Assessing the reactivity of sodium zincate [(TMEDA)Na(TMP)ZntBu2] towards benzoylferrocene: deprotonative metalationvs.alkylation reactions

Abstract

Exploring the reactivity of the mixed-metal reagent [(TMEDA)Na(TMP)Zn^tBu₂] (1) towards substituted metallocenebenzoylferrocene 2, this study has found that two competing reactivity pathways are available for the sodium TMP-zincate, namely (i) remote 1,6-nucleophilic addition of a tert-butyl group to the phenyl ring of 2, and (ii) simultaneous alpha-deprotonation of the substituted cyclopentadienyl ring of the metallocene and alkylation (1,2-addition) across the CO bond of the carbonyl group. A key organometallic intermediate [(TMEDA)Na(μ-TMP)Zn{OC(^tBu)(Ph)(η⁵-C₅H₃)Fe(η⁵-C₅H₅)}] (3), resulting from the latter reaction has been trapped and characterised by X-ray crystallography and multinuclear (¹H and ¹³C) NMR spectroscopy. Its molecular structure revealed a unique two-fold activation of the tert-butyl groups bonded to zinc in the bimetallic base 1, showing for the first time that each alkyl group can exhibit markedly different reactivities (deprotonation vs.1,2-addition) towards the same substrate molecule. Iodine interception of the organometallic intermediates of the reaction between 1 and 2 allowed the isolation and characterization (¹H, ¹³C NMR and X-ray crystallography) of the ferrocenyl derivatives [PhC(OH)(^tBu)(η⁵-C₅H₃I)Fe(η⁵-C₅H₅)] (4) and [4-^tBu-C₆H₄C(O)(η⁵-C₅H₄)Fe(η⁵-C₅H₅)] (5) in a 29% and 24% isolated yield respectively. The low yield observed for the formation of 5 (resulting from the 1,6-addition reaction followed by spontaneous aerobic oxidation during aqueous workup) could be increased to 41% when the reaction mixture was hydrolysed in the presence of the radical oxidant TEMPO.