Reactivity and structure of alkyl vinyl ethers. Part IV. Conformational effects of alkoxy-groups in alkyl vinyl ethers and alkyl aryl ethers on charge-transfer spectra with tetracyanoethylene, and on reactivity towards electrophiles

Abstract

Charge-transfer spectra of a series of alkyl vinyl ethers with tetracyanoethylene have been measured and indicate that ionisation potentials of ROCHCH₂ fall in the sequence R = Me > Et > Prⁱ > Bu^t. Measurement of u.v. absorption spectra in the gas phase and in hexane solution gives a similar sequence for decreasing energy of the olefinic π–π* transition. This sequence is identical to that previously reported of increasing reactivity towards electrophiles, and is the opposite of the expected resonance activation by the alkoxy-group. It is concluded that reactivity in alkyl vinyl ethers is governed by strain effects in the ground states.

For comparison, similar information has been collated for the related alkyl aryl ethers, ROPh. Charge-transfer spectra with tetracyanoethylene consist of two bands, the longer wavelength of which changes its position according to the nature of R. Ionisation potentials fall in the sequence R = Me > Et > Prⁱ < Bu^t and reactivity towards electrophiles increases in the same sequence. In contrast with alkyl vinyl ethers, resonance effects predominate in controlling reactivity and spectra.