Measurement of electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part VI. Further evidence for the direct field effect, and the invalidity of σ0+ values

Abstract

The relative rates of pyrolysis of 1-arylethyl acetates XC₆H₄CHMe·OAc (measured over a 50° range for each compound) are as follows at 600 °K: (X =) H, 1·00; 2-PhS, 0·89; 3-PhS, 0·805; 4-PhS, 1·32; 2-PhO, 1·20; 4-PhO, 2·14; 2-MeS, 1·02; 4-MeS, 1·48; 2-PhCH₂, 1·51; 4-PhCH₂, 1·51. The abnormally low activation by ortho- relative to para-oxygen- and -sulphur-containing substituents is shown to be general for reactions involving formation of carbonium ions at the side-chain α-positions. The results parallel those found for ortho-halogeno-substituents and show that the rules required to describe the effect of ortho-substituents in these reactions are different from those required for electrophilic substitutions; the difference is ascribed to the particular effectiveness of the direct field effect operating between the ortho- and the side-chain α-positions, and any attempt to determine σ⁺ values from these reactions for application to electrophilic substitutions is invalid. The abnormally high activation by the ortho-benzyl substituent is attributed to interaction between the substituent π-electrons and the incipient carbonium ion.

Values of σ₀⁺ have been evaluated as –0·27(4-PhCH₂), –0·53(4-PhO), and +0·145(3-PhS), but for the sulphur-containing para-substituents, correlation with electrophilic substitution data is poor, indicating substantial d_π–p_π conjugation in the gas-phase reaction, and this may arise from the absence of solvent.

Lithiation of diphenyl sulphide and coupling of the intermediate with acetaldehyde produces ca. 40% of meta-product in contrast to other coupling reactions. Caution is therefore recommended in using lithiation reactions for the preparation of specific isomers.