N-Heterocyclic Carbenes for Conjugate Additions of Arylboronic Acids

Significance

An air and moisture tolerant N-heterocyclic carbene (NHC) catalyst for the conjugate addition of arylboronic acids is described. This catalyst provides an improved procedure for the synthesis of a GABA analogue with the key addition adduct being produced with 99% ee. Enones with various ring sizes were tested and cyclohexenone gave the high enantioselectivies (>94% ee) while cyclopentenone and cycloheptenone were obtained in lower enantioselectivities (<90% ee). Importantly, both electron-withdrawing and electron-donating substituents on the arylboronic acid showed very high selectivity (>94% ee for addition to cyclohexenone).