Reactions of Vinyl Cations

The vinyl cations so far investigated are energetic intermediates and their reactions are too rapid for a direct study. The results discussed in detail in section II show that a variety of alternative paths are available to them. As electrophilic species, vinyl cations react by interaction with electron pairs which may be, following Bethell and Gold s (1967) classification (i) non-bonded electrons (ii) 7r-bonded electrons (iii) o-bonded electrons. 

Intramolecular rearrangements involving interaction of the positive vinyl carbon with a electrons of C—H and C—C bonds have been observed. Migration across the double bond (equation 32) is one type of 

Migration to the double bond (equation 33) is a second type of interaction and is well illustrated by the nature of rearranged products obtained in the hydrogen chloride addition to t-butylacetylene (section II,A,lc) and in the solvolysis of t-butylvinyl derivatives and a-cyclopropylvinyl derivatives (section II,C,2). 

Reactions of bridged unsaturated cations have not been investigated in detail and are currently interpreted in terms of ring opening to generate a back-shielded vinyl cation which may react with available electron pairs or as a direct nucleophilic attack at one of the carbon atoms of the cyclic cation to give the final products. 

The stereochemistry of the reactions of vinyl cations with nucleophiles is predictably different depending on their geometry, (a) High stereospecificity is expected from bridged ions (trans addition to acetylenes and retention of configuration in substitution reactions of vinyl derivatives) and is experimentally observed in the case of thiirenium ions, (b) From free linear cations with two j3 substituents of equal size, complete racemization is expected and is fully verified in the substitution products from l,2-dianisyl-2-phenylvinylbromide (section II,C,2). 

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The focus of Chapters 3 and 4 is on vinyl cations. In Chapter 3, T. Muller et al. discuss the preparation, isolation, and characterization of unusually stable vinyl cations whereas Chapter 4 by T. Okuyama and M. Fujita describes the generation and reactions of vinyl cations formed via solvolysis of vinyl iodonium salts. 

They are both reactions of vinyl cations, either as such 2.170, or highly stabilized vinyl cations in the case of ketene 2.167, where the carbonyl group is a stabilized carbocation. 

Kitamura, T., Kobayashi, S., and Taniguchi, H., Isoquinoline derivatives from the Ritter-type reaction of vinyl cations, Chem. Lett., 1351, 1984. 

The principal reaction of vinyl ethers to be considered in this article is cationic polymerization. 

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 ... 

Kinetic data on the carbonylation of vinyl cations have not been obtained so far, but it is likely to be a diffusion-controlled reaction as in the case of primary alkyl cations (Section IV, A). 

Products 19 and 20 result from the collapse of vinyl cation 23 with Cl to give 19, and the subsequent reaction of this product with HCl gives 20. Compounds 21 and 22 could arise from either a 1,2-methyl shift to vinyl cation 23 or... 

All of the above mentioned examples of vinyl cation intermediates have involved electrophilic additions to triple bonds or allenes or participation in solvolyses of such multiple bonds. In a sense, these reactions derive from analogies in normal... 

On the other hand, the involvement of vinyl cationic species in the reaction cannot be ruled out in some cases, as shown in Scheme 4. In this context, it was found that the reaction of 3-butyn-2-one with mesitylene can occur without Pd(OAc)2, clearly indicating the involvement of vinyl cations generated from alkynes and H+ in this reaction.47 1 The yield difference in the presence and in the absence of Pd(OAc)2 may be explained by the competition between [Pd(n)02CF3]+ and vinyl cationic species in the electrophilic substitution of aromatic G-H bonds. Recent kinetic isotope experiments suggest a mechanism involving alkyne coordination to Pd(n) followed by electrophilic aromatic substitution.476... 

Nucleophilic vinylic substitution and vinyl cation intermediates in the reactions of vinyl iodonium salts. 37, 1... 

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

General reactivities of vinyl iodonium salts are summarized, and reactions of cyclohexenyl, 1-alkenyl, styryl, and 2,2-disubstituted vinyl iodonium salts are discussed in relation to possible formation of vinyl cation intermediates. Primary vinyl cation cannot be generated thermally but rearrangement via neighboring group participation often occurs. Photosolvolysis to give primary vinyl cation is also discussed. 

Reactions of rhodium(III) porphyrins with olefins and acetylenes - Ogoshi et al. [326] have described the reactions of vinyl ether with rhodium (III) porphyrins which are depicted in reaction sequence (33). Step (a) appears to be an insertion of the olefin into the Rh-Cl bond followed by alcoholysis of a chlorosemiacetal to the acetal, step (b) is the hydrolysis of the acetal to the aldehyde. The insertion is thought to start by heterolysis of the Rh-Cl bond producing a cationic species which forms a 7i-complex with the electron-rich olefin. 

Intermolecular addition and addition-cyclization reactions of aminium cation radicals with electron-rich alkenes such as ethyl vinyl ether (EVE) allow an entry into products containing the N—C—C—O moiety of 13-amino ethers 70 or the equivalent of /3-amino aldehydes 71. The mild conditions under which aminium cation radicals are generated from PTOC carbamates makes the reactions described in Scheme 22 possible. In the absence of hydrogen atom donors, the /3-amino ethoxy(2-pyridylthio) acetal 71 was the major product. The mixed acetal can easily be converted... 

Formation of vinylic carbonium ions by various routes has been suggested in recent years by several workers. Addition of electrophiles, mostly protons, to various acetylenes is the most investigated pathway (Whitlock and Sandvick, 1966 Richey and Buckley, 1964 Noyce et ah, 1965 Letsinger et ah, 1965 Bott et ah, 1964, 1965 Peterson and Duddey, 1966 Peterson and Kamat, 1966 Fahey and Lee, 1966), but their formation was also suggested in the reaction of vinyltriazenes in acidic solution (Jones and Miller, 1966) or in the deamination of vinyl-amines (Curtin et al., 1965). However, solvolytic formation of vinyl cations has been investigated in very few cases. 

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