Electrochemical Rhodium-Catalyzed C–H Cyclodimerization of Alkynes to Access Diverse Functionalized Naphthalenes: Involvement of Rh^IV/V and Rh^I Dual Catalysis

Abstract

The first electrochemical rhodium-catalyzed C−H cyclodimerization of alkynes for the direct construction of functionalized naphthalenes was reported. The practicality and synthetic value of this strategy were demonstrated by the readily accessible scale-up synthesis and transformation of the products. Detailed mechanistic studies evidenced that electricity played an important role during the electrochemical disproportionation (ECD) process to generate and maintain the catalytically active Rh IV/V and Rh I species, which conducted the direct C−H activation.

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 . Subsequently, 1,4-diphenylbuta-1,3-diyne 1a insertion occurs to furnish the species IV , which quickly executes reductive elimination to give the rhodium( i ) sandwich species V .…”

“…9 b However, a 3-week reaction time, low reaction yield, and the preparation of five-membered C–Rh ring intermediates limit their further applications. Focusing on the electrochemically driven cyclotrimerization of 1,3-butadiynes and following our continuing research interest in electrochemical transition metal catalysis, 10 we hypothesized that an Rh( iii ) catalyst might be reduced into Rh( i ) species, 11 which then coordinates with two molecular 1,3-butadiynes, followed by oxidative addition to deliver a decisive five-membered C–Rh metallacycle species. Herein, we reported an electrochemically driven rhodium-catalyzed cyclotrimerization of 1,3-butadiynes, enabling the synthesis of structurally diverse hexasubstituted arenes in good yields with excellent regioselectivity and broad functional group tolerance (Scheme 1c).…”

Recent Advances in Rhodium‐Catalyzed Electrochemical C−H Activation

“…Recently, Zhang, Xie, and coworkers exploited this strategy for cyclodimerization of alkynes 23 to provide substituted naphthalenes 43 (Scheme 22). [116] This rhodium‐catalyzed transformation was smoothly realized in an undivided cell setup using platinum as both anode and cathode under CCE 4.0 mA. Based on the mechanistic studies, a possible mechanism involving rhodium(IV)/(V) and rhodium(I) dual catalysis was proposed (Figure 9).…”

“…Based on the mechanistic studies, a possible mechanism involving rhodium(IV)/(V) and rhodium(I) dual catalysis was proposed (Figure 9). [116] Thus, anodic oxidation of [Cp*RhCl 2 ] 2 to highvalent rhodium(IV/V) complex to afford the desired product 43 a and regenerate the rhodium(I) species 9-V.…”

“…Based on the mechanistic studies, a possible mechanism involving rhodium(IV)/(V) and rhodium(I) dual catalysis was proposed (Figure 9). [116] Thus, anodic oxidation of [Cp*RhCl 2 ] 2 to high‐valent rhodium(IV/V) complex 9 ‐ I followed by electrophilic metalation resulted aryl‐rhodium species 9 ‐ II . Migratory insertion of two alkynes one after another gives intermediate 9 ‐ IV .…”