Vinyl cations definition

Higher homologues than the butatrienyl cations are not known to the best of my knowledge. The summary given here provides strong evidence for the existence of the cumulated vinyl cations, i.e. allenyl and butatrienyl cations. A leap into the preparatory domain, harvesting the potentials of these cations, should definitely be a fruitful venture. 

The scope of this review is a detailed survey of reactions proceeding through vinyl cations and an attempt of a systematic definition of the properties of these intermediates with reference to those of saturated carbonium ions. Although attention will be particularly devoted to linear cations, bridged unsaturated species will be considered as alternative structures of vinyl cations rather than as a distinct type of reactive intermediates. The 77--complex terminology (Dewar, 1949) widely abused in the past decades to indicate especially cyclic cations and recently reassessed by Banthorpe (1970) will be generally avoided. The most recent Btudies not covered by published reviews on the subject (Rappoport, 1969 Richey and Richey, 1970 Richey, 1970 Hanack, 1970) are discussed in greater detail than others and data are collected in pertinent Tables. 

Reaction according to equation (22) is also distinguishable from that involving vinyl cations but it must be carefully taken into account before reaching a definite conclusion on the solvolysis mechanism (Hanack, 1970). In answer to a paper by Schubert and Barfnecht (1970) which is very critical of the current mechanistic interpretation of some solvolytic reactions of vinyl compounds, Rappoport et al., (1970) gave 20 distinctive criteria for mechanisms of equations (21) and (22), in particular the strong rate dependence upon proton concentration in the reaction media. 

Models (Hi) and (iv). Strictly, the only way of finding out definitely whether there is any complexation between the growing cation and the monomer or the polymer, or both, is to investigate whether (and if so, how) the apparent kp+ depends on monomer concentration [16, 17]. We have such evidence only for ACN and styrene and for these the value of kp does not depend on m. This is in accord with the prediction [15,17] that in a highly polar solvent the complexation of Pn+ by a Jt-donor monomer or its polymer is likely to be negligible. The likely behaviour of the w-donor vinyl ethers and their polymers is less clear, but a consideration of the dipole moments and concentrations involved makes it extremely unlikely that these monomers or their polymers could compete successfully for a place in the solvation shell of the growing cations. 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

Polymerization of vinyl ethers (VE) has been the subject of a considerable amount of theoretical studies. These monomers can be polymerized through radical initiation but the reaction is very slow and leads only to oligomers. Cationic polymerization initiated by a wide variety of Lewis acids is much more efficient and definitely preferred for homopolymer synthesis. Detailed theoretical aspects, and particularly recent developments concerning the controlled/living cationic polymerization of these monomers, have been discussed as well in previous exhaustive review [1,13,98,99] as in the present book (Chapters 4 and 5), and they will no longer be considered here. 

Halide anions have also been employed to facilitate the cyclization of weakly nucleophilic terminal vinyl ir-nucleophiles. For example, the butenylamine (85) undergoes Mannich cyclization in the presence of excess Nal to provide the 4-iodopiperidine (87) in excellent yield.74 The success of this cyclization should be contrasted with the failure of related amines to cyclize in formic acid with formaldehyde (Scheme 25). A detailed study74 of the effect that nucleophile concentration has on the outcome of Mannich cyclizations provides definitive evidence that the cyclization of iminium ions with alkenes is not a concerted process, but rather proceeds via a cationic intermediate capable of partitioning between product formation and reversal to the starting iminium ion. A bridged cation or ir-complex, e.g. (86) in equation (8), is a reasonable description of this intermediate. 

Vinylpynolidone/vinyl imidazolinium methochloride copolymer Classification Polymeric quaternary ammonium salt Definition Copolymer of methylvinylimidazolium chloride and vinylpyrrolidone Properties Cationic... 

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