Allyl cations halides

The addition of hydrogen halides to dienes can result in either 1,2- or 1,4-addition. The extra stability of the allylic cation formed by proton transfer to a diene makes the ion-... 

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

A similar transformation results when trimethylsilyloxy-substituted allylic halides react with silver perchlorate in nitromethane. The resulting allylic cation gives cycloaddition reactions with dienes such as cyclopentadiene. The isolated products result from desilyla-tion of the initial adducts ... 

Both cyclic and acyclic allylic cations have been produced in this way. Stable allylic cations have also been obtained by the reaction between alkyl halides, alcohols, or alkenes (by hydride extraction) and SbFs in SO2 or S02C1F. Divinylmethyl... 

This clearly reflects formation of the same, delocalised allylic cation (23, cf. p. 105) as an ion pair intermediate from each halide, capable of undergoing subsequent rapid nucleophilic attack by EtOH at either C, or C3 ... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

Certain allyl cations react quite efficiently with nonactivated alkenes to give cyclobutanes. Such cations can be generated by Lewis acid catalyzed dehalogenation of allyl halides, protonation of conjugated dienes and Lewis acid eomplexation of conjugated carbonyl derivatives. For example, 2-chloro-2,4-dimethylpent-3-ene ( ) reacts with alkenes in the presence of zinc(II) chloride to give the corresponding cyclobulanes.1 Alkyl substitution of the allyl cation at the 2-position results in [3 + 2]-cycloaddition products. 

Both cyclic and acyclic allylic cations have been produced in this way. Stable allylic cations have also been obtained by the reaction between alkyl halides, alcohols, or olefins (by hydride extraction) and SbF5 in SO2 or SOjCIF.26 Divinylmethyl cations27 are more stable than the simple allylic type, and some of these have been prepared in concentrated sulfuric acid.2 Arenium ions (p. 502) are important examples of this type. Propargyl cations (RCssCCR2 ) have also been prepared.29... 

The suprafacial thermal addition of an allylic cation to a diene (a 3 + 4 cycloaddition) is allowed by the Woodward-Hoffmann rules (this reaction would be expected to follow the same rules as the Diels-Alder reaction1095). Such cycloadditions can be carried out1096 by treatment of a diene with an allylic halide in the presence of a suitable silver salt, e.g,1097... 

In 1994, Buzek et al. (109) reported that the allyl cation could be prepared from a variety of halide precursors, e.g., allyl chloride or cyclopropyl bromide, on SbF5 at cryogenic temperature, based on the infrared spectrum of the products. Those workers challenged BGW s claim of the persistent allyl cation based on the discrepancy between the isotropic 13C shift in the zeolite and that calculated at MP2/6-31G. This was one of the first examples of the use of chemical shift calculations to interpret (and in this case challenge) an NMR study of a species on a solid acid. 

The best approach is to work through this reaction mechanistically. Addition of hydrogen halides always proceeds by protonation of one of the terminal carbons of the diene system. Protonation of C-l gives an allylic cation for which the most stable resonance form is a tertiary carbocation. Protonation of C-4 would give a less stable allylic carbocation for which the most stable resonance form is a secondary carbocation. 

Iron tricarbonyl allyls and dienyls have been reviewed (214). a. n-Allyl Cations. A variety of 77-allyliron tricarbonyl cations have been obtained by treatment of diene-Fe(CO)3 complexes with strong acids (71, 74). Alternatively, the 77-allyl cations are obtained by reaction of AgBF4 or AgC104 with covalent allyl halides of type (I). 

The side chain R in these enals contained functionalities such as a double bond, or an acetoxy group, which were unaffected. The reaction proceeds again through a vinyl iodonium intermediate which serves as a reactive allyl cation. In this way the umpolung of allylsilanes is achieved and, indirectly, the reactivity of allyl halides is considerably increased. 

In the second example, analysis of the neutral obtained via halide abstraction by the ionic intermediate allowed one to establish the structure (2-propenyl vs allyl) of the gaseous c3h5+ ionic species formed in the protonation of allene and propyne in radiolytic experiments7. Using 1,4-dibromobutane as the trapping agent it was demonstrated that 2-propenyl ions are formed almost exclusively and trapped before they can rearrange to the more stable allyl cation (Scheme 2)7 (See also Section II.B.l). 

The carbon atom next to a carbon-carbon double bond. The term is used in naming compounds, such as an allylic halide, or in referring to reactive intermediates, such as an allylic cation, an allylic radical, or an allylic anion, (p. 673)... 

In spite of the favourable steric arrangement, propargyl cations, although more stable than vinyl and ethyl cations, are less stable than allyl cations. This is indicated by the ionization potentials of the simple parent radicals, 8-25 and 8-16 eV (Lossing, 1963) respectively, and by the lower (by a factor of ca. 104) rate of unimolecular solvolysis in ethanol-water 4 1 of propargyl (174) than of allyl halides (175) (Burawoy and... 

Actually, the first example of torquoselectivity dates back to the 1960s, though it was not identified using this terminology. The solvolysis of cycloproyl halides and tosylates proceeds via a two-electron electrocyclic reaction to yield allyl cation. disrotatory pathways are allowed, only one occurs ... 

The most noticeable difference between NMR spectra of (135) and (138) is revealed in the Fe NMR. In these spectra the iron allyl cations (138) resonate at much higher field (800 -1000 ppm) than the halides (135) (123 5 1700 ppm). 

Nucleophilic selenenylation (RSe ) of allyl cation equivalents (ally halides, sulfonates, epoxides). 

The first reaction would produce allylic cations whose initiating capability must be very limited, as shown by Kennedy . The second one is an electron transfer which, as we have already pointed out, is possible with antimony (V) halides under special conditions 91,420) but unlikely with elements which do not possess lower oxidation states. 

Unimolecular solvolyses of allylic secondary halides and similar derivatives are often favoured by the capacity of the double bond to stabilise the intermediate carbonium ion as the mesomeric "allylic cation 4). This extra stability of the intermediate is reflected in the energy of the transition state for its formation whenever the path of departure of the anion... 

Substitution reactions of allylic halides show diverse character according to the reagent and the position of the allylic system. In buffered aqueous solutions the 3 a- and 3 3-chloro-A -compounds 7) and (8), and also the 4/5-chloro-AS-(10) and 6ji -chloro-A4 compounds (ii) clearly react through common allylic cations (9 and 12, respectively). The product patterns are what one would expect from nucleophilic attack upon these cations by water, accompanied in the case of the C<3) C(4)-C(5) cation (9) by proton abstraction to give a diene. This cation exhibits no preference for either 3 a- or 3jS-attack of the nucleophile but the C(4>-C(5) C(6) cation 12) is attacked... 

Silver(I) compounds are known to promote different kinds of cycloaddition. Reactions of 2-alkoxyallyl halides with 1,3-dienes in the presence of silver(I) compounds provide a beneficial route to cycloheptanones [2,3]. When a mixture of 2-(trimethyl-siloxy)allyl chloride 1 and cyclopentadiene (2) is treated with 2 equiv. AgC104 in THF-ether (1 2) at 0 °C, bicyclo[3.2.1]oct-6-en-3-one 3 is produced in 91 % yield [3] (Sch. 1). The 2-(trimethylsiloxy)allyl cation 4 is believed to be involved as a reactive species in the reaction. 

Cyclopropyl cations and allylic cations can open and close, respectively, by two-electron electrocyclic reactions, as can the corresponding halides. 

Ab initio calculations indicated that the cyclopropiminium ion is more stable than the 2-aminoallyl cation.As allyl cations can be generated from the reaction of allylic halides with silver salts, ° reactions of a-haloenamines with these salts have been investigated in order to increase the yield and chemoselectivity of the formation of bicyclo[3.1.0]hexan-6-one aminals. ° ... 

A transformation will be regarded as a thermolysis if the halocyclopropane is heated either neat, in the gas phase or in an inert solvent. This solvent should neither serve as a nucleophile HY, nor have a high dielectric constant (to favor cation formation). If the solvent does have one or both of the aforementioned properties, the transformation will be regarded as a solvolysis. Although all these methods in principle obey the same general rules vide supra), their synthetic results are clearly distinct. Conditions in thermolysis are harsh. Since there is no other nucleophile present, the allylic cation can only be trapped by the halide ion that has left the cyclopropane. At the elevated temperature of thermolysis, the resulting allylic halides... 

Depending on the reaction conditions, the allylic cation can be trapped by the halide ion expelled during cyclopropane ring opening or by an external nucleophile. For example, 3,3-dibromotricyclo[4.1.1.0 ]octane (14) rearranged to give 15. ... 

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