Iron-Catalyzed Dehydrogenative Coupling of Alcohols and Thiols with Wittig Reagents

The direct olefination of alcohols via Wittig reactions under dehydrogenative conditions remains rare, especially with catalysts based on earth-abundant metals. We report an iron-catalyzed dehydrogenative coupling of alcohols with phosphorus ylides using well-defined Knölker-type (η4-cyclopentadienone)iron complexes, under conditions that do not require an external hydrogen acceptor. Selected primary and secondary alcohols undergo efficient olefination with stabilized ylides to afford di- and trisubstituted alkenes in high yields with complete stereoselectivity, with concomitant release of molecular hydrogen and no detectable hydrogenation of the products. Control experiments and DFT studies support a homogeneous, nonradical mechanism in which the in situ-generated carbonyl compound is intercepted by the ylide. The data are consistent with a dehydrogenative pathway that may involve H₂ evolution, with the base promoting the Wittig step rather than alcohol dehydrogenation. A proof-of-concept extension to a thio-Wittig reaction from thiols is also demonstrated, avoiding the use of unstable thioaldehydes.