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4001861 
Journal Article 
Enantioselective synthesis of fluorene derivatives by chiral phosphoric acid catalyzed tandem double Friedel-Crafts reaction 
Sun, FL; Zeng, M; Gu, Q; You, SL 
2009 
Yes 
Chemistry: A European Journal
ISSN: 0947-6539
EISSN: 1521-3765 
15 
35 
8709-8712 
English 
19644985 
WOS:000270045500010 
A tandem double Friedel Crafts reaction of indoles with 2-formylbiphenyls, leading to 9-(3-indolyl) fluorene derivatives with up to 96% ee was reported. Several solvents, namely, benzene, dichioromethane, and carbon tetrachloride, were screened and the use of carbon tetrachloride which resulted in an excellent 92% ee. The dramatic decrease of both the ee value and the reaction time when compared with those of 2-methylindole, indicated that the reaction proceeded preferentially through intermediate II, in which the enantiocontrol is realized through a 3-benzylidene-3H-indolium intermediate. The desired fluorene derivatives were formed in good to excellent yields and with excellent ee values. This represents the first enantioselective synthesis of fluorene derivatives. The activation of both carbonyl and hydroxy groups by chiral phosphoric acid and the high enantiocontrol through the close proximity of the chiral phosphate counterion to the 3- benzylidene-3H-indolium ion also provides a new reaction model for chiral phosphoric acid catalysis.