Evidence of AlII Radical Addition to Benzene

Electrophilic Al-III species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low-valent Al-I aluminyl anions have showcased oxidative additions towards arenes C-C and/or C-H bonds. Herein, we communicate compelling evidence of an Al-II radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC8 in benzene furnishes a double addition to the benzene ring instead of a C-H bond activation, producing the corresponding cyclohexa-1,3(orl,4)-dienes as Birch-type reduction product. X-ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this reactivity proceeds through a stable Al-II radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection.

Automated summary

Nucleophilic low-valent Al-I aluminyl anions have been found to undergo oxidative additions to the benzene ring, rather than C-H bond activation. By electron reduction of a ligand stabilized precursor in benzene, a double addition to the benzene ring occurs, resulting in the formation of cyclohexa-1,3(orl,4)-dienes. X-ray crystallographic analysis, EPR spectroscopy, and DFT results confirm the involvement of a stable Al-II radical intermediate in this reaction, which is attributed to a rigid scaffold and strong steric protection.