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Kaleidoscope of Catalysis Heidelberg Forum of Molecular Catalysis 2005.

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Meeting Reviews
Kaleidoscope of Catalysis:
Heidelberg Forum of Molecular
Catalysis 2005**
Martin Oestreich*
The Heidelberg Forum of Molecular
Catalysis (HFMC) originated from the
Georg Wittig Lectureship in 1999 and
continues as a one-day symposium that
has been held biennially ever since.
The event is sponsored by the BASF corporation which reflects the considerable
importance this company sees in academic research and innovation. In the
tradition of the forum, the BASF Catalysis Award is presented to an outstanding young researcher in recognition of
her or his significant contributions to
the thriving field of molecular catalysis.
Frank Glorius (University of Marburg,
Germany) is the recipient of this year+s
award, which includes prize money of
E 10 000.
As in preceding years, the program
attracted well over 500 junior and
senior scientists from academia and
industry. In addition to four plenary lectures (three invited and one by the prize
winner), a strong poster session with
more than 100 contributions rounded
off the scientific program. The chairman
[*] Dr. M. Oestreich
Institut fr Organische Chemie und
Biochemie
Albert-Ludwigs-Universit#t Freiburg
Albertstrasse 21, 79104 Freiburg
(Germany)
Fax: (+ 49) 761-203-6100
E-mail: martin.oestreich@orgmail.
chemie.uni-freiburg.de
[**] The 3rd Heidelberg Forum of Molecular
Catalysis (July 8, 2005) was jointly organized by the University of Heidelberg,
BASF AG, and the Collaborative Research
Center (SFB 623) “Molecular Catalysts:
Structure and Functional Design” of the
German Research Foundation (DFG).
5166
of the local collaborative research
center, Peter Hofmann (University of
Heidelberg, Germany), moderated the
symposium.
The scientific program commenced
with a talk by David W. C. MacMillan
(California Institute of Technology) on
state-of-the-art asymmetric organocatalysis, which is hinged on two distinct
strategies for the reversible activation
of carbonyl groups by amines: activation
through lowering the lowest unoccupied
molecular orbital (iminium catalysis)
and activation through raising the highest occupied molecular orbital (enamine
catalysis). Starting with a brief outline of
the evolution of suitable chiral catalysts
based on the imidazolidinone framework, MacMillan presented an impressive number of applications, sometimes
outshining classic asymmetric Lewis
acid catalysis. Some of the recent accomplishments of his research group include
heteroconjugate addition reactions with
one amine as the catalyst and a second
amine as the reactant, unusual exoselective Diels–Alder reactions, aldehyde–aldehyde direct aldol reactions
(enabling rapid access to orthogonally
protected carbohydrates), and a-halogenations of carbonyl compounds.
Another part of his talk was devoted to
a formidable challenge in organic synthesis: the asymmetric construction of
quaternary carbons. The core architecture (“the heart of the beast”) of diazonamide A (and likewise, of pyrroloindoline alkaloids) was elegantly assembled
by Friedel–Crafts-type conjugate additions of indoles to a,b-unsaturated aldehydes coupled with the cyclization of a
pendant heteroatom donor. The fireworks ended with an alternative organocatalytic concept for complex molecule
synthesis. The efficiency of a synthetic
process decreases exponentially with
an increase in the number of sequential
operations; tandem catalysis provides a
strategic foundation to bypass this disadvantage. A highly stereoselective
tandem
iminium–enamine–enamine
one-pot process carried out for the first
time has already demonstrated promise
for facilitating asymmetric syntheses.
In his ceremonial address, Stefan
Marcinowski (BASF) introduced the
prizewinner, Frank Glorius, to the audience with a comprehensive summary of
his academic background and a list of
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
his significant scientific achievements.
This was followed by a lecture given by
the prizewinner himself. Glorius presented a blend of novel homogeneous
and heterogeneous catalytic processes.
Within the theme of homogeneous catalysis, part of his research program is
directed toward the synthesis and application of N-heterocyclic carbenes. His
research group devised a new family of
sterically demanding carbene ligands
(IBiox) with restricted flexibility and
applied them in transition-metal catalysis. The unique feature of flexible steric
constraints enables these ligands to
adapt to the changing requirements of
the catalytic cycle, thereby allowing the
intricate preparation of tetra-ortho-substituted biaryl compounds by Suzuki–
Miyaura cross-coupling of aryl chlorides. An additional research area for
Glorius is the use of carbenes in organocatalysis; thus a,b-unsaturated aldehydes can be transformed into homoenolate equivalents upon reaction with
N-heterocyclic carbenes. Through this
organocatalytic conjugate Umpolung
and the subsequent reaction with an
aldehyde, several g-butyrolactones
were made accessible in one step.
Within the theme of heterogeneous catalysis, Glorius reported on a highly diastereoselective hydrogenation of oxazolidinone-substituted pyridines accompanied by the “traceless” cleavage of the
chiral auxiliary. This single synthetic
operation affords highly enantiomerically enriched piperidine derivatives
with up to three stereocenters!
The afternoon session opened with a
talk by Amir H. Hoveyda (Boston College) about the quest for ligand–metal
combinations of broad utility in asymmetric synthesis. With a catalyst system
for the asymmetric alkylation of alkenes
as a starting point, Hoveyda nicely
delineated the subtle and unforeseeable
influences of incidental additives
(“impurities”) as well as the substrate
itself on reaction efficiency and selectivity. The targeted design of general chiral
ligands is an attractive alternative to the
painstaking search for the right substrate for a given ligand, and Hoveyda
has addressed this challenge by using
peptide-based ligands. These ligands
provide multiple sites for steric and electronic alterations, and libraries of considerable size are easily accessible by
Angew. Chem. Int. Ed. 2005, 44, 5166 – 5167
Angewandte
Chemie
parallel synthesis on solid support. With
a plethora of ligands and reasonable
mechanistic models at hand, several
transformations, such as titanium-catalyzed Strecker reactions, copper-catalyzed conjugate addition and allylic substitution reactions, and aluminum-catalyzed alkylation reactions, were carried
out with excellent enantioselectivity.
Application of the asymmetric Strecker
reaction to the synthesis of non-natural
a-amino acids led to the total synthesis
and stereochemical assignment of members of the complestatin family of natural products. Hoveyda closed his lecture
with the presentation of a novel silverbased chiral N-heterocyclic carbene
complex that has already been successfully applied toward enantioselective
ruthenium-catalyzed alkene metatheses
and copper-catalyzed allylic substitutions.
The scientific program ended with a
guided tour by Gregory C. Fu (Massachusetts Institute of Technology)
through palladium- and nickel-catalyzed
Angew. Chem. Int. Ed. 2005, 44, 5166 – 5167
coupling reactions. Equipped with a profound understanding of the importance
of electron-rich (facilitating oxidative
addition) and sterically demanding
(facilitating reductive elimination)
ligands at the transition-metal center,
Fu introduced broadly applicable tricyclohexylphosphine and tri-tert-butylphosphine ligands. These afford unusually active catalysts, which assist in a
number of cross-coupling reactions
(Suzuki, Stille, Negishi, and Heck reactions) of aryl and vinyl chlorides under
exceptionally mild reaction conditions
(C(sp2) Cl bonds appeared to be more
reactive than C(sp2) OSO2CF3 bonds).
Careful
mechanistic
investigations
revealed the pivotal influence of the
base in the resting state of the catalytic
cycle. Another focus of his research is
the development of capricious C(sp3)
C(sp3) cross-coupling reactions, which
are often thwarted by facile b-hydride
elimination. Nevertheless, Fu reported
powerful reaction protocols including
elucidation of the stereochemical
course of the oxidative addition at
C(sp3) OSO2C6H4CH3. This final part
culminated in the development of catalytic enantioselective Negishi couplings
of racemic secondary benzylic bromides
and alkyl zinc reagents.
The well-balanced content of these
four plenary lectures provided an excellent up-to-date overview of some firstrate facets of modern molecular catalysis. Interlacing the talks with poster sessions created a relaxed atmosphere in
which there was ample time for mutual
scientific and personal exchange. The
afternoon session ended with a social
get-together including complimentary
dinner for all participants. All those
infected with the “catalysis virus” will
have the opportunity to meet again in
two years at the 4th Heidelberg Forum
of Molecular Catalysis!
DOI: 10.1002/anie.200502426
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5167
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