Tolyl cations, stabilities

Effects of aryl and vinyl substituents on cation stability are quite large. The biphenylyl ions 75n and 75o have ca. 10 -fold longer lifetimes in aqueous solution than do their 4-tolyl counterparts, 75y and 75z. The 4-stilbenyl ions 75k and 751 have lifetimes that are ca. 300-fold longer than the corresponding 4-tolyl ions. 

Bulky groups such as methoxymethyl (which is also inductively destabilizing to a cation) resulted in mixtures of the two regioisomeric products from cyclization at the substituted and unsubstituted position. When the a substituent R is an aryl group such as p-tolyl, despite its cation-stabilizing properties, the regio- and stereocontrol of the reaction is lost due to its bulk thus three isomers 67,68, and 69, of the tricyclic product were isolated. While the first step of the cyclization process, the H abstraction, has been... 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

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