Gold-Catalyzed Cyclizations of cis-Enediynes: Insights into the Nature of Gold-Aryne Interactions

摘要

ortho--Benzynes and ortho-arynes are reactive and versatile intermediates in organic synthesis. Their diverse chemistry has been further enriched by the formation of metal aryne complexes. They are mostly generated by elimination of two adjacent groups or atoms on the arene ring. The elimination strategy, however, can be synthetically limiting because of difficulties in accessing suitably substituted arenes, demanding reaction conditions, and operational hazards. A notable exception is a de novo aryne formation by intramolecular hexadehydro Diels-Alder reactions. Considering that the coordination of alkynes to gold complexes often serves as the point of entry into versatile gold catalysis, it is notable that an aryne has seldom served as the substrate for gold catalysis. Although an aryne is highly electrophilic as a result of its low lying LUMO, the HOMO of benzyne was computed to have an energy level similar to that of 2-butyne. Hence, it is not unreasonable to anticipate that the interaction between electrophilic gold complexes and arynes would be plausible. In fact, the study by Zhang and coworkers has invoked a benzyne gold complex, but the interaction between in situ generated benzyne and [PhjPAuCl] is not clearly defined and leaves some doubt about its existence. In contrast, another study used in situ generated benzyne to trap a gold intermediate en route to anthracene derivatives, wherein no interaction between gold and benzyne is proposed. We envisioned that gold-aryne interactions, if understood, could serve to advance gold catalysis. Herein, we disclose a study inferring that the interaction between a cationic gold complex and an aryne (i.e., a gold aryne complex) is a transition state and leads to regioisomeric ortho-aurophenyl cation intermediates, which could either be trapped by weak nucleophiles or undergo intramolecular C-H insertions by a-carbene gold carbene mesomers.