Hydroamination has attracted considerable attention as a direct, atom-economical means of C-N bond construction. Although catalytic addition of N-H bond onto alkene or alkyne has been well investigated, the addition of tertiary amines onto unpolarized unsaturated bonds to form ammonium salt has not been investigated well. In view of recent surge of phase transfer catalysis mediated by chiral N-quat salts, it is highly desirable to have a concise access to structurally diverse ammonium salts. However, a catalytic process of non-substitution type has not been possible in the realm of cross-coupling reaction, presumably due to the enthalpic penalty.
We report herein two catalytic protocols for the formation of N-quaternary ammonium salts via intramolecular addition of tertiary amines onto alkynes. We found that the gold catalyzed addition of tertiary amines onto alkynes is highly dependent on the pKa of the solvent and appropriate balancing the acid-base property of the substrate-solvent led to a concise access to ammonium salts of various types proceeding at rt. Some of the preliminary studies on the derivatization at the alpha-position (to N-atom) will also be discussed. Alternatively, palladium catalyzed oxidative addition, aminopalladation, reductive elimination sequence provides N-quaternary ammonium salts having substituents at the beta-position.