Vinyl cation intermediates

Alkynes react when heated with trifluoroacetic acid to give addition products. Mixtures of syn and anti addition products are obtained. Similar addition reactions occur with trifluoromethanesulfonic acid. These reactions are analogous to acid-catalyzed hydration and proceed through a vinyl cation intermediate. 

The exact behavior and mechanism of electrophilic additions to alkynes is clearly strongly dependent upon the reaction conditions. In a highly polar and strongly acidic but weakly nucleophilic solvent such as trifluoroacetic acid, addition via a vinyl cation intermediate is favored whereas in less polar, more nucleophilic solvents such as acetic acid, a different mechanism prevails. 

Hydration of allene in sulfuric acid yields acetone, presumably via a vinyl cation intermediate (55, 56). Addition of HCl and HBr to allene results in... 

Participation by cumulative double bonds and vinyl cation intermediates has been suggested in the solvolyses of a number of homoallenyl systems. Hanack and Haffner (87) reported the solvolyses of 3,4-pentadienyl jS-naphthalene sulfonate, 110 (R = H) under a variety of conditions. The products... 

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... 

Solvolyses of these cyclic vinyl triflates at 100 in 50% aqueous ethanol, buffered with triethylamine, lead exclusively to the corresponding cyclo-alkanones. Treatment of 176 with buffered CH3COOD gave a mixture of cyclohexanone (85%) and 1-cyclohexenyl acetate (15%). Mass spectral analysis of this cyclohexanone product showed that the amount of deuterium incorporation was identical to that amount observed when cyclohexanone was treated with CH3COOD under the same conditions. This result rules out an addition-elimination mechanism, at least in the case of 174, and since concerted elimination is highly unlikely in small ring systems, it suggests a unimolecular ionization and formation of a vinyl cation intermediate in the solvolysis of cyclic triflates (170). The observed solvent m values, 174 m =. 64 175 m =. 66 and 16 m =. 16, are in accord with a unimolecular solvolysis. 

In both oxa- and aza-alkyne Prins cyclization an unexpected halide exchange with halogenated solvents presumably caused by the vinyl cation intermediates was observed [37]. From a synthetic point of view, it is important to use the correct combination of FeXs and X-containing solvent in order to avoid the undesired halide scrambling (Scheme 28). 

The reaction is formulated as an electrophilic attack by the aluminum halide, followed by hydride abstraction and transmetallation. A vinyl cation intermediate can account for both the regiochemistry and the stereochemistry. 

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

Not only the ring size but also the number of stabilising silyl groups in the -position is essential for the stability of the vinyl cations. Thus, reaction of alkyne 16 with tityl cation gave both stereoisomers of aikenylsilane 18 as the only products in 80-85% isolated yield (Scheme 3). This result suggests, that the generated / -silyl-substituted vinyl cation intermediate 17 did not persist under the applied reaction conditions but underwent a second hydride transfer with the formation of compound 18. 

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. 

Although formation of primary vinyl cation was disproved by the chirality probe approach, a vinyl cationic intermediate can be generated from a primary substrate via participation if a more stable cation could result. Unsymmetrically substituted 2,2-dialkylvinyl iodonium salt 24 gave mainly rearranged products on solvolysis.15 The products involve those of the 1,2-shift of either of the alkyl groups on the p position (Scheme 4). Those formed from migration of the alkyl... 

Conjugated ketones and esters react with allenylsilanes to yield acylcyclopentenes (Eq. 9.60) [63]. These products are formed by initial 1,4-addition to the conjugated double bond to afford a silyl-stabilized vinyl cation intermediate. 1,2-Silyl migration gives rise to a second silyl-stabilized vinyl cation which cyclizes to the acyl cyclopen-tene (Scheme 9.14). 

Alkynes, although not as prevalent as alkenes, have a number of important uses in synthesis. In general, alkynes are somewhat less reactive than alkenes toward many electrophiles. A major reason for this difference in reactivity is the substantially higher energy of the vinyl cation intermediate that is formed by an electrophilic attack on an alkyne. It is estimated that vinyl cations are about lOkcal/mol less stable than an alkyl cation with similar substitution. The observed differences in rate of addition in direct comparisons between alkenes and alkynes depend upon the specific electrophile and the reaction conditions.111 112 Table 4.4 summarizes some illustrative rate comparisons. A more complete discussion of the mechanistic aspects of addition to alkynes can be found in Section 6.5 of Part A. 

The alkylation of phenylacetylene with ferf-butyl chloride, benzyl chloride, and diphenylmethyl chloride follows an AdE2 mechanism57-59 with the involvement of the open vinyl cationic intermediate 17 ... 

Alkynes are readily acylated with acid chlorides under Friedel-Crafts conditions to form, in most cases, fram-p-chlorovinyl ketones through the corresponding vinyl cation intermediate [Eq. (8.16)]. The first study in 1935 reported low yields.11 Later in acylations with acyl triflates, p-keto vinyl triflates were obtained in satisfactory yields.123 When aroyl derivatives are used, the intermediate can undergo cyclization to form indenones. Chlorovinyl ketones formed from terminal alkynes may also react further losing hydrogen chloride to yield conjugated acetylenic ketones 11,13... 

The direct hydration of a C—C triple bond has been recently reviewed.302,303 Hydration is possible under acid or metal catalysis, as well as by photohydration. Hydration under acid catalysis is generally done with sulfuric acid, although other acid systems such as phosphoric acid/boron trifluoride have been reported.304 It is established that acid catalysis occurs by a vinyl cationic intermediate which reacts with... 

The ethoxycarbocation intermediate (363) produced by the action of acid on the cyclobutenedione monoacetal (362) has been found to react with bis(trimethylsilyl)-acetylene to afford a 2-methylenecyclopent-4-ene-l,3-dione derivative (365). The authors426 proposed that the rearrangement results from an unprecedented cationic 1,2-silyl migration on the alkynylsilane, subsequent ring expansion via a vinyl cation intermediate (364), and re-closure by intramolecular addition of an acyl cation to a silylallene in a 5-exo-trig mode (see Scheme 90). 

A new route to ( )-trachelanthamidine (6) and ( )-isoretronecanol (5) has been reported by Nossin and Speckamp.12 Cyclization of the ethoxy-lactam (28), which is formed on reduction of the imide (27), led to ring-closure with the acetylene to give, after hydrolysis, a 4 1 mixture of the epimers (29) and (30) (Scheme 7). The preferential formation of the five-membered ring is considered to be due to stabilization of an exocyclic vinyl cationic intermediate by the phenylthio group. Reduction of the separated diastereoisomers (29) and (30) yielded ( )-trachelanthamidine (6) and ( )-isoretronecanol (5), respectively. 

The addition of HC1 to alkynes proceeds through a vinyl cation intermediate. Explain which of the two possible vinyl cations that could be formed from the addition of HC1 to propyne is more stable. 

The addition of HC1 to 3-hexyne shows an unexpectedly different picture. The rate law is rather complicated and the reaction is accelerated by addition of chloride ion (as tetramethylammonium chloride). The product distribution ranges from that shown under 5 of Table 2 to the almost exclusive formation of ammonium chloride. The postulated mechanism excludes the formation of vinyl cations intermediates and indicates a concerted transition state for the reaction of type 16. 

However, at least in the case of the reaotion of the vinylamine 107 with isoamyl nitrite in cyclohexene the main reaction product is that expected from a carbene, 108, rather than from a vinyl cation intermediate. 

The formation of vinyl cations by deamination of vinyl amines in neutral media, is therefore not established and the possibility that a carbene intermediate is actually involved must be considered. Thus, the products of the alkaline decomposition of N-nitroso-oxazolidones 109 were earlier explained (Newman and Kutner, 1951 Newman and Weinberg, 1956) in terms of a mechanistic scheme involving vinyl cation intermediates (equation 19). Such a scheme has recently been... 

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