New insights into addition reactions of dialkylzinc reagents to trifluoromethyl ketones: structural authentication of a beta-hydride elimination product containing a tetranuclear zinc chain.
A systematic study of the stoichiometric alkylation reactions of 2,2,2-trifluoroacetophenone 1 with [ZnR(2)(TMEDA)] (R= Me, Et, (t)Bu, CH(2)SiMe(3); TMEDA= N,N,N',N'-tetramethylethylenediamine) monitored by (1)H and (19)F NMR spectroscopy is presented. For R = Me, Et the alkylation products alkyl(alkoxides) [(TMEDA)Zn(R){OC(CF(3))(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 = (t)Bu, the reduction product [(TMEDA)Zn((t)Bu){OC(CF(3))(H)Ph}] is obtained as a result of beta-hydride elimination from one of the (t)Bu groups of the organometallic reagent. (1)H 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 = CH(2)SiMe(3) which lacks hydrogen atoms at the beta position, no reaction is observed even under refluxing conditions. Two important intermediates from these reactions have been structurally elucidated: [(TMEDA)Zn(Me){OC(CF(3))(Me)Ph}] (2) which could be involved in the previously reported alkylation reaction of trifluoromethyl ketones by ZnR(2) catalysed by TMEDA and unprecedented tetranuclear [((t)Bu)(2)Zn(4){OC(CF(3))(H)Ph}(6)] (5) resulting from the reduction of 1 when reacted with (t)Bu(2)Zn, which displays a rare Zn...Zn...Zn...Zn linear chain arrangement for a zinc alkyl(alkoxide).
Hevia
Eva
E
WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK G1 1XL. eva.hevia@strath.ac.uk
Kennedy
Alan R
AR
Klett
Jan
J
Livingstone
Zoe
Z
McCall
Matthew D
MD
D000478:Alkylation; D018360:Crystallography, X-Ray; D005029:Ethylenediamines / Q000737:chemistry; D007659:Ketones / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*
D005029:Ethylenediamines; D007659:Ketones; D009942:Organometallic Compounds; C005798:N,N,N',N'-tetramethylethylenediamine; D015032:Zinc