TADDOL-Derived Cationic Phosphonites: Toward an Effective Enantioselective Synthesis of [6]Helicenes via Au-Catalyzed Alkyne Hydroarylation

A series
of cationic phosphonites, all sharing a TADDOL skeleton
but decorated with different positively charged substituents at phosphorus,
were synthesized and tested as chiral ancillary ligands on the Au-catalyzed
intramolecular hydroarylation of appropriate diynes toward carbo[6]­helicenes
with different substitution patterns. Our studies showed that the
Au complexes derived from phosphonites bearing 1,3-dimesityl-1,2,3-triazolium
and 1,4-dimesityl-1,2,4-triazolium substituents are the best precatalysts
for the desired cyclization in terms of regio- and enantioselectivity
of the products obtained. In contrast, all of our attempts to prepare
Au complexes from cationic phosphonites derived from CAACs failed,
and only ligand decomposition products could be isolated.