C O MMU N I C A T I O N S
alcohols in very high selectivity, yield, and enantioselectivity.
In comparison, when free ligand L was used instead of 4, a lower
4
ee (80%) was obtained for the addition of diethylzinc to 1-naph-
thaldehyde. The broad substrate scope for catalytic diethylzinc
i
additions using 4 and Ti(O Pr)
4
suggests that there is significant
flexibility in the dihydroxy groups to accommodate aldehydes of
various sizes. This result is entirely consistent with the CD data.
In summary, a family of chiral organometallic triangles has been
readily assembled on the basis of robust Pt-acetylide linkage.
Metallocycle 4 has been used for highly enantioselective catalytic
diethylzinc additions to aromatic aldehydes.
Figure 1. CD spectra of L4 and 4 in acetonitrile.
Acknowledgment. We thank NSF (CHE-0208930) for financial
support. W.L. is an A. P. Sloan Fellow, a Beckman Young
Investigator, a Cottrell Scholar of Research Corp., and a Camille
Dreyfus Teacher-Scholar.
Table 1. Diethylzinc Additions to Aldehydes Catalyzed by Ti(IV)
Supporting Information Available: Experimental procedures,
analytical data, and seven figures (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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All reactions were carried out with 7 equiv of Ti(O Pr)4 (on the basis
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b
1
c
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(
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similar to L1-4, but with red-shifts in energy and higher intensities.
Interestingly, a new intense CD band appeared at ∼202 nm,
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3 2
assignable to the transitions associated with cis-Pt(PEt ) moieties.
(
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apparently steered by chiral binaphthyl moieties.15 Enhanced lower
energy CD signals for 1-4 are consistent with the presence of
multiple ligands in each metallocycle, while their similarity with
those of L1-4 indicates that the dihedral angles between naphthyl
(
12) Pt-alkynyl bonds have been used to construct achiral metallocycles.
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9
rings in 1-4 are quite similar to those of L1-4
.
4
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With dihydroxyl groups present in 4, we expect it to be applicable
in asymmetric catalysis. We have thus carried out prototypical
diethylzinc additions to aromatic aldehydes using a combination
(
15) Such a chiral propeller-type arrangement of triethylphosphines has been
observed in a single-crystal structure of a related metallacyclophane built
from 3,3′-dialkynyl-1,1′-binaphthyl. Unpublished results.
i
16
of 4 and Ti(O Pr)
4
(eq 1). As shown in Table 1, the Ti(IV)
(16) (a) Pu, L.; Yu, H.-B. Chem. ReV. 2001, 101, 757. (b) Mori, M.; Nakai, T.
Tetrahedron Lett. 1997, 38, 6233.
complexes of 4 are excellent catalysts for the additions
of diethylzinc to aromatic aldehydes to generate chiral secondary
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