Sources of Vinyl Cations

Vinyl and aryl triflates have been much studied as important intermediates for the carbon-carbon bond formation, in addition to the use of vinyl triflates as a source of vinylic cations and unsaturated carbenes6. A recent good survey dealing with olefin synthesis via organometallic coupling reactions of enol triflates which covers the literature up to 1986 has appeared8 and here we intend only to add some new material. 

While some relative PA values have been measured experimentally from variable temperature equilibrium experiments, most experimental data have been measured at a single temperature. In this article, all experimental GB data have been adjusted to be consistent with the most recent GB and PA scales, and the entropies used to calculate PAs have been derived from scaled ab initio frequencies when experimental values are not available, Several compilations serve as a source for heats of formation of hydrocarbons and carbocations. Revised, heats of formation are available from the NIST web site and a detailed review of vinyl cations and many other carbocation heats of formation is found in Ref. 14. 

Evidence from a variety of sources however indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations and their involve ment m these additions has been questioned Eor example although electrophilic addi tion of hydrogen halides to alkynes occurs more slowly than the corresponding additions... 

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

Methylpropene can be made to continue the process to yield high polymers—cationic polymerisation—but most simple alkenes will go no further than di- or tri-meric structures. The main alkene monomers used on the large scale are 2-methyIpropene (— butyl rubber ), and vinyl ethers, ROCH=CH2 (— adhesives). Cationic polymerisation is often initiated by Lewis acid catalysts, e.g. BF3, plus a source of initial protons, the co-catalyst, e.g. traces of HzO etc. polymerisation occurs readily at low temperatures and is usually very rapid. Many more alkenes are polymerised by a radical induced pathway, however (p. 320). 

Azobisnitriles are efficient sources of free radicals for vinyl polymerizations and chain reactions, e.g., chlorinations. See also Initiators (Free-Radical) Initiators (Anionic) and Initiators (Cationic). These compounds decompose in a variety of solvenls al nearly first-order rates to give free radicals with no evidence of induced chain decomposition, They can be used in bulk, solution, and suspension polymerizations and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

Another version operating under neutral conditions (Scheme 3) uses vinyl epoxides (7) as the source of the i73-allylpalladium complex 8. Here, the alkoxide is generated by opening of the epoxide simultaneously with the formation of the cationic complex. The final compound (9) possesses an alcohol functional group. This variant is very important in the synthesis of carbanucleosides. 

Starting with the knowledge of carbenium ion stability and the understanding that it is necessary to decrease this stability enabling the reaction of these ions with nitriles102 (Section III.B.l), it should be assumed that acetals 209, oc-haloethers 211, enols 217 and vinyl ethers 218 are ineffective precursors since they are the sources of the highly stable hydroxy- (223) and alkoxycarbenium ions 224. In contrast, the carbonyl compound derivatives, which can produce destabilized ions (in comparison to ions 223 and 224) are the most interesting for reactions with nitriles. This includes acyloxycarbocations 225 (see Section III.B.3), halocarbocations 226, A-acyliminium ions 228 and vinyl cations 229. 

Nucleophilic attack on ( -alkene)Fp+ cations may be effected by heteroatom nucleophiles including amines, azide ion, cyanate ion (through N), alcohols, and thiols (Scheme 39). Carbon-based nucleophiles, such as the anions of active methylene compounds (malonic esters, /3-keto esters, cyanoac-etate), enamines, cyanide, cuprates, Grignard reagents, and ( l -allyl)Fe(Cp)(CO)2 complexes react similarly. In addition, several hydride sources, most notably NaBHsCN, deliver hydride ion to Fp(jj -alkene)+ complexes. Subjecting complexes of type (79) to Nal or NaBr in acetone, however, does not give nncleophilic attack, but instead results rehably in the displacement of the alkene from the iron residue. Cyclohexanone enolates or silyl enol ethers also may be added, and the iron alkyl complexes thus produced can give Robinson annulation-type products (Scheme 40). Vinyl ether-cationic Fp complexes as the electrophiles are nseful as vinyl cation equivalents. ... 

Ledwith has recently speculated about the possibility of initiating cationic polymerisation using common free radical sources coupled with oxidising cations. This idea has already been put into practice in his laboratory and the successful polymerisation of vinyl ethers and tetrahydrofuran has been achieved with two systems ... 

Evidence for the conjugative interaction of cyclopropyl with adjacent vinyl cations comes from a number of sources. Theoretical calculations for a variety of substituted vinyl cations (109), including the case where R = c-Pr, indicated that substituent effects in 109 were similar to those of the corresponding ethyl cations (110) . The rotational barrier calculated for cyclopropylvinyl cation was half that of cyclopropylethyl cation, a result tentatively attributed to conjugation of cyclopropyl with the 7r-bond of the vinyl cation when the cyclopropyl was twisted perpendicular to the vacant p orbitaP . ... 

The two perpendicular pi bonds of a triple bond can be considered separately, and thus this source becomes merely a subset of the double-bond sources. Alkynes are slightly poorer electron sources than alkenes because the vinyl cation formed upon electrophile attack is relatively unstable. Acetylide anions, RC=C M+, are treated as organometallic reagents (Section 5.3.1) and not as alkyne sources. 

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