The mechanism of thermal eliminations. Part 26. Substituent effects at each carbon of vinyl ethers: non-planarity of the transition state

Abstract

A range of 1-arylethyl vinyl ethers and 1-arylvinyl isopropyl ethers, 2-phenylethyl vinyl ether, isopropenyl isopropyl ether, and isopropyl cis- and trans-phenylvinyl ether have been synthesized and their rates of thermal elimination measured between 585.9–666.4 K. The Ei mechanism for vinyl ether pyrolysis has less E1 character than that for pyrolysis of the corresponding esters, so nucleophilic attack by the double bond upon the β-hydrogen is more important for the former. Electron supply to the double bond therefore increases the pyrolysis rate, so that isopropenyl isopropyl ether is more reactive than isopropyl vinyl ether (5.85-fold at 600 K), whereas acetates and formates are almost equally reactive. Likewise isopropyl 1-phenylvinyl ether is 2.2 times more reactive than isopropenyl isopropyl ether, slightly greater than the reactivity difference between the corresponding esters (benzoates and acetates). The rate-enhancing effect of a 2-phenyl substituent in ethyl vinyl ether (5.81 -fold per β-hydrogen) is a little less than the corresponding effect in ethyl acetate. In the 2-phenylvinyl ethers both cis and trans phenyl groups produce slight rate acceleration, due to the combined effects of loss of conjugation on going to products, and increase in electron supply to the double bond. The trans compound is more reactive than the cis, confirming that the six-membered transition state for β-elimination is not planar. Aryl ring substituents in 1-arylvinyl isopropyl ethers produce only very small rate effects; the effects could not be determined for electron-supplying substituents because these induced very rapid oxidation to isopropyl benzoates, and atmospheric hydrolysis also occurs. Isopropyl 2-phenylvinyl ethers were likewise oxidized to benzaldehyde and isopropyl formate. 1-Arylethyl vinyl ethers eliminate abnormally rapidly with concurrent rearragements to aldehydes and other products.