Dehydrative Coupling of Benzylic Alcohols Catalyzed by Bronsted Acid/Lewis Base

Traditional cross-coupling reactions show some disadvantages like the use of organohalides or the production of stoichiometric amounts of waste. The dehydrative homo- or heterocoupling of alcohols therefore arises as an interesting approach for a highly atom-economical formation of carbon-carbon bonds, since water is produced as the only by-product. We herein report a simple and direct, metal-free protocol for the synthesis of olefins by applying catalytic amounts of a sulfonic acid and triphenylphosphane under air. A variety of olefins could be synthesized from benzylic alcohols under relatively mild conditions. Additionally, dehydrative hydroarylation of benzylic alcohols with electron-rich arenes was possible by using only Bronsted acid under otherwise same reaction conditions. We could show that phosphane additives are essential to overcome oligomerization as main side reaction by the occupancy of the reactive carbocation intermediate.