Unveiling the Takai Olefination Reagent via Tris(tert-butoxy)siloxy Variants

The elusive Takai olefination reagent, namely, the iodo-methylidene Cr­(III) complex [Cr₂Cl₄(CHI)­(thf)₄], has been isolated by careful handling of the reaction between CrCl₂ and CHI₃ in THF at −35 °C. Alternatively, treatment of [Cr­(OSi­(OtBu)₃)₂] with CHI₃ gave the mixed-valent dihalido-methanide complex [Cr^II/III₂I₂(OSi­(OtBu)₃)₂(CHI₂)], featuring a Cr­(III)–CHI₂ moiety. In the presence of TMEDA nucleophilic attack at CHI₂ occurred generating the zwitterionic species [Cr^III(OSi­(OtBu)₃)₂(tmeda-CHI)]­[I]. Complexes [Cr₂Cl₄(CHI)­(thf)₄] and [Cr^II/III₂I₂(OSi­(OtBu)₃)₂(CHI₂)] were screened for their ability to induce monohalido olefination of benzaldehyde. Remarkably, both complexes promote olefination, with [Cr₂Cl₄(CHI)­(thf)₄] accomplishing the same E selectivity as Takai’s original mixture. Complex [Cr^II/III₂I₂(OSi­(OtBu)₃)₂(CHI₂)], however, appeared to give preferentially Z isomer, corroborating the monoiodo-methylidene species Cr­(III)–CHI–Cr­(III) as the active olefination component of the original in situ generated Takai reagent mixture.