Nickel-catalyzed olefination of cyclic benzylic dithioacetals by Grignard reagents. Scope and mechanism

The details of the first nickel-catalyzed olefination of cyclic dithioacetals to form substituted styrenes and aryl-substituted 1,4-pentadienes are described. The reaction represents a new synthetic use of the dithioacetal functionality. Only nickel complexes catalyzed these cross-coupling reactions; palladium complexes displayed no catalytic activity under the reaction conditions employed. Selective coupling occurred. A mechanism for the reaction is proposed. The experimental evidence indicates that, in these nickel-catalyzed couplings, cyclic dithioacetals are more reactive than their acyclic analogues. This increased reactivity appears to be the result of maintaining the two sulfur atoms in close proximity to each other by the use of a short chain of methylene groups