Electrochemical Cobalt-catalyzed Cyclotrimerization of Alkynes to 1,2,4-Substituted Arenes

Abstract

The transition-metal-catalyzed [2 + 2 + 2] cyclotrimerization of alkynes is one of the most straightforward routes for constructing six-membered ring motifs. Although this strategy possesses high atom/step economy and readily available substrates, the catalyst cycling is the key problem. Herein, we disclosed a facile and efficient cobalt-catalyzed cyclotrimerization of alkynes through electrochemical tools. Both terminal and internal alkynes were tolerated under the mild reaction conditions, affording the 1,2,4-trisubstituted and hexasubstituted benzenes with high regioselectivity. Electrochemical redox accurately tuned the valence of Co species to cycle the catalyst smoothly during the whole reaction.

Rhodium-catalyzed electrochemical [2 + 2 + 2] cyclotrimerization of 1,3-butadiynes toward hexasubstituted arenes

“…Based on the above-described experimental results and previous reports, 6–8,10,11 a convincing catalytic cycle for the exciting rhodium-catalyzed electrochemical cyclotrimerization mechanism is presented in Scheme 2. Initially, the catalyst precursor [Cp*RhCl 2 ] 2 is activated by NaOAc to generate Cp*Rh(OAc) 2 , followed by cathodic reduction to generate Cp*Rh( i ) species I , 11 which undergoes successive two molecular 1,4-diphenylbuta-1,3-diyne 1a coordination and oxidative addition to produce the five-membered C–Rh metallacycle species II .…”

“…In 2021, Lei, Yi, and co-workers first reported a facile and powerful approach for achieving the regioselective [2 + 2 + 2] cyclotrimerization of terminal or internal alkynes to deliver polysubstituted arenes by merging cobalt catalysis with electrocatalysis (Scheme 1b, up). 7 Therein, a Co( ii ) catalyst was initially reduced into the reactive Co(0) species by cathode reduction, which would sequentially undergo Co–alkyne coordination and oxidative addition to afford the key five-membered metallacycle species. However, only four examples of non-symmetrical aryl, alkyl, and alkyne groups were examined for 1,2,4-selective delivery of hexasubstituted arenes in lower yields (28–48% yields).…”

Ni‐Catalyzed Regioselective Cyclotrimerization of Internal Esteryl Alkynes towards Polysubstituted Benzene Rings

“…Cyclotrimerization of alkynes is an effective method for the construction of polysubstituted benzene derivatives and an important cornerstone for the synthesis of new materials and bioactive compounds with high atomic economy [1–10] . Since the first proposal by Schweckendiek and Reppe for transition metal‐catalyzed trimerization of alkynes in 1948, [11] a variety of polysubstituted benzene derivatives have been achieved in the presence of Ru, [12–16] Rh, [17–22] or Pd [23–27] catalysts, among others [28–35] .…”

Electrochemical Rhodium-Catalyzed C–H Cyclodimerization of Alkynes to Access Diverse Functionalized Naphthalenes: Involvement of RhIV/V and RhI Dual Catalysis

“…In particular, electrochemical C–H functionalization represents one of the most promising reaction types, since this logic not only avoids prefunctionalization of the substrate but also might provide a mechanistically distinct catalytic model from thermal conditions . Recent successes involving electrochemical cyclotrimerization ([2 + 2 + 2] cycloaddition) of alkynes under Co or Ni catalysis to synthesize 1,2,4- and 1,3,5-trisubstituted benzenes have been achieved by Lei and Park, , respectively, which further demonstrated the synthetic potential of electrochemistry (Figure b). Nevertheless, electrocatalytic C–H cyclodimerization ([4 + 2] cycloaddition) of aromatic alkynes is unknown thus far.…”