Alkylideneallyl cations acetals

Chapter 5 by M. Fujita and T. Okuyama examines the ringopening reactions of alkylidenecyclopropanone acetals for solvolytic generation and trapping of alkylideneallyl cations (resonance hybrids of 1-vinyl-substituted vinyl cations). In Chapter 6 by V. P. Reddy et al. stable ion and computational studies of cyclobutylmethyl cations are discussed. In Chapter 7, G. I. Borodkin and V. G. Shubin discuss and... 

Generation of Alkylideneallyl Cations from Alkylidenecyclopropanone Acetals Selectivity of Reaction with Nucleophiles... 

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. 

The product ratio of 8K/9K is similar to that of the Lewis acid-mediated reaction of 4a-c. These products of the C2-C3 bond cleavage (8K and 9K) may be formed via alkylideneallyl cation intermediate, which is formed by the oxygen protonation of 4. Thus, the product ratio of 10/(8K + 9K) is controlled by the protonations at the olefinic carbon and at the acetal oxygen of 4. 

Reaction of 4a with TiCl4 was carried out in the presence of siloxyalkene 3 as nucleophile and the results are summarized in Table III. In the reaction with ketene silyl acetals 3a and 3e at -78 °C, y-ketoesters 15a and 15e were obtained instead of chloride product 8 which is a major product in the absence of 3. Formation of product 15 is likely to result from trapping of alkylideneallyl cation 5 with 3 at the sp2 carbon. In contrast, the reactions with silyl enol ethers 3f and 3g gave no acyclic product 15, but gave cyclopentanone derivatives 16-18. The product distribution depends on the mode of addition of TiCl4 (entries 4-7). 

Product distribution in the reaction of 4 with furan depends on the reaction conditions as well as on the oxy group of the acetal substrates 4a-c. The diverse products formed in the reaction of 4a-c with furan are rationalized by the reaction pathways illustrated in Scheme 13. All products arise from nucleophilic addition of furan to alkylideneallyl cation intermediate 5M (5S), which is generated by acid-mediated ring opening of cyclopropanone acetals 4a-c (Scheme 5). The [4 + 3] cycloadduct 23 is simply formed via 27, and the furanyl... 

Cyclohexylidenecyclopropanone acetal 4a-c was treated with TiCU at -78 °C in dichloromethane in the presence of furan to give adducts of furan, 23 and 24 (Table IV). All products formed result from C2-C3 bond cleavage in 4a-c, and are rationalized by a reaction mechanism including alkylideneallyl cation intermediate 5. The reactions of 4a and 4c preferentially gave a furanyl product 24 (entries 1, 2, and 6) in contrast to preferential formation of [4 + 3] cycloadduct 23 in the reaction of the disilyl acetal substrate 4b (entries 3 and 4). The reaction also took place by using SnCl4 (entry 5). 

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