Unveiling the Takai Olefination Reagent via Tris(<i>tert</i>-butoxy)siloxy Variants

Published on 2018-10-11T13:36:41Z (GMT) by
The elusive Takai olefination reagent, namely, the iodo-methylidene Cr­(III) complex [Cr<sub>2</sub>Cl<sub>4</sub>(CHI)­(thf)<sub>4</sub>], has been isolated by careful handling of the reaction between CrCl<sub>2</sub> and CHI<sub>3</sub> in THF at −35 °C. Alternatively, treatment of [Cr­(OSi­(O<i>t</i>Bu)<sub>3</sub>)<sub>2</sub>] with CHI<sub>3</sub> gave the mixed-valent dihalido-methanide complex [Cr<sup>II/III</sup><sub>2</sub>I<sub>2</sub>(OSi­(O<i>t</i>Bu)<sub>3</sub>)<sub>2</sub>(CHI<sub>2</sub>)], featuring a Cr­(III)–CHI<sub>2</sub> moiety. In the presence of TMEDA nucleophilic attack at CHI<sub>2</sub> occurred generating the zwitterionic species [Cr<sup>III</sup>(OSi­(O<i>t</i>Bu)<sub>3</sub>)<sub>2</sub>(tmeda-CHI)]­[I]. Complexes [Cr<sub>2</sub>Cl<sub>4</sub>(CHI)­(thf)<sub>4</sub>] and [Cr<sup>II/III</sup><sub>2</sub>I<sub>2</sub>(OSi­(O<i>t</i>Bu)<sub>3</sub>)<sub>2</sub>(CHI<sub>2</sub>)] were screened for their ability to induce monohalido olefination of benzaldehyde. Remarkably, both complexes promote olefination, with [Cr<sub>2</sub>Cl<sub>4</sub>(CHI)­(thf)<sub>4</sub>] accomplishing the same <i>E</i> selectivity as Takai<i>’</i>s original mixture. Complex [Cr<sup>II/III</sup><sub>2</sub>I<sub>2</sub>(OSi­(O<i>t</i>Bu)<sub>3</sub>)<sub>2</sub>(CHI<sub>2</sub>)], however, appeared to give preferentially <i>Z</i> isomer, corroborating the monoiodo-methylidene species Cr­(III)–CHI–Cr­(III) as the active olefination component of the original <i>in situ</i> generated Takai reagent mixture.

Cite this collection

Werner, Daniel; Anwander, Reiner (2018): Unveiling

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

Variants. ACS Publications. Collection.