The scope of this reaction was then investigated with a
variety of N-tosylaldimines. In all cases the reactions proceeded
efficiently with good yields (61–96%) and good to high ee values
(53–95%) (Table 3). Alkyl or phenoxyl substituted aldimines
gave good ee values (84–85%). As fluoro substituted aromatic
compounds display special bioactivities, aldimines 3e, 3f, and
3g bearing a fluoro atom at the para, meta, or ortho position of
the aromatic ring, respectively, were tested under the optimal
reaction conditions. They were all good electrophiles to participate
in the transformations and provided high stereoselectivities
(91–92% ee) (entries 5–7). Aromatic aldimine 3h bearing –CF3
at the para position leads to a moderate ee value (80% ee,
entry 8). N-Tosylaldimine 3i with –COOMe at the para
position also gave a moderate ee value (70% ee, entry 9).
Aldimine 3j derived from 1-naphthaldehyde gave the highest
ee value (95% ee) and 85% chemical yield (entry 10). When
2-naphthyl aldimine 3k was employed, a decreased stereo-
selectivity (80% ee, entry 11) resulted. Notably, hetero-
aromatic 2-furan-substituted aldimine 3l was proved to be
an appropriate electrophile, affording high yield 93% and
88% ee value (entry 12). For the less reactive alkyl aldimines
3m and 3n, the reactions gave good yields and moderate
stereoselectivities (72% and 53% ee, entries 13 and 14). The
Friedel–Crafts reactions of other phenols were also explored.
The reactions of 3-methoxylphenol and 3-tert-butylphenol
only proceeded well above 0 1C and afforded the products
with moderate yields and ee values (entries 15 and 16).14
Unactivated phenols did not react in the current system. The
Friedel–Crafts reaction between more nucleophilic indole and
3a in the presence of 10 mol% catalyst 1k gave the corre-
sponding chiral amine in 89% yield and 90% ee.13
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In summary, the first enantioselective Friedel–Crafts reac-
tions between electron-rich phenols and N-tosylaldimines were
reported. The bifunctional catalyst used can be readily synthe-
sized from L-leucine in 5 steps with 81% overall yield. The
reactions provided chiral benzylic amines in high yields and up
to 95% ee.
This work was financially supported by the National
Natural Science Foundation of China (21072098), Program
for New Century Excellent Talents in University, and the 111
Project (B06005).
Notes and references
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13 For details, see ESIw.
14 The N-tosylaldimine prepared from pivalaldehyde did not react
with 3,5-dimethoxylphenol due to the steric hindrance of the tert-
butyl group.
c
5520 Chem. Commun., 2012, 48, 5518–5520
This journal is The Royal Society of Chemistry 2012