Vinyl carbonium ion

Van t Hoff t-factors 565 Vinylallenes rearrangement of 748 synthesis of 737 Vinyl carbonium ions 620 17a-Vinyl-17/f-hydroxysteroids, epimerization of 735 Vinyl sulphides, as alkyl sulphoxide reduction products 930, 932 Vinyl sulphones - see also Alkenyl... 

Recent studies on simple haloalkanes, especially iodoalkanes. suggest that the initial process is always homolytic carbon-halogen bond cleavage, but that iodo-systerns are especially susceptible to subsequent electron transfer from the alky) radical to the fairly unreactive iodine atom. This gives the alkyl carbonium ion that reacts with, for example, a nucleophilic solvent. The operation of this mechanism provides for the generation and reaction of vinyl carbonium ions from vinyl iodides fS.66), and this offers one of the lew ways of generating such intermediates. 

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. 

Vinyl cations 1 are derivatives of the ethenyl cation, H2C CH+, in which the hydrogen atoms may he replaced by other organic residues. The terms vinylium ion and, less correctly (Bethell and Gold, 1967), vinyl carbonium ion are also often used to indioate (1). The nomen-... 

The mechanism of this reaction has been formerly discussed in terms of a vinyl carbonium ion . It has been recently shown, however, that the reaction proceeds by a mechanism involving competition between vinyl carbonium ions and carbenes. This conclusion has been based upon the observations that on treatment of the nitrosooxazolidones with aqueous-methanolic KOH or with EtOLi in cyclohexene solution, yields of acetylenes 37 increase, and those of vinyl ethers 38 and alkylidene-bicycloheptanes 39 decrease, as R changes from methyl to cyclopropyl to phenyl (equation 113 and adjoining table) . 

Only the ethyl carbonium ion is produced under normal conditions. The vinyl carbonium ion, C2H3+,... 

It is found that a vinyl carbonium ion is slightly less stable than the corresponding alkyl carbonium ion. In particular, a secondary vinyl carbonium ion is less stable than a secondary alkyl carbonium ion. Bearing all these factors in mind, it is apparent that vinyl carbons tend not to undergo the SN1 reaction unless conditions are so arranged as to stabilise the resultant carbonium ion intermediate. Suggest two possible ways in which this could be done. 

Suggest what will be the shape of the intermediate vinyl carbonium ion, and then suggest what will be the stereochemical consequences of an SN1 reaction on a vinyl carbon. 

When a vinyl carbonium ion is formed it is linear, and so the stereochemistry of the final product after the addition of the electrophile will be randomised. 

Vinyl carbonium ion equivalent. This sulfoxide has been used as an equivalent of the vinyl carbonium ion as illustrated for the synthesis of (3) from (1). 

A choice between the two compounds is made on the basis that cleavage of the sp carbon-bromine bond is not a particularly favorable process (the resultant vinyl carbonium ion is not a stable species). 

Preliminary accounts of the reactions of HCl with complexes [Pt(C CH)2L2] (reported in Vol. 5, p. 386) have been superseded by a full paper. A feature of the reactions is the intermediacy of platinum-stabilized vinyl carbonium ions [Pt-(C+=CH2)] and another paper by the same authors is concerned with mechanistic studies of the formation and decomposition of alkoxycarbene complexes, reaction (2), which can also be rationalized in terms of a mechanism involving such species... 

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