New insights into addition reactions of dialkylzinc reagents to trifluoromethyl ketones: Structural authentication of a β-hydride elimination product containing a tetranuclear zinc chain

Abstract

A systematic study of the stoichiometric alkylation reactions of 2,2,2-trifluoroacetophenone 1 with [ZnR₂(TMEDA)] (R= Me, Et, ^tBu, CH₂SiMe₃; TMEDA= N,N,N′,N′-tetramethylethylenediamine) monitored by ¹H and ¹⁹F NMR spectroscopy is presented. For R = Me, Et the alkylation products alkyl(alkoxides) [(TMEDA)Zn(R){OC(CF₃)(R)Ph}] (R = Me, 2: Et, 3) are obtained as the single products of the reaction. When the steric bulk of the dialkylzinc reagent is increased the alkylation reaction is inhibited. Thus, for R = ^tBu, the reduction product [(TMEDA)Zn(^tBu){OC(CF₃)(H)Ph}] is obtained as a result of β-hydride elimination from one of the ^tBu groups of the organometallic reagent. ¹H NMR spectroscopic monitoring of the reaction allowed the detection of isobutene as a side product of this reduction process. For the highly sterically demanding group R = CH₂SiMe₃ which lacks hydrogen atoms at the β position, no reaction is observed even under refluxing conditions. Two important intermediates from these reactions have been structurally elucidated: [(TMEDA)Zn(Me){OC(CF₃)(Me)Ph}] (2) which could be involved in the previously reported alkylation reaction of trifluoromethyl ketones by ZnR₂ catalysed by TMEDA and unprecedented tetranuclear [(^tBu)₂Zn₄{OC(CF₃)(H)Ph}₆] (5) resulting from the reduction of 1 when reacted with ^tBu₂Zn, which displays a rare Zn⋯Zn⋯Zn⋯Zn linear chain arrangement for a zinc alkyl(alkoxide).