The mechanism of the gas-phase pyrolysis of esters. Part 13. The very strong activating effects of β-trialkylmetal groups

Abstract

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 (p→d)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. CH₂CHGeEt₃ 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.