J . Org. Chem. 2000, 65, 1215-1217
1215
catalytic activity of these complexes.8 Perhaps most
importantly, the metals themselves can be interchanged
to adjust the Lewis acidity. Ligand exchange phenomena
provide ample evidence that the early lanthanides, with
a lower charge-to-radius ratio, are less Lewis acidic than
the late lanthanides.6a-e
Our interest in studying these complexes as Lewis acid
catalysts for organic transformations was piqued by the
report from the Evans group detailing the facile and high-
yielding synthesis of lanthanide metallocene cations.6h
Although the extreme Lewis acidity of these complexes
created some concern about turnover in highly function-
alized substrates that might contain an array of Lewis
basic sites, we reasoned that the ability to tune the
complexes would afford sufficient adaptability to design
a suitable catalyst system. Herein, we report initial
results achieved in the application of lanthanocenium
cations to the synthesis of 4-pyranones via the hetero
Diels-Alder reaction.
Heter o Diels-Ald er Rea ction s Ca ta lyzed by
th e Meta llocen iu m Com p lex [Cp *2Ce][BP h 4]
Gary A. Molander* and Robert M. Rzasa
Department of Chemistry, University of Pennsylvania,
Philadelphia, Pennsylvania 19104
Received J une 7, 1999
In tr od u ction
Lewis acid promoted processes represent one of the
most useful families of reactions available to synthetic
chemists. As a testament to the importance of this class
of transformations, literally hundreds of different pro-
moters have been synthesized and utilized in diverse
organic reactions. As a class, cationic metallocene com-
plexes appear underrepresented in terms of their devel-
opment as potentially useful Lewis acid catalysts. Reetz
and co-workers were among the first to recognize the
potential of this group of complexes in selective organic
synthesis. In preliminary studies, his group described the
use of a cationic iron complex to promote a Mukaiyama
aldol reaction.1 More recently, cationic zirconium com-
plexes2 have been employed to promote Diels-Alder
reactions,3 a cationic ruthenium complex has been de-
veloped as an asymmetric Diels-Alder catalyst,4 and
cationic palladium(II) and platinum(II) species have been
employed in asymmetric hetero Diels-Alder processes.5
Cationic lanthanocenes6 have been utilized in poly-
merization reactions and related processes,6c,g but they
have not been employed as Lewis acid catalysts for the
selective synthesis of small organic molecules. This class
of Lewis acids possesses several features that would
appear to make them extremely attractive for this
purpose. In particular, one should be able to tune the
reactivity of the complexes to match the chemical conver-
sion desired. Diverse ligands with various steric proper-
ties, including chiral nonracemic platforms, might be
utilized to control reactivity and stereoselectivity.7 The
counterion might also be adjusted to modulate the
Resu lts a n d Discu ssion
The hetero Diels-Alder reaction between a diene and
an aldehyde was first demonstrated in the preparation
of 2,3-dihydropyran-4-ones.9 This method has provided
a useful strategy for the synthesis of saccharides10 and
complex natural products.11 Recent investigations12 have
led to the development of asymmetric13 and immobilized
catalysts.14
As a first step toward the development of novel
asymmetric catalysts for the hetero Diels-Alder reaction,
we report our results using achiral cationic lanthanide
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10.1021/jo9909195 CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/25/2000