Table 1 Diethylzinc additions to aldehydes catalyzed by Ti(IV) complexes
of 3
Aldehyde
Temp./K
Time/h
Conversion (%) Ee (%)
r.t.
16
16
> 95
> 95
77
84
0
°C
r.t.
16
16
> 95
> 95
91
94
0
°C
r.t.
16
16
> 95
> 95
75
78
0
°C
Fig. 2 UV-Vis spectra of 1–3 in acetonitrile.
major bisignate band corresponding to naphthyl p ? p*
transitions at ~ 245 nm and one minor band at ~ 290 nm due to
acetylenic p?p* transition. CD spectra of metallacyclophanes
r.t.
0 °C
16
40
> 95
~ 40
77
78
1
–3 exhibit a bisignate band at ~ 260 nm due to the naphthyl
r.t.
16
40
> 95
~ 80
76
77
p?p* transitions and an intense band at 320 nm assignable to
0
°C
the acetylenic p?p* transitions, along with a band at ~ 230 nm
which can be attributed to the chiral arrangment of the PEt
3
groups on the Pt centers (Fig. 3). Interestingly, the intensities of
the naphthyl p?p* CD bands of coordinated L1–3 in 1–3 have
decreased to ~ 1/4 of those of free L1–3, probably a consequence
of the reduction in their dihedral angles upon the formation of
metallacyclophanes.
r.t.
0 °C
16
16
> 95
> 95
75
78
aldehydes. Such a supramolecular approach will add a new
dimension to the rapidly expanding field of asymmetric
catalysis.
We acknowledge financial support from NSF (CHE-
208930). W. L. is an Alfred P. Sloan Fellow, an Arnold and
0
Mabel Beckman Young Investigator, a Cottrell Scholar of
Research Corp, and a Camille Dreyfus Teacher–Scholar.
Notes and references
‡
X-Ray single-crystal diffraction data for 3·EtAc·H
Siemens SMART CCD diffractometer. Crystal data: monoclinic, space
group P2 , a = 13.833(3), b = 15.047(3), c = 17.264(4) Å, b =
2
O were collected on a
1
3
23
9
4
2
c
2.105(5)°, U = 3591.1(14) Å , Z = 2, D = 1.51 g cm , m(Mo-Ka) =
21
0.3 cm . Least-squares refinement based on 13710 reflections with I >
s(I) and 802 parameters led to convergence, with a final R1 = 0.050, wR2
Fig. 3 Circular dichroism spectra of 1–3 in acetonitrile.
=
0.105, and GOF = 1.03. Flack parameter = 20.02(6). See http:/
The presence of chiral dihydroxy groups in 3 has prompted us
to examine its utility in asymmetric catalysis. We have carried
out prototypical diethylzinc additions to aromatic aldehydes
/
www.rsc.org/suppdata/cc/b2/b208324h/ for crystallographic data in CIF or
other electronic format.
i
11
using a combination of 3 and Ti(O Pr)
4
as the catalyst (eqn. 1).
1 J.-M. Lehn, Supramolecular Chemistry, Concepts and Perspectives,
VCH, New York, 1995.
2 P. H. Dinolfo and J. T. Hupp, Chem. Mater., 2001, 13, 3113.
As shown in Table 1, the Ti(IV) complexes of 3 are excellent
catalysts for the additions of diethylzinc to 1-naphthaldehyde
with 94% ee and > 95% conversion at 0 °C. The enantiose-
lectivity has however dropped significantly when other smaller
aromatic aldehydes were used as the substrates. This result
differs from the performance of BINOL and a BINOL-derived
organometallic triangle, both of which have a very broad
substrate scope.11 We believe that this difference is a direct
consequence of much more rigid structure of 3; the dihedral
angles of naphthyl rings in the Ti(IV) catalyst can no vary to
accommodate aldehydes of various sizes to give high enantiose-
lectivity. The chiral dihydroxy groups in 3 thus differ from
those of BINOL, and may prove useful for mechanistic work
owing to their rigid structure.
3
(a) S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100,
853–907; (b) B. J. Holiday and C. A. Mirkin, Angew. Chem., Int. Ed.,
2001, 40, 2022–2043; (c) M. Fujita, Chem. Soc. Rev., 1998, 27,
417–425.
4
(a) M. Yoshizawa, T. Kusukawa, M. Fujita, S. Sakamoto and K.
Yamaguchi, J. Am. Chem. Soc., 2001, 123, 10454–10459; (b) M.
Yoshizawa, Y. Takeyama, T. Kusukawa and M. Fujita, Angew. Chem.,
Int. Ed., 2002, 41, 1347–1349.
5 (a) L. Pu, Chem. Rev., 1998, 98, 2405; (b) R. Noyori, Angew. Chem. Int.
Ed., 2002, 41, 2008.
6
7
S. J. Lee and W. Lin, J. Am. Chem. Soc., 2002, 124, 4554–4555.
(a) K. Onitsuka, Y. Harada, F. Takei and S. Takahashi, Chem.
Commun., 1998, 643–644; (b) H. Sasai, T. Tokunaga, S. Watanabe, T.
Suzuki, N. Itoh and M. Shibasaki, J. Org. Chem., 1995, 60,
7
388–7389.
(1)
8 S. M. Al Qaisi, K. J. Galat, M. Chai, D. G. Ray, P. L. Rinaldi, C. A.
Tessier and W. J. Youngs, J. Am. Chem. Soc., 1998, 120, 12149.
9
J. W. Steed and J. L. Atwood, Supramolecular Chemistry, Wiley, New
York, 2000.
In summary, a family of novel chiral metallacyclophanes has
been readily assembled based on robust Pt–acetylide linkages.
Metallacyclophane 3 has been used as a chiral ligand for
enantioselective catalytic diethyl zinc additions to aromatic
1
0 V. W.-W. Yam, Acc. Chem. Res., 2002, 35, 555.
11 (a) L. Pu and H.-B. Yu, Chem. Rev., 2001, 101, 757; (b) S. J. Lee, A. Hu
and W. Lin, J. Am. Chem. Soc., 2002, 124, 12948.
CHEM. COMMUN., 2003, 96–97
97