New chiral amine ligands in the desymmetrization of prochiral phosphine boranes
Journal article, 2004
P-chirogenic phosphine ligands can be prepared via desymmetrization of prochiral phosphine boranes using s-BuLi.(-)-sparteine complexes. One limitation of this method, however, has been that (+)-sparteine is not easily accessible. Herein, we show that derivatives of another alkaloid, (-)-cytisine, are useful surrogates for (+)-sparteine in the desymmetrization of prochiral phenyl-, cyclohexyl- and tert-butyl dimethyl phosphine boranes, yielding chiral phosphine boranes in up to 92% ee. Other chiral diamines were also tested but did not give as high enantioselectivity as the (-)-cytisine derivatives.