Synthesis and structure of hypervalent diacetoxybromobenzene and aziridination of olefins with imino-λ3-bromane generated in situ under metal-free conditions

Article abstract of DOI:10.1021/ol201868n

Ligand exchange of p-CF₃C₆H₄BrF ₂ with acetoxy groups using AcOH and Ac₂O affords (diacetoxybromo)benzene in a high yield, which undergoes aziridination of alkenes in the presence of TfNH₂

Full text of DOI:10.1021/ol201868n

ORGANIC
LETTERS
2011
Vol. 13, No. 20
5428–5431
Synthesis and Structure of Hypervalent
Diacetoxybromobenzene and
Aziridination of Olefins with
Imino-λ3-bromane Generated in Situ
under Metal-Free Conditions
Md. Mahbubul Hoque,† Kazunori Miyamoto,† Norihiro Tada,† Motoo Shiro,‡ and
Masahito Ochiai*,†
Graduate School of Pharmaceutical Sciences, University of Tokushima, 1-78 Shomachi,
Tokushima 770-8505, Japan, and Rigaku Corporation, 3-9-12 Matsubara, Akishima,
Tokyo 196-8666, Japan
mochiai@ph.tokushima-u.ac.jp
Received July 12, 2011
ABSTRACT
Ligand exchange of p-CF3C6H4BrF2 with acetoxy groups using AcOH and Ac2O affords (diacetoxybromo)benzene in a high yield, which undergoes
aziridination of alkenes in the presence of TfNH2 and sulfamate esters in one pot under mild conditions. The aziridination with TfNH2 proceeds
stereospecifically with retention of stereochemistry of olefins at room temperature using limiting amounts of olefins under transition-metal-free
conditions. The one-pot aziridination procedure using sulfamate esters can be applied to the intramolecular versions.
(Diacetoxyiodo)benzene PhI(OAc)2 is among the most
popular and valuable oxidizing agents in a hypervalent
organo-λ3-iodane family.1 The commercially available
diacetoxy-λ3-iodane has enjoyed widespread application
as a powerful oxidant because of its environmentally
friendly nature and oxidizes a wide range of function-
alities including alkenes, alkynes, alcohols, phenols,
amines, sulfides, and carbonyl compounds under mild
conditions. On the other hand, the chemistry of the
nearest neighbors, (diacetoxybromo)arenes, remains
to be established.2 We report herein, for the first time,
the synthesis and characterization of diacetoxy(aryl)-
λ3-bromane (1) (Scheme 1). Diacetoxy-λ3-bromane (1)
undergoes aziridination of alkenes in the presence of
trifluoromethanesulfonamide (TfNH2). The aziridina-
tion proceeds stereospecifically at room temperature
using limiting amounts of alkenes under transition
metal-free conditions and probably involves in situ
generation of N-triflylimino-λ3-bromane (2) as an active
nitrenoid species.
Scheme 1
† Tokushima University.
‡ Rigaku Corporation.
(1) (a) Wirth, T., Ed. Hypervalent Iodine Chemistry. in Topics in Current
Chemistry; Springer: Berlin, 2003; Vol. 224. (b) Zhdankin, V. V.; Stang, P. J.
Chem. Rev. 2008, 108, 5299. (c) Tohma, H.; Kita, Y. Adv. Synth. Catal. 2004,
346, 111. (d) Ochiai, M. In Chemistry of Hypervalent Compounds; Akiba,
K.-y., Ed.; Wiley-VCH: New York, 1999; Chapter 12. (e) Varvoglis, A. The
Organic Chemistry of Polycoordinated Iodine; VCH: Weinheim, 1992.
(2) For the synthesis of bis(trifluoroacetoxy)(pentafluorophenyl)-λ3-
bromane, see: Frohn, H. J.; Giesen, M. J. Fluorine Chem. 1984, 24, 9.
Double ligand exchange of fluorine atoms of Frohn
reagent p-trifluoromethylphenyl(difluoro)-λ3-bromane3
with acetoxy groups takes place rapidly at room
r
10.1021/ol201868n
Published on Web 09/27/2011
2011 American Chemical Society

temperature: thus, mixing the difluoro-λ3-bromane with
2.2 equiv each of acetic acid and acetic anhydride in a
Teflon PFA vessel for 1 min under argon produced, after
removal of volatile materials liberated in vacuo, crystalline
p-trifluoromethyl(diacetoxybromo)benzene (1) in 90%
yield. Acetic anhydride probably traps hydrogen fluoride
generated in situ during the ligand exchange reaction, via
the formation of acetyl fluoride.4 Use of trimethylsilyl
acetate, instead of acetic anhydride, also afforded diace-
toxy-λ3-bromane 1 (83%). Diacetoxybromane 1 is rather
air sensitive but can be kept at room temperature under
argon in a PFA tube for more than 2 months without any
decomposition. The 1H NMR spectrum in CDCl3 showed
a set of characteristic signals assigned to the disubstituted
phenyl group [δ 8.07 (o) and 7.81 (m) ppm] (see Figure
S1(A), Supporting Information). The 13C resonance of the
ipso carbon atom appeared at δ 142.0 ppm. The polarized
hypervalent Oδꢀ Brδþ Oδꢀ bond tends to weaken the
two additional intramolecular close contacts around the
bromane(III) atom, which is reminiscent of the structure of
(diacetoxyiodo)benzene.6,7 The rms deviation of the six
atoms (Br1, C1, and O1ꢀO4) from the least-squares plane
˚
is 0.282(1) A, and the sum of the bromine centered bond
angles is Σ°Br = 365.08°. The plane of the benzene ring
makes a dihedral angle of 74.89° with this plane. The
Br1 O2 and Br1 O4 distances are considerably long-
3 3 3
3 3 3
˚
er than the computed covalent single bond length of 1.80 A
but definitively shorter than the sums of the van der Waals
8
˚
˚
radii of Br (1.85 A) and O (1.52 A). Cooperative donation
of the two lone pairs of electrons of the carbonyl oxygen
atoms to the vacant Br1ꢀC1 σ* orbital as shown in three-
center bonding model 1a, originally proposed by Alcock,6
will be responsible for these secondary Br O contacts.
3 3 3
The C1ꢀBr1ꢀO1 and C1ꢀBr1ꢀO3 bond angles are con-
siderably greater than those (81.4° and 82.6°) for PhI-
(OAc)2,6 probably because of the increased nonbonded
repulsionsbetween thesubstituentson bromane(III) with a
decreased atomic size.
3 3 3
3 3 3
CdO double bond, and hence, bromane 1 shows a lower IR
absorption by ca. 100 cmꢀ1 at 1684 cmꢀ1 compared to
acetyl hypobromite (Figure S1(C), Supporting Information).5
Aryl(sulfonylimino)-λ3-iodanes serve as excellent nitre-
noid progenitors in the aziridination of alkenes and in the
CꢀH amidation of alkanes in the presence of transition-
metal catalysts, where use of a metal catalyst (Cu, Rh, Mn,
Ru, or Ag) is crucial to the success of these nitrenoid
transfers to generate active metal-imido species.9,10 Re-
cently, we reported the synthesis of N-triflylimino-λ3-bro-
mane 2, which involves a facile ligand exchange of Frohn
reagent with triflylamide in acetonitrile.11 Triflylimino-λ3-
bromane 2 directly transfers the imino group to olefins and
heteroatom nucleophiles involving nitrogen heterocycles,
sulfur compounds, and iodoarenes at room temperature
undermetal-freeconditions.11,12 Highlyregioselectiveami-
nation of unactivated aliphatic CꢀH bonds also proceeds
smoothly by just mixing 2 with alkane without thermal
activation.13,14 These results are in marked contrast to the
reactions of sulfonylimino-λ3-iodanes, which requires
Figure 1. ORTEP drawing of diacetoxy-λ3-bromane 1 with
thermal ellipsoids at 50% probability and three-center second-
˚
ary bonding model 1a. Selected bond lengths (A) and angles
(8) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(9) For reviews of aryl(sulfonylimino)-λ3-iodanes, see: (a) Collet, F.;
Dodd, R. H.; Dauban, P. Chem. Commun. 2009, 5061. (b) Diaz-Requejo,
M. M.; Perez, P. J. Chem. Rev. 2008, 108, 3379. (c) Halfen, J. A. Curr.
Org. Chem. 2005, 9, 657. (d) Du Bois, J. Chemtracts: Org. Chem. 2005,
18, 1. (e) Dauban, P.; Dodd, R. H. Synlett 2003, 1571. (f) Muller, P.;
Fruit, C. Chem. Rev. 2003, 103, 2905.
(deg): Br1ꢀC1 1.914(5), Br1ꢀO1 2.009(4), Br1ꢀO3 2.064(3),
Br1 O2 2.796(2), Br1 O4 2.785(4), C1ꢀBr1ꢀO1 85.56(16),
3 3 3
3 3 3
C1ꢀBr1ꢀO3 84.85(16), O1ꢀBr1ꢀO3 170.31(14).
(10) For aziridination of olefins with sulfonylimino-λ3-iodanes, see:
(a) Evans, D. A.; Faul, M. M.; Bilodeau, M. T. J. Am. Chem. Soc. 1994,
116, 2742. (b) Li, Z.; Quan, R. W.; Jacobsen, E. N. J. Am. Chem. Soc.
1995, 117, 5889. (c) Au, S.-M.; Huang, J.-S.; Yu, W.-Y.; Fung, W.-H.;
Che, C.-M. J. Am. Chem. Soc. 1999, 121, 9120. (d) Guthikonda, K.; Du
Bois, J. J. Am. Chem. Soc. 2002, 124, 13672. (e) Cui, Y.; He, C. J. Am.
Chem. Soc. 2003, 125, 16202. (f) Llaveria, J.; Beltran, A.; Diaz-Requejo,
M. M.; Matheu, M. I.; Castillon, S.; Perez, P. J. Angew. Chem., Int. Ed.
2010, 49, 7092. (g) Moriarty, R. M.; Tyagi, S. Org. Lett. 2010, 12, 364.
(11) Ochiai, M.; Kaneaki, T.; Tada, N.; Miyamoto, K.; Chuman, H.;
Shiro, M.; Hayashi, S.; Nakanishi, W. J. Am. Chem. Soc. 2007, 129,
12938.
(12) (a) Ochiai, M.; Kawano, Y.; Kaneaki, T.; Tada, N.; Miyamoto,
K. Org. Lett. 2009, 11, 281. (b) Ochiai, M.; Naito, M.; Miyamoto, K.;
Hayashi, S.; Nakanishi, W. Chem.;Eur. J. 2010, 16, 8713. (c) Ochiai,
M.; Nakano, A.; Yoshimura, A.; Miyamoto, K.; Hayashi, S.; Nakanishi,
W. J. Chem. Soc., Chem. Commun. 2009, 959.
Recrystallization from pentaneꢀdichloromethane at
ꢀ78 °C afforded single crystals (colorless prisms) that were
suitable for X-ray crystallography (Figure 1). Diacetoxy-
λ3-bromane 1 essentially exhibits a T-shaped geometry
with the two acetoxy groups occupying apical positions
and is regarded as pentagonal planar arrangement with
(3) (a) Frohn, H. J.; Giesen, M. J. Fluorine Chem. 1998, 89, 59.
(b) Ochiai, M.; Nishi, Y.; Goto, S.; Shiro, M.; Frohn, H. J. J. Am. Chem.
Soc. 2003, 125, 15304. (c) Ochiai, M.; Yoshimura, A.; Mori, T.; Nishi,
Y.; Hirobe, M. J. Am. Chem. Soc. 2008, 130, 3742.
(4) Olah, G. A.; Kuhn, S. J. J. Org. Chem. 1961, 26, 237.
(5) Reilly, J. J.; Duncan, D. J.; Wunz, T. P.; Patsiga, R. A. J. Org.
Chem. 1974, 39, 3291.
(13) Ochiai, M.; Miyamoto, K.; Kaneaki, T.; Hayashi, S.; Nakanishi,
(6) Alcock, N. W.; Countryman, R. M.; Esperas, S.; Sawyer, J. F.
J. Chem. Soc., Dalton Trans. 1979, 854.
(7) Ochiai, M.; Suefuji, T.; Miyamoto, K.; Shiro, M. Chem. Commun.
2003, 1438.
W. Science 2011, 332, 448.
(14) For a review of triflylimino-λ3-bromane 2, see: Ochiai, M.;
Miyamoto, K.; Hayashi, S.; Nakanishi, W. Chem. Commun. 2010, 46,
511.
Org. Lett., Vol. 13, No. 20, 2011
5429

activation by a metal catalyst, and indicate that the λ3-
bromane 2 itself functions as a reactive organo nitrenoid at
room temperature without any transition metal activation.
N-Triflylimino-λ3-bromane 2, however, is thermally labile
and rather sensitive to atmospheric moisture, and should
be kept in a refrigerator under argon. In order to avoid
some difficulties associated with the long period of storage
and to enhance the synthetic value of these nitrenoid
transfer reactions, development of the methods for the in
situ generation of imino-λ3-bromane 2 seems to be highly
desirable.15
Table 1. One-Pot Aziridination of Olefins with in Situ Gener-
ated Imino-λ3-bromane 2 Using TfNH2
a
temp
time product
(h) yieldc (%)
entry
olefin
methodb
(°C)
Initial attempts for one-pot aziridination of (Z)-4-
octene using a combination of Frohn reagent and
triflylamide were found to be fruitless, because of the
modest yields of N-triflylaziridine 3g and of the lack of
reproducibility (Scheme S1, Supporting Information):
thus, yields of N-triflylaziridine 3g changed irregularly
from 11% to 53%. Reliable and efficient aziridination
method of olefins using triflylamide was developed by
changing Frohn reagent to diacetoxy(aryl)-λ3-bromane
1 under mild conditions without using a metal catalyst.
Representative results are summarized in Table 1. Pre-
mixing of each 1.2 equiv of diacetoxy-λ3-bromane 1 and
triflylamide in dichloromethane at 0 °C for 10 min
and the subsequent addition of (Z)-4-octene (method A)
produced a high yield (84%) of cis-aziridine 3g stereo-
selectively (Table 1, entry 9). Initial formation of
N-triflylimino-λ3-bromane 2 under the conditions
was suggested by its isolation in a high yield in a
separate experiment (Scheme S2, Supporting Inform-
ation). Very interestingly, addition of diacetoxy-λ3-
bromane 1 to a solution of triflylamide and (Z)-4-
octene in dichloromethane (method B) also gave rise
to cis-3g in a high yield (entry 10). This result indicates
that premixing of diacetoxybromane 1 with triflyla-
mide, i.e., preformation of imino-λ3-bromane 2, is
not essential for our one-pot aziridination of olefin
and that the rate of reaction of λ3-bromane 1 with
triflylamide will be considerably greater than that with
(Z)-4-octene.16
1
2
3
4
5
6
7
cyclohexene
cyclohexene
cycloheptene
cis-cyclooctene
cis-cyclooctene
norbornene
A
B
A
A
B
A
A
A
A
B
A
A
A
A
A
A
A
A
0
0
3
3
2
1
1
4
1
7
3
3
6
1
8
3a 72 (80)
3a 70 (78)
3b 88 (98)
3c 84 (96)
3c 80 (92)
3d 66 (73)
3e 3 (89)
3f 73 (75)
3g 84 (97)
3g 87 (98)
3h 73 (80)
3i 84 (91)
3j -e
0
0
0
0
Me2CdCMe2
8d n-C8H17CHdCH2
0
25
0
9
Z-PrCHdCHPr
10 Z-PrCHdCHPr
11d E-PrCHdCHPr
12 Z-PhCHdCHMe
13 E-PhCHdCHMe
14 Z-PhCHdCHPh
15 E-PhCHdCHPh
16 PhCHdCH2
0
25
0
25
0
3.5 3k 79 (78)
2.5 3l -
25
0
9
24
30
3m 78 (86)
3n 80
17 p-ClC6H4CHdCH2
18 p-CF3C6H4CHdCH2
0
0
3o 66 (70)
a Conditions: 1:1.2:1.2 olefin/TfNH2/diacetoxy-λ3-bromane 1, di-
chloromethane, Ar. b For details of methods A and B, see text and
Supporting Information. c Isolated yields. Numbers in parentheses
are 1H NMR yields. d 2 equiv of bromane 1 and TfNH2 were used.
e 1-Acetoxy-2-(triflylamino)-1-phenylpropane (syn/anti = 71:29) was
isolated in 45% yield.
dines 3 were produced using limiting amounts of starting
olefins, in contrast to most of the reported reactions
with preformed imino-λ3-iodanes, which rely on excess
amounts of substrate (3 or more equivalents) to effect high
product conversion.10 (2) The aziridination wasexclusively
stereospecific with retention of stereochemistry of both
Z- and E-olefins, which is a relatively rare example of
stereochemical retention in the aziridinations.9c (3) The
rates of aziridination of cis-1,2-disubstituted olefins
were larger compared to those of trans isomers. (4) The
reaction proceeded smoothly at 0 °C or at ambient tem-
perature under transition metal-free conditions. (5) No
formation of any allylic CꢀH amidation side product was
detected.10a,17
The results shown in Table 1 are comparable to those
obtained by the use of triflylimino-λ3-bromane 2 with
keeping the following scenario:11 (1) High yields of aziri-
(15) For the pioneering studies on the in situ generation method of
imino-λ3-iodanes in the metal-catalyzed aziridination and CꢀH amina-
tion, see: (a) Yu, X.-Q.; Huang, J.-S.; Zhou, X.-G.; Che, C.-M. Org. Lett.
2000, 2, 2233. (b) Espino, C. G.; Du Bois, J. Angew. Chem., Int. Ed. 2001,
40, 598. (c) Dauban, P.; Saniere, L.; Tarrade, A.; Dodd, R. H. J. Am.
Chem. Soc. 2001, 123, 7707.
(16) Oxidations of olefins with aryl-λ3-iodanes have been studied
extensively.1 For the reaction of olefins with trifluoro-λ3-bromane BrF3,
see: (a) Hagooly, A.; Rozen, S. Chem. Commun. 2004, 594. (b) Rozen, S.
Acc. Chem. Res. 2005, 38, 803.
Reductive cleavage of triflylamides, yielding parent
amines, seems to be a difficult process;18 however, use of
Red-Al under mild reaction conditions was found to be an
(17) For transition-metal-catalyzed allylic CꢀH amidations using
imino-λ3-iodanes, see: (a) Liang, C.; Collet, F.; Robert-Peillard, F.;
Muller, P.; Dodd, R. H.; Dauban, P. J. Am. Chem. Soc. 2008, 130, 343.
(b) Xu, Q.; Appella, D. H. Org. Lett. 2008, 10, 1497. (c) Guthikonda, K.;
Wehn, P. M.; Caliando, B. J.; Du Bois, J. Tetrahedron 2006, 62,
11331. (d) Caselli, A.; Gallo, E.; Ragaini, F.; Oppezzo, A.; Cenini, S.
J. Organomet. Chem. 2005, 690, 2142. (e) Kohmura, Y.; Katsuki, T.
Tetrahedron Lett. 2001, 42, 3339. (f) Au, S.-M.; Zhang, S.-B.; Fung,
W.-H.; Yu, W.-Y.; Che, C.-M.; Cheung, K.-K. Chem. Commun. 1998,
2677. (g) Mahy, J. P.; Bedi, G.; Battioni, P.; Mansuy, D. Tetrahedron
Lett. 1988, 29, 1927.
(18) (a) Malek, J. Org. React. 1988, 36, 249. (b) Gold, E. H.; Babad,
E. J. Org. Chem. 1972, 37, 2208. (c) Edwards, M. L.; Stemerick, D. M.;
McCarthy, J. R. Tetrahedron Lett. 1990, 31, 3417. (d) Taber, D. F.;
Wang, Y. J. Am. Chem. Soc. 1997, 119, 22. (e) Ho, W.-B.; Broka, C.
J. Org. Chem. 2000, 65, 6743. (f) Arnott, G.; Brice, H.; Clayden, J.;
Blaney, E. Org. Lett. 2008, 10, 3089. (g) Wang, X.; Mei, T.-S.; Yu, J.-Q.
J. Am. Chem. Soc. 2009, 131, 7520. (h) Ochiai, M.; Miyamoto, K.;
Kaneaki, T.; Hayashi, S.; Nakanishi, W. Science 2011, 332, 448.
5430
Org. Lett., Vol. 13, No. 20, 2011

efficient method for the reductive deprotection of N-
triflylaziridines. For instance, treatment of cis-diphenyla-
ziridine 3k with sodium bis(2-methoxyethoxy)aluminum
dihydride (10 equiv) in toluene at 0 °C for 3 h under argon
afforded the corresponding amine in a high yield (82%),
without formation of ring-opened byproduct (Scheme 2). In
this deprotection, isomerization to trans-2,3-diphenylaziri-
dine was not detected.
reaction of trichloro-, trifluoro-, or unsubstituted ethyl
sulfamate with a limiting amount of (Z)- and (E)-4-octene
(Table 2). In these sulfamate-transfer reactions, however,
loss of stereochemistry of olefins was detected to a great extent,
especially in the reaction of ethyl sulfamate with (Z)-octene
(Table 2, entry 5), which is in marked contrast to the reactions
of triflylamide (Table 1). Formation of N-(alkoxysulfonyl)-
imino-λ3-bromane species is assumed to be involved in the N-
atom transfer process; however, we could not detect their
1
intermediacy by H NMR tube experiments (Scheme S3,
Supporting Information). Alkoxysulfonyl moieties serve as
convenient amine protecting groups and alkoxysulfonylated
aziridines are known to be effective substrates in ring-opening
reactions with S-, N-, and O-centered nucleophiles.10d
The one-pot aziridination procedure using sulfamate
esters can be applied to the intramolecular versions
(Scheme 3): thus, olefinic sulfamate esters 5a and 5b by
the reaction with diacetoxy-λ3-bromane 1 under our con-
ditions afforded bicyclic 6a10d,20b and tricyclic aziridine
6b21 in good yields.
Scheme 2
Thus, a hitherto unknown kind of diacetoxy(aryl)-λ3-
bromane was synthesized and well characterized. The λ3-
bromane makes it possible to undergo aziridinations of
Table 2. Aziridination of 4-Octene with Sulfamate Estersa
product
Scheme 3
temp time
entry olefin
ROSO2NH2
(°C)
(h)
yieldb (%) ratioc
1
2
3
4
5
6
Z
E
Z
E
Z
E
CCl3CH2OSO2NH2
0
5
3
8
3
2
4
4a 77 (81) 83:17
4a 62 (71) 26:74
4b 76 (88) 82:18
4b 75 (90) 30:70
4c 78 (84) 43:57
4c 73(100) 27:73
CCl3CH2OSO2NH2 25
CF3CH2OSO2NH2
CF3CH2OSO2NH2
EtOSO2NH2
0
25
0
EtOSO2NH2
25
a Conditions: method B, 1:1.2:1.2 4-octene/ROSO2NH2/diacetoxy-
λ3-bromane 1, dichloromethane, Ar. b Isolated yields. Numbers in
parentheses are 1H NMR yields. c Cis/trans ratios of 4 determined by
1H NMR of crude reaction mixtures.
alkenes with triflylamide and sulfamate esters in one-pot
under transition metal-free, mild conditions. The aziridina-
tion method probably relies on in situ generation of active
N-sulfonylimino-λ3-bromanes as well as the vastly enhanced
hypernucleofugality of aryl-λ3-bromanyl groups com-
pared to that of aryl-λ3-iodanyl groups.22 Oxidation
reactions of other functional groups such as carbonyl
compounds and amides with diacetoxy-λ3-bromane 1
are under active investigation.
Instead of triflylamide, use of other amides such
as p-nitro-, 2,4-dinitro-, and 3,5-bis(trifluoromethyl)-
benzenesulfonamides19 and methanesulfonamide showed
no evidence for formation of aziridines under the condi-
tions of method A and B; however, the bromane(III)-
induced aziridination of double bonds in the absence of
transition metal catalystscan be extended tothe reaction of
alkoxysulfonamides (sulfamates).10d,20 Thus, method B
resulted in the good yield formation of aziridines 4 by the
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research (B) (JSPS). We thank Central
Glass Co. Ltd., Japan for a generous gift of BrF3.
(19) We supposed that these arenesulfonamides with electron-with-
drawing substituents might condense with diacetoxybromane 1 to
produce imino-λ3-bromane because they do not undergo λ3-bromane-
induced Hofmann rearrangement. See: Ochiai, M.; Okada, T.; Tada, N.;
Yoshimura, A.; Miyamoto, K.; Shiro, M. J. Am. Chem. Soc. 2009, 131,
8392.
(20) (a) Espino, C. G.; Wehn, P. M.; Chow, J.; Du Bois, J. J. Am.
Chem. Soc. 2001, 123, 6935. (b) Duran, F.; Leman, L.; Ghini, A.; Burton,
G.; Dauban, P.; Dodd, R. H. Org. Lett. 2002, 4, 2481. (c) Liang, J.-L.;
Yuan, S.-X.; Huang, J.-S.; Yu, W.-Y.; Che, C.-M. Angew. Chem., Int.
Ed. 2002, 41, 3465. (d) Zalatan, D. N.; Du Bois, J. J. Am. Chem. Soc.
2009, 131, 7558.
Supporting Information Available. Experimental pro-
cedures, Figure S1, Schemes S1ꢀS3, and X-ray crystal-
lographic data for 1 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
(21) Wehn, P. M.; Du Bois, J. Org. Lett. 2005, 7, 4685.
(22) Ochiai, M.; Tada, N.; Okada, T.; Sota, A.; Miyamoto, K. J. Am.
Chem. Soc. 2008, 130, 2118.
Org. Lett., Vol. 13, No. 20, 2011
5431