A series of Pd(II) complexes containing chiral
diamine ligands were investigated as asymmetric
catalysts for the rearrangement of allylic imidates to allyl amides.
The best catalysts were cations
obtained by dechlorination of
dichloro[(S)-2-(isoindolinylmethyl)-N-methylpyrrolidine]palladium(II) (17) with silver salts in
CH2Cl2. Catalyst 18 was
studied thoroughly and shown by 1H NMR
and X-ray crystallography analysis to be a C
1
symmetric dimer (Figure ). A series of related
catalysts 22−27 having various counterions and
anionic ligands were also prepared and studied
as asymmetric catalysts for the rearrangement of allylic
N-(4-trifluorophenyl)benzimidate 29
to
allylic benzamide 30 (eq 4). Rearrangement of
29 in CH2Cl2 (48 h at 40 °C)
in the presence of 5
mol % of 18 affords (−)-30 in 69% yield and
55% ee. Enantioselection is increased to 60% when
an
isomerically pure sample of 18 is employed. Chemical
correlation of allylic benzamide 30 with
(R)-norvaline established that (−)-30 has the
R absolute configuration (Scheme ). A
cyclization-induced rearrangement mechanism (Scheme ) requires that in the major
pathway the imidate
nitrogen attacks the re face of the olefin with Pd
coordinated to the si face. These studies
constitute
the first report of asymmetric catalysis of the rearrangement of
allylic imidates to allylic amides.
However, significant hurdles remain to be overcome before the
enantioselective rearrangement of
allylic imidates becomes a practical route to enantioenriched nitrogen
compounds.
Pyrimidine derivatives of the cinchona alkaloids function as excellent asymmetric catalysts for the "Interrupted" Feist-Bénary Reaction. This reaction produces highly substituted hydroxydihydrofurans from simple starting materials under mild conditions. The asymmetric reaction gives high enantioselectivities with unsubstituted bromoketones, and high enantio- and diastereoselectivities with substituted substrates. Mechanistic experiments suggest that the hydrobromide salt of the alkaloid derivative is the active catalyst for the reaction.
[reaction: see text] The cinchona alkaloid-catalyzed dimerization of monosubstituted ketenes generated in situ from the reaction of acid chlorides and diisopropylethylamine yields ketene dimers in high yields and enantioselectivities. This reaction tolerates sterically demanding and functionally diverse substituents. Kinetic studies suggest that the rate-determining step for the reaction is the deprotonation of the acid chloride by the tertiary amine to form ketene and that the stereochemistry-forming step is addition of an ammonium enolate with ketene.
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