Enantioselective cyclopropenation of alkynes with acceptor/acceptor- substituted diazo reagents via Co(II)-based metalloradical catalysis

Article abstract of DOI:10.1021/ja111334j

The cobalt(II) complex of a new D₂-symmetric chiral porphyrin 3,5-diMes-ChenPhyrin, [Co(P2)], has been shown to be a highly effective chiral metalloradical catalyst for enantioselective cyclopropenation of alkynes with acceptor/acceptor-substituted diazo reagents, such as α- cyanodiazoacetamides and α-cyanodiazoacetates. The [Co(P2)]-mediated metalloradical cyclopropenation is suitable to a wide range of terminal aromatic and related conjugated alkynes with varied steric and electronic properties, providing the corresponding trisubstituted cyclopropenes in high yields with excellent enantiocontrol of the all-carbon quaternary stereogenic centers. In addition to mild reaction conditions, the Co(II)-based metalloradical catalysis for cyclopropenation features a high degree of functional group tolerance.

Full text of DOI:10.1021/ja111334j

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pubs.acs.org/JACS
Enantioselective Cyclopropenation of Alkynes with
Acceptor/Acceptor-Substituted Diazo Reagents via Co(II)-Based
Metalloradical Catalysis
Xin Cui, Xue Xu, Hongjian Lu, Shifa Zhu, Lukasz Wojtas, and X. Peter Zhang*
Department of Chemistry, University of South Florida, Tampa, Florida 33620-5250, United States
S Supporting Information
b
Scheme 1. Synthesis of Cyclopropenes Bearing Geminal
Nitrile and Carbonyl Functionalities and Their Further
Potential Transformations
ABSTRACT: The cobalt(II) complex of a new D2-symmetric
chiral porphyrin 3,5-diMes-ChenPhyrin, [Co(P2)], has been
shown to be a highly effective chiral metalloradical catalyst for
enantioselective cyclopropenation of alkynes with acceptor/
acceptor-substituted diazo reagents, such as R-cyanodiazoace-
tamides and R-cyanodiazoacetates. The [Co(P2)]-mediated
metalloradical cyclopropenation is suitable to a wide range of
terminal aromatic and related conjugated alkynes with varied
steric and electronic properties, providing the corresponding
trisubstituted cyclopropenes in high yields with excellent
enantiocontrol of the all-carbon quaternary stereogenic cen-
ters. In addition to mild reaction conditions, the Co(II)-based
metalloradical catalysis for cyclopropenation features a high
degree of functional group tolerance.
As stable metalloradicals with well-defined open-shell doublet
d7 electronic structure, cobalt(II) complexes of porphyrins,
[Co(Por)], have emerged as a new class of carbene transfer
catalysts for olefin cyclopropanation.11 With the introduction of
D2-symmetric chiral porphyrins as supporting ligands,11a,12 the
Co(II)-based metalloradical catalysts [Co(D2-Por*)] have been
demonstrated to be highly effective for asymmetric cyclopropa-
nation of a broad combination of olefin substrates and diazo
reagents with excellent diastereo- and enantioselectivity,13 in-
cluding electron-deficient olefins13b and acceptor/acceptor-sub-
stituted diazo reagent.13d,13g It is evident that Co(II)-based
metalloradical cyclopropanation possesses a distinct reactivity
profile from the widely studied Rh2- and Cu-based closed-shell
systems. Recent detailed mechanistic study confirmed the in-
volvement of an unusual Co(III)-carbene radical as the key
intermediate and elucidated an unprecedented stepwise radical
addition-substitution pathway for the Co(II)-catalyzed olefin
cyclopropanation.14 To further validate the concept of metallo-
radical catalysis (MRC), we envisioned the possibility of Co(II)-
based catalytic process for alkyne cyclopropenation if the radical
addition-substitution pathway of the Co(III)-carbene radical
intermediate could be also operative for alkynes in a similar way
to alkenes. To address the aforesaid challenges in the area, we
decided to target R-cyanodiazoacetates and R-cyanodiazoacet-
amides, two types of common acceptor/acceptor-substituted
diazo reagents that have not been previously applied for asym-
metric cyclopropenation (Scheme 1).10 These catalytic processes
would be synthetically attractive as the resultant multifunc-
tionalized cyclopropenes bearing an all-carbon quaternary
yclopropenes are a unique class of carbocyclic compounds
C
with unsaturated, highly strained three-membered ring
structures. The combination of high strain and unsaturation
renders cyclopropenes as versatile synthons for a wide variety of
synthetic organic transformations.1 Consequently, significant
efforts have been devoted toward the synthesis of this class of
molecules, especially optically active chiral cyclopropenes.1,2 Of
different methods, catalytic asymmetric cyclopropenation of
alkynes with diazo reagents constitutes one of the most direct
and general methods for stereoselective construction of this type
of strained ring structure.1,2 A number of catalytic systems based
on dirhodium(II) complexes of chiral carboxamidate and car-
boxylate ligands have been successfully developed to catalyze
enantioselective cyclopropenation using several different types of
diazo reagents as carbene sources, including diazoacetates,3,4
diazosulfones,5 aryldiazoacetates,6 and styryldiazoacetates.7-9
While the existing chiral Rh2 catalysts were shown to be highly
effective with both acceptor- and donor/acceptor-substituted
diazo regents, asymmetric cyclopropenation with acceptor/ac-
ceptor-substituted diazo reagents remains to be developed.10
Owing to the presence of two electron-withdrawing groups at the
R-carbon, this class of diazo reagents has inherent low reactivity
with Lewis acidic metal catalysts toward formation of the
corresponding metallocarbene intermediates. Even when they
can be formed under forcing conditions, their subsequent reac-
tions with substrates are often difficult in terms of controlling
enantioselectivity due to the high electrophilicity of the accep-
tor/acceptor-substituted metallocarbenes.
Received: December 16, 2010
Published: February 18, 2011
r
2011 American Chemical Society
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Table 1. Reaction Conditions for Cyclopropenation of
Phenylacetylene with r-Cyano(N,N-dimethyl)diazoacetamide
by Cobalt(II) Porphyrinsa
Table 2. [Co(P2)]-Catalyzed Enantioselective Cycloprope-
nation of Phenylacetylene with Acceptor/Acceptor-Substituted
Diazo Reagentsa
entry
[Co(Por)]b
solvent
yield (%)c
ee (%)d
1
2
3
4
5
6
7
8
9
[Co(TPP)]
[Co(P1)]
[Co(P2)]
[Co(P2)]
[Co(P2)]
[Co(P2)]
[Co(P2)]
[Co(P2)]
[Co(P2)]
PhCF3
PhCF3
PhCF3
PhC1
PhF
<5
10
95
40
38
26
45
<5
70
-
71
82
nde
nde
nde
nde
nde
77
PhMe
PhH
CH2C12
CC14
a Reactions were carried out at room temperature for 10 h in one-time
fashion without slow addition of the diazo reagent using 1 mol %
[Co(Por)] under N2 with 1.0 equiv of R-cyanodiazo(N,N-dimethyl)
acetamide and 1.5 equiv of phenylacetylene. Concentration: 0.10 mmol
diazo/mL. b See Figure 1 for structure. c Isolated yields. d Enantiomeric
excess determined by chiral HPLC. e Not determined.
a Reactions were carried out in one-time fashion without slow addition
of the diazo reagent using 1 mol % [Co(P2)] under N2 with 1.0 equiv of
diazo reagent and 1.5 equiv of phenylacetylene. Concentration: 0.10
c
mmol diazo/mL PhCF3. b Isolated yields. Enantiomeric excess deter-
mined by chiral HPLC. d At room temperature for 12 h. e At 40 °C for 24
h. f [R] absolute configuration determined by anomalous-dispersion
effects in X-ray diffraction measurements on the crystal.
Figure 1. Structures of D2-symmetric chiral cobalt(II) porphyrins.
stereogenic center can serve as invaluable chiral synthons for a
range of stereoselective synthetic applications (Scheme 1).1,2
As the outcome of the effort, we report herein a highly efficient
catalytic system based on a new chiral Co(II) metalloradical
catalyst for enantioselective cyclopropenation of alkynes with
both R-cyanodiazoacetates and R-cyanodiazoacetamides. In ad-
dition to high enantioselectivity, the Co(II)-catalyzed cyclopro-
penation can operate at room temperature using a stoichiometric
ratio of reactants without the need of slow addition of the diazo
reagents. Furthermore, the metalloradical catalytic process fea-
tures a remarkable degree of tolerance toward various function-
alities, including CHO, OH, and NH2 groups.
Initial experiments were focused on the evaluation of ligand
and solvent effects on cyclopropenation of phenylacetylene (1a)
with R-cyano(N,N-dimethyl)diazoacetamide (2a) by [Co(Por)]
under conditions that were deemed most practical: 1 mol %
catalyst loading; room temperature; stoichiometric ratio of reac-
tants; and one-time protocol without slow addition (Table 1).
While the ineffectiveness of [Co(TPP)] for the reaction might be
expected due to the absence of the H-bonding donor amide units
(entry 1), we were somewhat surprised by the inferior performance
of [Co(P1)] (Figure 1; entry 2), which has been previously shown
to be highly effective for various cyclopropanation reactions.12,13
This result indicated different requirements of catalyst
environment for the two carbene transfer processes and prompted
us to develop new catalysts by taking advantage of the modular
design and tunability of the D2-symmetric chiral porphyrin
system.11a,12 To this end, replacement of the aliphatic t-butyl
substituent in P1 with aromatic mesityl group led to the design
and synthesis of new D2-symmetric chiral porphyrin 3,5-diMes-
ChenPhyrin (P2; Figure 1). Gratifyingly, [Co(P2)] was found
to be a highly effective catalyst for the reaction, leading to the
formation of the desired 1,1-cyclopropeneamidonitrile 3aa in 95%
yield and 82% ee (entry 3). During the process of optimizing the
reaction conditions, trifluorotoluene was shown to be the solvent of
choice and performed significantly better than other solvents
screened (entries 4-9).
The [Co(P2)]-based metalloradical cyclopropenation system
was found to be applicable for different acceptor/acceptor-
substituted diazo reagents under the similar practical conditions
(Table 2). Like the N,N-dimethyl diazo 2a (entry 1), N,N-diethyl
analog 2b could also function as effective carbene source for
Co(II)-catalyzed enantioselective cyclopropenation of 1a (entry 2).
Notably, the catalytic system went equally well with N-methoxy-N-
methyl R-cyanodiazoacetamide 2c, providing the corresponding
chiral cyclopropenyl Weinreb amide 3ac (entry 3), which can serve
as a potential synthon for preparation of chiral cyclopropenyl
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Table 3. [Co(P2)]-Catalyzed Asymmetric Cyclopropenation
of Different Combinations of Aryl/Vinyl Alkynes and A/A-
Type Diazo Reagentsa
Table 4. Diastereoselective Thiol Addition to Cyclopropenesa
a Reactions were carried out overnight in toluene using 10 mol % DBU
with 1.0 equiv of cyclopropene 3af (98% ee) and 1.5 equiv of thiol. The
reaction temperature was initially at -30 °C and gradually warmed to
c
room temperature. b Isolated yields of single diastereomers. Enantio-
meric excess determined by chiral HPLC. d [1S,2R,3S] absolute config-
uration determined by anomalous-dispersion effects in X-ray diffraction
measurements on crystal. e 100 mol % DBU; room temperature; 48 h .
of phenylacetylenes substituted with alkyl groups at different
positions proceeded equally well as phenylacetylene (entries 1 and
2). In addition, various halogenated phenylacetylenes could be
enantioselectively cyclopropenated with both R-cyanodiazoaceta-
mides and -acetates (entries 3-8). Among the chiral cyclopropene
products, the absolute configuration of the all-carbon quaternary
stereogenic center in 3df (entry 5) was established to be [R] byX-ray
crystal structural analysis (see Supporting Information). Chiral
cyclopropene derivatives from reactions of aromatic alkynes contain-
ing both electron-withdrawing and -donating groups could also be
obtained in good yields and high enantioselectivities (entries 9
and 10). The Co(II)-catalyzed reaction could be extended for
nonaromatic conjugated alkynes as demonstrated with cyclopro-
penation reaction of cyclohexenylethyne with diazo 2d for the
high-yielding formation of enantioenriched 1,1-cyclopropene-
amidonitrile 3jd (entry 11).
Consistent with the proposed radical mechanism of non-
electrophilic carbene radical intermediate,14 the Co(II)-catalyzed
carbene transfer process was found to tolerate functional groups
that wouldotherwise undergoylide-typechemistryassociatedwith
electrophilic metallocarbenes. For example, the aldehyde function-
ality could be tolerated without complication from potential ylide-
mediatedepoxidation (entry12). The functional group tolerability
of [Co(P2)]-catalyzed asymmetric cyclopropenation was further
highlighted by the reactions of phenylacetylene derivatives con-
taining hydroxyl and amino substituents (entries 13-15). In all
the cases, no O-H or N-H insertion products were observed.
The above demonstrated cyclopropenation process via [Co(P2)]-
based metalloradical catalysis presents a viable route to access
densely functionalized cyclopropenes containing enantioenriched
all-carbon quaternary stereogenic centers, which should find a range
of applications as chiral building blocks for stereoselective organic
syntheses through further functional group transformations.
For instance, they can be transformed to highly functionalized
a Reactions were carried out in one-time fashion without slow addition
of the diazo reagent using 1 mol % [Co(P2)] under N2 with 1.0 equiv
of diazo reagent and 1.5 equiv of phenylacetylene. Concentration:
0.10 mmol diazo/mL PhCF3; Isolated yields; Enantiomeric excess
determined by chiral HPLC. b At room temperature for 12 h. c At
40 °C for 24 h. d [R] absolute configuration; see footnote f of Table 2.
cyanoaldehyde and cyanoketone derivatives.15 In addition to the
tertiary analogs, secondary R-cyanodiazoacetamides could also be
productively used as exemplified with N-isopropyl R-cyanodiazo-
acetamide 2d, forming the desired cyclopropene 3ad in 96% yield
and 96% ee (entry 4). Besides R-cyanodiazoacetamides, [Co(P2)]
was shown to enable cyclopropenation with R-cyanodiazoacetates
as well. For example, both ethyl and t-butyl R-cyanodiazoacetates
(2e and 2f) could be effectively utilized to cylopropenate 1a to form
1,1-cyclopropeneesternitrile 3ae and 3af, respectively, in good
yields and excellent enantioselectivities (entries 5 and 6). The
absolute configuration of the all-carbon quaternary stereogenic
center in 3af was established as [R] by X-ray crystal structural
analysis (see Supporting Information).
The [Co(P2)]-catalyzed asymmetric cyclopropenation
could be successfully expanded for a wide range of terminal
aromatic and related conjugated alkynes in combination
with various acceptor/acceptor-substituted diazo reagents
(Table 3). For example, using N-isopropyl diazo 2d as the
carbene source, enantioselective cyclopropenation reactions
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cyclopropane derivatives by nucleophilic or electrophilic addition to
the activated π-bonds as a result of the high ring strain.1,2 In
particular, nucleophilic addition with soft nucleophiles would
provide an entry to chiral heterosubstituted cyclopropanes, which
would be difficult or impossible to access via direct asymmetric
cyclopropanation of alkenes. As an initial effort toward this type of
applications, we demonstrated that cyclopropene 3af could under-
go highly diastereoselective addition reactions with thiol nucleo-
philes to furnish heterosubstituted cyclopropane derivatives
(Table 4).16 For example, when 3af in 98% ee was treated with
1.5 equiv of n-propanethiol, the corresponding 1,1,2,3-tetra-sub-
stituted cyclopropane 4a could be isolated in 98% yield as a sole
diastereomer in the same high optical purity (entry 1). The absolute
configuration of the three continuous stereogenic centers in 4a was
established to be [1S,2R,3S] by X-ray crystal structural analysis (see
Supporting Information). Highly diastereoselective addition reac-
tions of 3af could be similarly accomplished with isopropanethiol
and tert-butylthiol, affording enantiopure thiolated cyclopropanes
4b and 4c, respectively, albeit in relatively lower yields due to the
higher steric hindrance (entries 2 and 3).
Nakamura, M.; Isobe, H.; Nakamura, E. Chem. Rev. 2003, 103, 1295. (e)
Carter, F. L.; Frampton, V. L. Chem. Rev. 1964, 64, 497. (f) Prosser, A. R.;
Banning, J. E.; Rubina, M.; Rubin, M. Org. Lett. 2010, 12, 3968. (g) Banning,
J. E.; Prosser, A. R.; Rubin, M. Org. Lett. 2010, 12, 1488. (h) Alnasleh, B. K.;
Sherrill, W. M.; Rubina, M.; Banning, J.; Rubin, M. J. Am. Chem. Soc. 2009,
131, 6906. (i) Sherrill, W. M.; Rubin, M. J. Am. Chem. Soc. 2008, 130, 13804.
(j) Alnasleh, B. K.; Sherrill, W. M.; Rubin, M. Org. Lett. 2008, 10, 3231. (k)
Xie, X. C.; Yang, Z.; Fox, J. M. J. Org. Chem. 2010, 75, 3847. (l) Tarwade, V.;
Liu, X. Z.; Yan, N.; Fox, J. M. J. Am. Chem. Soc. 2009, 131, 5382. (m) Yan, N.;
Liu, X. Z.; Fox, J. M. J. Org. Chem. 2008, 73, 563. (n) Fisher, L. A.; Fox, J. M. J.
Org. Chem. 2008, 73, 8474.
(2) Marek, I.; Simaan, S.; Masarwa, A. Angew. Chem., Int. Ed. 2007,
46, 7364.
(3) (a) Protopopova, M. N.; Doyle, M. P.; M€uller, P.; Ene, D. J. Am.
Chem. Soc. 1992, 114, 2755. (b) Doyle, M. P.; Protopopova, M.; M€uller,
P.; Ene, D.; Shapiro, E. A. J. Am. Chem. Soc. 1994, 116, 8492. (c) Imogai,
H.; Bernardinelli, G.; Granicher, C.; Moran, M.; Rossier, J. C.; M€uller, P.
Helv. Chim. Acta 1998, 81, 1754. (d) M€uller, P.; Imogai, H. Tetrahedron:
Asymmetry 1998, 9, 4419. (e) Lou, Y.; Horikawa, M.; Kloster, R. A.;
Hawryluk, N. A.; Corey, E. J. J. Am. Chem. Soc. 2004, 126, 8916. (f) Lou,
Y.; Remarchuk, T. P.; Corey, E. J. J. Am. Chem. Soc. 2005, 127, 14223.
(4) For asymmetric intramolecular cyclopropenation with diazoacetates,
see:(a) Doyle, M. P.; Ene, D. G.; Forbes, D. C.; Pillow, T. H. Chem. Commun.
1999, 1691. (b) Doyle, M. P.; Ene, D. G.; Peterson, C. S.; Lynch, V. Angew.
Chem., Int. Ed. 1999, 38, 700. (c) Doyle, M. P.; Hu, W. H. Tetrahedron Lett.
2000, 41, 6265. (d) Doyle, M. P.; Hu, W. H. Synlett 2001, 1364. (e) Doyle,
M. P.; Weathers, T. M.; Wang, Y. H. Adv. Synth. Catal. 2006, 348, 2403.
(5) Weatherhead-Kloster, R. A.; Corey, E. J. Org. Lett. 2006, 8, 171.
(6) (a) Davies, H. M. L.; Lee, G. H. Org. Lett. 2004, 6, 1233. (b)
Chuprakov, S.; Gevorgyan, V. Org. Lett. 2007, 9, 4463.
Furthermore, we showed that the thiolated cyclopropane 4a
could be oxidatively converted to cyclopropyl sulfone 5a in a high
yield without loss of its optical purity upon simple treatment with
m-chloroperbenzoic acid (CPBA) at room temperature (eq 1).
(7) Briones, J. F.; Hansen, J.; Hardcastle, K. I.; Autschbach, J.;
Davies, H. M. L. J. Am. Chem. Soc. 2010, 132, 17211.
(8) Panne, P.; Fox, J. M. J. Am. Chem. Soc. 2007, 129, 22 Although
the issue of enantioselectivity was not addressed, it was shown that Rh2
complexes of sterically demanding carboxylate ligands could effectively
catalyze cyclopropenation with R-alkyldiazoacetates possessing β-hydrogens,
a class of donor/acceptor-substituted diazo reagents that are challenging for
metal-mediated carbene transfer reactions due to competitive β-elimination..
(9) For a recent nonasymmetric synthesis of trifluoromethyl-sub-
stituted cyclopropenes via Rh2-catalyzed cyclopropenation of alkynes,
see: Morandi, B.; Carreira, E. M. Angew. Chem., Int. Ed. 2010, 49, 4294.
(10) While this manuscript was in the process of submission, Katsuki
and coworkers reported a Ir(salen)-catalyzed asymmetric cyclopropenation
system that is effective with a cyclic R-cyanodiazoacetamide in addition
to several donor/acceptor-substituted diazo reagents, see: Uehara, M.;
Suematsu, H.; Yasutomi, Y.; Katsuki, T. J. Am. Chem. Soc. 2011, 133, 170.
(11) (a) Doyle, M. P. Angew. Chem., Int. Ed. 2009, 48, 850. (b)
Huang, L. Y.; Chen, Y.; Gao, G. Y.; Zhang, X. P. J. Org. Chem. 2003, 68,
8179. (c) Penoni, A.; Wanke, R.; Tollari, S.; Gallo, E.; Musella, D.;
Ragaini, F.; Demartin, F.; Cenini, S. Eur. J. Inorg. Chem. 2003, 1452.
(12) Chen, Y.; Fields, K. B.; Zhang, X. P. J. Am. Chem. Soc. 2004, 126,
14718.
(13) (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, 129, 12074.
(c) Zhu, S. F.; Ruppel, J. V.; Lu, H. J.; Wojtas, L.; Zhang, X. P. J. Am.
Chem. Soc. 2008, 130, 5042. (d) Zhu, S. F.; Perman, J. A.; Zhang, X. P.
Angew. Chem., Int. Ed. 2008, 47, 8460. (e) Fantauzzi, S.; Gallo, E.; Rose,
E.; Raoul, N.; Caselli, A.; Issa, S.; Ragaini, F.; Cenini, S. Organometallics
2008, 27, 6143. (f) Ruppel, J. V.; Gauthier, T. J.; Snyder, N. L.; Perman,
J. A.; Zhang, X. P. Org. Lett. 2009, 11, 2273. (g) Zhu, S. F.; Xu, X.;
Perman, J. A.; Zhang, X. P. J. Am. Chem. Soc. 2010, 132, 12796.
(14) Dzik, W. I.; Xu, X.; Zhang, X. P.; Reek, J. N. H.; de Bruin, B.
J. Am. Chem. Soc. 2010, 132, 10891.
In summary, we have developed a highly enantioselective process
based on the new metalloradical catalyst [Co(P2)] for cyclopro-
penation of aryl/vinyl alkynes with both R-cyanodiazoacetamides
and R-cyanodiazoacetates. It represents the first successful applica-
tions of these two types of acceptor/acceptor-substituted diazo
reagents for asymmetric cyclopropenation,10 providing a practical
method for the preparation of multifunctionalized cyclopropenes
bearing enantioenriched all-carbon quaternary stereogenic centers
that may serve as useful chiral synthons for stereoselective synthesis
(Scheme 1). Among several salient features, the Co(II)-based
system enjoys an unusual degree of functional group tolerance,
which is believed to have close relevance to the radical pathway of
Co(II)-based metalloradical catalysis.14
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and ana-
b
lytical data for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
xpzhang@usf.edu
’ ACKNOWLEDGMENT
We are grateful for financial support by the National Science
Foundation (CAREER award: CHE-0711024).
(15) (a) Balasubramaniam, S.; Aidhen, I. S. Synthesis-Stuttgart 2008,
3707. (b) Khlestkin, V. K.; Mazhukin, D. G. Curr. Org. Chem. 2003, 7, 967.
(16) (a) Martinez-Grau, A.; Blasco, J. M.; Ferritto, R.; Espinosa, J. F.;
Mantecon, S.; Vaquero, J. J. Arkivoc 2005, 394. (b) Shapiro, E. A.;
Kalinin, A. V.; Nefedov, O. M. Org. Prep. Proced. Int. 1992, 24, 517.
’ REFERENCES
(1) (a) Rubin, M.; Rubina, M.; Gevorgyan, V. Chem. Rev. 2007, 107,
3117. (b) Rubin, M.; Rubina, M.; Gevorgyan, V. Synthesis-Stuttgart 2006,
1221. (c) Fox, J. M.; Yan, N. Curr. Org. Chem. 2005, 9, 719. (d)
3307
dx.doi.org/10.1021/ja111334j |J. Am. Chem. Soc. 2011, 133, 3304–3307