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

摘要

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 degrees C. Alternatively, treatment of [Cr(OSi(OtBu)(3))(2)] with CHI3 gave the mixed-valent dihalido-methanide complex [(Cr2I2)-I-II/III(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 [(Cr2I2)-I-II/III(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 [(Cr2I2)-I-II/III(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.