(Table 4). All reactions proceeded in good to excellent yields
and, generally, excellent enantioselectivities. Importantly, the
Friedel–Crafts reaction of an aliphatic aldehyde derived imine
was also examined and afforded the expected product in good
yield and enantioselectivity (75% yield, 81% ee, 92 h, 5 mol%
catalyst loading, entry 14). Current efforts are focused on the
optimization of catalyst and conditions for aliphatic imine
substrates, and the results will be reported in due course.
The results above show that the chiral squaramide family of
hydrogen bond donors are highly effective catalysts for the
enantioselective Friedel–Crafts reaction of indoles with
imines. The ease of preparation combined with their modular
nature allows for convenient tuning of the chiral environment
and the pKa of the donor hydrogens of squaramides.
This work is supported by the National Basic Research
Program of China (2010CB923303), the National Science
Foundation of China (NSFC) (Grants no. 20902045). VHR
thanks the National Institutes of Health (R01GM069990) for
partial support.
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estimated to be 33% larger than in thioureas. See: J. P. Malerich,
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