Highly asymmetric intramolecular cyclopropanation of acceptor-substituted diazoacetates by Co(II)-based metalloradical catalysis: Iterative approach for development of new-generation catalysts

Article abstract of DOI:10.1021/ja2062506

3,5-Di^tBu-QingPhyrin, a new D₂-symmetric chiral porphyrin derived from a chiral cyclopropanecarboxamide containing two contiguous stereocenters, has been developed using an iterative approach based on Co(II)-catalyzed asymmetric cyclopropanation of alkenes. The Co(II) complex of 3,5-Di^tBu-QingPhyrin, [Co(P2)], has proved to be a general and effective catalyst for asymmetric intramolecular cyclopropanation of various allylic diazoacetates (especially including those with α-acceptor substituents) in high yields with excellent stereoselectivities. The [Co(P2)]-based intramolecular metalloradical cyclopropanation provides convenient access to densely functionalized 3-oxabicyclo[3.1.0]hexan-2-one derivatives bearing three contiguous quaternary and tertiary chiral centers with high enantiomeric purity.

Full text of DOI:10.1021/ja2062506

COMMUNICATION
pubs.acs.org/JACS
Highly Asymmetric Intramolecular Cyclopropanation of
Acceptor-Substituted Diazoacetates by Co(II)-Based Metalloradical
Catalysis: Iterative Approach for Development of New-Generation
Catalysts
Xue Xu, Hongjian Lu, Joshua V. Ruppel, Xin Cui, Silke Lopez de Mesa, Lukasz Wojtas, and X. Peter Zhang*
Department of Chemistry, University of South Florida, Tampa, Florida 33620-5250, United States
S Supporting Information
b
enantioselectivity remains a major challenge and is currently
ABSTRACT: 3,5-DitBu-QingPhyrin, a new D2-symmetric
chiral porphyrin derived from a chiral cyclopropanecarbox-
amide containing two contiguous stereocenters, has been
developed using an iterative approach based on Co(II)-
catalyzed asymmetric cyclopropanation of alkenes. The Co(II)
complex of 3,5-DitBu-QingPhyrin, [Co(P2)], has proved to
be a general and effective catalyst for asymmetric intramolecular
cyclopropanation of various allylic diazoacetates (especially
including those with α-acceptor substituents) in high yields
with excellent stereoselectivities. The [Co(P2)]-based intra-
molecular metalloradical cyclopropanation provides convenient
access to densely functionalized 3-oxabicyclo[3.1.0]hexan-
2-one derivatives bearing three contiguous quaternary and
tertiary chiral centers with high enantiomeric purity.
limited to the reactions of α-diazo-β-ketone sulfones.9
Allyl diazoacetates with α-acceptor substituents such as cyano,
nitro, ketone, and ester groups [X = CN, NO2, C(O)R, and
CO2R, respectively, in eq 1] are a class of common unsaturated
acceptor/acceptor-substituted diazo reagents that have not been
successfully demonstrated to be effective substrates for asym-
metric intramolecular cyclopropanation. Since the resultant
3-oxabicyclo[3.1.0]hexan-2-one derivatives possess three contig-
uous chiral centers, including one multifunctionalized quaternary
stereogenic carbon (eq 1), this family of enantioselective trans-
formations would be highly attractive for synthetic applications.
There have been only a few previous reports on catalytic systems
for asymmetric cyclopropanation of α-acceptor-substituted allyl
diazoacetates. While there has been no report on allyl α-nitro-
diazoacetates (X = NO2),10 the only report on asymmetric intra-
molecular cyclopropanation of allyl α-cyanodiazoacetates (X = CN)
attained a highest enantioselectivity of 91% ee with a 57% yield
using a Rh2-based catalyst.11 The enantiocontrol of the catalytic
system was shown to be greatly influenced by the substituents
around the allyl group, ranging from 29 to 91% ee.11 Whereas the
catalytic process for allyl α-ketodiazoacetates [X = C(O)R] has
not been reported,12 prior efforts were made for asymmetric intra-
molecular cyclopropanation of allyl α-esterdiazoacetates (X = CO2R)
using both Rh2- and Cu-based catalysts, which reached a highest
enantioselectivity of 83% ee but with a yield of only 24%.13 Once
again, the enantioselectivity was found to be highly substrate-
dependent, ranging from 11 to 83% ee.13 It is evident that
asymmetric intramolecular cyclopropanation of α-acceptor-
substituted diazoacetates is largely an unsolved problem and faces
formidable challenges with respect to both reactivity and selectivity.
etal-catalyzed asymmetric olefin cyclopropanation with
M
diazo reagents stands as one of the most general approaches
for the synthesis of optically active cyclopropane derivatives.1 Its
intramolecular variant offers a powerful strategy for stereoselec-
tive construction of complex [n.1.0]bicyclic ring systems directly
from linear unsaturated diazo precursors.1 The resulting bicyclic
molecules have been demonstrated to be versatile intermediates
for syntheses of a wide range of important compounds such as
pharmaceuticals, peptidomimetics, and natural products.2 Since
the feasibility of intramolecular cyclopropanation was first de-
monstrated in 1961,3 significant efforts have been devoted to
the development of chiral metal catalysts for controlling the
enantioselectivity of the double cyclization reaction, in parallel
with the development of intermolecular cyclopropanation.1,2 Results
seem to indicate that a chiral catalyst that is capable of highly
asymmetric intermolecular cyclopropanation may not necessarily
be effective for enantioselective intramolecular cyclopropanation
because of additional constraints in the transition state of the
unimolecular process. In fact, asymmetric intramolecular cyclo-
propanation is a significantly less developed process in com-
parison with the enormous advancements in asymmetric intermole-
cular cyclopropanation.1,2 To date, most achievements in asymmetric
intramolecular cyclopropanation have been made for reactions of
unsaturated acceptor-substituted diazo reagents, such as diazoa-
cetates and diazoacetamides.4À7 While there have been several
reports on asymmetric reactions of donor/acceptor-substituted
diazo reagents with partial success,8 intramolecular cyclopropa-
nation of acceptor/acceptor-substituted diazo reagents with high
Since they were first introduced in 2004,14 Co(II) complexes of
D2-symmetric chiral amidoporphyrins [Co(D2-Por*)] have emerged
as a new class of catalysts for asymmetric cyclopropanation.15
These metalloradical catalysts have been shown to be highly effective
for asymmetric intermolecular cyclopropanation of a broad scope
of substrates with different classes of carbene sources, particularly
including electron-deficient olefins and acceptor/acceptor-substituted
Received: July 6, 2011
Published: August 26, 2011
r
2011 American Chemical Society
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Table 1. Ligand Effect on Asymmetric Intramolecular
Cyclopropanation of Cinnamyl α-Cyanodiazoacetate (1a)
Scheme 1. Iterative Approach for the Development of
[Co(D2-Por*)] and Application for Synthesis of [Co(P2)]
diazo reagents, with excellent diastereoselectivity and enantio-
selectivity.16,17 Increasing amounts of evidence support an
unprecedented reaction mechanism for the Co(II)-catalyzed cyclo-
propanation that involves an unusual Co(III)Àcarbene radical
intermediate undertaking a stepwise radical additionÀsubstitution
pathway.18 This radical mechanism, which is fundamentally different
from the electrophilic metallocarbene mechanism shared by the
commonly used Rh2 and other closed-shell systems, is consistent
with the distinct cyclopropanation reactivity profile.16,17 To date, the
Co(II)-based metalloradical cyclopropanation has been demon-
strated only for intermolecular reactions. In view of the more
demanding steric requirements associated with the double cyclization
process, it was unclear whether the confined chiral cavity in
[Co(D2-Por*)] could accommodate intramolecular cyclopropana-
tion reactions. Since the metalloradical catalysis operates in a
stepwise radical mechanism, would its diastereoselectivity be com-
parable to that of the existing concerted catalytic systems? To
answer these and related questions and, more importantly, to
address the aforesaid challenges in the area, we embarked on a
project to study asymmetric intramolecular cyclopropanation. As
the outcome of this effort, we report herein a highly effective
catalytic system based on a new generation of chiral Co(II)
metalloradical catalysts for asymmetric intramolecular cyclopropa-
nation of α-acceptor-substituted diazoacetates (eq 1). In addition to
substrates bearing two various acceptor groups, the Co(II)-cata-
lyzed intramolecular cyclopropanation is also suitable for acceptor-
and donor/acceptor-substituted diazo compounds, producing the
corresponding bicyclic compounds in high yields with excellent
diastereo- and enantioselectivity. Furthermore, the metalloradical
catalytic process features a practical one-portion protocol that
operates at room temperature without slow addition of substrates.
The unsaturated acceptor/acceptor-substituted diazo compound
cinnamyl α-cyanodiazoacetate (1a) was used as the initial substrate
for intramolecular asymmetric cyclopropanation by [Co(D2-Por*)]
(Table 1). Since the metalloradical catalyst [Co(P1)] was shown pre-
viously to be highly enantioselective for intermolecular cyclo-
propanation with α-cyanodiazoacetates,16f it was first evaluated
as a potential catalyst for the reaction of 1a. Under practical con-
ditions (room temperature without slow addition), it was pleasing to
observe that 1a could be quantitatively transformed to the desired
bicyclic product 2a as a single diastereomer under the catalysis of
2 mol % [Co(P1)] (Table 1). However, the enantioselectivity of
the [Co(P1)]-catalyzed reaction reached only 55% ee, indicating a
significant difference between the asymmetric intra- and inter-
molecular cyclopropanation processes. No major improvements
in enantioselectivity were achieved even though extensive attempts
employing other existing [Co(D2-Por*)] catalysts were made.14,17
Results from these early efforts prompted us to develop new-
generation catalysts by exploiting an iterative approach (Scheme 1)
based on the combination between the modular design of the
D2-symmetric chiral porphyrins14 and the high stereoselectivities
of [Co(D2-Por*)]-catalyzed intermolecular cyclopropanation.16
Starting from optically pure chiral amide 3, which is available
inexpensively from commercial sources, this iterative approach
allows for the effective generation of diverse cyclopropanecar-
boxyamides with high enantiomeric purity that can be further
utilized as chiral building blocks for construction of new-generation
[H2(D2-Por*)] ligands. As one application of this approach, the
metalloradical catalyst [Co(P2)], which represents the first
example of a [Co(D2-Por*)] complex bearing chiral amides with
two contiguous stereocenters, was designed and synthesized from
α-methylstyrene [Scheme 1; also see the Supporting Information
(SI)]. Gratifyingly, [Co(P2)] was found to be a superior chiral
catalyst in comparison with [Co(P1)] for the asymmetric
intramolecular cyclopropanation of 1a, resulting in the quanti-
tative formation of the desired product 2a as a single diaster-
eomer with 96% ee (Table 1). It is fascinating to note that the
chirality in [Co(P2)] originated from [Co(P1)].
The new metalloradical catalyst [Co(P2)] was shown to be
generally effective for asymmetric intramolecular cyclopropana-
tion of allyl diazoacetates with different α-substituted groups
(Table 2). In addition to 1a (entry 1), cinnamyl α-nitrodiazoa-
cetate (1b) could also be effectively catalyzed by [Co(P2)] under
the same practical conditions to give the corresponding cyclo-
propanation product 2b in high yield and enantioselectivity (entry 2).
This represents the first successful example of asymmetric intra-
molecular cyclopropanation of allyl α-nitrodiazoacetates.10 The
absolute configurations of the three contiguous stereogenic
centers in 2b were established as (1S,5S,6R) by X-ray crystal
structure analysis (Figure S1 in the SI). Under the catalysis of
[Co(P2)], unsaturated α-ketodiazoacetates could for the first
time be asymmetrically cyclopropanated, as exemplified by the
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Table 2. Asymmetric Intramolecular Cyclopropanation of
Table 3. Catalytic Asymmetric Intramolecular Cyclopropana-
tion of Various Allyllic α-Cyanodiazoacetates
Allyl Diazoacetates with Different α-Substituted Groupsa
a [1] = 0.20 M; Isolated yields; Enantiomeric excess of major trans
diastereomer determined by chiral HPLC. b [1S,5S,6R] Absolute con-
figuration determined by X-ray crystal structural analysis. c In the
presence of 0.5 eq DMAP. d At 60 °C. e Determined by chiral GC.
successful reaction of cinnamyl α-ketodiazoacetate (1c).12 The
desired cyclopropanation product 2c was formed with almost
perfect enantioselectivity, albeit in a lower yield (entry 3). The
stereocontrol capability of catalyst [Co(P2)] was further demon-
strated with the highly asymmetric intramolecular cyclopropana-
tion of cinnamyl α-esterdiazoacetate (1d). In addition to high
enantioselectivity, the corresponding cyclopropanation product 2d
could be obtained in almost quantitative yield (entry 4). Besides
acceptor/acceptor-substituted diazo substrates, the [Co(P2)]-
based system could be also applied to acceptor- and donor/
acceptor-substituted diazo compounds, as illustrated by the asym-
metric intramolecular cyclopropanations of cinnamyl diazoacetate
(1e) and α-methyldiazoacetate (1f), respectively (entries 5 and 6).
In addition to cinnamyl diazoesters, the [Co(P2)]-based metal-
loradical catalysis could effectively enable asymmetric intramolecular
cyclopropanation of other unsaturated diazoesters, as demon-
strated using a series of allylic α-cyanodiazoacetates (Table 3). For
example, cinnamyl derivatives having alkyl groups substituted at
different phenyl positions, such as 1gÀi, could be intramolecularly
cyclopropanated with similarly high yields and stereoselectivities
as for 1a (entries 1À4). Besides alkyl substituents, the catalytic
system worked equally well with derivatives containing electron-
withdrawing groups such as Br (1j; entry 5) and CF3 (1k; entry 6).
The Co(II)-catalyzed cyclopropanation could also be success-
fully applied to heteroaryl allylic α-cyanodiazoacetates such as 1l
and 1m (entries 7 and 8). In addition, conjugated allylic α-cyano-
diazoacetates, as illustrated with substrates 1n and 1o, could be
productively catalyzed to form 3-oxabicyclo[3.1.0]hexan-2-one
derivates2n and2o, respectively, although with decreasedenantio-
selectivities (entries 9 and 10). The absolute configurations of
the three contiguous stereogenic centers in the major enantiomer
of 2n were established as (1R,5S,6S) by X-ray crystal structure
analysis (Figure S2 in SI). It is worthy of note that two of the
three contiguous chiral centers in 2o are all-carbon quaternary
stereocenters. Catalyst [Co(P2)] was found to be similarly active
for alkyl-substituted allylic α-cyanodiazoacetates, as demon-
strated by the reactions of 1p and its cis-isomer 1q, which formed
the corresponding bicyclic products 2p and 2q in excellent yields
but with moderate enantioselectivities (entries 11 and 12).
aSee footnotes of Table 2. b At 0 °C. c [1R,5S,6S] Absolute configuration
determined by X-ray crystal structural analysis. d GC yield. e Determined
by chiral GC. f Determined via derivatization. g cis isomer.
It is noteworthy that all of the above catalytic reactions except for
two generated the 3-oxabicyclo[3.1.0]hexan-2-one products as a
single diastereomers (Tables 1À3). While diastereoselectivity is not
an issue for intramolecular cyclopropanation by concerted catalytic
systems, this fact suggests that the last ring-closure step (intra-
molecular radical substitution) in the stepwise radical additionÀ
substitution pathway of Co(II)-catalyzed intramolecular cyclopro-
panation is a low-barrier or barrierless process, the same as shown in
Co(II)-catalyzed intermolecular cyclopropanation.18 The two excep-
tions are the catalytic reactions of conjugated 1n and cis alkene
1q, which provided the products 2n and 2q as mixtures of two
diastereomers (Table 3, entries 9 and 12). These results are
consistent with the stepwise radical mechanism.18
The demonstrated asymmetric intramolecular cyclopropana-
tion by the [Co(P2)]-based catalytic system paves a practicable
way to access optically pure 3-oxabicyclo[3.1.0]hexan-2-one deri-
vatives bearing multiple stereocenters with dense functionalities,
which should find a myriad of applications in stereoselective
synthesis.2 As an initial exploration of their synthetic applications,
we demonstrated that the γ-butyrolactone unit in enantioen-
riched bicyclo[3.1.0]hexanenitrile 2a could be selectively opened
with nitrogen-based nucleophiles such as aniline in the presence
of lithium diisopropylamide (LDA) to produce multifunctionalized
cyclopropane derivative 8 as a single diastereomer in high yields
without loss ofopticalpurity(eq2). Itis notedthatenantioenriched
cyclopropane derivatives such as 8 would be difficult or impossible
to prepare directly via asymmetric intermolecular cyclopropanation.
As another demonstration of a stereoselective transformation, the
cyclopropane unit in enantioenriched bicyclo[3.1.0]hexane ester 2d
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could be enlarged through [3 + 2] dipolar cycloaddition with di-
polarophiles such as benzaldehyde to form hexahydrofuro[3,4-c]-
furan derivative 9 as a single diastereomer in good yield without a
change in the optical purity under Lewis acid-catalyzed condi-
tions (eq 3).19 The absolute configurations of the four contig-
uous stereogenic centers in 9 were established as (1S,3S,3aS,6aS)
by X-ray crystal structure analysis (Figure S3 in SI).
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In summary, we have successfully applied an iterative approach
for the development of the new-generation chiral Co(II) metal-
loradical catalyst [Co(P2)], which proved to be generally effective
for highly asymmetric intramolecular cyclopropanation. The
[Co(P2)]-based catalytic system permits for the first time the
efficient transformation of α-acceptor-substituted diazoacetates
into enantioenriched 3-oxabicyclo[3.1.0]hexan-2-one derivatives
bearing three contiguous stereocenters with multiple functional-
ities that may serve as valuable intermediates for stereoselective
synthesis. The demonstration of Co(II)-based metalloradical
catalysis for asymmetric intramolecular cyclopropanation may
open the door for the discovery of other enantioselective radical
cyclization processes, including polycyclization reactions.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, analytical
b
data for all new compounds, and crystallographic data (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
xpzhang@usf.edu
(15) Doyle, M. P. Angew. Chem., Int. Ed. 2009, 48, 850.
’ ACKNOWLEDGMENT
(16) (a) Chen, Y.; Zhang, X. P. J. Org. Chem. 2007, 72, 5931.
(b) Chen, Y.; Ruppel, J. V.; Zhang, X. P. J. Am. Chem. Soc. 2007,
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We are grateful for financial support by the National Science
Foundation (CHE-0711024).
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