Bifunctional iminophosphorane organocatalysts for enantioselective synthesis: Application to the ketimine nitro-Mannich reaction

Article abstract of DOI:10.1021/ja409121s

The design, synthesis, and development of a new class of modular, strongly basic, and tunable bifunctional Bronsted base/H-bond-donor organocatalysts are reported. These catalysts incorporate a triaryliminophosphorane as the Bronsted basic moiety and are readily synthesized via a last step Staudinger reaction of a chiral organoazide and a triarylphosphine. Their application to the first general enantioselective organocatalytic nitro-Mannich reaction of nitromethane to unactivated ketone-derived imines allows the enantioselective construction of β-nitroamines possessing a fully substituted carbon atom. The reaction is amenable to multigram scale-up, and the products are useful for the synthesis of enantiopure 1,2-diamine and α-amino acid derivatives.

Full text of DOI:10.1021/ja409121s

Communication
pubs.acs.org/JACS
Bifunctional Iminophosphorane Organocatalysts for Enantioselective
Synthesis: Application to the Ketimine Nitro-Mannich Reaction
Marta G. Nunez, Alistair J. M. Farley, and Darren J. Dixon*
́
̃
The Department of Chemistry, Chemistry Research Laboratory, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.
S
* Supporting Information
address some of these limitations, we proposed to develop a new
ABSTRACT: The design, synthesis, and development of
a new class of modular, strongly basic, and tunable
bifunctional Brønsted base/H-bond-donor organocatalysts
are reported. These catalysts incorporate a triarylimino-
phosphorane as the Brønsted basic moiety and are readily
synthesized via a last step Staudinger reaction of a chiral
organoazide and a triarylphosphine. Their application to
the first general enantioselective organocatalytic nitro-
Mannich reaction of nitromethane to unactivated ketone-
derived imines allows the enantioselective construction of
β-nitroamines possessing a fully substituted carbon atom.
The reaction is amenable to multigram scale-up, and the
products are useful for the synthesis of enantiopure 1,2-
diamine and α-amino acid derivatives.
class of bifunctional organocatalysts that possessed a much
stronger and tunable Brønsted basic group. Our hope was that
through the synergistic effects of the stronger Brønsted base and
the H-bond donor, good reactivity and selectivity in new and
challenging enantioselective organocatalytic addition reactions
would be obtained. Herein we report our findings.
From the outset, we wanted a design that relied on a clean and
efficient last step generation of the Brønsted basic moiety of the
catalyst; this would greatly simplify their synthesis and handling.
To this end we considered the Staudinger reaction10 of a
triarylphosphine and an enantiopure organoazide possessing an
effective H-bond donor group (Scheme 1). Our hope was that
Scheme 1. Concept and Design of a New Class of Bifunctional
Iminophosphorane (BIMP) Organocatalysts
he addition of a carbon- or heteroatom-centered acid to an
Telectron-deficient carbon−carbon (CC) or carbon−
heteroatom (CX) double bond is a reaction of fundamental
importance in organic synthesis. Such reactions offer perfect
atom economy, are often energetically favorable, and can
generate products that are chiral.1 Brønsted basic reagents can
be employed to catalyze the addition by generating the ion-
paired conjugate base as the key nucleophilic entity, and
asymmetry in the Brønsted base can relay through to the
product via energetically discriminated transition states.2 To this
end, a plethora of chiral single-enantiomer Brønsted bases have
been developed for a wide range of enantioselective addition
reactions. These range from relatively weak chiral tertiary amines
to organic superbases3 such as amidines,4 guanidines,5
phosphazenes,6 and cyclopropenimines.7
Within the past decade, one particular class of base that has
received considerable attention is that of the bifunctional
Brønsted base/H-bond donor organocatalyst.8 These catalysts
typically possess both a tertiary amine group and a hydrogen-
bond donor group appropriately positioned on a chiral scaffold.
Additional organization of the transition structure through
stabilization of developing negative charge by the H-bond donor
can result in increased reaction rates and/or enantioselectivity
relative to H-bond donor free analogues.9 Although this class
continues to demonstrate synthetic utility, it is not without its
limitations; reaction times are often long even with the most
reactive reagent combinations, and arguably the range of pro-
nucleophiles and electrophiles amenable to asymmetric union is
relatively narrow. This low catalytic activity often stems from the
relatively weak Brønsted basicity of the tertiary amine moiety,
providing insufficient activation of the pro-nucleophile. To
the strong nucleophilicity of the triarylphosphine would create a
strongly Brønsted basic iminophosphorane moiety through
favorable loss of dinitrogen gas.11 As many triarylphosphine
reagents are readily available, this simple design could allow the
synthesis of a range of bifunctional catalysts with electronic and/
or steric variations at the iminophosphorane moiety. Further
variations to the chiral scaffold and the H-bond donor group
would add favorably to the diversity of catalysts accessible
through this design.
The premise of our catalyst design was the enhanced Brønsted
basicity of the triaryliminophosphorane functionality relative to
the weak tertiary amine group. Interestingly, whereas the
basicities of triaminoiminophosphoranes are reportedsuch as
P1-tBu with a pKBH+ (MeCN) of 26.98 (Figure 1)the Brønsted
basicities of triaryliminophosphoranes are not. Accordingly, to
quantify the basicity of triaryliminophosphoranes, pKBH+
measurements were carried out on a model triphenylphos-
phine-derived iminophosphorane, 1a (derived from cyclohexyl
azide; see Supporting Information). Indeed its pKBH+ (MeCN)
of 22.7 was determined to be 4 orders of magnitude greater than
Received: September 3, 2013
Published: October 9, 2013
© 2013 American Chemical Society
16348
dx.doi.org/10.1021/ja409121s | J. Am. Chem. Soc. 2013, 135, 16348−16351

Journal of the American Chemical Society
Communication
and the appropriate positioning of an effective H-bond donor
group over a suitable chiral scaffold could provide both the
potency and control required.16
To expedite screening, a library of BIMP catalysts (4a, 4d−i)
was made in situ from the corresponding catalytically inactive
azides and triphenylphosphine by simply stirring equimolar
amounts in diethyl ether at rt for 24 h (see Supporting
Information). This library was assessed for performance in the
reaction of nitromethane with acetophenone-derived N-
diphenylphosphinoyl (DPP) ketimine 5a (Table 1).17 Pleasingly,
Figure 1. pKBH+ (MeCN) measurements of a tertiary amine and some
common organic superbases and two triaryliminophosphorane super-
bases 1a and 1b. PMP = para-methoxyphenyl.
that of triethylamine and comparable to that of guanidines and
other organic superbases, thus validating our design concept.3,12
Importantly, tris(4-methoxyphenyl)phosphine-derived imino-
phosphorane 1b was found to be a further hundred-fold more
basic (pKBH+ (MeCN) of 25.0), thus demonstrating that the
basicity of the triaryliminophosphorane moiety can be readily
modified by varying the electronics of the triarylphosphine
component.
Table 1. Proof of Concept and Optimization Studies in the
Nitro-Mannich Reaction of Nitromethane with N-Diphenyl-
a
phosphinoyl Ketimine 5a
L-tert-Leucine-derived azide 2 was then prepared on a gram
scale (see Supporting Information) and reacted separately with
equimolar quantities of both triphenylphosphine and tris(4-
methoxyphenyl)phosphine in anhydrous diethyl ether (Scheme
2). In both cases after stirring for 24 h bifunctional
Scheme 2. Synthesis of Two Representative BIMPs and
Single-Crystal X-ray Structures of 4b and 4c (P, orange; N,
blue; S, yellow; O, red; F, green; H, white)
b
c
b
c
conversion, ee
conversion, ee
entry catalyst
(%)
entry catalyst
(%)
1
2
3
4
5
6
7
8
4a
4d
4e
4f
98, 85
93, 78
94, 77
9
4k
4l
98, 79
98, 84
98, 70
99, 68
98, 73
98, 11
92, 0
10
11
12
13
14
15
16
4m
4n
4o
4p
4q
4r
d
95, 77
d
4g
4h
4i
93, 70
d
48, 34
d
98, 20
4j
97, 79
99, 0
a
Reactions performed using 0.2 mmol of ketimine 5a in 0.2 mL of
b
MeNO2 at rt. Conversion was determined by 1H NMR analysis of the
crude reaction mixtures. Enantiomeric excess (ee) was determined by
HPLC analysis on a chiral stationary phase. Enantiomer (S)-6a was
c
d
obtained.
using neat nitromethane (20 equiv) and 10 mol % catalyst, all
BIMPs demonstrated excellent reactivity in the formation of
addition product 6a; conversion after 24 h was near full with all
but catalyst 4h. Furthermore, enantioselectivities were good
except with catalysts 4h and 4i (Table 1, entries 6, 7). L-tert-
Leucine-derived catalyst 4a outperformed the other catalysts in
terms of enantioselectivity (85% ee at rt, Table 1, entry 1) and
was therefore selected as the scaffold of choice for the remainder
of the optimization studies.
BIMPs 4j−r possessing a range of H-bond donor groups
including thiourea, urea, amide, sulfonamide, and carbamate
were investigated for performance. In all cases good conversion
to addition product 6a was observed after 24 h at rt (Table 1,
entries 8−16). Pleasingly both urea and thioureas with electron-
donating or electron-withdrawing substituents (4a, 4j−o)
attached to the aromatic ring imparted high levels of
enantioselectivity in the reaction (Table 1, entries 1, 8−13).
However, electron-deficient aryl amide 4p afforded the product
iminophosphoranes 4a and 4b were isolated as bench-stable
solids by precipitation and filtration.13 In the case of tris(4-
methoxyphenyl)phosphine-derived catalyst 4b, crystals of
suitable quality for single-crystal X-ray diffraction were obtained
(Scheme 2), allowing the unambiguous determination of its
solid-state structure. Furthermore, in the enantiomeric series, the
single-crystal X-ray structure of D-phenylglycine-derived imino-
phosphorane 4c was also obtained.
To demonstrate the synthetic potential of this new class of
bifunctional organocatalysts, we selected the challenging
enantioselective organocatalytic nitro-Mannich (or aza-Henry)
reaction of nitromethane with unactivated ketone-derived imines
(ketimines). Unlike the many variations of the well-developed
organocatalytic enantioselective nitro-Mannich reaction of
aldehyde-derived imines (aldimines),14 the corresponding
ketimine variant has no reported general solution.15 We believed
that the strong Brønsted basicity of the triaryliminophosphorane
16349
dx.doi.org/10.1021/ja409121s | J. Am. Chem. Soc. 2013, 135, 16348−16351

Journal of the American Chemical Society
Communication
with poor enantiocontrol (Table 1, entry 14), and both
sulfonamide 4q and carbamate 4r yielded the product in racemic
form (Table 1, entries 15, 16). From these results, it was clear
that the H-bond donor group was playing a key role in
controlling the stereochemical outcome of the reaction and that
the Schreiner-type 3,5-bis(trifluoromethyl)phenyl thiourea
moiety of catalyst 4a was optimal.18
Table 2. Scope of Asymmetric Nitro-Mannich Reaction of
Ketimines
a
With the best scaffold and H-bond donor group identified, we
turned our attention to the Brønsted base moiety and studied
how the reaction rate varied as a function of the electronics of the
triarylphosphine component. Catalyst 4a (R = H) and its
analogues 4b (R = MeO) and 4s (R = Cl) were used in the
reaction of ketimine 5a with nitromethane, and conversion to
addition product 6a was measured by 1H NMR analysis (Figure
2). The reaction rate was indeed governed by the aryl
Figure 2. Comparison of reaction rates with BIMP catalysts 4a, 4b, and
4s and cinchonine-derived bifunctional organocatalyst I.
substituents of the iminophosphorane moiety. The reaction
with catalyst 4s was slower than that with 4a or 4b, in line with a
more electron-deficient triarylphosphine generating a weaker
base. As a direct comparison, when cinchonine-derived bifunc-
a
Reactions were carried out on 0.2 mmol of 5 at the indicated
b
1
temperature. Enantiomeric excess (ee) was determined by HPLC
analysis on a chiral stationary phase. *Conversion was 50%.
tional organocatalyst I was used as the catalyst in the same H
NMR kinetic experiment, no product was detected even after a
prolonged reaction time of 32 h. The above experiments clearly
demonstrate that the enhanced basicity of iminophosphoranes
compared to tertiary amines is responsible for the increase in
catalytic activity.
crude product in 98% conversion and 86% ee. After
recrystallization, 8.3 g of addition product 6a (70% yield) was
obtained in 98% ee, and a further recrystallization afforded 6a in
>99% ee. In a short demonstration of synthetic utility this
enantiopure β-nitroamine was converted to the corresponding
1,2-diamine derivative 720 via a nickel boride-mediated reduction
of the nitro group followed by Cbz protection and then DPP
removal. Furthermore, 6a was transformed into fully substituted
α-amino acid 821 by a two-step sequence involving a Nef
oxidation followed by DPP removal (Scheme 3).
In summary, we have designed and developed a new and effective
class of modular bifunctional iminophosphorane superbase
organocatalysts and have applied it to the first metal-free
catalytic enantioselective addition of nitromethane to unreactive
ketone-derived imines, a reaction where existing tertiary amine
organocatalysts are impotent. The synthesis of the catalysts from
catalytically inactive and readily synthesized azide precursors and
triarylphosphines allows for rapid catalyst screening and
optimization. The nitro-Mannich reaction of ketimines can be
performed on a multigram scale and allows ready access to
synthetically relevant, nitrogen-containing chiral building blocks
possessing a fully substituted carbon atom. Work to uncover the
full capabilities of this new catalyst design is ongoing, and the
results will be disclosed in due course.
Although catalyst 4b was marginally superior to 4a in terms of
reaction rate, 4a was more enantioselective and was therefore
chosen as the catalyst to investigate the substrate scope in the
nitro-Mannich addition of nitromethane to ketimines. To
maximize enantioselectivity, reactions were performed in chilled
(0 or −15 °C) nitromethane. Good to excellent enantioselectiv-
ities were obtained with aromatic ketone-derived imines bearing
either electron-withdrawing or electron-donating substitutents
(up to 95% yield, up to 95% ee; Table 2, entries 1−12). Phenyl
ethyl ketone-derived imine 5m also afforded the product with
high enantioselectivity, as did the tetralone-derived imine
substrate 5n albeit in lower yield in the latter case (Table 2,
entries 13, 14). Heteroaromatic ketimines 5o and 5p gave the
desired products in near quantitative yields and with good
enantioselectivities (Table 2, entries 15, 16). Pleasingly, the
reaction was also applicable to aliphatic ketimine 5q (Table 2,
entry 17).19
The performance of our new BIMP catalysts for enantiose-
lective catalysis on a preparative scale was investigated next. To
minimize reaction time, the most active catalyst, 4b, was selected
for this purpose. Using 1 mol % 4b, 10 g of ketimine 5a was
reacted with 10 equiv of nitromethane at rt over 21 h to afford the
16350
dx.doi.org/10.1021/ja409121s | J. Am. Chem. Soc. 2013, 135, 16348−16351

Journal of the American Chemical Society
Communication
1675. For reviews see: (h) Ishikawa, T.; Kumamoto, T. Synthesis 2006,
737. (i) Leow, D.; Tan, C.-H. Chem. Asian J. 2009, 4, 488. (j) Leow, D.;
Tan, C.-H. Synlett 2010, 1589. (k) Ishikawa, T. Chem. Pharm. Bull. 2010,
58, 1555. (l) Fu, X.; Tan, C.-H. Chem. Commun. 2011, 47, 8210.
(6) (a) Uraguchi, D.; Sakaki, S.; Ooi, T. J. Am. Chem. Soc. 2007, 129,
12392. (b) Uraguchi, D.; Yoshioka, K.; Ueki, Y.; Ooi, T. J. Am. Chem. Soc.
2012, 134, 19370. (c) Corbett, M. T.; Uraguchi, D.; Ooi, T.; Johnson, J.
S. Angew. Chem., Int. Ed. 2012, 51, 4685.
Scheme 3. Preparative-Scale Synthesis of 6a and
Derivatization into Enantiopure Diamine 7 and Quaternary
a
α-Amino Acid 8
(7) (a) Bandar, J. S.; Lambert, T. H. J. Am. Chem. Soc. 2012, 134, 5552.
(b) Bandar, J. S.; Lambert, T. H. J. Am. Chem. Soc. 2013, 135, 11799.
(8) Seminal work: (a) Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am.
Chem. Soc. 2003, 125, 12672. (b) Li, H. M.; Wang, Y.; Tang, L.; Deng, L.
J. Am. Chem. Soc. 2004, 126, 9906. (c) Vakulya, B.; Varga, S.; Csam
́
pai,
a
Reagents and conditions: (a) 1 mol % 4b (231 mg), 17 mL of
A.; Soos, T. Org. Lett. 2005, 7, 1967. (d) Li, B. J.; Jiang, L.; Liu, M.; Chen,
́
MeNO2 (10 equiv), 21 °C, 21 h, recrystallization (propan-2-ol), 70%;
(b) NiCl2·6H2O, NaBH4, MeOH, 84%; (c) CbzCl, Na2CO3, H2O/
dioxane, 90%; (d) HCl, MeOH, 73%; (e) KMnO4, KOH, KH2PO4,
tBuOH; (f) HCl, MeOH, 57% over 2 steps.
Y. C.; Ding, L. S.; Wu, Y. Synlett 2005, 603. (e) McCooey, S. H.;
Connon, S. J. Angew. Chem., Int. Ed. 2005, 44, 6367. (f) Ye, J. X.; Dixon,
D. J.; Hynes, P. S. Chem. Commun. 2005, 4481. (g) Okino, T.; Hoashi,
Y.; Furukawa, T.; Xu, X. N.; Takemoto, Y. J. Am. Chem. Soc. 2005, 127,
119. (h) Marcelli, T.; van der Haas, R. N. S.; van Maarseveen, J. H.;
́
Hiemstra, H. Angew. Chem., Int. Ed. 2006, 45, 929. (i) Andres, J. M.;
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
Manzano, R.; Pedrosa, R. Chem.Eur. J. 2008, 14, 5116. For reviews
see: (j) Takemoto, Y. Org. Biomol. Chem. 2005, 3, 4299. (k) Connon, S.
J. Chem. Commun. 2008, 2499. (l) Marcelli, T.; Hiemstra, H. Synthesis
2010, 1229.
S
(9) (a) Hamza, A.; Schubert, G.; Soos
2006, 128, 13151. Review: (b) Cheong, P. H.-Y.; Legault, C. Y.; Um, J.
M.; Celebi-Olcum, N.; Houk, K. N. Chem. Rev. 2011, 111, 5042.
́ ́
, T.; Papai, I. J. Am. Chem. Soc.
AUTHOR INFORMATION
Corresponding Author
Darren.dixon@chem.ox.ac.uk
Notes
■
̧
̧
̈
(10) (a) Staudinger, H.; Meyer, J. Helv. Chim. Acta 1919, 2, 635.
Review: (b) Gololobov, Y. G.; Kasukhin, L. F. Tetrahedron 1992, 48,
1353. (c) Johnson, A. W. Ylides and Imines of Phosphorus; Wiley: New
York, 1993.
The authors declare no competing financial interest.
(11) Iminophosphoranes as Lewis bases: Steiner, A.; Zacchini, S.;
Richards, P. I. Coord. Chem. Rev. 2002, 227, 193.
ACKNOWLEDGMENTS
■
(12) Kaljurand, I.; Kutt, A.; Soovali, L.; Rodima, T.; Maemets, V.;
̈
̈
̈
This work was supported by the EPSRC (Leadership Fellowship
to D.J.D., Postdoctoral Fellowship to M.G.N., and Studentship to
A.J.M.F.), the EC (IEF to M.G.N. [PIEF-GA-2009-254637]),
and AstraZeneca (Studentship to A.J.M.F.). We also thank
Alison Hawkins for X-ray structure determination and the
Oxford Chemical Crystallography Service for use of the
instrumentation. X-ray crystallographic data have been deposited
in the Cambridge Crystallographic Data Centre database
(http://www.ccdc.cam.ac.uk/) under accession code 966013-
966014.
Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019.
(13) Further tests demonstrated that the catalysts showed remarkable
stability to water; partitioning ( )-4c between CH2Cl2 and water for 2
min, then drying with MgSO4 and treatment with excess PS-BEMP
afforded essentially pure catalyst in 94% yield.
(14) (a) Westermann, B. Angew. Chem., Int. Ed. 2003, 42, 151.
́ ́
(b) Marques-Lopez, E.; Merino, P.; Tejero, T.; Herrera, R. P. Eur. J. Org.
Chem. 2009, 2401. (c) Noble, A.; Anderson, J. C. Chem. Rev. 2013, 113,
2887.
(15) (a) García Ruano, J. L.; Topp, M.; Lop
́ ́
ez-Cantarero, J.; Aleman, J.;
Remuinan, M. J.; Belen Cid, M. Org. Lett. 2005, 7, 4407. (b) Pahadi, N.
́
́
̃
K.; Ube, H.; Terada, M. Tetrahedron Lett. 2007, 48, 8700. (c) Tan, C.;
Liu, X.; Wang, L.; Wang, J.; Feng, X. Org. Lett. 2008, 10, 5305. (d) Wang,
L. W.; Tan, C.; Liu, X. H.; Feng, X. M. Synlett 2008, 2075. (e) Xie, H.;
Zhang, Y.; Zhang, S.; Chen, X.; Wang, W. Angew. Chem., Int. Ed. 2011,
50, 11773.
(16) Triethylamine is ineffective in promoting the reaction; see ref 15b.
(17) Weinreb, S. M.; Orr, R. K. Synthesis 2005, 1205.
(18) Seminal work: Wittkopp, A.; Schreiner, P. R. Chem.Eur. J. 2003,
9, 407. For reviews see: Taylor, M. S.; Jacobsen, E. N. Angew. Chem., Int.
Ed. 2006, 45, 1520. Doyle, A. G.; Jacobsen, E. N. Chem. Rev. 2007, 107,
5713.
(19) Currently the reaction is limited to nitromethane; analogous
reactions with nitroethane were slow and proceeded with poor
diastereo- and enantioselectivity.
(20) Kotti, S. R. S. S.; Timmons, C.; Li, G. Chem. Biol. Drug Des. 2006,
67, 101.
REFERENCES
■
(1) Trost, B. M.; Fleming, I. Comprehensive Organic Synthesis;
Pergamon Press: Oxford, 1991.
(2) (a) Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2004, 43, 5138.
(b) Berkessel, A.; Groger, H. Asymmetric Organocatalysis: Wiley VCH:
̈
Weinheim, 2005. (c) List, B.; Yang, J. W. Science 2006, 313, 1584.
(d) Dondoni, A.; Massi, A. Angew. Chem., Int. Ed. 2008, 47, 4638.
(e) Bertelsen, S.; Jørgensen, K. A. Chem. Soc. Rev. 2009, 38, 2178.
́
(f) Palomo, C.; Oiarbide, M.; Lopez, R. Chem. Soc. Rev. 2009, 38, 632.
(3) Ishikawa, T. Superbases for Organic Synthesis: Guanidines, Amidines,
Phosphazenes and Related Organocatalysts; Wiley: New York, 2009.
(4) (a) Nugent, B. M.; Yoder, R. A.; Johnston, J. N. J. Am. Chem. Soc.
2004, 126, 3418. (b) Davis, T. A.; Wilt, J. C.; Johnston, J. N. J. Am. Chem.
Soc. 2010, 132, 2880. (c) Davis, T. A.; Johnston, J. N. Chem. Sci. 2011, 2,
1076.
(5) (a) Iyer, M. S.; Gigstad, K. M.; Namdev, N. D.; Lipton, M. J. Am.
Chem. Soc. 1996, 118, 4910. (b) Corey, E. J.; Grogan, M. J. Org. Lett.
1999, 1, 157. (c) Ishikawa, T.; Araki, Y.; Kumamoto, T.; Seki, H.;
Fukuda, K.; Isobe, T. Chem. Commun. 2001, 245. (d) Terada, M.; Ube,
H.; Yaguchi, Y. J. Am. Chem. Soc. 2006, 128, 1454. (e) Sohtome, Y.; Shin,
B.; Horitsugi, N.; Takagi, R.; Noguchi, K.; Nagasawa, K. Angew. Chem.,
Int. Ed. 2010, 49, 7299. (f) Sohtome, Y.; Tanaka, S.; Takada, K.;
Yamaguchi, T.; Nagasawa, K. Angew. Chem., Int. Ed. 2010, 49, 9254.
(g) Misaki, T.; Jin, N.; Kawano, K.; Sugimura, T. Chem. Lett. 2012, 41,
(21) Viso, A.; Fernan
Flores, A. Chem. Rev. 2011, 111, PR1.
́
dez de la Pradilla, R.; Tortosa, M.; García, A.;
16351
dx.doi.org/10.1021/ja409121s | J. Am. Chem. Soc. 2013, 135, 16348−16351