Cobalt-catalyzed asymmetric cyclopropanation with diazosulfones: Rigidification and polarization of ligand chiral environment via hydrogen bonding and cyclization

Article abstract of DOI:10.1021/ja7106838

A new D₂-symmetric chiral porphyrin P6 (2,6-DiMeO-ZhuPhyrin) with enhanced chiral rigidity and polarity was designed and synthesized through incorporation of hydrogen bonding and cyclic structure. Its cobalt(II) complex [Co(P6)] is a highly active and selective catalyst for asymmetric cyclopropanation of alkenes with diazosulfones. The [Co(P6)]-based catalytic system is suitable for various aromatic olefins as well as electron-deficient olefins, including α,β-unsaturated esters, ketones, and nitriles, forming the corresponding cyclopropyl sulfones under mild conditions in high yields and high selectivities. In most cases, both excellent diastereo- and enantioselectivities were achieved. Copyright

Full text of DOI:10.1021/ja7106838

Published on Web 03/22/2008
Cobalt-Catalyzed Asymmetric Cyclopropanation with
Diazosulfones: Rigidification and Polarization of Ligand Chiral
Environment via Hydrogen Bonding and Cyclization
Shifa Zhu, Joshua V. Ruppel, Hongjian Lu, Lukasz Wojtas, and X. Peter Zhang*
Department of Chemistry, UniVersity of South Florida, Tampa, Florida 33620-5250
Received November 28, 2007; E-mail: pzhang@cas.usf.edu
Cyclopropane derivatives are a unique class of compounds with
fundamental importance of being the smallest all-carbon cyclic mol-
ecules as well as having practical significance as recurring units in
numerous natural products and as valuable synthons for many chem-
ical transformations.1 Of different methods, metal-catalyzed cyclo-
propanation of alkenes with diazo reagents is considered one of the
most versatile methods for the stereoselective construction of the
three-membered ring structures.1,2 Among known catalytic systems,1,2
Cu-,3 Rh-,4 and Ru-5catalyzed asymmetric processes have been
successfully developed to permit olefin cyclopropanation in high
yields and high selectivities. While the vast majority of those
catalytic systems employed diazocarbonyls, mostly diazoacetates,
as carbene sources, metal-catalyzed asymmetric cyclopropanation
reactions with other types of diazo reagents are underdeveloped.1–5
Following our original discovery of [Co(Por)]’s unique catalytic
capability for cyclopropanation,6a,7 a family of D2-symmetrical chi-
ral porphyrins was designed and synthesized via a versatile, modu-
lar approach for the development of the asymmetric variant of the Co-
catalyzed process.6b Among them, [Co(P1)] (Figure 1) has proved
to be one of the most selective catalysts for asymmetric cyclopro-
panation of both electron-sufficient (styrene derivatives)6c and elec-
tron-deficient (R,ꢀ-unsaturated carbonyls and nitriles)6d olefins with
diazoacetates. To further augment its substrate generality, we decid-
ed to explore the effectiveness of the Co-based catalytic system
for asymmetric cyclopropanation with diazo reagents, rather than
diazoacetates. As a result of this effort, we wish to describe herein
a highly effective catalytic system for asymmetric cyclopropanation
employing diazosulfones. This is a class of known diazo reagents
that has not been previously employed for asymmetric cyclopro-
panation except via a Cu-based intramolecular system reported by
Nakada and co-workers.8–11 Asymmetric olefin cyclopropanation
with diazosulfones would be highly desirable as the resulting
cyclopropyl sulfones have found a variety of applications.8–13
Figure 1. Structures of D2-symmetric chiral porphyrins.
Table 1. Asymmetric Cyclopropanation of Styrene with N2CHTs
Catalyzed by Cobalt(II) Complexes of Different Chiral Porphyrinsa
entry
[Co(Por)]b
DMAPc
yield (%)d
trans:cise
ee (%)f
configg
1
2
3
4
5
6
7
[Co(P1)]
[Co(P1)]
[Co(P2)]
[Co(P3)]
[Co(P4)]
[Co(P5)]
[Co(P6)]
+
-
-
-
-
-
-
∼6h
86
78
60
99
30
99
>99:01
>99:01
>99:01
>99:01
>99:01
>99:01
>99:01
3
14
56
23
61
54
92
[1R,2S]-(-)
[1S,2R]-(+)
[1S,2R]-(+)
[1S,2R]-(+)
[1S,2R]-(+)
[1R,2S]-(-)
[1R,2S]-(-)
a Performed in CH2Cl2 at room temperature for 24 h using 1 mol % of
[Co(Por)] under N2 with 1.0 equiv of styrene and 1.2 equiv of N2CHTs;
[styrene] ) 0.25 M. b See Figure 1 and Scheme S1 for structures and
syntheses. c With (+) or without (-) 0.5 equiv of DMAP. d Isolated
yields. e Determined by NMR. f Trans isomer ee was determined by chi-
ral HPLC. g Absolute configuration of major enantiomer determined by
X-ray crystal structural analysis and optical rotation. h Estimated by NMR.
Under the conditions optimized for asymmetric cyclopropanation
with diazoacetates,6 which required a substoichiometric amount of
DMAP due to a positive trans effect,14 our initial attempts to apply
[Co(P1)] as a catalyst to cyclopropanate styrene with tosyldiazo-
methane met with surprising disappointment (Table 1, entry 1).
Concurring with the assumption of a competitive carbene transfer to
DMAP, removal of DMAP resulted in a dramatic increase of the
cyclopropane formation but still exhibited poor enantioselectivity
(entry 2). Employment of a bulkier ligand P2 bearing meso-2,6-
dimethoxyphenyl groups improved the enantioselectivity substan-
tially (entry 3). Alteration of the chiral units with acyclic amides but
possessing intramolecular O · · ·H-N hydrogen bonding interactions
provided chiral porphyrins P3 and P4 (Figure 1),15 Co complexes
of which [Co(P3)] and [Co(P4)] gave better results than the respec-
tive [Co(P1)] and [Co(P2)] (entries 2–5). To create an even more
rigid and polar chiral environment, the combined incorporation of
intramolecular O · · ·H-N hydrogen bonding interactions and cyclic
structures led to the design and synthesis of chiral porphyrins P5
and P6 through the use of (S)-(-)-2-tetrahydrofurancarboxamide
(Figure 1). This design strategy was evidenced by X-ray crystal-
lographic analysis (Figure 1). While [Co(P5)] provided a better
enantioselectivity than the respective [Co(P1)] and [Co(P3)] (entry
6), [Co(P6)] proved to be the optimal catalyst, furnishing the desired
product in 99% yield and 92% ee (entry 7). Varying with enantio-
selectivity, all the catalysts exhibited excellent diastereoselectivity
(entries 1–7). It was noted that [Co(P5)] and [Co(P6)] gave a sense
of asymmetric induction opposite that of the other catalysts, despite
having the same (S) absolute configuration (Table 1).
9
5042 J. AM. CHEM. SOC. 2008, 130, 5042–5043
10.1021/ja7106838 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 2. [Co(P6)]-Catalyzed Diastereo- and Enantioselective
catalyst for asymmetric olefin cyclopropanation with diazosulfones.
The new catalytic system is general and can be applied to various
aromatic olefins as well as electron-deficient olefins, leading to
high-yielding formations of the corresponding cyclopropyl sulfones
in both high diastereoselectivity and high enantioselectivity. Further-
more, the [Co(P6)]-based asymmetric cyclopropanation can be
operated effectively in a one-pot fashion with alkenes as limiting
reagents and requires no slow addition of diazo reagents. This prac-
tical protocol is atypical for many other catalytic cyclopropanation
systems, due to the competitive carbene dimerization side reaction,1,2
but is a common feature for [Co(Por)]-catalyzed cyclopropanation.6
Acknowledgment. We are grateful to USF (Startup Funds),
ACS-PRF (AC Grant), NSF (CAREER Award), and NSF (CRIF:
MU-0443611, Mass Facility) for financial support.
Cyclopropanation of Different Alkenes with Various Diazosulfonesa
Supporting Information Available: Experimental procedures
and analytical data for all compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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a See footnote of Table 1. b Isolated yields. c The cis:trans ratio
determined by NMR. d The trans isomer ee was determined by chiral
HPLC. e Sign of optical rotation. f In CH2Cl2 at room temperautre for 24 h
using 1 mol % of [Co(P6)]. g In CH2Cl2 at -20 °C for 48 h using 1 mol %
of [Co(P6)]. h In ClC6H5 at room temperature for 24 h using 2 mol % of
[Co(P6)]. i In ClC6H5 at -20 °C for 24 h using 2 mol % of [Co(P6)].
j After one recrystallization. k [1R,2S] absolute configuration; see Table 1.
l Ms: 4-methoxybenzenesulfonyl; Ns: 4-nitrobenzenesulfonyl.
In addition to cyclopropanation of styrene with N2CHTs, [Co-
(P6)] was shown to be a general catalyst for a range of aromatic
and electron-deficient terminal olefins and with different diazoar-
ylsulfones (Table 2).16 For example, N2CHMs and N2CHNs served
equally well as carbene sources as compared to N2CHTs (entries
2–4). Both aromatic olefins with different substituents (entries 5–9)
and electron-deficient olefins, such as R,ꢀ-unsaturated esters (entries
10–12), ketones (entry 13), and nitriles (entry 14), could be effec-
tively cyclopropanated with N2CHTs by [Co(P6)]. Except for the
case of an R,ꢀ-unsaturated nitrile (entry 14), all the corresponding
cyclopropyl sulfones were formed in high enantioselectivity and
excellent trans diastereoselectivity (Table 2). Cyclopropyl sulfones
that are almost enantiomerically pure (>98% ee) were obtained
through a simple recystallization procedure due to the high
crystalline nature of this class of compounds, as exemplified in
the styrene and methyl vinyl ketone reactions (entries 1 and 13).
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tions and the use of cyclic structures. With P6 as a supporting
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