Electrochemical Enol Ether/Olefin Cross‐Metathesis in a Lithium Perchlorate/Nitromethane Electrolyte Solution

Abstract

Transition-metal-catalyzed olefin cross-metathesis plays an important role in the construction of various carbogenic skeletons by allowing the exchange of substituents between different olefins via an activated key intermediate consisting of a four-membered ring coordinated to a transition metal. [1] Alternatively, electrochemical reactions have proven to be a viable method in triggering reactions between different olefins by reversing the polarity of alkenes [2] and initiating radical anion- [3] (or cation- [4] ) based reactions.[5] Electrochemically activated olefin cyclodimerization, for example, can effectively lead to the intermolecular formation of fourmembered rings.[6] Moreover, we reported an electrocatalytic intermolecular formal [2 + 2] cross-coupling of electron-rich olefins (Scheme 1).[7] The anodic oxidation of enol ethers [8] that bear a methoxyphenyl group can be used to trigger an intermolecular formal [2 + 2] cycloaddition with varied alkenes. In this case, the intermolecular olefin coupling was assisted by a unique lithium perchlorate/nitromethane electrolyte solution [9] and the methoxyphenyl group of the substrates served as an intramolecular electron donor [10] to form the cyclobutane ring. Herein, we describe novel electrochemical olefin cross-metatheses between anodically activated enol ethers and aliphatic alkenes.Initially, the electrolytic intermolecular reaction between 4-methoxybut-3-enylbenzene (1) and hex-1-ene (2) was investigated. The anodic oxidation of enol ether 1 involved a carbon felt anode, a constant potential of 1.43 V (vs. Ag/ AgCl), a carbon felt counter electrode, and a lithium perchlorate/nitromethane electrolyte solution (1.0 m) in an undivided cell under argon. Remarkably, the anodic activation of enol ether 1 in the presence of 2 resulted in the formation of but-3-enylbenzene (3) in 78 % yield (Scheme 2).In our experiments, the reaction was deemed complete after the passage of approximately 1.0 F mole À1 of electricity. Under different electrolytic conditions, the desired product was obtained in trace or in unacceptable yields (Table 1). In the absence of electrolysis, a mixture of 1 and 2 in lithium perchlorate/nitromethane electrolyte solution at ambient temperature did not react, even after more than 10 h. Furthermore, the reaction was responsive to a stepwise application of the electricity. Because the amount of the product for each stage remained essentially unchanged during the intervals of electrolysis, an external switch of the electrolysis allows the regulation of the reaction process.The electrochemical enol ether/olefin cross-metathesis can also be extended to the combination of simple alkyl enol ethers and alkenes. For example, anodic oxidation of 1-methoxy-tridec-1-ene (4) in the presence of 2 afforded the desired tridec-1-ene (5) in 76 % yield (Scheme 3).To gain insight into the reaction mechanism, the anodic activation of enol ether 1 was carried out with deuteriumScheme 1. Anodic intermolecular formal [2 + 2] cross-coupling of olefins.Scheme 2....

Interplay of arene radical cations with anions and fluorinated alcohols in hole catalysis

“…Hole catalysis is known in peri-cyclization reactions including [2+1] 13,14 , [2+2] [15][16][17][18][19][20] , and [3+2] 21 cycloadditions and Diels-Alder reactions [22][23][24] . Olefin metathesis reactions 25 and polymerizations 26 mediated by radical cations have also been reported. Recently, researchers proposed that radical cations from styrene derivatives react with alcohols and carbonyl groups to realize radical addition via hole catalytic manner, where carbonyl group works as a intermolecular radical acceptor for the first time 27 .…”

“…In such reactions, radical cations are generated by various methods, including photoredox catalysts 14,17,19,21,23 , semiconducting nanomaterials 30,31 , chemical oxidants 7,20 and electrochemistry 16,24,25 . Due to the high reactivity of the radical cation intermediate, the efficiency in hole catalysis is highly dependent on the reaction media.…”

Cationic Polymerization: From Photoinitiation to Photocontrol

“…Chemie organocatalyzed photoredox transformation. [9] Inspired by the studies of Chiba on electrochemical [2+ +2] cycloadditions [54] and the emergence of photocontrolled polymerization, [5][6][7][8] they combined the vinyl ether initiator 19,a n organic photocatalyst [2,4,6-tri(4-methoxyphenyl)pyrylium tetrafluoroborate (20)],and blue-light irradiation to promote the polymerization of norbornene (21), an archetypal ROMP monomer (Figure 12 a).…”

Stereoretention in styrene heterodimerisation promoted by one-electron oxidants

“…Since the rst report of photochemical [2 + 2] cycloaddition accessing a 4-membered carbocycle by Liebermann in 1877, 6 the construction of 4membered cyclobutyl rings from photo-cycloaddition of alkenes has become arguably the most employed technique for cyclobutanation. [7][8][9][10][11][12] Cyclobutanation can now also be readily achieved via organocatalysis, [13][14][15] organometallic catalysis, 7,[16][17][18] as well as electrocatalysis [19][20][21][22] (Scheme 1). Until recently when the synthesis of substituted cyclobutanes could be realised via selective C-H functionalisation of unsubstituted cyclobutanes, [23][24][25][26] formal [2 + 2] cycloaddition remained the main strategy for the synthesis of complex, tetra-substituted cyclobutyl rings.…”