Vinyl cations generation

Highly unstable vinyl cations, generated in situ from vinyl triflates have also been arylated (the triflate group is replaced by the aromatics) to give vinyl aromatics under Friedel-Crafts conditions (28). 

This seeming unreactivity of vinyl halides in solvolytic processes and the lack of availability of more reactive precursors, such as sulfonate esters, until recently has discouraged early attempts at mechanistic investigations of vinyl cations generated by solvolyses. However, vinyl cations have been generated via vinyl diazonium ions derived from various precursors. 

More recently, such vinyl cations generated by the alkaline decomposition of 3-nitroso-2-oxazolidones have been trapped by halogens to give vinyl halides as products (108). It has been suggested that unsaturated carbenes, RjC=C , may be the intermediates in the basic decomposition of 132 (109). Indeed, when 132 (Ri=R2=CH3, R3=H) was treated with lithium ethoxide in the... 

It seems then that Schubert and Barfknecht (126) may have had not a-bromo-p-aminostyrene as their substrate, but some other less reactive compound, and the original vinyl cation hypothesis of Grob (121) for the solvolysis of this substrate may hold. Furthermore, Grob s pioneering study has served as a major catalyst for further investigations into vinyl cations generated by solvolysis. 

The first example of a carbon migrating across a double bond of a vinyl cation generated by solvolysis was reported by Hanack, Schleyer, Stang, and co-workers (177) with the vinyl substrate 230. Solvolysis of 230 in 80% aqueous ethanol gave, besides a small amount of allene, exclusively the rearranged ketone... 

Summary of Kinetic (3-Deuterium Isotope Effects in Vinyl Cations Generated... 

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... 

In 1969 Sherrod and Bergman and Bassler and Hanack independently reported that cyclopropyl groups strongly stabilize adjacent vinyl cations generated by solvolysis. Thus, 1-cyclopropylvinyl iodide (105) reacted with silver acetate in acetic acid at room temperature at a rate which is about 10 times faster than that of the corresponding 1-isopropylvinyl iodide (106). 

Vinyl cations, generated from 3-arylvinyl bromides by silver(I)-assisted solvolysis or by photolysis, are trapped by nitriles to yield isoquinolines (equation 24). ... 

Ionic reactions occur preferentially when aliphatic alkenyl iodides and aromatic alkenyl chlorides and bromides are irradiated. The trapping of the vinyl cations generated by irradiation readily takes place in nucleophihc solvents such as alcohols. In these cases, enol ethers or acetals are the main products (Equation 11.8 and Equation 11.9).3.20 4,30... 

An interesting class ot covalent Inflates are vin l and ar>/ or heteroaryl Inflates Vinyl inflates are used for the direct solvolytic generation of vinyl cations and for the generation of unsaturated carbenes via the a-elimination process [66] A triflate ester of 2-hydroxypyridine can be used as a catalyst for the acylation of aromatic compounds with carboxylic acids [109] (equation 55)... 

Metalated epoxides can react with organometallics to give olefins after elimination of dimetal oxide, a process often referred to as reductive alkylation (Path B, Scheme 5.2). Crandall and Lin first described this reaction in their seminal paper in 1967 treatment of tert-butyloxirane 106 with 3 equiv. of tert-butyllithium, for example, gave trans-di-tert-butylethylene 110 in 64% yield (Scheme 5.23), Stating that this reaction should have some synthetic potential , [36] they proposed a reaction pathway in which tert-butyllithium reacted with a-lithiooxycarbene 108 to generate dianion 109 and thence olefin 110 upon elimination of dilithium oxide. The epoxide has, in effect, acted as a vinyl cation equivalent. 

Vinyl cations have been generated in three ways ... 

Vinyl cations also have been invoked as intermediates in the addition of carbonium ions generated in strong acid to acetylene (50-53). Sasaki et al (50) observed 1-adamantyl methyl ketone, 25, as the sole product in the reaction of acetylene with 1-bromoadamantane in concentrated H2SO4 at 5°. Bott (51), on the other hand, reported a mixture consisting of 75% 1-adamantylacetaldehyde,... 

The first genuine example of a solvolytic generation of vinyl cations was the pioneering work of Grob and co-workers (121) on substituted a-bromostyrenes... 

Common-ion rate depressions and the involvement of vinyl cation ion pairs similar to those observed by Miller and Kaufman (131) were observed by Rappoport and Gal (136) in the solvolysis of 150 in acetic acid. Further interesting evidence for ion-pair involvement in the solvolytic generation of some vinyl cations comes from the investigation of cis- and trans-1,2-dianisyl-2-phenylvinyl halides, 151 (1937). In 80% ethanol, solvolysis... 

It is clear from all of the above evidence that despite earlier doubts, vinyl cations can be generated via solvolysis and bond heterolysis, especially in cases... 

Finally, participation by a remote double bond in the solvolytic generation of a vinyl cation has also been observed recently (190). Solvolysis of cis and trans triflate, 234a and 234b, in trifluoroethanol buffered with lutidine gave, besides acyclic products, 20% cyclic products 235abc in the case of the cis triflate 234a and 35% cyclic products in the case of the trans isomer 234b (190). 

A number of kinetic /J-deuterium isotope effects in the solvolytic generation of vinyl cations have been measured. Stang and co-workers (193) observed a kn /kp = 1.43 in the solvolysis of 237 in 80% aqueous ethanol at 25° C. This effect is considerably larger than the corresponding j8-deuterium... 

Other than the stabilized allenyl 242 and related 248 cations, there is also to date no report on the solvolytic generation of a primary 256 vinyl cation. Such species have, however, been implicated in the decomposition of some... 

Finally, in a recent study by Walling and El-Taliawi (216) it was shown that solvolytically generated vinyl cations may be reduced by Fe ions in solution to the corresponding vinyl radical. When 2-buten-2-yl triflate was solvolyzed in concentrated ferrous perchlorate solution in the presence of acrylonitrile monomer, polymerization of the acrylonitrile was observed. No such polymerization occurred under identical conditions in the absence of Fe ions. It seems that the polymerization of acrylonitrile was initiated by the vinyl radicals formed by reduction of the intermediate vinyl cation by Fe as follows (216) ... 

The mechanism of reaction of primary vinyl triflates and the possibility of solvolytic generation of primary vinyl cations needs further exploration. Along these lines, an examination of the behavior of the simplest vinyl system CH2=CHOTf and the possibility of generating the parent vinyl cation needs to be done. 

The allyl-resonance stabilized E- and Z-pent-l,3-dienyl-2-cations (22 and 23) are the smallest member of vinyl cations observed as persistent species in superacid solution 49 These are difficult to generate experimentally50 but structures with only five heavy atoms are suitable candidates for coupled cluster model calculations. A challenging task of quantum chemistry was to assign the 13C NMR spectrum of the mixture of isomers (Fig. 3), which exhibits pairs of signals of 22 and 23 which differ only by a few ppm, to the chemical shifts for the specific carbon atoms of the E- and Z-isomers, respectively. 

Vinyl trifluoromethanesulfonates (triflates) are a new class of compounds, unknown before 1969, that have been used most extensively in solvolytic studies to generate vinyl cations.2,3,812 Three methods have been used to prepare these sulfonic esters. The first, involving the preparation and decomposition of acyltriazines,4 requires several steps to prepare the acyltriazines and is limited to the preparation of fully substituted vinyl triflates. The second method involves the electrophilic addition of trifluoromethanesulfonic acid to acetylenes5,8,15 and, consequently, is not applicable to the preparation of trisubstituted vinyl triflates and certain cyclic vinyl triflates. However, this second procedure is relatively simple and often gives purer products in higher yield than the subsequently discussed reaction with ketones. Table I lists vinyl triflates that have been prepared by this procedure. ... 

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. 

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... 

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