Development of Titanocene-Catalyzed Multicomponent Coupling Reactions

A titanocene catalyst system is described herein for the synthesis of tertiary carbon centers. Using catalytic titanocene, phosphine, and zinc dust, zinc acetylides can be generated from the corresponding iodoalkynes to affect sequential nucleophilic additions to aromatic aldehydes. The intermediate propargylic alkoxides are trapped in situ with acetic anhydride, which are susceptible to a second nucleophilic displacement upon treatment with a variety of electron rich species, including acetylides and electron-rich aromatics. The use of two acetylide units as the nucleophilic components provides symmetrical and unsymmetrical 1,4-diynes. This adaptable methodology also includes electron-rich arenes as the third nucleophilic component, to provide substituted butenolides and diarylethynyl methanes in good yields. Additional investigation into the catalytic metalation capability of titanocene provides novel entries to chalcones from α-halo ketones and aldehydes, and homopropargyl alcohols from propargylic acetates and aldehydes. Finally, an alternative approach to diarylethynyl methanes from terminal alkynes and Brønsted acid is described using indium catalysis.