Computational Mechanistic Study on Intramolecular Triple Cyclization of 1-Biphenylethynyl-2-phenylethynylbenzenes Giving Spiro Fluorene-Phenylenevinylenes by Dual Catalysis

Cationic Au(I) complex-catalyzed double cyclizations of 1,2-dialkynylbenzene derivatives are valuable for the straightforward synthesis of ladder-type π-conjugated compounds. We have previously developed a method for synthesizing ladder-type π-conjugated molecules with spiro skeleton (spiro fluorene-phenylene vinylenes) by the triple cyclization of 1-biphenylethynyl-2-phenylethynylbenzenes using an AuCl(SMe2)/tBuXPhos/AgNTf2 catalyst system. Experimental mechanistic studies revealed that an Ag(I) complex or HNTf2, as well as a cationic Au(I) complex, catalyzes different reaction steps. However, detailed insight into which of the Ag(I) complex or HNTf2 functions as the actual cocatalyst and the origin of the selectivity of the reaction have not been elucidated. Here we report a detailed reaction mechanism for this triple cyclization by dual catalysis based on computational studies. A cationic Au(I) complex catalyzes the first and second cyclization steps, whose regioselectivity is thermodynamically and kinetically controlled. The third spirocyclization step to construct a spirocenter is catalyzed by in situ generated HNTf2 rather than an Au(I) or Ag(I) complex via a benzyl cation intermediate. The present Au(I)/Brønsted acid dual-catalyzed mechanism of the triple cyclization of 1-biphenylethynyl-2-phenylethynylbenzenes provides complimentary activation modes to the conventionally used single Au(I)-catalyzed one.