ically pure imidazolium salts, which opens up the possibility of
synthesising a new class of N-heterocyclic carbenes for
asymmetric catalysis.
We thank Professor A. Fürstner for generous support. F. G.
thanks the Fonds der Chemischen Industrie for a Liebig
fellowship.
Notes and references
‡
General imidazolium triflate formation: to a mixture of bioxazoline 1
(5.0 g, 22.2 mmol) and AgOTf (6.8 g, 26.6 mmol) were added CH2Cl2 (75
mL) and then chloromethyl pivalate (4.6 mL, 31.2 mmol). The tube was
sealed and stirred in the dark at 40 °C for 24 h. After the solution was cooled
to r.t. the mixture was filtered, the solvent evaporated in vacuo and the
resulting oil was chromatographed on silica gel (4 3 10 cm, CH2Cl2–MeOH
20+1). Subsequent crystallisation from a solvent mixture comprising THF
(30 mL), toluene (150 mL) and pentane (50 mL) gave 6.85 g (80%) of
imidazolium triflate 2 as colorless crystals.
Rf = 0.37 (CHCl3–MeOH 93+7); [a]20 = 55.0 (c 1.0, CH2Cl2); IR
D
Fig. 1 Structure of 12. Selected distances (Å) and angles (°): Pd–C1
2.035(3), Pd–I 2.6139(5), C8…C8* 3.838(11), C11…C11* 3.819(12); C1–
Pd–C1* 179.0(3), mean plane (C1–C6, N1, N2, O1, O2, Pd)/mean plane
(C1*–C6*, N1*, N2*, O1*, O2*, Pd) 50(1).
(KBr): 3110, 2970, 1731, 1532, 1481, 1285, 1261, 1154, 1032, 966, 917,
1
882, 826, 755, 638; H NMR (400 MHz, CDCl3): d 8.73 (s, 1H, NCHN),
5.07 (dd, J 7.9, 9.0 Hz, 2H, CH2), 4.98–4.93 (m, 2H, CHCH2), 4.83 (dd, J
4.1, 9.0 Hz, 2H, CH2), 2.33 (m, 2H, CHCH3), 1.03 (d, J 6.9 Hz, 6H, CH3),
0.99 (d, J 6.9 Hz, 6H, CH3); 13C NMR (100 MHz, CDCl3): d 125.6 (NCO),
120.6 (q, J 320 Hz, CF3), 116.3 (NCHN), 79.1 (CH2), 63.9 (CHCH2), 31.1
(CHCH3), 17.6 (CH3), 16.7 (CH3); 19F NMR (300 MHz, CDCl3): d 278.7
(CF3); MS (EI), m/z (%) 386 (0.4) [M+], 237 (100), 169 (5), 69 (7); MS
(ESI+): m/z (%) 237 (100). HRMS (EI): calc. for C13H21N2O2 (cation):
237.1603, found 237.1605. Anal. Calc. for C14H21F3N2O5S: C, 43.52; H,
5.48. Found C, 43.44; H, 5.55%.
chromatography yielded the air-stable complex 12 in 87% yield.
Unlike related palladium complexes,11 12 was formed solely as
the trans isomer. The structure was validated by an X-ray
structure analysis (Fig. 1).6 In the crystal the molecule adopts a
conformation in which the two imidazolylidene ligands are
twisted relative to one another, resulting in short H…H contacts
(2.06, 2.15 Å)12 between the methyne H atoms of isopropyl
groups on adjacent ligands, presumably caused by steric
interaction between the chirally positioned isopropyl groups
and the iodide ligands.
Lee and Hartwig recently reported on the enantioselective a-
arylation of amides, giving oxindole 14 in an isolated yield of
74% with 57% ee using 5 mol% of catalyst.13 We chose 13 as
substrate to test the imidazolium triflates 2, 4 and 6 (Table 1).
Using imidazolium salt 2 as a NHC precursor and a Pd(0) or
Pd(2) source in a 1+1 ratio gave a very good yield of oxindole
14 with an ee of around 30% (entries 1 and 2). Complex 12
could also be used as a catalyst precursor; however, higher
temperature and a longer reaction time were needed (entry 3).
Interestingly, 40% of complex 12 were recovered after
completion of the reaction. Slightly better results were obtained
with the tBu-substituted imidazolium triflate 4. With Pd2(dba)3
the reaction was run at r.t. and gave the product 14 in a very
good yield and with 43% ee (entry 4). At a slightly higher
temperature the reaction could also be brought to completion
using only 1 mol % of the catalyst (entry 6).
1 W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1291; D. Bourissou,
O. Guerret, F. P. Gabbaï and G. Bertrand, Chem. Rev., 2000, 100, 39.
2 M. T. Powell, D.-R. Hou, M. C. Perry, X. Cui and K. Burgess, J. Am.
Chem. Soc., 2001, 123, 8878; T. J. Seiders, D. W. Ward and R. H.
Grubbs, Org. Lett., 2001, 3, 3225; J. J. Van Veldhuizen, S. B. Garber, J.
S. Kingsbury and A. H. Hoveyda, J. Am. Chem. Soc., 2002, 124,
4954.
3 Comprehensive Asymmetric Catalysis, ed. E. N. Jacobsen, A. Pfaltz and
H. Yamamoto, Springer, Berlin, 1999; A. K. Ghosh, P. Mathivanan and
J. Cappiello, Tetrahedron: Asymmetry, 1998, 9, 1; T. G. Gant and A. I.
Meyers, Tetrahedron, 1994, 50, 2297.
4 D. Müller, G. Umbricht, B. Weber and A. Pfaltz, Helv. Chim. Acta,
1991, 74, 232; G. Helmchen, A. Krotz, K.-T. Ganz and D. Hansen,
Synlett, 1991, 257.
5 A. J. Arduengo III, R. Krafczyk, R. Schmutzler, H. A. Craig, J. R.
Goerlich, W. J. Marshall and M. Unverzagt, Tetrahedron, 1999, 55,
14523.
6 Crystal data: for 2: [C13H21N2O2]+[CF3O3S]2, from THF/toluene/
pentane, orthorhombic, space group P212121 (no. 19), M = 386.39, a =
9.6882(3), b = 11.0578(3), c = 16.3800(5) Å, U = 1754.8(1) Å3, T =
100 K, Z = 4, m(Mo-Ka) = 0.242 mm21, 22778 refl. meas., 3622
unique (Rint = 0.081), R = 0.037 [I > 2s(I)], wR(F2) = 0.084 (all
In conclusion, we report a new straightforward method to
transform bioxazolines and oxazolineimines into enantiomer-
data), Flack param.
= 20.05(7). For 12·2(CHCl3): [C26H40I2-
N4O4Pd]·2[CHCl3], from chloroform–heptane, orthorhombic, space
group P21212 (no. 18), M = 1071.56, a = 12.257(3), b = 13.001(3), c
= 12.305(3) Å, U = 1960.9(7) Å3, T = 200 K, Z = 2, m(Mo-Ka) =
2.49 mm21, 68069 reflections measured, 6714 unique (Rint = 0.087),
chloroform disordered, R = 0.043 [I > 2s(I)], wR(F2) = 0.085 (all
data), Flack parameter = 20.02(3). CCDC 191920 and 1919201. See
in CIF format.
Table 1 Enantioselective a-arylation of amide 13
7 A. J. Arduengo III, H. V. Rasila Dias, R. L. Harlow and M. Kline, J. Am.
Chem. Soc., 1992, 114, 5530; W. A. Herrmann, C. Köcher, L. J.
Goossen and G. R. J. Artus, Chem. Eur. J., 1996, 2, 1627.
8 L. N. Pridgen and G. Miller, J. Heterocycl. Chem., 1983, 20, 1223; J. V.
Allen and J. M. J. Williams, Tetrahedron: Asymmetry, 1994, 5, 277.
9 For a related triazolium salt, see: D. Enders and U. Kallfass, Angew.
Chem., Int. Ed., 2002, 41, 1743.
Entry
Catalyst
T/°C
Time/h
Yielda (%) % Eeb
1
2
3
4
5
6c
7
2 + Pd2(dba)3
2 + Pd(OAc)2
12
4 + Pd2(dba)3
4 + Pd(OAc)2
4 + Pd2(dba)3
6 + Pd2(dba)3
50
50
90
20
50
50
50
5
5
85
92
95
95
90
97
90
28 (+)
32 (+)
30 (+)
43 (+)
35 (+)
37 (+)
11 (+)
20
14
14
14
16
10 A. J. Arduengo III, Acc. Chem. Res., 1999, 32, 913.
11 W. A. Herrmann, M. Elison, J. Fischer, C. Köcher and G. R. J. Artus,
Angew. Chem., Int. Ed. Engl., 1995, 34, 2317; D. Enders and H. Gielen,
J. Organomet. Chem., 2001, 617–618, 70.
12 Using a calculated C–H distance of 0.98 Å. For a discussion of short
H…H contacts, see: J. D. Dunitz and A. Gavezzotti, Acc. Chem. Res.,
1999, 32, 677.
a Isolated yield. b Determined by HPLC with Chiralcel OD–H. c 1 mol%
catalyst.
13 S. Lee and J. F. Hartwig, J. Org. Chem., 2001, 66, 3402.
CHEM. COMMUN., 2002, 2704–2705
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