The Mechanism of the Gas-Phase Pyrolysis of Esters. Part 13. the Very Strong Activating Effects of Β-Trialkylmetal Groups

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Journal Title, Volume, Page: 
Colin Eaborn, Foad M. S. Mahmoud and Roger Taylor J. Chem. Soc., Perkin Trans. 2, 1313-1319
Year of Publication: 
Foad M. S. Mahmoud
Current Affiliation: 
Department of Chemistry, Faculty of Science, An-Najah National University, Nablus, Palestine
Colin Eaborn
Roger Taylor
Preferred Abstract (Original): 

The rates of gas-phase pyrolysis of β-substituted ethyl acetates AcOCH2CH2X where X = SiMe3, SiEt3, GeEt3, and SiMe2Ph, and of 1-aryl-2-trimethylsilylethyl acetates have each been measured over a minimum of 50 °C, between 282 and 397 °C; the rates of pyrolysis of 2-aryldimethylsilylethyl acetates have been measured at 396.9 and 378.2 °C. The β-organometallic substituents are not themselves eliminated (except at considerably higher temperatures) but strongly accelerate the normal elimination of acetic acid, the relative rates per β-hydrogen at 327 °C for X being: H, 1.0; SiMe3, 125; SiEt3, 179; GeEt3, 108; SiMe2Ph, 144. These groups appear to activate by a combination of increasing the acidity of the β-hydrogen via stabilisation of the forming β-carbanion through (pd)π bonding, stabilisation of the incipient α-carbocation by C–X hyperconjugation, and steric acceleration. The effect of substituents in the aryl ring of 1-aryl-2-trimethylsilylethyl acetates gave an excellent correlation with σ+ values with ρ–0.52 at 327 °C. The lower ρ-factor for this reaction compared to that for 1-arylethyl acetates (–0.66) is consistent with either conjugative stabilisation of the α-carbocation or increased β-hydrogen acidity. Substituents in the aryl ring of 2-aryldimethylsilylethyl acetates gave a very small positive ρ-factor indicating that overall their effect on β-hydrogen acidity is larger than that on the forming carbocation. The product of pyrolysis of AcO·CH2·CH2·GeEt3viz. CH2[double bond, length half m-dash]CHGeEt3 underwent increasingly rapid elimination of successive moieties, believed to be ethylene, in a reaction of stoicheiometry 4.0, which did not occur with the silicon analogue.

The_Mechanism_of_the_Gas-Phase_Pyrolysis_of_Esters._Part_13._the_Very_Strong_Activating_Effects_of_Β-Trialkylmetal_Groups.pdf1.42 MB