Solvolysis allylic halides

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. 

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... 

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... 

Isomerization reactions of allylic halides ((2), X = halogen) are well known (ref. 99, p, 720), and occur in both aprotic and protic solvents. Isomerizations which accompany solvolysis reactions of allylic chlorides are discussed in Section 2.2.2. 

Rates Allylic halides are more reactive in S l processes than nonallylic halides. For example, the tertiary allylic chloride shown undergoes solvolysis in ethanol over 100 times faster than tert-huiy chloride. 

With secondary and tertiary allylic halides or under solvolysis conditions, 8 1 reactions can compete with 8 2, and a mixture of direct displacement and allylic rearrangement products results. 

Chloro-1,3-cyclopentadiene (bdow) undergoes 1 solvolysis in the presence of silver ion extremdy slowly even though the chlorine is doubly allylic and allylic halides normally ionize readily (Section 15.15). Provide an explanation for this behavior. 

As mentioned earlier, the allyl cation can be captured at either carbon sharing the positive charge. Table 12.3 gives some average rates of SnI solvolysis for a few common structural types. Primary allylic halides react much faster than primary alkyl halides. Secondary and tertiary allylic halides also ionize faster than their non-allylic counterparts. Resonance stabilization does have an accelerating effect on the ionizations. 

Other Reactions of Allenes.—Allylic halides undergo solvolysis at rates greater than those of their saturated analogues owing to charge delocalization through the Tr-system. The vinyl cation,it may be anticipated, could be... 

The ready dissociation of allylic halides has important chemical consequences. Different allylic halides may give identical products upon solvolysis if they dissociate to the same... 

Stabilized vinyl cations also can be formed by allylic double-bond participation in vinyl halide solvolysis. Grob and co-workers (151) have investigated the solvolysis of a number of substituted 2-bromo-l,3-butadienes, 165. Bromodienes 165 solvolyzed via first-order rates in 80% aqueous ethanol... 

The equivalent to allylic oxidation of alkenes, but with allylic transposition of the carbon-carbon double bond, can be carried out by an indirect oxidative process involving addition of an electrophilic arylselenenyl reagent, followed by oxidative elimination of selenium. In one procedure, addition of an arylselenenyl halide is followed by solvolysis and oxidative elimination. 

Polystyrene-bound allylic or benzylic alcohols react smoothly with hydrogen chloride or hydrogen bromide to yield the corresponding halides. The more stable the intermediate carbocation, the more easily the solvolysis will proceed. Alternatively, thionyl chloride can be used to convert benzyl alcohols into chlorides [7,25,26]. A milder alternative for preparing bromides or iodides, which is also suitable for non-benzylic alcohols, is the treatment of alcohols with phosphines and halogens or the preformed adducts thereof (Table 6.2, Experimental Procedure 6.1 [27-31]). Benzhy-dryl and trityl alcohols bound to cross-linked or non-cross-linked polystyrene are particularly prone to solvolysis, and can be converted into the corresponding chlorides by treatment with acetyl chloride in toluene or similar solvents (Table 6.2 [32-35]). 

The earliest report of the solvolysis of a halocyclopropane to an allyl alcohol was presented by Roberts and Chambers in 1951.10 These early examples did not utilize silver to assist in halide departure. Instead, the halocyclopropane was simply dissolved in solvent and allowed to react at elevated temperatures. For example, in... 

In many instances, however, solvolysis of a halocyclopropane is deliberately accomplished in order to install an essential vinyl halide or hindered olefin. In 2000, Murphy and coworkers performed a silver ion-mediated ring expansion of gem-dibromocyclopropane 18 in wet acetone to afford the allylic alcohol 19 in 82% yield (Scheme 4.5).16 Under these conditions the desired cyclononene product was obtained as an inseparable mixture of E- and Z-isomers (7 93). Interestingly, two sets of peaks observed in the1H NMR spectrum indicated that the Z-isomer existed as two separate conformers at room temperature. This intermediate was subsequently used in Murphy s approach to the radical-based preparation of tricyclic indoles. 

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 acetate unit is particnlarly convenient, since it is obtained from the corresponding alcohol and is relatively stable toward solvolysis, Sn2 reactions, and so on. Many other electronegative units are also effective, including halide and tosylate. Less obvious is the high reactivity of allylic sulfones and allylic nifro compounds toward the oxidative addition. Three special cases are outlined here. First, an allylic epoxide can undergo rapid oxidative addition of the allylic C-O bond under mild conditions, leading to the zwitterion or equivalent (Scheme 13). Proton abstraction can lead... 

In contrast, the less-strained four-rr-electron cyclopentadienyl cation is very unstable. The p R+ has been estimated as -40, using an electrochemical cycle. Solvolysis of cyclopentadienyl halides assisted by silver ion is extremely slow, even though the halide is in an allylic position. " When the bromide and antimony pentafluoride react at -78°C, the product has an EPR spectrum indicating that the cyclopentadienyl cation is a triplet, in agreement with HMO and more sophisticated MO calculations. The pentachloro derivative is also a triplet, but the pentaphenyl ion is a singlet. ... 

Formation of allylic products is characteristic of solvolysis reactions of cyclopropyl halides and arenesulfonates. Similarly, diazotization of cyclopropylamine in aqueous media gives only allyl alcohol. In a few instances, products containing an intact cyclopropyl ring have been reported, but these are special cases in which either the cyclopropyl cation is especially stabilized or the ring-opened allyl cation especially destabilized. Apart from these, the ring-opening of cyclopropyl cations to... 

Due to the strain imposed by the three-membered ring, the cyclopropyl cation is not a stable intermediate and electrocyclic ring opening occurs readily. Therefore, in the solvolysis of cyclopropyl tosylate in acetic acid, allyl acetate is obtained rather than cyclopropyl acetate. Solvolysis reactions of other cyclopropyl halides, sulphonates, and diazotization of cyclopropylamine in aqueous solution also give the allylic products. 

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