Asymmetric Conjugate Addition of Arylboronic Acids to Enones Catalyzed by Rhodium-Monodentate Phosphoramidite Complexes in the Presence of Bases

Abstract

Rhodium(I)-catalyzed 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds was carried out in the presence of a chiral phosphoramidite ligand based on (R)-binol and dialkylamines. The reaction was significantly accelerated in the presence of a base such as KOH and Et₃N, allowing the reaction to be completed within 6 hours at 50 °C. The addition to 2-cyclohexenone achieved enantioselectivities up to 99%, though they were less effective for 2-cyclopentenone (79% ee), 2-cycloheptenone (77% ee) and acyclic enones (31-43% ee).

arylboronic acids - rhodium catalyst - phosphoramidite - asymmetric - conjugate addition

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9

Addition of phenylboronic acid to 3-nonen-2-one (entry 12) showed the enantioselectivities suggesting the superiority of bulky dialkylamino ligands; 4a (1% ee), 4b (43% ee), 4c (7% ee), 4d (26% ee) and 4e (12% ee).