Asymmetric intramolecular C-H insertion of α-diazoacetamides in water by dirhodium(II) catalysts derived from natural amino acids

Article abstract of DOI:10.1002/adsc.201200101

The asymmetric dirhodium(II)-catalyzed intramolecular C-H insertion of α-diazo acetamides in water is described for the first time. The use of natural α-amino acids as chiral ligands allowed the preparation of novel dirhodium(II) homochiral complexes by a

Full text of DOI:10.1002/adsc.201200101

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DOI: 10.1002/adsc.201200101
À
Asymmetric Intramolecular C H Insertion of a-Diazoacetamides
in Water by Dirhodium(II) Catalysts Derived from Natural
Amino Acids
Nuno R. Candeias,a,b,* Carolina Carias,c Luis F. R. Gomes,c Vꢀnia Andrꢁ,d
M. Teresa Duarte,d Pedro M. P. Gois,a and Carlos A. M. Afonsoa,c,*
a
iMed.UL, Faculdade de Farmꢂcia da Universidade de Lisboa, Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal
Fax : (+351)-21-794-6476; phone: (+351)-217-946-400 (ext. 14614); e-mail: carlosafonso@ff.ul.pt
Department of Chemistry and Bioengineering, Tampere University of Technology, Korkeakoulunkatu 8, 33101 Tampere,
b
Finland
E-mail: nuno.candeias@tut.fi
c
CQFM, Centro de Quꢃmica-Fꢃsica Molecular and IN - Institute of Nanosciences and Nanotechnology, Universidade de
Lisboa, 1049-001 Lisboa, Portugal
CQE – Centro de Quꢃmica Estrutural, Instituto Superior Tꢁcnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
d
Received: February 6, 2012; Revised: June 22, 2012; Published online: October 10, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200101.
Abstract: The asymmetric dirhodium(II)-catalyzed
intramolecular C H insertion of a-diazo acetamides
as the reaction medium. Due to the nature of the typ-
ical ligands used in the preparation of homochiral dir-
hodium complexes (proline derivatives[4], phthaloyl-
protected amino acids[5] mandelic acid derivatives[6],
amongst others) the catalysts are usually water insolu-
ble and the reutilization of the catalyst became com-
promised due to the costs associated with purification
À
in water is described for the first time. The use of
natural a-amino acids as chiral ligands allowed the
preparation of novel dirhodium(II) homochiral
complexes by a simple procedure consisting of the
in situ ligand exchange starting from dirhodium tet-
raacetate. The catalytic system was further reused
up to 7 cycles and b-lactams were obtained in good
yields and enantiomeric excess.
À
Keywords: C H insertion; a-diazo compounds; dir-
hodium(II); b-lactams; water
Dirhodium(II) complexes are efficient catalysts in car-
À
À
À
benoid cyclopropanation, C H, N H and O H acti-
vation reactions derived from a-diazo carbonyl com-
pounds. Due to the presence of two rhodium atoms,
these metallic complexes promote the formation as
well as the stabilization of an organic carbene moiety
that can consequently undergo an insertion reaction.[1]
À
The use of homochiral catalysts in C H insertion re-
actions in organic solvents is well documented in the
literature (Figure 1) and good to excellent enantiose-
lectivities have been achieved both for intramolecu-
lar[2] and intermolecular[1g,3] reactions. However, and
despite the widely available homochiral dirhodium
complexes, to the best of our knowledge, the dirho-
Figure 1. Some reported (1–3) homochiral dirhodium(II)
catalysts and new ones (7a, 7b and 7c) derived from unpro-
À
dium(II)-catalysed asymmetric intramolecular C H
insertion reaction has not been reported using water tected amino acids.
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procedures. This catalyst reutilization is always desira-
ble in a more sustainable chemistry context[7] but this
Despite the excellent results obtained in the a-di-
AHCTUNGTREGaNNUN zoacetamide cyclization in water, namely using N-
becomes even more important considering the high phthaloyl-protected amino acid dirhodium complexes
cost of rhodium derivatives. Dirhodium catalysts were derivatives (1a and 1b), most of the known chiral dir-
recently reported as water tolerant complexes and hodium catalysts are poorly soluble in water. Due to
several examples of their use in water have been de- the high availability of enantiomerically pure a-amino
scribed.[8] The solubility and tolerance of some dirho- acids in nature, this type of compound can be ex-
dium complexes, in particular Rh2ACTHNUTRGNEN(UG OAc)4, allowed the tremely important in the preparation of enantioen-
reutilization of the catalytic system.[7] If a solvent is riched organic molecules. Taking advantage of the
required for a reaction, water is the cheapest, most chiral pool of natural products and derivatives, the
abundant solvent available and also with unique prop- use of water as the reaction medium, and the known
erties. Despite the widely explored use of water as protocol for the formation of new dirhodium(II) com-
solvent in organic reactions, the full potential of this plexes by ligand exchange starting from dirhodium(II)
solvent has not been completely brought to light and tetraacetate,[10] we developed a protocol for the for-
many works have been done to understand the chemi- mation of new catalysts. Such a protocol was based on
cal and physical properties of water and how they in- the Rh
2ACHTUNGTERN(NUNG OAc)4 ligand exchange with an excess of a-
fluence organic reactions.[9] Here we present the first amino acids, and after ligand exchange the complex
À
À
asymmetric intramolecular C H insertion in water, was tested on the intramolecular C H insertion of a-
promoted by a dirhodium(II) catalyst obtained from diazoacetamide 4a for 24 h. After screening a range
reaction with natural a-amino acids. The success of in- of a-amino acids and derivatives (see the Supporting
À
tramolecular C H insertions in water is strongly de- Information for the complete table), l-phenylalanine
pendent on the substrate and catalyst solubility.[8g] In derived complex was one of the most promising ones.
order to evaluate this effect on the reaction’s stereo- Rh2ACTHNUGTRNE(UNG OAc)4 ligand exchange reaction was performed
selectivity, we tested several known hydrophobic at 808C for 56 h and the catalytic activity of the newly
chiral dirhodium catalysts (Figure 1) on the intramo- complex formed was tested in the cyclization of diazo
À
lecular C H insertion of a-diazoacetamide 4a in di- compound 4a at 808C for 48 h. Lactam 5a was ob-
chloromethane (Scheme 1) and in water and we were tained in 54% yield and 62% enantiomeric excess.
glad to observe that after replacing dichloromethane Under these ligand exchange conditions, the starting
by water, similar enantioselectivities could be ob- Rh
tained (Table 1, entries 1 and 2).
2ACHTUNGTERN(NUNG OAc)4 was completely consumed after 56 h, as
determined by HPLC (Figure 2 in the Supporting In-
formation).
By using l-phenylalanine analogues as chiral li-
gands in the preparation of new chiral dirhodium
complexes, it was possible to determine structures of
the complexes 7b and 7c by X-ray analysis.[11] Howev-
er, the structure of the l-phenylalanine derived com-
plex 7a was only determined after preparative HPLC
purification, due to a side product of the complex
preparation that made the crystal formation difficult.
The new complexes maintained two acetate bridge
À
Scheme 1. Rh(II)-catalyzed intramolecular C H insertion of
a-diazoacetamide 4a.
À
Table 1. C H insertion of a-diazoacetamide 4a.
Entry
Rh(II) catalyst
Conditions[a]
Yield [%][b]
cis:trans[c]
ee [%][d]
1
2
3
4
5
6
7
8
1a
1a
1a
1b
2a
2b
3b
3a
CH2Cl2, 258C, 3.5 h
H2O, 25 8C, 21 h
H2O, 80 8C, 1 h
H2O, 80 8C, 0.5 h
H2O, 80 8C, 0.5 h
H2O, 80 8C, 0.5 h
H2O, 80 8C, 1 h
93
93
82
84
75
65
80
83
1:0
1:0
1:0
1:0
1:0
1:0.1
1:0
1:0
66
60
56
72
15
28
0
H2O, 80 8C, 0.5 h
0
[a]
All reactions were carried out using 4[5c] (0.15 mmol), Rh(II) catalyst (2 mol%), solvent (1.5 mL).
Isolated yield after purification by flash chromatography.
[b]
[c]
[d]
Observed ratio by 1H NMR of the crude reaction mixture.
Determined by chiral HPLC analysis, after epimerization to trans diastereoisomer.
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Asymmetric Intramolecular C H Insertion of a-Diazoacetamides in Water by Dirhodium(II) Catalysts
Figure 2. X-ray structure representation of 7a (see the Supporting Information for details).
À
Table 2. Asymmetric C H insertion of a-diazoacetamides 4a
and 4b with Rh(II) catalysts 7a–c derived from a-amino
acids.
units and two amino acid units (Figure 2). However,
the amino acid units do not maintain the expected
bridge geometry but rather bond one of these to each
rhodium atom. The amino acid unit bonds to the rho-
dium atom by the oxygen of the carboxylate moiety
and the nitrogen atom of the amine functional group.
One should also notice the different orientations of
the amino acid units where the nitrogen of one amino
acid unit is superimposed to the oxygen of the other
amino acid unit.[12] A closed geometry has been dis-
closed for complexes derived from nitrogen-based
chelate ligands such as phenanthroline and 2,2’-bipyri-
dine[13] as well as for oxothioethers,[14] nevertheless to
the best of our knowledge, this is the first example of
such a peculiar geometry with amino acids as dirhodi-
um ligands. In this way, three novel water soluble dir-
hodium complexes were prepared (7a–c). Recently,
we observed that the complex 7a is strongly active to-
wards human colon adenocarcinoma cells.[11]
Such new water soluble catalysts (7a–c) were tested
in the cyclization of a-diazoacetamide 4a (Scheme 2)
and it was possible to decrease the reaction tempera-
ture to 608C (Table 2, entries 1–4). A 1 mol% loading
of catalyst 7a provided the desired lactam in a lower
yield and identical enantioselectivity (Table 2,
entry 2), whilst a decrease to 0.5 mol% resulted in
low diazo compound consumption (Table 2, entry 3).
Entry 4a/ Rh(II)
Reaction
Yield cis:trans[c] ee
4b catalyst time [h][a] [%][b]
[%][d]
1
2
3
4
5
6
7
4a 7a
4a 7a[e]
4a 7a[f]
4a[h] 7a
4a 7b
4a 7c
4b 7a
6
85
70
1:0.2
1:0.1
66
68
54
60
53
32
46
48
48
24
24
72
12
(20)[g] 1:0
77
80
67
81
1:0.2
1:0.2
1:4.6
1:0.1
All reactions were carried out using 4[5c] (0.15 mmol),
Rh(II) catalyst (2 mol%), water (1.5 mL), except en-
tries 2 and 3.
[a]
[b]
[c]
Isolated yield after purification by flash chromatography.
1
Observed ratio by H NMR of the crude reaction mix-
ture.
[d]
Determined by chiral HPLC analysis, after epimerization
to trans diastereoisomer.
1 mol% of catalyst loading.
0.5 mol% of catalyst loading.
[e]
[f]
[g]
1
Conversion determined by H NMR of the reaction mix-
ture.
[h]
Reaction conducted at a 3.5 mmol scale (1.06 g) of diazo
compound.
Due to the heterogeneous nature of the reaction compound allowed the isolation of lactam 5a after
medium, the reaction scale up to one gram of diazo a simple filtration of the reaction medium. However,
this required longer reaction times and a slight ee
drop to 60% (Table 4, entry 4).
Longer reaction times were needed due to the pres-
ence of the methoxy electron-donating substituent in
the aryl rings of the catalyst. While the phenylala-
nine-derived catalyst 7a allowed a complete reaction
after 6 h (Table 2, entry 1), the introduction of elec-
tron-donating substituents lead to complexes 7b and
7c having decreased reactivity and 24 h were needed
Scheme 2. Rh(II)-catalyzed intramolecular C-H insertion of
a-diazoacetamides 4a and 4b in water.
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Table 3. Reuse of the 7a catalyst after cyclizations of 4a.
Run
Yield [%][b]
cis:trans[c]
ee [%][d]
Run
Yield [%][b]
cis:trans[c]
ee [%][d]
1
2
3
4
81
82
79
79
1:0.3
1:0.2
1:0.2
1:0.3
58
68
70
72
5
85
82
88
1:0.4
1:0.4
1:0.6
74
74
74
6
7[e]
[a]
All reactions were carried out using 4[5c] (0.15 mmol), Rh(II) catalyst (5 mol%) and water (1.5 mL) for 5 h. The reaction
mixture was extracted with Et2O (3ꢅ2 mL) and reloaded with 4a.
[b]
[c]
[d]
[e]
Isolated yield after purification by flash chromatography.
Observed ratio by 1H NMR of the crude reaction mixture.
Determined by chiral HPLC analysis, after epimerization to trans diastereoisomer.
7 h were needed to reach complete diazo consumption.
À
Table 4. C H insertion of a-diazoacetamides 4d and 4e.
Entry
Diazo compound
Conditions[a]
Yield [%][b]
cis:trans[c]
ee [%][d]
1
2
3
4
4d
4d
4e
4e
C2H4Cl2, reflux, 1.5 h
H2O, 80 8C, 1.5 h
C2H4Cl2, reflux, 4 h
H2O, 80 8C, 12 h
58 (80)
45 (59)
86 (95)
57 (82)
1:0.14
1:1
1:6
66
64
62
60
1:10
[a]
All reactions were carried out using 4[17] (0.15 mmol), 7a catalyst (2 mol%), DCE (1.8 mL) or water (1.5 mL).
[b]
Isolated yield after purification by chromatography. The observed conversion determined by 31P NMR is shown in paren-
theses.
[c]
[d]
Observed ratio by 31P NMR of the crude reaction mixture.
Determined by chiral HPLC analysis after epimerization to the trans diastereoisomer in basic alumina.
to reach complete consumption of the diazo com- the product and the catalyst have to be purified by
pound (Table 2, entry 5). This aspect was more obvi- chromatography. Since Rh2A(l-PheAla)2A(OAc)2 (7a) is
C
CHTUNGTRENNUNG
ous when 3,4-dimethoxyphenylalanine derivative 7c soluble in water and insoluble in diethyl ether, this
was tested whereby even after 24 h reaction the diazo metallic complex can be easily reused as demonstrat-
compound was not completely consumed (Table 2, ed for substrate 4a (Table 3).
entry 6).
After removal of product 5a by diethyl ether ex-
Once we observed that a side product was present traction, more 4a was added and allowed to react.
after purification of 7a, we proceeded to the prepara- The catalytic system was reused six times without
tive HPLC isolation of both complexes. From these yield decay and a small ee enhancement was observed
two complexes, only 7a was catalytically active lead- with the system reuse. This enhancement is probably
ing to formation of 5a in 69% ee and no product was attributed to the presence of vestigial diethyl ether
observed after reaction with the unknown side prod- that promotes the substrate solubilization. For a total
uct.
Several chiral dirhodium complexes were reported
to perform the intramolecular C H insertion reac- complex 7a, other a-diazoacetamides were submitted
of 7 cycles a TON of 115 was obtained.
In order to further evaluate the catalytic activity of
À
À
tions of a-diazoacetamides in excellent yields and to our intramolecular C H insertion conditions.
enantioselectivities.[2b,d,6,16] For instance, the dirhodi- Keeping the same a-alkoxycarbonyl substituent,
ACHTUNGTRENNUNGum(II) catalysts derived from N-phthalimide-protect- a more sterically hindered N-[bis(trimethylsilyl)meth-
ed amino acids have been used for the enantioselec- yl]-substituted acetamide was evaluated (Scheme 3).
tivity preparation of lactams in up to 96% ee.[16b,c,d] By Surprisingly, despite the excellent yields and regiose-
the use of catalyst 1b, b-lactam 5b can be obtained in lectivities, whereby only g-lactam 6c was observed,
74% ee in dichloromethane after 6 h at room temper- the product was obtained in very low enantioselectivi-
ature.[5c] Typically, these catalysts have a hydrophobic ty. On changing from dichloromethane to water, an
character and after reaction with a-diazoacetamides opposite enantioselectivity was observed.
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Asymmetric Intramolecular C H Insertion of a-Diazoacetamides in Water by Dirhodium(II) Catalysts
À
Scheme 5. 7a-catalyzed intramolecular C H insertion of a-
diazo acetamide 4f. All reactions were carried out using 4f[6]
(0.08 mmol), 7a catalyst (5 mol%), DCE (0.9 mL) or water
(0.8 mL).
À
Scheme 3. 7a-catalyzed intramolecular C H insertion of a-
diazoacetamide 4c.
À
Scheme 4. 7a-catalyzed intramolecular C H insertion of a-
diazoacetamides 4d and 4e.
À
Scheme 6. 7d-catalyzed intramolecular C H insertion of a-
diazoacetamides 4a.
After the somewhat disappointing results obtained
À
in the intramolecular C H insertion of sterically hin- insertion of a-phosphono-a-diazo acetamides. Cata-
dered acetamide 4c, we decided to explore the effect lysts 1–3 have been previously tested in this transfor-
of the a-substituent. Hence, and due to our interest in mation resulting in the a-phosphono lactam forma-
finding a suitable catalyst for the enantioselective for- tion in only 40% ee.[6]
mation of a-phosphonoacetamides, we tested several
Finally, in order to demonstrate the viability of this
of these compounds in water and dichloroethane.[18] method in the preparation of enantioenriched lac-
Dirhodium complexes derived from mandelic acid tams, catalyst 7d, based on the use of enantiomer d-
were previously reported to provide a-phosphono b- phenylalanine was prepared and tested in the prepa-
and g-lactams in moderate enantioselectivities (up to ration of lactam 5g, which was achieved in 75% yield
40% ee).[6,17] Keeping the same acetamide substituent, and 70% ee (Scheme 6).
À
compound 4d was submitted to aqueous C H inser-
tion (Scheme 4) and b-lactam 5d was obtained in 64% Rh2ACHTUNRTGNEUNG(OAc)4 in water allows the creation of a new dir-
In summary, the simple amino acid exchange on
ee (Table 4, entry 2). Additionally, g-lactam 6e was hodium catalyst framework that can be used in the in-
À
obtained in 60% ee and in better yields under the tramolecular C H insertion of a-diazoacetamides in
same conditions (Table 4, entry 4). When using di- water. b-Lactams were obtained in good yields and in
chloroethane as the reaction solvent, similar enantio- some cases in better enantioselectivity than the ones
selectivities were achieved (Table 4, entries 1 and 3) observed for the reaction in a usual organic solvent.
and the desired lactams were obtained in up to 66% These new complexes can be promising as catalysts
ee.
for other dirhodium(II)-catalyzed asymmetric reac-
Envisioning that the presence of a more rigid tions, using water or organic solvents as the reaction
system in the acetamide substituent would contribute medium. With the use of water as the reaction media,
to better enantioselectivities, a-diazoacetamide 4f was and due to the low solubility of the prepared catalyst
submitted to the same reaction conditions in organic solvents, it was possible to recover the
À
(Scheme 5). However, in order to achieve over 75% C H insertion catalytic system in up to 7 runs without
consumption of the starting diazo compound, 5 mol% enantiomeric excess or yield erosion. The newly de-
of the catalyst had to be used. b-Lactam 5f was ob- veloped catalyst 7a derived from phenylalanine al-
tained in up to 64% ee in reasonable yields, which lowed the preparation of a-(dialkoxyphosphoryl)-
makes this catalyst the best one reported so far as lactams in moderate enantioselectivities.
À
what concerns the stereoselective intramolecular C H
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Experimental Section
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À
General Procedure for the Intramolecular C H
Insertion
A suspension of diazo compound (0.15 mmol) and dirhodi-
ACHTUNGTRENNUNGum(II) complex (2 mol%) in water (1.5 mL) was left stirring
at room temperature or under heating (see Tables in the
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A suspension of 4a (46 mg, 0.15 mmol) and the complex Rh2
ACHTUNGTRENNUNG(S-PheAla)2ACHTUNGTRENNUNG(OAc)2 (5.0 mg, 5 mol%) was heated at 608C
during 5 h in water (1.5 mL). The product was extracted
with Et2O (3ꢅ2 mL) and more N-(benzyl)-N-(tert-butyl)-2-
(ethoxycarbonyl)-2-diazoacetamide 4a (46 mg, 0.15 mmol)
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Acknowledgements
This work was supported by the Fundażo para a CiÞncia
e Tecnologia and FEDER (Ref. PTDC/QUI/66695/2006,
PTDC/QUI-QUI/099389/2008,
PTDC/QUI-QUI/101187/
2008 SFRH/BPD/46589/2008, SFRH/BD/61220/2009a and
strategic project: PEst-OE/SAU/UI4013/2011), and the Portu-
guese NMR Network (IST-UTL Center) is acknowledged for
providing access to the NMR facility.
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