4.8 Article

Asymmetric para-Selective aza-Friedel-Crafts Reaction of Phenols Catalyzed by Bulky PyBidine-Ni(OAc)2

Journal

ACS CATALYSIS
Volume 13, Issue 14, Pages 9495-9501

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acscatal.3c01961

Keywords

Friedel-Crafts reaction; phenol; sulfonylaldimine; asymmetric catalyst; regioselective; nickel; strontium

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The conventional acid-catalyzed aza-Friedel-Crafts reaction of phenols with imines typically shows ortho-selectivity. However, a chiral bis(imidazolidine) pyridine (PyBidine)-Ni(OAc)(2) catalyst can switch the regioselectivity to para-selectivity. This switch is facilitated by the bulky PyBidine-Ni(OAc)(2) catalyst with a Ph2CHCH2 substituent, resulting in highly para-selective aza-Friedel-Crafts reactions (up to 99:1 para/ortho selectivity).
The conventional acid-catalyzed aza-Friedel-Craftsreactionof phenols with imines typically proceeds in an ortho-selective manner. A chiral bis(imidazolidine) pyridine (PyBidine)-Ni(OAc)(2) catalyst switches the regioselectivity to para-selectivity. By replacement of the benzyl substituent of the parentPyBidine with a bulky Ph2CHCH2 substituent,the bulky PyBidine-Ni(OAc)(2) catalyst enabledhighly para-selective aza-Friedel-Crafts reactions(up to 99:1 para/ortho selectivity).With the assistance of a Sr(OAc)(2) additive, a wide rangeof 3,5-dialkoxyphenols with sulfonylaldimines gave para-substituted products with up to 93% ee. DFT calculations suggestthat the cooperative activation from nickel phenoxide formation (HOMOactivation) and the hydrogen bonding activation of the sulfonylaldimine(LUMO activation) is the key for switching the regioselectivity.

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