Enantioselective intramolecular carbene C-H insertion catalyzed by a chiral iridium(III) complex of D4-symmetric porphyrin ligand

Article abstract of DOI:10.1021/cs4001656

The synthesis of iridium(III) complexes containing bulky porphyrin ligands is described. The chiral iridium(III) complex of the D₄-symmetric Halterman porphyrin ligand [Ir((+)-D₄-Por)Me(L)] (L = solvent) is an effective catalyst for enantioselective intramolecular carbene insertion into saturated C-H bonds of α-diazoesters, giving the corresponding cis-β-lactones in good isolated yields (up to 87%), excellent stereoselectivities (cis products exclusively), and good enantioselectivities (up to 78% ee).

Full text of DOI:10.1021/cs4001656

Letter
pubs.acs.org/acscatalysis
Enantioselective Intramolecular Carbene C−H Insertion Catalyzed by
a Chiral Iridium(III) Complex of D4‑Symmetric Porphyrin Ligand
Jing-Cui Wang,† Yan Zhang,† Zhen-Jiang Xu,† Vanessa Kar-Yan Lo,‡,§ and Chi-Ming Che*,†,‡,§
†Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, 354 Feng Ling Road,
Shanghai, China
‡State Key Laboratory of Synthetic Chemistry and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong
Kong, China
§HKU Shenzhen Institute of Research and Innovation, Shenzhen 518053, China
S
* Supporting Information
ABSTRACT: The synthesis of iridium(III) complexes con-
taining bulky porphyrin ligands is described. The chiral
iridium(III) complex of the D4-symmetric Halterman porphyr-
in ligand [Ir((+)-D4-Por)Me(L)] (L = solvent) is an effective
catalyst for enantioselective intramolecular carbene insertion
into saturated C−H bonds of α-diazoesters, giving the
corresponding cis-β-lactones in good isolated yields (up to
87%), excellent stereoselectivities (cis products exclusively),
and good enantioselectivities (up to 78% ee).
KEYWORDS: iridium porphyrin, C−H insertion, enantioselectivity, intramolecular, cis-β-lactone
Since the report on intermolecular carbene insertion to C−H
bonds of n-alkanes by Callot7 in 1982 and the characterization
of ruthenium porphyrin carbene intermediate by Collman8 in
1985, a number of metalloporphyrin catalysts, such as that of
iron,9 ruthenium,10 rhodium,11 and osmium,12 have been
developed and were found to be catalytically active toward
carbene C−H bond insertion. Recently, intermolecular cyclo-
irect C−H functionalization continues to be an area of
1
Dinterest in catalysis and organic synthesis. Because of the
huge abundance of C−H bonds in organic compounds, this
strategy is appealing in streamlining the routes for the synthesis
of organic molecules with complexity and diversity. However,
the large bond energy of and selectivity among the saturated
C−H bonds in organic compounds present major challenges in
achieving selective C−H functionalization with practical
interest. To achieve direct C−H functionalization, various
transition metal catalysts, such as metalloporphyrin,1i,j,2 metal-
Schiff base,3 metal-PyBOX,1h,4 and dirhodium(II,II) complex-
es,1b−d,g,h,k,l,5 have been developed over the past decades. We
are interested in developing metalloporphyrin catalysts for
oxidative C−H functionalization and for organic oxidation
reactions for the following reasons: (1) Metalloporphyrins have
been widely studied as mimics of biological oxidation reactions
of cytochrome P450 enzymes.2d,6 (2) The shape of metal-
loporphyrin catalysts can be fine-tuned by tailoring the
functional group(s) or bulkiness of the substituent(s) on the
periphery of the porphyrin ligand, thereby allowing control of
the product selectivity of metalloporphyrin catalyzed organic
transformations. (3) A high product turnover number can be
achieved with metalloporphyrin catalysts, particularly the ones
containing halogeno substituents on the porphyrin ligand.1i,j,2a,d
(4) The isolation of reactive metal−ligand multiple bonded
species of metalloporphyrins is instrumental to the study of the
mechanisms of metalloporphrin-catalyzed atom/group transfer
reactions.1i,j,2a,d (5) Chiral substituents can be incorporated
onto the porphyrin ligand for catalytic enantioselective organic
transformation reactions.
propanation and carbene C−H insertion reactions have been
15
reported by Katsuki,13 Woo,14 and Rodriguez-Garcia using
iridium(III) salen, iridium(III) porphyrin, and iridium(I)
porphyrin catalysts, respectively. Previously, we reported highly
enantioselective intermolecular carbene C−H insertion reac-
tions catalyzed by [Ir(D4-Por)Me]16 supported on a D4-
Halterman porphyrin ligand.17 Following this work, we would
like to expand the scope of reactions of [Ir(D4-Por)Me] catalyst
to intramolecular carbene C−H insertion.
́
́
Transition-metal-catalyzed decomposition of diazoesters for
intramolecular carbene C−H insertion has been studied by us18
and by others.1b,g−j Herein, we report the synthesis of
iridium(III) complexes supported on bulky porphyrin ligands.
The methylated iridium(III) porphyrin complex [Ir(TTP)Me-
(L)] (H2TTP = meso-tetrakis(p-tolyl)porphyrin; L = H2O or
solvent; Figure 1) was found to be catalytically active toward
stereoselective intramolecular carbene C−H insertion of α-
diazoesters, giving lactones exclusively in yields up to 87%.
With chiral [Ir((+)-D4-Por)Me] catalyst (Figure 1), cis-β-
Received: March 2, 2013
Revised: April 22, 2013
© XXXX American Chemical Society
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Scheme 2. Synthesis of Methylated Iridium(III) Porphyrin
Catalysts
Figure 1. Structures of iridium(III) porphyrin catalysts used in this
work.
insertion was examined (Table 1). By referring to the reaction
conditions employed in the analogous intermolecular carbene
lactones have been obtained with good stereoselectivity (cis
product exclusively) and enantioselectivities (up to 78% ee).
A series of bulky porphyrin ligands with structures depicted
in Scheme 1, including the bis-pocket porphyrin H2TTPPP,19
Table 1. Intramolecular C−H Insertion of α-Diazoester
a
Catalyzed by [Ir(TTP)Me]
Scheme 1. Synthesis of Iridium(Iii) Complexes Supported
on Bulky Porphyrin Ligands
were synthesized according to literature procedures19,20 and the
D4-symmetric Halterman ligand was synthesized according to
the procedure reported by Halterman.17 We found that
iridium(III) ion could be efficiently incorporated to these
bulky porphyrin ligands by reacting [Ir(COD)Cl]2 (COD =
1,5-cyclooctadiene) with the corresponding free porphyrin
ligand in 1,2,4-trichlorobenzene at 190 °C. The iridium(III)
porphyrin complexes [Ir(Por)(CO)Cl] were obtained in 65−
85% yields (Scheme 1). Oxygen from air is necessary to oxidize
iridium(I) to iridium(III),21 and a high reaction temperature of
190 °C and the use of high-boiling solvent 1,2,4-trichlor-
obenzene (bp = 214 °C) are essential. When xylene (bp =
137−140 °C) was used as the solvent, no iridium(III)
porphyrin complex was observed.
a
Reaction conditions: A mixture of α-diazoester 1 (0.2 mmol) and
[Ir(TTP)Me] (1 mol %) in 2 mL of DCM was stirred at 25 °C for 1 h
under Ar atmosphere.
The methylation of iridium(III) porphyrin complexes was
performed by reductive alkylation of [Ir(Por)(CO)Cl] with
NaBH4/MeI (Scheme 2).16 [Ir(TTP)Me(L)] and [Ir((+)-D4-
Por)Me(L)] (L = H2O or solvent) were obtained in 84% and
80% yields, respectively. The solvent molecule L could be H2O
from the reaction mixture. In the case of [Ir((−)-D4-
Por)Me(EtOH)], the coordinated EtOH, that was incorpo-
rated into the Ir(III) center during recrystallization in THF/
EtOH, has been revealed by X-ray crystallographic analysis.16
The catalytic activity of the achiral iridium porphyrin
complex [Ir(TTP)Me] toward intramolecular carbene C−H
C−H insertion,16 α-diazoester 1a (0.2 mmol) was treated with
1 mol % of [Ir(TTP)Me] in 2 mL of DCM at 25 °C for 1 h
under Ar atmosphere. On the basis of 1H NMR analysis of the
crude reaction mixture, a complete substrate conversion was
observed, and β-lactone 2a (carbene insertion to 3° C−H
bond) was isolated in 82% yield (Table 1, entry 1). This
reaction worked well for a variety of p-substitued α-diazoesters
(p-Br, 1b; p-F, 1c; and p-Me, 1d), giving the corresponding β-
lactones 2b−d in moderate to good isolated yields (53−86%;
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entries 2−4, Table 1). α-Diazoester 1e with a m-Cl substituent
also underwent cyclization to form β-lactone 2e in 76% isolated
yield (entry 5, Table 1). From these results, there is a high
tendency for carbene insertion into the 3° C−H bond geminal
to the ester oxygen atom22 when [Ir(TTP)Me] is used as the
catalyst, and in this work, β-lactones with 4-membered ring
structure were formed as the only products.
trimethylsilane as the internal standard, and 2i was isolated in
87% yield with 76% ee (entry 1, Table 2). Lowering the
reaction temperature to 0 °C or −40 °C significantly reduced
the yield of 2i to <5% (entries 2−3, Table 2). Increasing the
reaction temperature to 40 °C did not impose any significant
effect on the yield and enantioselectivity of 2i (entry 4, Table
2).
The effect of solvent was examined. The intramolecular
carbene C−H insertion of α-diazoester 1i catalyzed by [Ir((+)-
D4-Por)Me] was favored in a chlorinated solvent, such as DCM
or DCE, giving cis-β-lactone 2i in 85−90% NMR yields with
72−76% ee (entries 1 and 5, Table 2). The yield of 2i was
significantly reduced when a nonpolar solvent was used (19−
27% NMR yields; entries 6−7 and 9, Table 2). When toluene
was used as the solvent, 2i was obtained in 61% NMR yield
with 73% ee (entry 8, Table 2).
We also compared the catalytic activities of chiral dirhodium-
(II,II) carboxylate complexes, which are well-known catalysts
for carbene transfer reactions,1b−d,g,h,l,5 toward this intra-
molecular carbene C−H insertion as depicted in Table 2.
When [Rh2(S-PTAD)4] (tetrakis[(S)-(1-adamantyl)-(N-
phthalimido)acetate]dirhodium(II)) or [Rh2(S-MEPY)4]
(tetrakis[methyl 2-pyrrolidone-5(S)-carboxylate]dirhodium-
(II)) was used as the catalyst for intramolecular carbene C−
H insertion of α-diazoester 1i, both the yields (<5−25% NMR
yields) and enantioselectivity (30% ee) of cis-β-lactone 2i were
unsatisfactory, even when 1i was slowly added to the reaction
mixture via syringe pump over 80 min (entries 10−11, Table
2).
Decomposition of α-diazoester 1f bearing a tetrahydro-2H-
pyran moiety led to carbene insertion into the 3° C−H bond
geminal to the ester oxygen atom, giving β-lactone 2f
containing a spiro structure in 53% isolated yield (entry 6,
Table 1); however, changing the tetrahydro-2H-pyran group to
a tetrahydrofuran moiety favored carbene insertion to 2 °C−H
geminal to the tetrahydrofuran oxygen atom rather than that of
the ester oxygen atom, giving cis-γ-lactone 2g, bearing a fused
tetrahydrofuro[3,2-b]furan-2(5H)-one system in 81% isolated
yield (entry 7, Table 1). The stereochemistry of 2g was
confirmed by X-ray crystallography (see Figure S1, Supporting
Information). The same directed regioselectivity by the geminal
tetrahydrofuran oxygen atom was also observed for carbene C−
H insertion of 1h bearing a (tetrahydrofuran-2-yl)methyl
group, giving γ-lactone 2h containing a spiro structure in
74% isolated yield (entry 8, Table 1). Cyclization of the α-
diazoester 1i gave cis-β-lactone 2i in 68% isolated yield, without
detection of a trans isomer (entry 9, Table 1).
With the success of intramolecular carbene C−H insertion of
α-diazoesters catalyzed by [Ir(TTP)Me], we further expanded
our study to asymmetric catalytic reactions. The intramolecular
carbene C−H insertion of the α-diazoester 1i was selected as a
model reaction for the optimization of reaction conditions
using [Ir((+)-D4-Por)Me(L)] (L = H2O or solvent) as catalyst
(Table 2). By treating 0.2 mmol of α-diazoester 1i with 1 mol
% of [Ir((+)-D4-Por)Me] in 2 mL of DCM at 25 °C for 1 h, a
Because 1 mol % of [Ir((+)-D4-Por)Me] gave the best result
in terms of product yield and enantioselectivity in DCM at 25
°C, these reaction conditions were adopted for subsequent
studies.
With the optimized reaction conditions, we examined the
substrate scope of the [Ir((+)-D4-Por)Me]-catalyzed enanatio-
selective intramolecular C−H insertion of α-diazoesters (Table
3). α-Diazoester 1j bearing p-Br substitution on Ar2 gave cis-β-
lactone 2j in 81% yield and with 78% ee for a reaction time of
20 min (entry 2, Table 3). Substitution on the o position
significantly lengthened the reaction time to 10 h, but the
product yield was reduced (54%; entry 3, Table 3). This could
be explained by the steric hindrance exerted by the o
substitution to the benzylic C−H where the carbene insertion
takes place. α-Diazoester 1l with electron-withdrawing m-Cl
substitution on Ar2 required a further extension of the reaction
time to 24 h to give cis-β-lactone 2l in 58% yield and with 77%
ee (entry 4, Table 3). From these results, the substitution on
the Ar2 group of α-diazoester did not have a significant effect on
the enantioselectivity of the intramolecular carbene C−H
insertion reaction.
We then proceeded to examine the effect of substitution onto
the Ar1 of α-diazoesters (Table 3). The intramolecular carbene
C−H insertion of α-diazoester 1m bearing an electron-
donating p-Me group onto the Ar1 required a reaction time
of 24 h to give cis-β-lactone 2m in 63% yield and 78% ee (entry
5, Table 3). Changing the substituent(s) of Ar1 to electron-
withdrawing halogen group(s) (p-F, 1n; m-Cl; 1o; m-Br, 1p)
significantly shortened the reaction time to 10 min with the
desired cis-β-lactones obtained in 80−88% yields (entries 6−8,
Table 3); however, the enantioselectivities were all reduced
(39−76% ee) (entries 6−8, Table 3). From these results, the
substitution on the Ar1 of α-diazoester significantly affects both
the rate and enantioselectivity of the intramolecular carbene
1
90% yield of cis-β-lactone 2i was found, on the basis of H
NMR analysis of the crude reaction mixture using phenyl-
Table 2. Condition Optimization and Catalyst Screening for
a
Enantioselective C−H Insertion of α-Diazoester 1i
temp
b
c
entry
catalyst
°C
solvent
DCM
yield,
%
d
ee, %
1
2
3
4
5
6
7
8
9
[Ir((+)-D4-por)Me]
[Ir((+)-D4-por)Me]
[Ir((+)-D4-por)Me]
[Ir((+)-D4-por)Me]
[Ir((+)-D4-por)Me]
[Ir((+)-D4-por)Me]
[Ir((+)-D4-por)Me]
[Ir((+)-D4-por)Me]
[Ir((+)-D4-por)Me]
[Rh2(S-PTAD)4]
25
90 (87)
5
76
0
−40
40
DCM
DCM
<5
85
DCM
72
72
69
61
73
80
30
25
DCE
85
25
cyclohexane
n-hexane
toluene
benzene
DCM
22
25
27
25
61
25
19
e
10
25
25
e
11
[Rh2(S-MEPY)4]
25
DCM
<5
a
Reaction conditions: A mixture of 1i (0.2 mmol) and catalyst (1 mol
%) in 2 mL of solvent was stirred for 1 h under Ar atmosphere.
b
Determined by 1H NMR using PhTMS as internal standard.
c
d
Determined by HPLC with Chiralpak AD-H column. Datum in
e
parentheses was isolated yield. 1i was added via syringe pump over 80
min.
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Table 3. Substrate Scope for Enantioselective Intramolecular
AUTHOR INFORMATION
■
C−H Bond Insertion of α-Diazoesters Catalyzed by [Ir((+)-
Corresponding Author
*E-mail: cmche@hku.hk.
a
D4-Por)Me]
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by grants from the Innovation and
Technology Commission (HKSAR, China) to the State Key
Laboratory of Synthetic Chemistry, The External Cooperation
Program of the Chinese Academy of Sciences (CAS-
GJHZ200816), NSFC (21272197 and 21102162), and CAS-
Croucher Funding Scheme for Joint Laboratories.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details, characterization of compounds, and
Figure S1. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
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