Aluminum Redox Catalysis: Reppe Cyclotrimerization of Alkynes

Aluminum, as the third most abundant element comprising over 8% of the Earth's crust, is the most prevalent metallic constituent in nature. ¹ Historically, aluminum catalysis has predominantly exploited the inherent Lewis acidity associated with its stable +III oxidation state. ² Due to its uniquely low electronegativity (1.61)—the lowest among p-block elements—and the absence of an inert pair effect, aluminum presents formidable intrinsic challenges for engaging in catalytic redox transformations. Herein, we disclose the unprecedented redox catalytic capability of a low-valent aluminum species, carbazolylaluminylene, ³ which executes 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 2290. Through X-ray crystallographic and quantum chemical analyses, we elucidate how the dynamic nitrogen geometry within the carbazolyl ligand framework precisely modulates the aluminum coordination environment, thereby facilitating the catalytic cycle. This work not only fundamentally advances the conceptual understanding of main-group redox catalysis but also sets a compelling precedent for future catalyst design and sustainable synthetic methodologies centered around aluminum redox transformations.