Alkylideneallyl cations 4+2 cycloaddition

Some reactions via intermediate alkylideneallyl cations have been reported. Solvolysis of 3-bromo-2,5-dimethyl-2,4-hexadiene in ethanol at 100 °C for 80 min gives 5-ethoxy-2,5-dimethyl-2,3-hexadiene in quantitative yield (Scheme 2) (5). This indicates that ethanol selectively attacks the sp2 carbon of the intermediate alkylideneallyl cation. A similar selectivity has been observed in the solvolysis of 2,3-dienyl alcohols (6), and is in agreement with the charge distribution. A cycloaddition reaction via an alkylideneallyl cation intermediate has been reported as illustrated in Scheme 3(7). 

We have recently developed a novel method for the generation of alkylideneallyl cations from alkylidenecyclopropanone acetals (8, 9). This method provides a nice opportunity to examine the selectivity of reactions of the ambident cation with various nucleophiles including siloxyalkenes (10) and furans (11). The reaction of the cation with the carbon nucleophiles gives [4 + 3] and [3 + 2] cycloaddition products as well as simple nucleophilic addition products. These results are summarized in this chapter. 

To obtain further insight into the [3 + 2] cycloaddition of alkylideneallyl cation 5, the reaction with 2,3-benzofuran was carried out as shown in Scheme 15. The reaction of 4a gave a simple [3 + 2] cycloadduct 30 as a single regioisomer in 76 % isolated yield. The regioisomeric structure of 30 confirmed by NOE measurements is well compatible with the cyclic structure of 29. These... 

Modes of cycloaddition of alkylideneallyl cation are also controlled by the reaction conditions. [4 + 3] Cycloaddition occurs in the reaction with furan. The [4 + 3] cycloaddition with furan was observed for the siloxy-substituted allyl cation 5S, but not for the methoxy-substituted allyl cation 5M. The lower electrophilicity of 5S may prefer the concerted pathway of [4 + 3] cycloaddition in competition with the stepwise pathway to yield a [3 + 2] cycloadduct and an electrophilic substitution product. 

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