Nucleophile-vinyl cation solvolysis scheme

Evidence for the occurrence of vinyl cations as short-lived intermediates in solvolysis and other reactions has accumulated in the last few years (reviewed by Hanack, 1970, by Richey and Richey, 1970, and by Modena and Tonellato, 1971), but they have not been observed spectroscopically. It has been shown possible to intercept some vinyl cations—prepared in a system of extremely low nucleophilicity (EHSO3—SbEj 1 1-1 10) by protonation of propyne and 2-butyne— by carbon monoxide (Hogeveen and Roobeek, 1971b). The oxocarbo-nium ions formed in these cases are shown in the following scheme ... 

In contrast to the thermal solvolysis, a rearranged enol ether 45 (and also the hydrolysis product, acetophenone) is formed in addition to the unrearranged product 44. The rearrangement is more apparent in less nucleophilic TFE. The results are best accounted for by heterolysis to give the open primary styryl cation 46 (Scheme 8). This cation gives products of substitution 44 and elimination 30 by reaction with the solvent. Alternatively, 46 can rearrange to the a-phenyl vinyl cation 47 via 1,2-hydride shift, which gives rise to 45 and 30. 

The typical primary vinylic substrate ( )-l-decenyl(phenyl)iodonium tetrafluor-oborate (11) displays unexpected reactivity under solvolytic conditions/ Its solvolysis was expected to be sluggish due to the instability of the possible primary vinyl cation intermediate, but it proceeded smoothly and as rapidly as that of the cyclohexenyl derivative 30 in alcoholic and aqueous solvents. However, the rates of solvolysis of 11 depend on the nucleophilicity of the solvent and not on the ionizing power. Both substitution (enol ether or aldehyde) and elimination products (19) are formed, in ratios which depend on the basicity of the medium (Scheme 46). 

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