI
EtOH, r.t.
67–72% yield
Scheme 2
Screening of solvent revealed that toluene was the best
solvent for this reaction. Other solvents such as dichloro-
methane, ethanol, and acetone led to less than 50% sub-
strate conversion for a reaction time of one hour. When
the reaction was performed in THF or acetonitrile, no β-
lactam was observed with ca. 88% substrate recovery.
At the outset, α-diazoacetamide 1a was treated with
1 mol% of [Rh(TTP)Me] (H2TTP = meso-tetrakis(p-tolyl)-
porphyrin) in 10 mL refluxing toluene for 30 minutes. On
the basis of 1H NMR analysis of the crude reaction mix-
ture, complete substrate conversion was found, and β-lac-
tam 2a was isolated in 99% yield with a cis/trans ratio of
83:17 (Table 1, entry 1). Prolonged reaction for 12 hours
resulted in the exclusive formation of trans-β-lactam
(cis/trans = 1:99; Table 1, entry 2), suggesting an epimer-
ization of the initial cis product to the trans stereoisomer
catalyzed by the rhodium(III) porphyrin catalyst. Lower-
ing the reaction temperature to 80 °C improved the stereo-
selectivity to a cis/trans ratio of 95:5, despite a reduction
of substrate conversion to 25% (Table 1, entry 3). In the
absence of rhodium(III) porphyrin catalyst [Rh(TTP)Me],
no 2a was observed, and 1a was recovered quantitatively.
As 1 mol% of [Rh(TTP)Me] gave the best result in terms
of conversion, yield, and stereoselectivity in refluxing tol-
uene, these reaction conditions were adopted for sub-
sequent studies.
With the optimized reaction conditions, we then expanded
the substrate scope of this intramolecular carbene C–H-
insertion reaction.12 We also compared the catalytic activ-
ities of these rhodium(III) porphyrin complexes with that
of dirhodium(II,II) carboxylate. The latter is a well-
known catalyst for carbene-transfer reactions.9 The results
are depicted in Table 2. α-Diazoacetamides 1a–c were cy-
clized in the presence of [Rh(TTP)Me] catalyst, giving
cis-β-lactams 2a–c in excellent yields and good selectivi-
ties (Table 2, entries 1, 3 and 5). Cyclization of 1a–c also
proceeded with high product yields when [Rh2(OAc)4]
was employed as the catalyst, despite a reduction in stereo-
selectivity with trans-β-lactams as the major products
(Table 2, entries 2, 4 and 6). The [Rh(TTP)Me]-catalyzed
carbene insertion with 1d as substrate was directed to the
3° C–H bond, affording 2d in 98% yield (Table 2, entry
7). No product from carbene insertion to 1° C–H bond was
Table 1 Condition Optimization and Catalyst Screeninga
O
O
EtO2C
O
[Rh(Por)Me] (1 mol%)
toluene, reflux
Ph
N
OEt
N
N2
t-Bu
Ph
t-Bu
1a
2a
Entry Catalyst
Time (h) Conv.b Yield (%)c cis/transb
1
[Rh(TTP)Me]
[Rh(TTP)Me]
[Rh(TTP)Me]
0.5
12
0.5
100
100
25
99
99
99
94
93
–
83:17
1:99
95:5
24:76
37:63
–
1
detected by H NMR analysis of the crude reaction mix-
2
ture. The same product 2d was also obtained exclusively
from the [Rh2(OAc)4]-catalyzed decomposition of 1d
with a lower isolated yield (76%; Table 2, entry 8).
3d
4
[Rh(TDCPP)Me] 12
48
With 1e bearing an N-p-methoxyphenyl group and 1f with
N-p-nitrophenyl group as the substrate, the [Rh(TTP)Me]-
catalyzed reaction in both cases resulted in the formation
of a mixture of trans-β-lactams 2e and 2f and 2-(3H)-in-
dolinones 3e and 3f (derived from aromatic C–H inser-
tion), with moderate preference towards the formation of
trans-β-lactams (Table 2, entries 9 and 11). Changing the
catalyst to [Rh2(OAc)4] for the reaction of 1e gave trans-
β-lactam 2e as the major product (Table 2, entry 10),
while cyclization of 1f led to the formation of trans-β-lac-
tam 2f exclusively (Table 2, entry 12).
5
[Rh(TMP)Me]
12
12
54
6
[Rh(TTPPP)Me]
<10
a Reaction conditions: A mixture of 1a (0.1 mmol) and catalyst
(1 mol%) was stirred in refluxing toluene under an N2 atmosphere.
b Determined by 1H NMR analysis of the crude reaction mixture.
c Isolated yield based on conversion.
d Reaction temperature was 80 °C.
The effect of the porphyrin ligand was studied. Sterically
bulky [Rh(TDCPP)Me] (H2TDCPP = meso-tetrakis(2,6-
dichlorophenyl)porphyrin)
and
[Rh(TMP)Me]
To probe the regioselectivity for β- vs. γ-lactam forma-
tion, we examined the catalytic cyclization of α-diazo-
acetamide 1g bearing both N-tethering phenylethylene
(for the formation of γ-lactams) and benzyl groups (for the
formation of β-lactams) using [Rh(TTP)Me] or
[Rh2(OAc)4] as catalyst.12 The results are depicted in Ta-
ble 3. Treating 1g with [Rh(TTP)Me] (1 mol%) in reflux-
ing toluene gave trans-γ-lactam 4g exclusively in 93%
isolated yield (Table 3, entry 1). The use of [Rh2(OAc)4]
for the same reaction gave a mixture of γ- and β-lactams
(60:40) in 85% total yield (Table 3, entry 1). Changing the
(H2TMP = meso-tetrakismesitylporphyrin) were found to
be less reactive catalysts for the cyclization reaction, lead-
ing to less than 60% substrate conversion over a reaction
time of 12 hours with trans-β-lactam as the major product
(Table 1, entries 4 and 5). Rhodium catalyst bearing the
bulky
Suslick’s
bispocket
porphyrin
ligand
[Rh(TTPPP)Me]
[H2TTPPP = meso-tetrakis(2,4,6-tri-
phenylphenyl)porphyrin] only led to less than 10% con-
version of 1a over a period of 12 hours. These results
suggest that the ortho-substituents on the four meso-aryl
Synlett 2012, 23, 2753–2757
© Georg Thieme Verlag Stuttgart · New York

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Stereoselective Intramolecular Carbene C–H Insertion
2755
substrate to N,N-dialkyl diazoacetamides 1h and 1i fa- entry 5). Reacting 1k with [Rh2(OAc)4] gave γ-lactam 4k
vored the formation of γ-lactams over β-lactams when as the major product (4k/3k = 84:16; Table 3, entry 5).
[Rh(TTP)Me] was used as the catalyst (Table 3, entries 2 Cyclization of 1m bearing an N-p-nitrophenyl group cat-
and 3). The same result was also observed for the alyzed by [Rh(TTP)Me] resulted in a mixture of γ-lactam
[Rh2(OAc)4]-catalyzed cyclization of 1h (Table 3, entry 4l, indolinone 3l, and β-lactam 2l in 83% overall yield
2), but the reverse selectivity was detected for substrate 1i (Table 3, entry 6), while the use of [Rh2(OAc)4] catalyst
(Table 3, entry 3).
favored the formation of γ-lactam 4l in moderate yield
(Table 3, entry 6).
The effect of the N-substituents on the γ-lactam formation
was studied using substrates 1j–l having different N-pro- α-Diazoacetamides derived from β-amino acid deriva-
tecting groups (Table 3, entries 4–6). Substrate 1j with an tives, which are commercially available chiral starting
N-tert-butyl group in the presence of [Rh(TTP)Me] cata- materials, were investigated for the synthesis of chiral
lyst led to the formation of γ-lactam 4j and β-lactam 2j in γ-lactams (Scheme 3).12 Treating substrates 1m and 1n
93% total yield with a regioselectivity of 65:35 favoring with [Rh(TTP)Me] (1 mol%) in refluxing toluene for 3–5
γ-lactam formation (Table 3, entry 4). The use of hours led to the formation of bicyclic γ-lactams 4m and 4n
[Rh2(OAc)4] as catalyst for the same reaction gave ap- in 98% and 96% yield, respectively. 1H NMR analysis of
proximately equimolar of 4j and 2j (Table 3, entry 4). the crude reaction mixture showed the signals of only one
Changing the N-substituent to N-p-methoxyphenyl group diastereomer, indicating diastereoselectivity of >99% for
(1k) resulted in a competitive reaction between γ-lactam the rhodium(III) porphyrin catalyzed reaction. The nucle-
formation and aromatic C–H insertion when ar Overhauser and exchange spectroscopy (NOSEY)
[Rh(TTP)Me] was employed as catalyst, leading to the spectral correlation of γ-lactam 4m revealed a trans ste-
formation of 4k and 3k in equimolar amounts (Table 3, reochemistry (see Supporting Information).
Table 2 Rh-Catalyzed β-Lactam Formation from α-Diazoacetamidesa
Entry Substrate
Product
Catalyst
Yield (%)b
cis/transc
EtO2C
O
O
O
N
N
OEt
t-Bu
t-Bu N2
R
R
1
2
3
4
5
6
1a R = H
2a R = H
[Rh(TTP)Me]
[Rh2(OAc)4]
[Rh(TTP)Me]
[Rh2(OAc)4]
[Rh(TTP)Me]
[Rh2(OAc)4]
99
97
96
98
90
96
83:17
35:65
84:16
45:55
75:25
30:70
1b R = MeO
1c R = Cl
2b R = MeO
2c R = Cl
EtO2C
O
O
N
O
OEt
N
7
8
[Rh(TTP)Me]
[Rh2(OAc)4]
98
76
N2
1d
2d
O
O
EtO2C
O
MeO
p-MeOC6H4
O
yield
95
52
2/3d
55:45
75:25
N
OEt
OEt
N
9
10
N
[Rh(TTP)Me]
[Rh2(OAc)4]
Ph
C6H4-p-OMe
N2
Bn
Ph
2e
3e
1e
O
O
O2N
EtO2C
Ph
O
p-O2NC6H4
O
N
11
12
[Rh(TTP)Me]
[Rh2(OAc)4]
78
44
60:40e
100:0
N
N
N2
C6H4-p-NO2
Bn
Ph
2f
3f
1f
a Reaction conditions: A mixture of diazo compound (0.1 mmol) and catalyst (1 mol%) was stirred in refluxing toluene under an N2 atmosphere.
b Isolated yield.
c Determined by 1H NMR analysis of the crude reaction mixture.
d Only trans-β-lactam was obtained.
e p-Nitroaniline was obtained in 12% yield.
© Georg Thieme Verlag Stuttgart · New York
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V. K.-Y. Lo et al.
CLUSTER
Table 3 Rh-Catalyzed γ-Lactam Formation from α-Diazoacetamidesa
Entry
Substrate
Product
Rhodium catalyst
Yield (%)b 4/2 or 3c
CO2Et
O
Ph
EtO2C
Ph
O
O
O
Ph
[Rh(TTP)Me]
[Rh2(OAc)4]
93
85
100:0
60:40
N
OEt
N
1
N
Bn
N2
O
Ph
Bn
1g
4g
2g
CO2Et
O
C3H7
O
EtO2C
O
C5H11
N
OEt
N
[Rh(TTP)Me]
[Rh2(OAc)4]
80
72
90:10
80:20
N
2
3
4
N2
C4H9
C5H11
C4H9
C5H11
4h
2h
1h
CO2Et
O
O
EtO2C
Me
O
N
OEt
O
[Rh(TTP)Me]
[Rh2(OAc)4]
93
46
85:15
5:95
N
N
N2
Et
Et
1i
4i
2i
Ph
CO2Et
EtO2C
O
O
O
Ph
1j
N
N
OEt
O
[Rh(TTP)Me]
[Rh(OAc)2]2
93
96
65:35
50:50
N
t-Bu
t-Bu N2
t-Bu
Ph
4j
2j
O
N
O
Ph
CO2Et
O
MeO
p-MeOC6H4
O
OEt
N
[Rh(TTP)Me]
[Rh2(OAc)4]
97
92
50:50
84:16
N2
N
5
6
C6H4-p-OMe
Ph
Ph
4k
3k
1k
O
N
O
Ph
CO2Et
O2N
p-O2NC6H4
O
OEt
38:42d
80:20
N
[Rh(TTP)Me]
[Rh2(OAc)4]
83
51
O
N
N2
C6H4-p-NO2
Ph
Ph
4l
3l
1l
a Reaction conditions for [Rh(TTP)Me]: A mixture of diazo compound (0.1 mmol) and [Rh(TTP)Me] (1 mol%) was stirred in refluxing toluene
under N2 atmosphere. Reaction conditions for [Rh2(OAc)4]: A mixture of diazo compound (0.1 mmol) and [Rh2(OAc)4] (2 mol%) was stirred
in refluxing CH2Cl2 under an N2 atmosphere.
b Isolated yield.
c Determined by 1H NMR analysis of the crude reaction mixture.
d trans-β-Lactam 2l was also obtained as minor product (4l/3l/2l = 38:42:20).
In summary, [Rh(TTP)Me] is an effective catalyst for ste-
reoselective intramolecular carbene insertion into saturat-
O
O
[Rh(TTP)Me] (1 mol%)
N
H
E
E
O
N
ed C–H bonds of α-diazoacetamides, giving the
corresponding cis-β-lactams and trans-γ-lactams in excel-
lent yields (up to 99%), excellent regioselectivities (up to
100%) and good stereoselectivities (cis/trans ratio up to
83:17).
O
toluene, 70 °C
3–5 h
N2
R
R
>99% de
1m R = Ph, E = CO2Et
1n R = Ph, E = COMe
4m 98%
4n 96%
Scheme 3
Synlett 2012, 23, 2753–2757
© Georg Thieme Verlag Stuttgart · New York

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Stereoselective Intramolecular Carbene C–H Insertion
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(8) Wang, J.-C.; Xu, Z.-J.; Guo, Z.; Deng, Q.-H.; Zhou, C.-Y.;
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Acknowledgment
This work was supported by The University of Hong Kong and
Hong Kong Research Grants Council (HKU1/CRF/08).
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
maotrin
r
t
References and Notes
(1) Current address: State Key Laboratory of Chirosciences and
Department of Applied Biology and Chemical Technology,
The Hong Kong Polytechnic University, Hung Hom,
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(12) General Procedure
Diazo ester 1 (0.1 mmol) dissolved in toluene (5 mL) was
added to a toluene solution (5 mL) containing [Rh(Por)Me]
(1 mol%) under an N2 atmosphere. The reaction mixture was
refluxed until 1 was completely consumed. The mixture was
concentrated under reduced pressure, and the residue was
purified by flash column chromatography using EtOAc–
hexane as the eluent to obtain the desired products.
Data of β-Lactam 2f
Yellow oil. 1H NMR (300 MHz, CDCl3): δ = 8.15 (d, J = 5.1
Hz, 2H), 7.43–7.37 (m, 7 H), 5.41 (d, J = 2.8 Hz, 1 H), 4.31
(q, J = 7.1 Hz, 2 H), 3.65 (d, J = 2.8 Hz, 1 H), 1.34 (t, J = 7.1
Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ = 168.2, 161.8,
137.8, 134.6, 129.6, 129.5, 128.6, 126.1, 125.2, 117.1, 64.1,
62.5, 58.3, 14.1. MS (EI): m/z = 340 [M+]. HRMS (EI): m/z
calcd for C18H16N2O5: 340.1059; found: 340.1065.
Data of γ-Lactam 4h
(4) Recent review: Fantauzzi, S.; Caselli, A.; Gallo, E. Dalton
Trans. 2009, 5434.
(5) Recent review: Che, C.-M.; Huang, J.-S. Chem. Commun.
2009, 3996.
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Y.; Huang, J.-S.; Zhou, Z.-Y.; Che, C.-M. J. Am. Chem. Soc.
2001, 123, 4843. (c) Li, Y.; Huang, J.-S.; Zhou, Z.-Y.; Che,
C.-M.; You, X.-Z. J. Am. Chem. Soc. 2002, 124, 13185.
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Y.; Che, C.-M. Dalton Trans. 2006, 4845.
Colorless oil. 1H NMR (400 MHz, CDCl3): δ = 4.28–4.19
(m, 2H), 3.58–3.48 (m, 1H), 3.15–3.11(m, 2H), 3.05–2.91
(m, 1H), 2.79–2.70 (m, 1H), 1.78–1.60 (m, 1H), 1.54–1.45
(m, 2H), 1.31–1.25 (m, 7H), 1.20–1.15 (m, 1H), 0.94–0.85
(m, 9 H). 13C NMR (100 MHz, CDCl3): δ = 169.4, 168.3,
61.5, 55.5, 52.1, 52.0, 48.9, 49.0, 36.3, 33.0, 26.9, 20.4, 16.8,
14.0, 11.2. MS (EI): m/z = 269 [M+]. HRMS (EI): m/z calcd
for C19H18N2O5: 269.1991; found: 269.1992.
(7) Thu, H.-Y.; Tong, G. S.-M.; Huang, J.-S.; Chan, S. L.-F.;
Deng, Q.-H.; Che, C.-M. Angew. Chem. Int. Ed. 2008, 47,
9747.
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Synlett 2012, 23, 2753–2757

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