Aryl radical cyclization with alkyne followed by tandem carboxylation in methyl 4-tert-butylbenzoate-mediated electrochemical reduction of 2-(2-propynyloxy)bromobenzenes in the presence of carbon dioxide

Sequential Vinyl Radical Cyclization/Fixation of Carbon Dioxide through Electrochemical Reduction of Vinyl Bromide in the Presence of an Electron‐Transfer Mediator

“…The thus‐generated aryl radicals were found to undergo aryl radical cyclization with a carbon–carbon double bond, and one‐electron reduction of the resulting cyclized radicals followed by reaction of the generated anion species with carbon dioxide took place efficiently to give cyclized carboxylic acids in good yields and with good selectivity . Recently, we also reported a unique electrochemical fixation of two molecules of carbon dioxide in an aryl radical cyclization reaction by using a carbon–carbon triple bond as an aryl radical acceptor . In the course of our studies on electroorganic synthesis, electrochemical reduction of vinyl bromide by using methyl 4‐ tert ‐butylbenzoate as an electron‐transfer mediator was found to generate a vinyl radical with high selectivity even in the presence of carbon dioxide.…”

“…An increase in the amount of 1 a in the CV of methyl 4‐ tert ‐butylbenzoate resulted in an increase in reduction peak current and a decrease in oxidation peak current of methyl 4‐ tert ‐butylbenzoate. Similar phenomena were observed in the electroreductive generation of aryl radicals from bromoarenes by using the same mediator system . These results strongly suggest that methyl 4‐ tert ‐butylbenzoate plays an important role and also works as an electron‐transfer mediator in the present electroreductive generation of vinyl radicals from vinyl bromide.…”

“…[27] Recently,w ea lso reported au nique electrochemical fixation of two molecules of carbon dioxide in an aryl radicalc yclization reactionb yu sing ac arbon-carbon triple bond as an aryl radical acceptor. [28] In the course of our studies on electroorganic synthesis, [29] electrochemical reduction of vinyl bromide by using methyl 4-tert-butylbenzoate as an electron-transfer mediatorw as found to generate av inyl radicalw ith high selectivity even in the presence of carbon dioxide. Moreover,t he thus-generated vinyl radical underwent radicalc yclization followedb yf ixation of carbon dioxide with carbon-carbon bond formation.T ot he best of our knowledge, this is the first example of electrochemical vinyl radicalc yclization generatedf rom vinyl bromides withoutat ransition-metal catalysta nd the sequential fixation of carbon dioxide with carbon-carbon bond formation as at ermination step.…”

“…[31] The fact that vinyl radicalc yclization products werea ctually obtained by the present electrochemical carboxylation of vinyl bromidew ith an appropriate radical acceptor under similar conditions is clear evidence of our previous assertion. On the other hand, similar electrolysis of 1a in the presence of 0.5 equivalents of methyl 4-tert-butylbenzoate as an electrontransfer mediator [27,28] gave 2a with higher selectivity,9 2% (entry 3). In this case, as malla mount of bicyclic carboxylic acid 4a,w hich was produced by tandemr adicalc yclization followed by fixation of carbon dioxide, was also detected by 1 HNMR spectroscopy.B oth 2a and 4a were produced by the reactiono ft he vinyl radical, whereas 3a was formed by the reaction of the vinyl anion.B ased on these points, the ratio of vinyl radical reaction/vinyl anion reaction can be determined as 97:3.…”

Organic Electrosynthesis: Applications in Complex Molecule Synthesis

“…The resulting radical undergoes a second reduction to afford a carbanion that reacts with CO 2 to afford the cyclized carboxylic product. The strategy was recently expanded to alkynes (Scheme ) …”