A series of electrostatically enhanced phosphoric acid catalysts were synthesized and studied. These compounds possess two positively charged N-octylpyridinium or triarylphosphonium ion centers at the 3,3′-positions of the (R)-BINOL backbone to enhance reactivity and provide needed steric bulk for enantioselective transformations. Catalytic activities for Friedel-Crafts alkylations of indoles with trans-β-nitrostyrenes were studied. Both types of catalysts accelerate reaction conversions relative to noncharged analogues, and good enantioselectivities up to 90% ee are observed with the phosphonium-ion-tagged phosphoric acids. This transformation also can be scaled up to synthetically useful amounts, affording >250 mg of product without losing any reactivity or selectivity.
Bibliographical noteFunding Information:
Generous support from the National Science Foundation (CHE-1665392) and the Petroleum Research Fund (55631-ND4) as administered by the American Chemical Society is gratefully acknowledged.
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