Unveiling the Takai Olefination Reagent via Tris(<i>tert</i>-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 [CrII/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 [CrIII(OSi­(OtBu)3)2(tmeda-CHI)]­[I]. Complexes [Cr2Cl4(CHI)­(thf)4] and [CrII/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 [CrII/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.