Journal
ACS CATALYSIS
Volume 12, Issue 12, Pages 6874-6886Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.2c01404
Keywords
cyclotrimerization; regioselectivity; mediator; electrochemical; reduction; terminal alkynes; leaching
Categories
Funding
- National Research Foundation of Korea (NRF) [2019R1A2C2004902, 2022R1A4A2000778]
- Samsung Science & Technology Foundation [SSTF-BA190113746]
- National Science Foundation (NSF) [CHE-1352663]
- National Research Foundation of Korea [2019R1A2C2004902, 2022R1A4A2000778] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)
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Electrochemical access to organometallic redox reagents allows for selective synthesis of various benzene derivatives with different regioisomers. A unified electrochemical strategy is presented for the selective cyclotrimerization of terminal alkynes to access both 1,3,5- and 1,2,4-regioisomeric trisubstituted benzene derivatives. The regiocontrol can be switched by the addition of a carboxylic acid.
Electrochemical access to organometallic redox reagents can broaden the potential and impact of organic syntheses, as a selective reaction pathway could be achieved. Herein, we present a unified electrochemical synthetic strategy for the paired electrochemical [2 + 2 + 2] cyclotrimerizations of terminal alkynes to selectively access both 1,3,5- and 1,2,4-regioisomeric trisubstituted benzene derivatives. The regiocontrol in our process can be simply switched by the addition of a carboxylic acid. In the presence of an acid, 1,3,5-isomers were synthesized exclusively, whereas in the absence of acid, 1,2,4-isomers were formed predominantly. The scope of this electrochemical cyclotrimerization was surprisingly broad and mildly effective, tolerating both electron-rich and electron-poor substrates, including redox-sensitive bromide substitutions. Detailed mechanistic investigations involving cyclic voltammetry, electron paramagnetic resonance spectroscopy, deuterium exchange, and quantum mechanical computations were performed. Mechanistically, the pivot between the 1,3,5- and 1,2,4-selectivity appears to be governed by the protonation of the reduced alkyne which shifts the radical distribution from a terminal to an internal position and thus in turn dictates the regiocontrol of the subsequent radical coupling processes. The data are consistent with a mechanism where the reduction of the alkyne occurs by Ni(I) in both the 1,3,5- and 1,2,4-processes.
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