Looking for a synergic effect between NHCs and chiral P-ligands

Article abstract of DOI:10.1021/ol800225t

(Graph Presented) The issue of the added value of NHCs in asymmetric catalysis with respect to trusted chiral P-ligands was addressed: considering a prototypical asymmetric allylic alkylation reaction as a model, the association of a priori inhibiting and

Full text of DOI:10.1021/ol800225t

ORGANIC
LETTERS
2008
Vol. 10, No. 7
1453-1456
Looking for a Synergic Effect between
NHCs and Chiral P-Ligands
Nicolas Toselli, David Martin,* and Ge´rard Buono*
UniVersite´ Aix-Marseille, ECM, Institut des Sciences Mole´claires de Marseille (ISM2),
CNRS UMR 6263, AVenue Escadrille Normandie Niemen, case A 62,
F-13397 Marseille Cedex 20, France
gerard.buono@uniV-cezanne.fr; daVid.martin@uniV-cezanne.fr
Received January 31, 2008
ABSTRACT
The issue of the added value of NHCs in asymmetric catalysis with respect to trusted chiral P-ligands was addressed: considering a prototypical
asymmetric allylic alkylation reaction as a model, the association of a priori inhibiting and achiral NHCs with modular P-ligand resulted in
enhancement of er up to 508% and increased rates.
During the past decade, N-heterocyclic carbenes (NHCs)
have emerged as extremely powerful and versatile ligands
for transition metals in organometallic catalysis, with tre-
mendous applications in organic synthesis.1 This popularity
is commonly attributed to an unprecedented set of proper-
ties: their electron-rich nature, their tight binding to the
metal, their original shape and resulting steric bulk.1 Al-
though they are sometimes presented as “superior equivalents
of phosphines”, which are more labile and/or π-acceptor
ligands, a more objective statement would rather emphasize
the complementarities of these two families. In 1998, Ro¨sch
et al. already pointed out the theoritical potentiality of
bidentate ligands featuring these two functionalities.2a Since
then bidentate P-NHC ligands, as well as catalytic systems
mixing both a monodentate phosphine and a NHC, have been
extensively applied in catalysis.2-4 The NHC is usually
considered as the key partner. The P-ligand is mainly proned
(2) (a) Albert, K.; Gisdakis, P.; Ro¨sch, N. Organometallics 1998, 17,
1608-1616. (b) Yang, C.; Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3,
1511-1514. (c) Wang, A.-E.; Xie, J.-H.; Wang, L.-X.; Zhou, Q. L.;
Tetrahedron 2005, 61, 259-266. (d) Wang, A.-E.; Zhong, J.; Xie, J.-H.;
Li, K.; Zhou, Q.-L. AdV. Synth. Catal. 2004, 346, 595-598. (e) Lee, H.
M.; Chiu, P. L.; Zeng, J. Y. Inorg. Chim. Acta 2004, 357, 4313-4321. (f)
Shi, J.-C.; Yang, P.-Y.; Tong, Q.; Wu, Y.; Peng, Y. J. Mol. Catal. A 2006,
259, 7-10. (g) Zhong, J.; Xie, J.-H.; Wang, A.-E.; Zhang, W.; Zhou,
Q.-L. Synlett 2006, 18, 1193-1196. (h) Wolf, J.; Labande, A.; Daran,
J.-C.; Poli, R. J. Organomet. Chem. 2006, 691, 433-443. (i) Hahn, F. E.;
Jahnke, M. C.; Pape, T. Organometallics 2006, 25, 5927-5936. (j) Field,
L. D.; Messerle, B. A.; Vuong, K. Q.; Turner, P. Organometallics 2005,
24, 4241-4250.
(1) (a) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913-921. (b)
Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39-92. (c) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1291-
1309; Angew. Chem. 2002, 114, 1342-1363. (d) Carbene Chemistry. From
Fleeting Intermediates to Powerful Reagents; Bertrand, G., Ed.; Dekker:
New York, 2002. (e) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004,
37, 534-541. (f) Peris, E.; Crabtree, R. H. Coord. Chem. ReV. 2004, 248,
2239-2246. (g) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 1815-
1828. (h) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348-1352. (i)
N-Heterocyclic Carbenes in Synthesis; Nolan, S. P., Ed.; Wiley-VCH:
Weinheim, 2006. (j) N-Heterocyclic Carbenes in Transition Metal Catalysis,
Topics in Organometallic Chemistry; Glorius, F., Ed.; Springer: Berlin,
2007; Vol. 21. (k) Kantchev, R. A. B.; O’Brien, C. J.; Organ, M. G. Angew.
Chem., Int. Ed. 2007, 46, 2768-2813.
(3) (a) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953-956. (b) Herrmann, W. A.; Bo¨hm, V. P. W.; Gsto¨ttmayr, C. W. K.;
Grosche, M.; Reisinger, C. P.; Weskamp, T. J. Organomet. Chem. 2001,
617-618, 616-628. (c) Matsubara, K.; Ueno, K.; Shibata, Y. Organome-
tallics 2006, 25, 3422-3427.
10.1021/ol800225t CCC: $40.75
© 2008 American Chemical Society
Published on Web 02/27/2008

to leave the metal center in order to initiate the catalytic
cycle. However it can play a more active role, by joining
back and assisting the metal in another key elemental step,
such as reductive elimination. The reality of such a synergic
effect was recently unambiguously demonstrated in nickel-
catalyzed reactions.5
efficiency in AAA, we focused on modular 1,3,2-diaza-
phospholidines12-13 and chose ligands 4a-f as standards. We
completed that small library with a set of classical mono-
dentate NHCs 5a-e and related bidentate P-NHCs 6a,b
(Scheme 1).
As recently stated,6c NHCs are far from outclassing the
popular P-ligands in asymmetric organometallic catalysis.6
One reason for this low success rate may lie in the particular
optimization of chiral catalysts, which cannot usually be
dissociated from a serendipitous search for key structural
features through screening of collections of ligands. Indeed,
NHCs are far from matching the enormous structural
diversity of their phosphorus analogues.7 Thus, bidentate
P-NHCs, which would meet the attractive electronic and
steric properties of NHCs and would also benefit from
successful decades of innumerable studies in designing chiral
phosphines, should have emerged as a powerful class of
chiral ligands. But strikingly, they have generally yielded
poor results.8
In this paper, we address the issue of the added value of
NHCs in asymmetric catalysis, with respect to trusted chiral
P-ligands. Focusing on the critical need of chiral NHCs for
structural diversity, we considered an alternative strategy
based on the combination9 of monodentate NHCs with highly
tunable monodentate chiral P-ligands. Herein, we validate
this approach for a specific model reaction by reporting the
proof of a synergic effect between these two partners.
As a model reaction, we chose the prototypical asymmetric
palladium-catalyzed allylic alkylation (AAA)10 of 1,3-diphen-
ylallylacetate 1 by dimethyl malonate 2 because it perfectly
illustrates the paradoxical statement that initiated our study.
Indeed P-NHC ligands have already been tested in AAA,8g-i
but they afforded poor enantiomeric excesses (ee’s), whereas
a synergic effect between a phosphine and such a strong
σ-donor partner would have been expected.11 Among the
numerous chiral P-ligands that have already proven their
Scheme 1
Monodentate chiral ligands 4a-f were engaged in the
palladium-catalyzed AAA in standard conditions. As ex-
pected, the reaction was completed within 1-3 h at 0 °C in
dichloromethane. Compound 3 was obtained with excellent
isolated yields and with ees ranging from 26% to 82% (Table
1, entries 1-6). In marked contrast, classical NHCs 5a,b
are unefficient because their strong σ-donation decreases the
(9) Mixtures of monodentate P-ligands have been recently developed as
a new tool for combinatorial asymmetric Rh-catalysis: (a) Reetz, M. T.;
Sell, T.; Meiswinkel, A.; Mehler, G. Angew. Chem. 2003, 115, 814-817;
Angew. Chem., Int. Ed. 2003, 42, 790-793. (b) Reetz, M. T.; Mehler, G.
Tetrahedron Lett. 2003, 44, 4593-4596. (c) Pen˜a, D.; Minnaard, A. J.;
Boogers, J. A. F.; de Vries, A. H. M.; de Vries, J. G.; Feringa, B. L. Org.
Biomol. Chem. 2003, 1, 1087-1089. (d) Monti, C.; Gennari, C.; Piarulli,
U. Tetrahedron Lett. 2004, 45, 6859-6862. (e) Reetz, M. T.; Li, X. Angew.
Chem. 2005, 117, 3019-3021; Angew. Chem., Int. Ed. 2005, 44, 2959-
2962. (f) Hoen, R.; Boogers, J. A. F.; Bernsmann, H.; Minnaard, A. J.;
Meetsma, A.; Tiemersma-Wegman, T. D.; de Vries, A. H. M.; de Vries, J.
G.; Feringa, B. L. Angew. Chem. 2005, 117, 4281-4284; Angew. Chem.,
Int. Ed 2005, 44, 4209-4212. (g) Duursma, A.; Pen˜a, D.; Minnaard, A. J.;
Feringa, B. L. Tetrahedron: Asymmetry 2005, 16, 1901-1904. (h) Monti,
C.; Gennari, C.; Piarulli, U.; de Vries, J. G.; de Vries, A. H. M.; Lefort, L.
Chem. Eur. J. 2005, 11, 6701-6717. (i) Reetz, M. T.; Fu, Y.; Meiswinkel,
A. Angew. Chem. 2006, 118, 1440-1443; Angew. Chem., Int. Ed. 2006,
45, 1412-1415. (j) Reetz, M. T.; Bondarev, O. Angew. Chem., Int. Ed.
2007, 119, 4607-4610; Angew. Chem., Int. Ed. 2007, 46, 4523-4526.
(10) (a) Pfaltz, A.; Lautens, M. In ComprehensiVe Asymmetric Catalysis;
Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: Heidelberg,
1999; pp 833-886. (b) Trost, B. M.; Van Vranken, D. L. Chem. ReV. 1996,
96, 395-422. (c) Trost, B. M.; Lee C. In Catalytic Asymmetric Synthesis,
2nd ed.; Ojima, I., Ed.; Wiley-VCH: New York, 2000; pp 593-649. (d)
Lu, Z.; Ma, S. Angew. Chem., Int. Ed. 2007, 46, 2-42; Angew. Chem.
2007, 119, 2-42.
(4) Other classes of stable carbenes could arise as alternative to NHCs;
a bidentate aminocarbene-phosphine ligand has been recently reported:
Vignolle, J.; Donnadieu, B.; Bourissou, D.; Soleilhavoup, M.; Bertrand, G.
J. Am. Chem. Soc. 2006, 128, 14810-14811.
(5) (a) Chiu, P. L.; Lai, C.-L.; Chang, C.-F.; Hu, C.-H.; Lee, H. M.
Organometallics 2005, 24, 6169-6178. (b) Lee, C.-C.; Ke, W.-C.; Chan,
K.-T.; Lai, C.-L.; Hu, C.-H.; Lee, H. M. Chem. Eur. J. 2007, 13, 582-
591. (c) Ho, C.-Y.; Jamison, T. F. Angew. Chem., Int. Ed. 2007, 46, 782-
785; Angew. Chem. 2007, 119, 796-799.
(6) (a) Perry, M. C.; Burgess, K. Tetrahedron: Asymmetry 2003, 14,
951-961. (b) Cesar, V.; Bellemin-Laponnaz, S.; Gade, L. H. Chem. Soc.
ReV. 2004, 33, 619-636. (c) Mauduit, M.; Clavier H. In N-Heterocyclic
Carbenes in Synthesis; Nolan, S. P., Ed.; Wiley-VCH: Weinheim, 2006;
pp 183-239.
(7) (a) ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz,
A., Yamamoto H., Eds.; Springer: Heidelberg, 1999. (b) Catalytic Asym-
metric Synthesis, 2nd ed.; Ojima, I., Ed.; Wiley-VCH: New York, 2000.
(c) Grabulosa, A.; Granell, J.; Muller, G. Coord. Chem. ReV. 2007, 251,
25-90.
(8) (a) Seo, H.; Park, H.-J.; Kim, B. Y.; Lee, J. H.; Son, S. U.; Chung,
Y. K. Organometallics 2003, 22, 618-620. (b) Gischig, S.; Togni, A.
Organometallics 2004, 23, 2479-2487. ( c) Bappert, E.; Helmchen, G.;
Synlett 2004, 16, 1789-1793. (d) Focken, T.; Raabe, G.; Bolm, C.
Tetrahedron: Asymmetry 2004, 15, 1693-1706. (e) Nanchen, S.; Pfaltz,
A. HelV. Chim. Acta 2006, 89, 1559-1573. (f) Focken, T.; Rudolph, J.;
Bolm, C. Synthesis 2005, 3, 429-436. (g) Hodgson, R.; Douthwaite, R. E.
J. Organomet. Chem. 2005, 690, 5822-5831. (h) Flahaut, A.; Roland, S.;
Mangeney, P. J. Organomet. Chem. 2007, 692, 5754-5762. (i) Visenti,
F.; Togni, A. Organometallics 2007, 26, 3746-3754.
(11) The resulting electronic distortion upon the allyl moiety of the
palladium complex should afford good ee’s, because the enantioselectivity
is governed by the regioselectivity of the attack of the soft nucleophile on
the coordinated π-allyl. See for example: Helmchen, G.; Pfaltz, A. Acc.
Chem. Res. 2000, 33, 336-345.
(12) (a) Brunel, J.-M.; Constantieux, T.; Labande, A.; Lubatti, F.; Buono,
G. Tetrahedron Lett. 1997, 38, 5971-5974. (b) Constantieux, T.; Brunel,
J.-M.; Labande, A.; Buono, G. Synlett 1998, 1, 49-50. (c) Muchow, G.;
Brunel, J.-M.; Maffei, M.; Pardigon, O.; Buono, G. Tetrahedron 1998, 54,
10435-10448. (d) Brunel, J.-M.; Constantieux, T.; Buono, G. J. Org. Chem.
1999, 64, 8940-8942. (e) Brunel, J.-M.; Tenaglia, A.; Buono, G.
Tetrahedron: Asymmetry 2000, 11, 3585-3590. (f) Ansell, J.; Wills, M.
Chem. Soc. ReV. 2002, 31, 259-268. (g) Gavrilov, K. N.; Bondarev, O.
G.; Polosukhin, A. I. Russ. Chem. ReV. 2004, 73, 671-699. (h) Tsarev, V.
N.; Lyubimov, S. E.; Shiryaev, A. A.; Zheglov, S. V.; Bondarev, O. G.;
Davankov, V. A.; Kabro, A. A.; Moiseev, S. K.; Kalinin, V. N.; Gavrilov,
K. N. Eur. J. Org. Chem. 2004, 2214-2222.
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1454
Org. Lett., Vol. 10, No. 7, 2008

Table 1. Chiral 1,3,2-Diazaphospholidines 4a-f and Bidentate
P-NHCs 6a,b as Ligands for the Palladium-Catalyzed AAA
entrya
ligand
temp (°C)
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
6a
6b
6a
6b
0
0
0
0
0
0
0
0
25
25
86
91
89
98
92
99
47d
21d
95
92
82
62
23
74
53
77
89
79
80
71
Figure 1. Synergic effect between monodentate NHCs and
P-Ligand 4a: kinetics of the palladium-catalyzed allylic alkylation
of 1 by 2 in CH2Cl2 at 0 °C.
10
a See Supporting Information for experimental details. b Yield after
purification. c Determined by chiral HPLC on a OD-H Chiracel column.
The combination of 1-mesityl-3-methylimidazol-2-ylidene 5c
with 4a (series d) afforded a slow rate, similar to bidentate
P-NHC 6a (series g). On the other hand, when replacing the
methyl group of 5c by bulkier substituents, the AAA
proceeded in few hours in CH2Cl2 at 0 °C (series b, c, e, f),
so that it could even exceed the rate observed with 4a alone
(series a). With 1,3-di-tert-butyl- or 1,3-di-1-adamantyl
imidazolidenes 5b,d, the addition of a priori deactivating
σ-donor NHCs resulted in fine in doubling the reaction rate.
Interestingly, 5e inhibited the reaction (series f). This is in
agreement with the general assumption that imidazolidin-2-
ylidenes are more σ-donating than corresponding imidazol-
2-ylidenes.16-17
1
d Conversion after 48 h, determined by H NMR.
electrophilicity of the key π-allylpalladium intermediate.14
In our condition, they afforded negligible conversions (less
than 1%) after 2 days, even at room temperature. This
deactivating effect can be partly compensated by associating
the NHC with a more acceptor partner15 or by increasing its
steric bulk.14 Thus, NHC-substituted diazaphospholidines
6a,b, which were generated in situ from the corresponding
imidazolium or imidazolidinium salts and butyllithium,
proved to be active ligands for the formation of 3. However,
despite an improved ee of 89% (ligand 6a, entry 7), the
reaction rates remained modest when compared with the
parent monodentate diazaphospholidines 4a-f. Indeed, the
half reaction times were at minimum 2 days at 0 °C (Table
1, entries 7 and 8). At 25 °C, total conversions were obtained
within 2 days (entries 9 and 10). The enantioselectivities
(71-80% ee) were the highest ever obtained to date for that
reaction with a chiral P-NHC ligand. However, they did not
exceed the 82% ee obtained with ligand 4a.
We then turned our attention to enantioselectivities, and
investigated 22 combinations between diazaphospholidines
4a-f and NHCs 5a,b,d,e.
As shown in Table 2, compound 3 was obtained in few
hours at 0 °C in CH2Cl2, with good isolated yields and
enantioselectivities ranging from 47% (ligands 4c and 5e,
entry 21) to 95.2% (ligands 4d and 5a, entry 4). In every
case, (-)-S-3 remained the major enantiomer. Importantly,
the combination of a NHC with a diazaphospholidine resulted
in better ee’s, compared to the same P-ligand alone. The
structural diversity, which arose from the replacement of
bidentate P-NHCs by combinations of corresponding mono-
dentate ligands, is then a key advantage. Starting from m
cheap and easily available NHCs and n chiral modular
P-ligands, up to n‚m chiral catalysts bearing NHCs could be
screened, thus increasing the probability of discovering a
“hit”.
We considered that NHCs 6a,b do not feature a so
significant steric hindrance because they are related to
1-mesityl-3-methylimidazol-2-ylidene 5c. This assumption
was supported by the report of Hodgson et al. that tuning
the single N-substituent of a bidentate P-NHC ligand was
not
suffi-
cient to affect the reaction rate.8g Thus, expecting a more
significant synergic effect, we associated monodentate
NHCs 5a-e, generated in situ from the corresponding tetra-
fluoroborate imidazolium (or imidazolidinium) salt, with
P-ligand 4a.
As a matter of fact, although enantiomeric excess is a
popular descriptor of enantiomer composition, er is by far
more suitable because it directly reflects the relative rates
As shown in Figure 1, the use of monodentate NHCs
clearly exacerbated the effect of the steric hindrance on rates.
(16) Denk, K.; Sirsch, P.; Herrmann, W. A. J. Organomet. Chem. 2002,
649, 219-224.
(14) Bulky NHCs can be used as single ligands in Pd-catalyzed allylic
alkylation, although elevated temperatures are required: (a) Sato, Y.;
Yoshino, T.; Mori, M. Org. Lett. 2003, 5, 31-33. (b) Sato, Y.; Yoshino,
T.; Mori, M. J. Organomet. Chem. 2005, 690, 5753-5758.
(15) (a) Bonnet, L. G.; Douthwaite, R. E.; Hodgson, R. Organometallics
2003, 22, 4187-4189. (b) Douthwaite, R. E. Coord. Chem. ReV. 2007, 251,
702-717.
(17) A similar trend was observed for P-NHCs 6a and 6b (see Table 1,
entries 7 and 8). However, differences between imidazolidin-2-ylidenes and
imidazol-2-ylidenes are not always so significant, and their interpretation
could be puzzling; see for example: (a) Herrmann, W. A.; Schu¨tz, J.; Frey,
G. D.; Herdtweck, E. Organometallics 2006, 25, 2437-2448. (b) Diez-
Gonzales, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251, 874-883.
Org. Lett., Vol. 10, No. 7, 2008
1455

Table 2. A Combinatorial Approach: Mixtures of Chiral
P-Ligands 4a-f with NHCs 5a,b,d,e
entrya
P-ligand
NHC
yield (%)b
erc
ee (%)
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
4a
4b
4c
4d
4e
4f
4a
4b
4c
4d
4e
4a
4a
4b
4c
4d
4e
5a
5a
5a
5a
5a
5a
5b
5b
5b
5b
5b
5b
5d
5d
5d
5d
5d
5e
96
93
84
84
88
92
78
94
94
89
82
89
96
98
96
93
78
93
90
82
81
70
92
13.0
6.4
2.8
40.7
5.8
8.5
14.6
8.0
3.1
13.8
8.7
12.3
26.4
7.6
3.1
8.8
8.9
12.0
11.7
8.2
2.8
19.6
11.3
86
73
48
95.2
71
79
87
78
51
86
79
85
93
77
51
80
80
85
84
78
47
90
84
Figure 2. Synergic effect on enantioselectivity: enhancement of
enantiomeric ratio.
9
Importantly, 5a and 4d afforded also the best absolute
enantioselectivity (95.2% ee, which corresponds to 40.7 er),
whereas initially 4d was not the best chiral 1,3,2-diazaphos-
pholidine (see Table 1, entries 1-6). In addition, the most
efficient combinations of NHCs and diazaphospholidines are
displayed in Figure 2 in an apparent random fashion, so that
the effect of added NHCs on enantioselectivity clearly
resulted in subtle stereoelectronic factors that could not be
easily modelized. Such statements indicate a posteriori the
relevance of a combinatorial approach.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
5ed
5ed
5ed
5ed
5ed
In conclusion, we have demonstrated the paradoxical
added value of a priori deactivating and achiral NHCs in a
particular case study. The association of a strong σ-donor
ligand with chiral 1,3,2-diazaphospholidines led to an
improvement of enantioselectivity, and the resulting inhibi-
tion of the catalytic system could be compensated by a
significant steric hindrance around the metal center. In
principle, this combinatorial approach could be generalized
to a broad range of asymmetric organometallic catalytic
processes that would benefit from the key features of NHCs
(strong σ-donation and/or the steric bulk) and the structural
diversity of modular chiral P-ligands. Indeed, because of the
lability of the latter, compared to the tight binding of NHCs
to metals, the main prerequisite for such cooperation is the
ability of the chiral P-ligand to coordinate the metal at the
enantioselective step of the mechanism.
a See Supporting Information for experimental details. b Yield after
purification. c Determined by chiral HPLC on a OD-H Chiracel column.
d NHC generated in situ from the corresponding imidazolidinium hexafluo-
rophosphate salt.
that determine the enantioselectivity of asymmetric reac-
tions.18 In order to address more precisely the issue of the
added value of the NHC ligand, we considered τ, the
enhancement of enantiomeric ratio of product 3, as a measure
of the synergy between NHCs 5 and diazaphospholidines 4
on enantioselectivity (τ ) [{er with ligands 4 and 5}-{er
with ligand 4}]/{er with ligand 4} × 100). As shown in
Figure 2, addition of a NHC frequently resulted in an
enhancement of er ranging from 50% to 120%. Seven
combinations afforded τ > 150%, a maximal synergic effect
(τ ) 508%) being reached with ligands 5a and 4d.
Acknowledgment. N.T. acknowledges MRT for fund-
ings.
(18) (a) Selke, R.; Facklam, C.; Foken, H.; Heller, D. Tetrahedron:
Asymmetry 1993, 4, 369-382. (b) Seebach, D.; Beck, A. K.; Schmidt, B.;
Wang, Y. M. Tetrahedron 1994, 50, 4363-4384. (c) Gallagher, D. J.; Du,
H.; Long, S. A.; Beak, P. J. Am. Chem. Soc. 1996, 118, 11391-11398. (d)
Kagan, H. B. Rec. TraV. Chim. Pays-Bas 1995, 114, 203-205. (e) Gawley,
R. E. J. Org. Chem. 2006, 71, 2411-2416.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL800225T
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Org. Lett., Vol. 10, No. 7, 2008