Unexpected hydrobromic acid-catalyzed C-C bond-forming reactions and facile synthesis of coumarins and benzofurans based on ketene dithioacetals.

Hydrobromic acid was found to be a unique catalyst in C-C bond-forming reactions with ketene dithioacetals. Distinctly different from other acids (including Lewis and Brønsted acids), the remarkable catalytic performance of hydrobromic acid in catalytic amounts was observed in the "acid"-catalyzed reactions of readily available functionalized ketene dithioacetals 1 with various electrophiles. Under the catalysis of 0.1 equivalents of hydrobromic acid, the reaction of 1 with carbonyl compounds 2a-l gave polyfunctionalized penta-1,4-dienes 3 or conjugated dienes 4 in good to excellent yields. The reaction tolerated a broad range of substituents on both the ketene dithioacetals 1 and the carbonyl compounds 2. Application of this efficient C-C bond-forming method generated coumarins 5 and benzofurans 7 under mild, metal-free conditions by hydrobromic acid-catalyzed reactions of 1 with salicylaldehydes 2m-o and p-quinones 6a-d, respectively. A new reactive species, a sulfur-stabilized carbonium ylide, formed depending on the nature of the counterion, and this was proposed as the key intermediate in the unique catalysis of hydrobromic acid.