Nucleophile-vinyl cation

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

Evidence for the occurrence of vinyl cations as short-lived intermediates in solvolysis and other reactions has accumulated in the last few years (reviewed by Hanack, 1970, by Richey and Richey, 1970, and by Modena and Tonellato, 1971), but they have not been observed spectroscopically. It has been shown possible to intercept some vinyl cations—prepared in a system of extremely low nucleophilicity (EHSO3—SbEj 1 1-1 10) by protonation of propyne and 2-butyne— by carbon monoxide (Hogeveen and Roobeek, 1971b). The oxocarbo-nium ions formed in these cases are shown in the following scheme ... 

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. 

In all of the above cases involving decompositions of vinyl diazonium ions, the observed products are consistent with a vinyl cation formulation, but extensive mechanistic studies of these reactions have not been reported. It is difficult, for instance, to establish to what extent reaction proceeds through the diazonium ion via a backside nucleophilic attack and concerted loss of nitrogen rather than through the free vinyl cation. In the absence of kinetic data, it is also difficult to rule out competing or alternative mechanisms not involving vinyl cations. 

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

In general, allenyl cations 38 attack at the sp2-carbon atom of 1,3-dienes and form vinyl cations 39 and 40 (R = H, alkyl) or (R = aryl). Although a concerted cycloaddition mechanism is possible, a stepwise mechanism is preferred34. If a nucleophilic attack at the sp-carbon atom of the allenyl cation takes place, then cation 41 and the resulting cations 42 and 43 are formed. Some examples of bicyclic products obtained from cyclic 1,3-dienes and propargyl chlorides are given in equation 1534. 

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

Vinyl cations of type 28 with a-aryl or a-alkyl substituents and two P-silyl groups and with an anion of very low nucleophilicity can be generated at room temperature in non-coordinating solvents from 30 by a Si-H to C-H hydride transfer reaction. For 29 (R = t-butyl), an X-ray structure determination has been reported (43, 52, 53). 

In solution, the TPFPB salt of the methyl-substituted vinyl cation 19 has only limited stability. Decomposition into unidentified products took place during several days. In contrast, the steric and electronic effects provided by the /-butyl substituent confer indefinite stability to the vinyl cation 20 in the absence of nucleophiles. That is, according to the NMR experiments, a solution of 20 TPFPB in benzene showed no sign of decomposition even after weeks at room temperature. With the carborane anion [CBnH6Br6] suitable crystals for... 

Under more basic conditions, a-elimination predominates and insertion of the carbene 40 to the solvent gives racemic 22. Non-basic and poorly nucleophilic conditions allow neighboring group participation to form the rearranged substitution product 23 with complete chirality transfer. The participation can be considered as an intramolecular nucleophilic substitution, and does occur only when it is preferable to the external reactions. Under slightly basic conditions with bases in HFIP, participation is allowed, and the weak base can react with the more electrophilic vinylic cation 21 (but not with the iodonium ion 19). A suitably controlled basicity can result in the formation of cycloalkyne 39, which is symmetrical and leads to racemization. These reactivities are illustrated in Scheme 6. 

In contrast to the thermal solvolysis, a rearranged enol ether 45 (and also the hydrolysis product, acetophenone) is formed in addition to the unrearranged product 44. The rearrangement is more apparent in less nucleophilic TFE. The results are best accounted for by heterolysis to give the open primary styryl cation 46 (Scheme 8). This cation gives products of substitution 44 and elimination 30 by reaction with the solvent. Alternatively, 46 can rearrange to the a-phenyl vinyl cation 47 via 1,2-hydride shift, which gives rise to 45 and 30. 

Alkylideneallyl cations can be described as resonance hybrids of 1-vinyl-substituted vinyl cations and allenylmethyl cations, and thus contain two reactive sites (the sp- and sp2-hybridized carbons) for nucleophilic addition (Scheme 1) (7,2). Hybridization affects the electronic and steric character of these reaction sites. The electronic property was deduced from the l3C NMR chemical shifts of alkylideneallyl cations measured under superacidic conditions (3) and also from the charge distribution calculated (4). The charge distributions are affected by substituents on the cation the sp2 carbon is more positive than the sp carbon when two methyl groups are introduced at the sp2 carbon. 

Photolysis of vinyl halides can induce both heterolysis of the C-X bond, thereby generating vinyl cations, and homolysis giving vinyl radicals. This competition between the two mechanisms was studied for 3-vinyl halides, 1,2,2-triphenylbromoethane (136) and 1-phenyl-2,2-bis(o-methoxyphenyl)-l-bromoethene and /3-styrene. Incursion of the photo-induced SrnI process, through the intermediate vinyl radical, is verified in the presence of reducing nucleophiles, such as the enolate ions of ketones and in part with (EtO)2PO . Incursion of the heterolytic pathway and the intermediacy of the radical cation, occurs in the presence of weak electron-donor anions, such as N02, Ns and Cl . The vinyl cation of /3-styrene gives phenylacetylene via an El-type elimination. 

Protonation of the alkyne produces the more favourable secondary vinyl cation, which is then attacked by water, since water is the predominant nucleophile available. Loss of a proton from this produces an enol, which is transformed into a more stable isomeric form, the ketone. This transformation is termed tautomerism, and we shall meet it... 

Because of their high reactivity, these complex salts react rapidly and regiospecifically, at low temperature, with a number of carbon and heteroatomic nucleophiles, including thiols, amines, and alcohols. Finally, exposure of the double bond takes place under particularly mild conditions so that isomerization of the (3,Y-unsaturated carbonyl system may be avoided. The present scope of reactions with these vinyl cation synthons is summarized in [able I. 

Nucleophilic additions to mesitylphenylketene [Ph(Mes)C=C=0, Mes = 2,4,6-Me3C6H2] and the related vinyl cation, Ph(Mcs)C=d)Mcs, proceed as if the mesityl group was effectively smaller than the phenyl group.20 The effect is explained by calculations that show that the phenyl is coplanar with the carbon-carbon double bond, while the mesityl is twisted the in-plane nucleophilic attack prefers the mesityl side. 

The presence of cyclohexenyl cation intermediates was firmly established by the observation of carbocation rearrangement during the solvolysis of 25, in which an initially generated bent vinyl cation 26 with sp2 hybridization rearranges to a more stable linear vinyl cation 27 with sp hybridization [Eq. (16)]. The ratio of the rearranged to the unrearranged ketones depends on the nature of solvents used and changed from 14 86 (27a 26a) in 60% aqueous ethanol to 46 54 in the less nucleophilic 2,2,2-trifluoroethanol. 

The exceptional nucleofugality of the phenyliodonio group has been determined in an alkenyl salt and it is about 106 times greater than that of triflate [30]. This remarkable property makes alkenyl iodonium salts excellent vinyl cation equivalents in nucleophilic substitutions. The chemistry of alkenyl iodonium salts is dominated by the transfer of their aliphatic moiety to a variety of nucleophiles other important reactions involve Michael-type addition and alkylidenecarbene generation, along with elimination to alkynes which is actually an undesirable side-reaction. 

The alicyclic vinyl halide 1-iodocyclooctene yields not only the direct nucleophilic trapping product 86 (and its acid-catalysed decomposition product 87) but also 88 and 89 (equation 74)291, which are produced via 1,2-cyclooctadiene as intermediate, as demonstrated by deuterium labelling for a related vinyl iodide. Deprotonation of the photogenerated vinyl cation is clearly more efficient than nucleophilic trapping in methanol. A... 

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