Highly selective catalyst-dependent competitive 1,2-C→C, -O→C, and -N→C migrations from β-methylene-β-silyloxy-β-amido- α-diazoacetates

Article abstract of DOI:10.1021/ja311392m

Transition-metal catalysts direct 1,2-C→C, -O→C, and -N→C migrations from β-methylene-β-silyloxy-β-amido-α- diazoacetates with high selectivity. The key to achieving this unique display of differential selectivities relies on steric and stereoelectronic c

Full text of DOI:10.1021/ja311392m

Communication
pubs.acs.org/JACS
Highly Selective Catalyst-Dependent Competitive 1,2-C→C, -O→C,
and -N→C Migrations from β‑Methylene-β-silyloxy-β-amido-α-
diazoacetates
Xichen Xu,† Yu Qian,† Peter Y. Zavalij, and Michael P. Doyle*
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
S
* Supporting Information
(Scheme 1). Azomethine imines appeared to be promising
candidates since they are stable, easily accessible dipolar
ABSTRACT: Transition-metal catalysts direct 1,2-C→C,
-O→C, and -N→C migrations from β-methylene-β-
silyloxy-β-amido-α-diazoacetates with high selectivity.
The key to achieving this unique display of differential
selectivities relies on steric and stereoelectronic control by
their catalytically generated metal carbenes.
Scheme 1. Strategy for the Synthesis of α-Diazoacetates
Bearing β-Quaternary Carbons and Chemoselectivity of
Competitive 1,2-C→C, -O→C, and -N→C Migrations
1,2-Migration to an electrophilic carbon center is a common
transformation in organic chemistry that has been widely
investigated for its intriguing mechanistic features1 and
extensive occurrence in catalysis.2 Among the reactions that
encompass these 1,2-migrations are the semipinacol rearrange-
ment and its multiple variants3 and solvolysis reactions,1d which
include Wagner−Meerwein rearrangements,4 ring-enlargement
reactions,5 and, more recently, gold-catalyzed migrations.6 With
free rotation around the C−C bond to the electrophilic carbon,
group migration is dependent upon the migratory aptitude as
well as the spatial positioning of the migrating group relative to
the electrophilic center.1a,b,4a,c In general, although highly
selective migrations are most desirable, competition between
two migrating groups is commonly observed.1c,5b,c In these
reactions, the product that is formed is dependent on the
structure of the reactant,4a,b,e and external control of which
group migrates has long been problematic.3,4 Catalytically
generated metal carbenes from α-diazo carbonyl compounds
are highly electrophilic, and 1,2-migration occurs when a
saturated carbon is directly bonded to the carbene center.7
However, examples of catalyst-controlled migrations are rarely
encountered,8 and effective catalyst-controlled selectivity in
these transformations remains unsolved.9
We have been intrigued by catalyst control of reaction
outcomes emanating from the same reactants, and we have
previously presented diverse catalyst-dependent outcomes from
reactions of enol diazoacetates with α,β-unsaturated alde-
hydes10 and nitrones.11 We asked the question “could a
catalytically generated metal carbene intermediate with an
adjacent saturated carbon atom having three different
substituents be induced by different catalysts to undergo
selective 1,2-migration of each of the substituents?” To answer
this question, obtaining facile access to α-diazo compounds
bearing β-quaternary carbons was our first objective. Inspired
by our recent efforts in using enol diazoacetates in Lewis acid-
catalyzed reactions,12 we envisioned that the construction of α-
diazo compounds could be accomplished by [3 + 2]
cycloaddition of dipolar species and an enol diazoacetate
compounds and precursors for the synthesis of dinitrogen-
fused heterocyclic rings,13 which display broad biological
activities.14 As expected, the synthesis of the aforementioned
α-diazoacetates was achieved by diastereoselective [3 + 2]
cycloaddition reactions of azomethine imines 1 with enol
diazoacetate 2 under Lewis acid catalysis. The β-quaternary
carbon is directly bound to carbon, nitrogen, and oxygen
substituents that are poised to rearrange to an adjacent
electrophilic carbon. Herein we report that different metal
catalysts direct the migration by carbon, nitrogen, and oxygen
substituents with a high degree of selectivity to generate a
diverse array of highly functionalized fused-ring heterocyclic
compounds in an efficient and controllable manner.
We initiated our study of the Lewis acid-catalyzed reaction
between azomethine imines 1 (R2 = H) and enol diazoacetate 2
by using Sc(OTf)3 (Table 1, entries 1−7). Highly diaster-
eoselective [3 + 2] cycloaddition occurred with the pendant
phenyl and TBSO groups cis to each other, as confirmed by X-
ray diffraction (XRD) analysis. Further investigations with 5-
phenylazomethine imines 1 (R2 = Ph) revealed that In(OTf)3
afforded a higher level of diastereoselectivity than Sc(OTf)3
(entries 8−13). Azomethine imines containing electron-with-
drawing (entries 3, 9, 11, and 12) and electron-donating
(entries 2, 4, 5, and 10) substituents on the phenyl rings
afforded the desired products in high isolated yields.
Received: November 20, 2012
Published: January 14, 2013
© 2013 American Chemical Society
1244
dx.doi.org/10.1021/ja311392m | J. Am. Chem. Soc. 2013, 135, 1244−1247

Journal of the American Chemical Society
Communication
Table 1. Lewis Acid-Catalyzed Diastereoselecive [3 + 2]
Cycloaddition of Azomethine Imines 1 with Enol
Diazoacetate 2
Table 2. Catalyst Screening with 3a for Selective 1,2-C→C,
-O→C, and -N→C Migrations
time
(h) 4a:5a :6a
yield
f
a
d
e
entry
catalyst
temp
solvent
(%)
a
d
e
entry
R1
R2
3
yield (%)
b
1
Rh2(OAc)4
Rh2(tfa)4
Rh2(pfb)4
Rh2(cap)4
Rh2(OAc)4
Rh2(piv)4
Rh2(esp)2
Cu(OTf)2
Cu(hfacac)2
AgBF4
40 °C
40 °C
40 °C
80 °C
80 °C
40 °C
40 °C
rt
CH2Cl2
CH2Cl2
CH2Cl2
(CH2Cl)2
(CH2Cl)2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
3
3
58:25:17
78:22:−
71:29:−
78:9:13
90
94
91
93
85
82
87
70
88
40
86
b
1
Ph
H
3a
3b
3c
3d
3e
3f
90
74
78
65
69
80
67
81
66
71
78
72
56
b
2
b
2
4-MeC6H4
4-ClC6H4
H
b
3
3
b
3
H
b
4
3
b
4
2-MeOC6H4
4-MeOC6H4
cyclohexyl
(E)-Ph−CHCH
Ph
H
b
5
3
61:25:14
12:39:49
20:39:41
−:−:100
9:−:91
b
5
H
b
6
3
b
6
H
b
7
3
b
7
H
3g
3h
3i
c
8
12
12
12
12
c
8
Ph
Ph
Ph
Ph
Ph
Ph
c
9
rt
c
9
4-NO2C6H4
4-MeOC6H4
4-BrC6H4
c
10
rt
−:−:100
−:−:100
c
10
3j
c
11
CuPF6
rt
c
11
3k
3l
a
b
c
Reactions were performed with 0.1 mmol of 3a. 1 mol % Rh2Ln was
12
3-BrC6H4
c
d
c
used as the catalyst. 5 mol % catalyst was used. 5a was obtained as
13
cyclohexyl
3m
e
the E isomer exclusively. Ratios were determined by 1H NMR analysis
a
Reactions were performed with 0.25 mmol of 1 (1.0 equiv) and 2
(1.8 equiv) in CH2Cl2 for 12 h at room temperature. 5 mol %
Sc(OTf)3 used as the catalyst. 5 mol % In(OTf)3 used as the catalyst.
A single diastereomer was obtained. Isolated yields.
f
of the reaction mixtures. Combined yields of 4a−6a.
b
c
d
e
migration (entries 8−11) compared with the dirhodium
complexes. Thus, catalysts derived from different metals (Rh
and Cu or Ag) direct competitive 1,2-C→C and -N→C
migration pathways with high selectivities, and the 1,2-C→C
migration product 4a or the 1,2-N→C migration product 6a
could be obtained in high yield under catalysis of Rh2(cap)4 or
CuPF6, respectively. The 1,2-C→C and -O→C migrations
appear to be linked, but highly selective catalyst-directed 1,2-
O→C migration could not be achieved with 3a.
Furthermore, substituents at the para (entries 2−3, 5, and 9−
11), meta (entry 12), and ortho (entry 4) positions on the
phenyl rings were well-tolerated. Azomethine imines derived
from cinnamaldehyde (entry 7) and cyclohexylcarboxaldehyde
(entries 6 and 13) showed reactivity profiles similar to those
derived from aromatic aldehydes, and all of the substrates
shown in Table 1 afforded the cycloaddition products with
complete diastereocontrol.
The influence of R1 on the selective 1,2-C→C migration
catalyzed by Rh2(cap)4 and the 1,2-N→C migration catalyzed
by CuPF6 was investigated next (Table 3). Reactions performed
Having prepared highly functionalized α-diazoacetates 3
bearing quaternary carbons at the β-positions, we next
investigated their transition-metal-catalyzed dinitrogen extru-
sion and subsequent rearrangement of the catalytically
generated metal carbene intermediate (Table 2). In the
reaction of 3a with Rh2(OAc)4, three products were obtained
in 90% combined yield (eq 2), and these were determined by
XRD and/or spectroscopic analysis to be those from 1,2-C→C
migration (4a, 58%), stereoselective 1,2-O→C migration (5a,
25%), and 1,2-N→C migration (6a, 17%). Using the more
Lewis acidic Rh2(tfa)4 (entry 2) and Rh2(pfb)4 (entry 3) under
the same conditions showed that the 1,2-N→C migration
pathway was inhibited in favor of the 1,2-C→C and -O→C
migrations with approximately 3:1 selectivity. In contrast,
performing this reaction with the less Lewis acidic Rh2(cap)4 in
ClCH2CH2Cl at 80 °C revealed that both the 1,2-N→C and
-O→C migration pathways were suppressed and 1,2-C→C
migration was dominant (entry 4). In a control experiment
where the same reaction was conducted with Rh2(OAc)4 under
conditions otherwise identical to those with Rh2(cap)4 (entry
5), the selectivity was comparable to that for the reaction
performed in CH2Cl2 at 40 °C (entry 1) and much lower than
that with Rh2(cap)4 (entry 4). The use of Rh2(piv)4 and
Rh2(esp)2 containing sterically bulky ligands gave high
selectivities for both 1,2-N→C and -O→C migrations (entries
6 and 7). In contrast, copper and silver catalysts displayed
distinctive and virtually exclusive selectivity for 1,2-N→C
Table 3. Selective 1,2-C→C and -N→C Migrations of 3
Catalyzed by Rh2(cap)4 and CuPF6, Respectively
b
e
Rh2(cap)4
CuPF6
yield
d
yield
f
a
c
3
R1
4
4:5
(%)
6
(%)
3a
3b
3c
3d
3e
3f
Ph
4a
4b
4c
4d
4e
4f
85:15
89:11
88:12
81:19
88:12
87:13
86:14
85
81
83
79
86
88
85
6a
6b
6c
6d
6e
6f
86
85
80
86
78
77
84
4-MeC6H4
4-ClC6H4
2-MeOC6H4
4-MeOC6H4
cyclohexyl
3g
(E)-Ph−CHCH
4g
6g
a
b
Reactions were performed with 0.1 mmol of 3. Reactions were
performed with 1 mol % Rh2(cap)4 in ClCH2CH2Cl at 80 °C for 3 h.
c
Determined by 1H NMR analysis of the crude reaction mixtures.
d
e
Combined yields of 4 and 5. Reactions were performed with 5 mol
f
% CuPF6 in CH2Cl2 for 12 h at room temperature. Isolated yields of
6.
1245
dx.doi.org/10.1021/ja311392m | J. Am. Chem. Soc. 2013, 135, 1244−1247

Journal of the American Chemical Society
Communication
with CuPF6 as the catalyst uniformly afforded the 1,2-N→C
migration product, and Rh2(cap)4 was found to promote the
1,2-C→C migration process with high selectivities but without
a significant dependence on R1.
Table 5. Substrate Scope for Ligand-Induced Divergent 1,2-
C→C and -O→C Migrations
In an effort to achieve 1,2-C→C, 1,2-O→C, and 1,2-N→C
migrations selectively with different catalysts, 3h, which has an
additional phenyl group on the pyrazolidinone ring, was
subjected to the same series of catalytic reactions (Table 4).
b
c
Rh2(cap)4
Rh2(esp)2
yield
e
yield
e
Table 4. Catalyst Screening with 3h for Selective 1,2-C→C,
-O→C, and -N→C Migrations
a
d
f
d
3
R1
4
4:5
(%)
5
4:5
(%)
3h
3i
Ph
4h
4i
91:9
97:3
96:4
93:7
88:12
75:25
91
90
92
93
91
86
5h
5i
15:85
16:84
17:83
10:90
14:86
15:85
88
86
85
85
82
92
4-NO2C6H4
4-MeOC6H4
4-BrC6H4
3j
4j
5j
3k
3l
4k
4l
5k
5l
3-BrC6H4
3m
cyclohexyl
4m
5m
a
d
e
f
a
entry
catalyst
solvent
time (h) 4h:5h :6h
yield (%)
Reactions were performed with 0.1 mmol of 3 with 1 mol % Rh2Ln as
b
b
1
Rh2(OAc)4
Rh2(Oct)4
Rh2(OBz)4
Rh2(piv)4
Rh2(esp)2
Rh2(cap)4
Rh2(tfa)4
Rh2(pfb)4
CuPF6
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
(CH2Cl)2
CH2Cl2
CH2Cl2
CH2Cl2
3
3
60:40:−
43:57:−
25:75:−
12:88:−
15:85:−
91:9:−
91
88
82
74
88
86
81
79
72
the catalyst. Reactions were performed in ClCH2CH2Cl at 80 °C for
c
b
1 h. Reactions were performed in CH2Cl2 at room temperature for 3
2
d
h. Determined by 1H NMR analysis of the reaction mixtures.
b
3
3
e
f
b
Combined yields of 4 and 5. Obtained as the E isomer exclusively.
4
3
b
5
3
b
employed. This heightened selectivity for 6 may be due to
coordination of the copper carbene intermediate16 with the
carbonyl oxygen of the pyrazolidinone ring (7 in Scheme 2), a
6
1
b
7
3
69:31:−
64:36:−
b
8
3
c
9
12
−:−:100
a
b
Scheme 2. Directing Effect in Cu-Catalyzed 1,2-N→C
Reactions were performed with 0.1 mmol of 3h. Reaction was
performed with 1 mol % Rh2Ln as the catalyst. Reaction was
c
Migration
d
performed with 5.0 mol % CuPF6 as the catalyst. 5h was obtained as
e
the E isomer exclusively. Ratios were determined by 1H NMR analysis
f
of the crude reaction mixtures. Combined yields of 4h−6h.
With 1 mol % Rh2(OAc)4, the formation of the 1,2-C→C
migration product 4h and the stereoselective 1,2-O→C
migration product 5h occurred without evidence for the
formation of the 1,2-N→C migration product 6h (entry 1).
Once again, Rh2(cap)4 selected the 1,2-C→C migration
pathway (entry 6), and CuPF6 effected exclusive 1,2-N→C
migration (entry 9). Consistent with entries 6 and 7 but more
dramatic, increasing the steric size of the ligand by using
Rh2(piv)4 (entry 4) significantly enhanced the reaction
selectivity toward the formation of the 1,2-O→C migration
product 5h, and this outcome was mirrored by the use of
Rh2(esp)2 (entry 5) in higher overall yield. Hence, with a
simple change in the ligand attached to the dirhodium centers,
the competing 1,2-C→C or 1,2-O→C migration could be made
dominant through catalysis by Rh2(esp)2 [or Rh2(piv)4] or
Rh2(cap)4, respectively; the 1,2-N→C migration product 6h,
which was not detected in reactions catalyzed by dirhodium
complexes, was the sole migration outcome under CuPF6
catalysis. In addition, as shown in Table 5, these reactions
exhibited the same selectivities irrespective of the electronic
nature of the substituents on the phenyl ring of R1 and even
when cyclohexyl was used in place of an aryl group.
complexation that would not be expected with the
coordinatively saturated dirhodium complexes.17 Consequently,
the overall 1,2-N→C migration is facilitated by transition-metal
catalysts with open coordination sites.
In summary, we have discovered catalyst-controlled highly
selective 1,2-C→C, -O→C, and -N→C migrations of β-
methylene-β-silyloxy-β-amido-α-diazoacetates. These rear-
rangement reactions produce a variety of highly functionalized
dinitrogen-fused heterocyclic compounds. Further studies of
these intriguing migrations are underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, optimization of reaction conditions,
characterization data, and crystallographic data for 3a, 4h, 5k,
and 6c (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
The 1,2-C→C migration (to form 4) and the 1,2-O→C
migration (to form 5) are linked in dirhodium-catalyzed
reactions, and the causes for selectivity may be attributed to
steric [e.g., results with Rh2(piv)4 and Rh2(esp)2] and
electronic [results with Rh2(cap)4] factors. However, the
unexpected 1,2-N→C migration from 3 to form 6,15 which is
formally an amide nitrogen migration, stands out as exception-
ally favorable when copper rather than dirhodium catalysts are
AUTHOR INFORMATION
Corresponding Author
mdoyle3@umd.edu
■
Author Contributions
†X.X. and Y.Q. contributed equally.
Notes
The authors declare no competing financial interest.
1246
dx.doi.org/10.1021/ja311392m | J. Am. Chem. Soc. 2013, 135, 1244−1247

Journal of the American Chemical Society
Communication
(d) Suga, H.; Funyu, A.; Kakehi, A. Org. Lett. 2007, 9, 97. (e) Shintani,
R.; Murakami, M.; Hayashi, T. J. Am. Chem. Soc. 2007, 129, 12356.
(f) Chan, A.; Scheidt, K. A. J. Am. Chem. Soc. 2007, 129, 5334.
(g) Chen, W.; Du, W.; Duan, Y.; Wu, Y.; Yang, S.; Chen, Y. Angew.
Chem., Int. Ed. 2007, 46, 7667. (h) Sibi, M. P.; Rane, D.; Stanley, L.
M.; Soeta, T. Org. Lett. 2008, 10, 2971. (i) Shapiro, N. D.; Shi, Y.;
Toste, F. D. J. Am. Chem. Soc. 2009, 131, 11654. (j) Perreault, C.;
Goudreau, S. R.; Zimmer, L. E.; Charette, A. B. Org. Lett. 2008, 10,
689. (k) Hashimoto, T.; Maeda, Y.; Omote, M.; Nakatsu, H.;
Maruoka, K. J. Am. Chem. Soc. 2010, 132, 4076.
(14) (a) Varvounis, G.; Fiamegos, Y.; Pilidis, G. Adv. Heterocycl.
Chem. 2001, 80, 75. (b) Eicher, T.; Hauptmann, S. The Chemistry of
Heterocycles, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2003.
(c) Konaklieva, M. I.; Plotkin, B. J. Curr. Med. Chem.: Anti-Infect.
Agents 2003, 2, 287.
(15) This 1,2-migration may be stepwise (e.g., cleavage of the C−
Namide bond followed by reattachment at carbene carbon) rather than
concerted.
(16) Recent review of copper carbene complexes: Kirmse, W. Angew.
Chem., Int. Ed. 2003, 42, 1088.
(17) (a) Drago, R. S.; Long, J. R.; Cosmano, R. Inorg. Chem. 1981,
20, 2920. (b) Drago, R. S. Inorg. Chem. 1982, 21, 1697. (c) Drago, R.
S.; Long, J. R.; Cosmano, R. Inorg. Chem. 1982, 21, 2196. (d) Doyle,
M. P.; Mahapatro, S. N.; Caughey, A. C.; Chinn, M. S.; Colsman, M.
R.; Harn, N. K.; Redwine, A. E. Inorg. Chem. 1987, 26, 3070.
ACKNOWLEDGMENTS
■
Support for this research from the National Institutes of Health
(GM 46503) and the National Science Foundation (CHE-
0748121 and CHE-1212446) is gratefully acknowledged.
REFERENCES
■
(1) (a) Nickon, A. Acc. Chem. Res. 1993, 26, 84. (b) Liu, M. T. H.
Acc. Chem. Res. 1994, 27, 287. (c) Farlow, R. A.; Thamattoor, D. M. J.
Org. Chem. 2002, 67, 3257. (d) Okuyama, T. Acc. Chem. Res. 2002, 35,
12.
(2) Recent examples of 1,2-migrations in catalytic reactions:
(a) Sromek, A. W.; Kel’in, A. V.; Gevorgyan, V. Angew. Chem., Int.
Ed. 2004, 43, 2280. (b) Kirsch, S. F.; Binder, J. T.; Liebert, C.; Menz,
H. Angew. Chem., Int. Ed. 2006, 45, 5878. (c) Dudnik, A. S.;
Gevorgyan, V. Angew. Chem., Int. Ed. 2007, 46, 5195. (d) Dudnik, A.
S.; Sromek, A. W.; Rubina, M.; Kim, J. T.; Kel’in, A. V.; Gevorgyan, V.
J. Am. Chem. Soc. 2008, 130, 1440. (e) Dudnik, A. S.; Xia, Y.; Li, Y.;
Gevorgyan, V. J. Am. Chem. Soc. 2010, 132, 7645.
(3) (a) Snape, T. J. Chem. Soc. Rev. 2007, 36, 1823. (b) Wang, B.; Tu,
Y. Acc. Chem. Res. 2011, 44, 1207. (c) Song, Z.; Fan, C.; Tu, Y. Chem.
Rev. 2011, 111, 7523.
(4) Recent examples of Wagner−Meerwein rearrangements:
(a) Trost, B. M.; Yasukata, T. J. Am. Chem. Soc. 2001, 123, 7162.
(b) Langer, P.; Bose, G. Angew. Chem., Int. Ed. 2003, 42, 4033.
(c) Trost, B. M.; Xie, J. J. Am. Chem. Soc. 2008, 130, 6231.
(d) Leboeuf, D.; Huang, J.; Gandon, V.; Frontier, A. J. Angew. Chem.,
Int. Ed. 2011, 50, 10981. (e) Leboeuf, D.; Gandon, V.; Ciesielski, J.;
Frontier, A. J. J. Am. Chem. Soc. 2012, 134, 6296.
(5) (a) House, H. O.; Grubbs, E. J.; Gannon, W. F. J. Am. Chem. Soc.
1960, 82, 4099. (b) Namyslo, J. C.; Kaufmann, D. E. Chem. Rev. 2003,
103, 1485. (c) Leemans, E.; Matthias, D.; Kimpe, N. D. Chem. Rev.
2011, 111, 3268. (d) Hashimoto, T.; Naganawa, Y.; Maruoka, K. J. Am.
Chem. Soc. 2011, 133, 8834. (e) Li, W.; Liu, X.; Hao, X.; Cai, Y.; Lin,
L.; Feng, X. Angew. Chem., Int. Ed. 2012, 51, 8644.
(6) (a) Marion, N.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46,
2750. (b) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2008, 47, 6754.
(c) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239. (d) Arcadi,
A. Chem. Rev. 2008, 108, 3266. (e) Gorin, D. J.; Sherry, B. D.; Toste,
F. D. Chem. Rev. 2008, 108, 3351. (f) Rudolph, M.; Hashmi, A. S. K.
Chem. Commun. 2011, 47, 6536. (g) Lu, B.; Dai, L.; Shi, M. Chem. Soc.
Rev. 2012, 41, 3318. (h) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2012,
49, 5232.
(7) (a) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic
Methods for Organic Synthesis with Diazo Compounds; Wiley: New
York, 1998. (b) Ye, T.; McKervey, M. A. Chem. Rev. 1994, 94, 1901.
(8) One recent example of catalyst-controlled 1,2-migration: Vitale,
M.; Lecourt, T.; Sheldon, C. G.; Aggarwal, V. K. J. Am. Chem. Soc.
2006, 128, 2524.
(9) (a) Aggarwal, V. K.; Sheldon, C. G.; MacDonald, G. J.; Martin,
W. P. J. Am. Chem. Soc. 2002, 124, 10300. (b) Jiang, N.; Ma, Z.; Qu,
Z.; Xing, X.; Xie, L.; Wang, J. J. Org. Chem. 2003, 68, 893. (c) Shi, W.;
Jiang, N.; Zhang, S.; Wu, W.; Du, D.; Wang, J. Org. Lett. 2003, 5, 2243.
(d) Shi, W.; Xiao, F.; Wang, J. J. Org. Chem. 2005, 70, 4318. (e) Xu, F.;
Shi, W.; Wang, J. J. Org. Chem. 2005, 70, 4191.
(10) Xu, X.; Hu, W.; Zavalij, P. Y.; Doyle, M. P. Angew. Chem., Int.
Ed. 2011, 50, 11152.
(11) (a) Wang, X.; Xu, X.; Zavalij, P. Y.; Doyle, M. P. J. Am. Chem.
Soc. 2011, 133, 16402. (b) Qian, Y.; Xu, X.; Wang, X.; Zavalij, P. J.;
Hu, W.; Doyle, M. P. Angew. Chem., Int. Ed. 2012, 51, 5900. (c) Wang,
X.; Abrahams, Q. M.; Zavalij, P. Y.; Doyle, M. P. Angew. Chem., Int. Ed.
2012, 51, 5907.
(12) (a) Xu, X.; Ratnikov, M. O.; Zavalij, P. Y.; Doyle, M. P. Org.
Lett. 2011, 13, 6122. (b) Xu, X.; Hu, W.-H.; Doyle, M. P. Angew.
Chem., Int. Ed. 2011, 50, 6392. (c) Jaber, D.; Burgin, R. N.; Helper, P.;
Zavalij, P.; Doyle, M. P. Org. Lett. 2012, 14, 1676.
(13) (a) Shintani, R.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 10778.
(b) Suar
́
ez, A.; Downey, C. W.; Fu, G. C. J. Am. Chem. Soc. 2005, 127,
11244. (c) Shintani, R.; Hayashi, T. J. Am. Chem. Soc. 2006, 128, 6330.
1247
dx.doi.org/10.1021/ja311392m | J. Am. Chem. Soc. 2013, 135, 1244−1247