Aluminium redox catalysis enables cyclotrimerization of alkynes

Aluminium comprises over 8% of Earth's crust and is the most abundant metallic constituent¹. Historically, aluminium catalysis has predominantly exploited the inherent Lewis acidity associated with its stable +III oxidation state². Owing to its uniquely low electronegativity (1.61)-the lowest among p-block elements-and the absence of an inert-pair effect, aluminium presents formidable intrinsic challenges for engaging in catalytic redox transformations. Here we report the redox catalytic capability of a low-valent aluminium species, carbazolylaluminylene³, which carries out a complete Al(I)/Al(III) catalytic cycle encompassing oxidative addition, double insertion, intramolecular isomerization and reductive elimination-fundamental mechanistic steps conventionally exclusive to transition-metal catalysis. Leveraging this Al(I)/Al(III) redox cycle, we achieve highly efficient and regioselective Reppe cyclotrimerization of alkynes^4,5, producing diverse benzene derivatives with a turnover number of up to 2,290. Through X-ray crystallographic and quantum chemical analyses, we elucidate how the dynamic nitrogen geometry within the carbazolyl ligand framework precisely modulates the aluminium coordination environment, thereby facilitating the catalytic cycle. This work fundamentally advances the conceptual understanding of main-group redox catalysis. It further sets a compelling precedent for future catalyst design and sustainable synthetic methodologies centred on aluminium redox transformations.