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realistic possibility of resolving the racemic mixture by
chemical or chromatographic means.
In contrast to 1a, the spectrum of the planar 2a contains only
two diastereotopic isopropyl methyl protons and remains also
unchanged over the temperature range studied.
Preliminary data show that 1 and 2 are active catalysts in
9
Heck coupling reactions (Table S1, ESI†). The catalysts show
excellent long term stability at high temperatures. Furthermore,
the activity is sensitive to the steric congestion at the reactive
site (1b is more active than 1a). It is interesting that aryl
chlorides can also be used as substrates. The mechanism and the
nature of the active species in these reactions is far from clear.
Heck reaction catalysed by ‘pincer’ phosphine ligated palla-
dium complexes have been reported.13
Isolation of the enantiomerically pure complexes and exten-
sion of this methodology to other transition metals and their
catalytic reactions are currently under way.
We thank Dr J. E. Anderson for a helpful discussion, and
acknowledge Mr S. Winston, Mr J. Gonzalez-Outeirino and
Mrs J. M. Street for assistance in solving the structure of 1a and
VT NMR spectroscopy, respectively.
Fig. 2 Molecular structure of the cation in 2a. Selected bond lengths (Å) and
angles (°): Pd1–N3 1.977(16), Pd1–C13 2.02(2), Pd1–C21 2.03(2); N3–
Pd1–C13 79.1(8), N3–Pd1–C21 79.0(8).
Notes and references
‡
Crystal data: for 1a: crystals were obtained by layering a CH
of 1a with light petroleum (bp = 40–60 °C); C40 45AgCl
129.13, monoclinic, space group P2 /c, a = 11.559(2), b = 42.043(8), c
10.631(2) Å, b = 112.51(3)°, U = 4772.6(16) Å , T = 150(2) K, Z =
2
Cl
2
solution
H
N Pd, M =
9 5
1
=
4
1
3
21
, m(Mo-Ka) = 1.325 mm , 28393 reflections collected, 10436 unique
reflections, Rint = 0.0867), Final R indices: wR = 0.1185 and R = 0.0514
2
1
2
2
[F > 2s(F )] and 0.1340 and 0.0897 for all data.
For 2a: crystals were obtained by layering a CH
2
Cl
Pd, M = 809.66, triclinic,
space group P1, a = 8.4782(4), b = 11.8790(6), c = 20.8146(11) Å, a =
2
solution of 2a with
light petroleum (bp = 40–60 °C), C36H41Cl4.5N
5
¯
3
7
1
6
R
5.077(3), b = 83.364(3), g = 71.322°, U = 1917.65 (17) Å , T =
21
50(2) K, Z = 2, m(Mo-Ka) = 0.829 mm , 14147 reflections collected,
2
524 unique reflections, Rint = 0.0839), Final R indices: wR = 0.2213 and
2
2
1
= 0.0838 [F > 2s(F )] and 0.2562 and 0.1316 for all data.
The refinement of both structures was hampered by severe disorder of
dichloromethane solvent molecules, and, not unrelated, limited quality data
from poor crystals, especially for compound 2a. At the present stage, the
modelling of the disorder is not complete, but the definition of the cation is
reliable in each case, with reasonable positional esds and good displacement
parameters for all component atoms. CCDC reference numbers 162457 and
1
Fig. 3 The isopropyl methyl- and bridging methylene-regions of the H
NMR spectrum of 1a before (a) and after (b) the addition of Pirkle’s
1
62458.
acid.
See http://www.rsc.org/suppdata/cc/b1/b103330c/ for crystallographic
data in CIF or other electronic format.
similar to 1a. However, the whole molecule is virtually planar,
with the Ar groups twisted 76.68(11) and 89.63(22)° out of the
plane for rings C12 and C24, respectively. All other geometrical
data are very similar to 1a. It is instructive to view the
generation of 1 by desymmetrisation of 2 through a twist
1 For recent reviews see: D. Bourisou, O. Guerret, F. P. Gabbai and G.
Bertrand, Chem. Rev., 2000, 100, 39; W. A. Herrmann and C. Kocher,
Angew. Chem., Int. Ed. Engl., 1997, 36, 2162.
2
3
4
J. C. Green, R. G. Scurr, P. L. Arnold and F. G. N. Cloke, Chem.
Commun., 1997, 1963.
Catalytic Asymmetric Synthesis, ed. I. Ojima, Wiley-VCH, 2nd edn.,
2
deformation around the C axis. The direction of the twist
2
000.
F. Fache, E. Schulz, M. L. Tommasino and M. Lemaire, Chem. Rev.,
000, 100, 2159; A. H. Hoveida and J. P. Moren, in Metallocenes, ed.
A. Togni and R. L. Halterman, Wiley-VCH, Weinheim, 1998.
determines the chirality generated. A compound related to 2a
with methyl substituted carbene functionalities has also recently
been reported.11
2
The structures of 1a and 2a observed in the solid state are
5 A. W. Coleman, P. B. Hitchcock, M. F. Lappert, R. K. Maskell and J. H.
Muller, J. Organomet. Chem., 1985, 296, 173; W. A. Herrmann, L. J.
Goosen, C. Koecher and G. R. J. Artus, Angew. Chem., Int. Ed. Engl.,
1
persistent in solution. The H NMR (CD
2
Cl
2
) spectrum of 1a
(
ESI†) shows four anisochronous doublets which can be
1
996, 35, 2805.
M. Scholl, S. Ding, C. W. Lee and R. H. Grubbs, Org. Lett., 1999, 1,
53.
W. A. Herrmann, L. J. Goosen and M. Spiegler, Organometallics, 1998,
7, 2162.
assigned to the isopropyl methyl protons. Furthermore, the
methylene-bridge protons appear as an AB pattern in the range
d 5.5–6.3 [Fig. 3(a)]. In order to demonstrate the chiral nature of
6
7
8
9
1
1
a in solution Pirkle’s acid. [TFAE, S-(+)-2,2,2-trifluoro-
1
-(9-anthryl)ethanol] was used as a chiral discriminating
D. S. Clyne, J. Jin, E. Genest, J. C. Gallucci and T. V. RajanBabu, Org.
Lett., 2000, 2, 1125.
12
agent. The isopropyl methyl groups and the bridging methy-
lene regions of the spectrum obtained after addition of 3.4
9 A. A. D. Tulloch, A. A. Danopoulos, R. P. Tooze, S. M. Cafferkey S.
Kleinhenz and M. B. Hursthouse, Chem. Commun., 2000, 1247; D. S.
McGuiness and K. J. Cavell, Organometallics, 2000, 19, 741.
0 A. A. D. Tulloch, A. A. Danopoulos, S. Winston, S. Kleinhenz and G.
Eastham, J. Chem. Soc., Dalton Trans., 2000, 4499.
2 2
equivalents of TFAE in CD Cl solution of 1a are shown in Fig
3
(b). The doubling of the four anisochronous doublets and one
1
1
1
of the doublets due to the methylene linkers originates from the
strong interaction of the chiral discriminating agent with one of
the two enantiomers of 1a (degree of non-equivalence Dd ca.
1 E. Peris, J. A. Loch, J. Mata and R. H. Crabtree, Chem. Commun., 2001,
2
01.
2 W. H. Pirkle and M. S. Hoekstra, J. Am. Chem. Soc., 1976, 98,
832.
0
.2 ppm). The conformation of 1a is rigid at least up to 80 °C as
1
evidenced by variable temperature H NMR spectroscopy
Cl). This supports the contention that a high activation
barrier exists for enantiomer interconversion, thus leading to the
1
(C
6
D
5
13 M. Ohff, A. Ohff, M. E. van der Boom and D. Milstein, J. Am. Chem.
Soc., 1997, 119, 11 687.
Chem. Commun., 2001, 1270–1271
1271