Electrochemical Cycloaddition Reactions of Alkene Radical Cations: A Route toward Cyclopropanes and Cyclobutanes

Herein, we describe a mild and efficient electrochemical method for cycloaddition reactions of alkene radical cations. Anodic oxidation of olefins produces electrophilic alkene radical cations, which further react with either diazo compounds in a [2 + 1] cycloaddition toward cyclopropane synthesis, or styrene derivatives in a [2 + 2] cycloaddition producing cyclobutanes. Both processes are green, metal- and catalyst-free, and scalable and tolerate a broad range of electron-rich olefins.


Electrochemical Radical Synthesis of Cyclobutanes from olefins and styrenes
General procedure: A electrolyte (c = 0.125) was placed in a 5 mL vial with a septum and dissolved in MeCN (c = 0.15 M). Then, the reaction mixture was then degassed for 10 min with Ar and then transanethole (1k, 1.0 equiv.) and styrene (7.5 equiv.) were added and stirred under constant electrical power (I = 10 mA) for 5 hours. As both anode and cathode graphite electrodes were used with rapid altering polarity (1 min). The electrical power was was turned off, the reaction mixture was concentrated and subjected to flash column chromatography (hexane/AcOEt).   NBu4ClO4
Results: The addition of TEMPO (at the beginning of the reaction) stops the reaction completely, thus confirming the radical nature of the transformation.
[2+2] cycloaddition: General procedure: A electrolyte (c = 0.125) and TEMPO (2 equiv.) was placed in a 5 mL vial with a septum and dissolved in MeCN (c = 0.15 M). Then, the reaction mixture was then degassed for 10 min with Ar and then trans-anethole (1.0 equiv.) and styrene (7.5 equiv.) were added and stirred under constant electrical power (I = 10 mA) for 5 hours. As both anode and cathode graphite electrodes were used with rapid altering polarity (1 min). The electrical power was was turned off, the reaction mixture was concentrated and and checked for product.
Results: The addition of TEMPO (at the beginning of the reaction) stops the reaction completely, thus confirming the radical nature of the transformation.

5b. Cis vs trans selectivity
General procedure: A electrolyte (0.5 mmol in 4 mL MeCN, c = 0.125), olefin (A or B) were placed in a 5 mL vial with a septum and dissolved in dry MeCN (c = 0.125 M). The reaction mixture was then degassed for 10 min with Ar and then ethyl diazoacetate (1.2 equiv.) was added and stirred under constant electrical power (I = 5 mA) for 10h. As both anode and cathode graphite electrodes were used with rapid altering polarity (1 min). The electrical power was turned off, the reaction mixture was concentrated and checked for product.
GC chromatograms after reactions for A (black) and B (green): NMR data for the both products is also the same.
Results: Both olefins with an E or Z configuration gave only trans-cyclopropane, suggesting that the reaction proceeds via a radical-cation intermediate. The only difference between reaction for A and B is the yield of the reaction (A -82%, and B -67%), suggesting that trans-olefin undergoes cyclopropanation faster and isomer cis.