Vinylic Cations from Solvolysis. 33. Solvolysis of (E)- and (Z)-1-Anisyl-2-(p-nitrophenyl)propen-1-yl chlorides

Article abstract of DOI:10.1021/jo01310a011

The solvolysis of (E)- and (Z)-1-anisyl-2-p-nitrophenylpropen-1-yl chlorides (4-E and 4-Z) in buffered aqueous EtOH, TFE, and aqueous TFE was studied.The products in aqueous EtOH are a 6:4 ratio of the ethers 9-E and 9-Z and 1-anisyl-2-(p-nitrophenyl)-1-propanone (6), and in TFE a 7:3 ratio of the trifluoroethyl ethers 11-E and 11-Z and 6 are formed. The Grunwald-Winstein m values at 120 deg C in aqueous EtOH are 0.53 (4-E) and 0.62 (4-Z) and in aqueous TFE are 0.98 (4-E) and 0.88 (4-Z).In aqueous EtOH, common-ion rate depression and a 4-E <=> 4-Z isomerization during the solvolysis are negligible, whereas in TFE and less so in aqueous TFE strong common-ion rate depression and an extensive isomerization, which give an equilibrium ratio of 66:34 4-E/4-Z, accompany the solvolysis.In 80percent EtOH/p-MeC6H4S(1-) the reaction is of second order, the products are a 6:4 ratio of the vinyl thiolates 12-E and 12-Z, and a rapid 4-E <=> 4-Z isomerization takes place.In TFE containing CF3CH2O(1-) or p-MeC6H4S(1-) neither isomerization nor common-ion rate depression were observed, and the reactions were several times faster than in TFE/2,6-luitidine.An ionization to a cationoid species is suggested in all cases except in 80percent EtOH/RS(1-).In aqueous EtOH the reaction probably proceeds via the ion pairs 24a and 24b, whose capture by the solvent is slower than their interconversion but faster than internal return. In TFE a long-lived and selective free vinyl cation, 23, is formed. Preferential capture of the Cl(1-) over the solvent occurs from the less hindered side of 23, resulting in common-ion rate depression and isomerization. In the presence of the stronger nucleophiles RO(1-) and RS(1-), the capture occurs at the ion-pair stage. In 80percent EtOH/p-MeC6H4S(1-) the mechanism is probably a nucleophilic substitution via a carbanionic intermediate, but the isomerization follows a different route, probably a reversible electron-transfer process.