Allylic cations gives

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 ... 

Treatment of cyclohexenol with HBr gives the corresponding allylic bromide. Only one compound is formed because attack at either end of the allylic cation gives the same product. 

The reverse reaction of this general class—an allyl cation giving a cyclopropyl cation—is found in Favorskii rearrangements. The diastereoisomeric Q-chloro enolates 6.334 and 6.337 give the cyclopropanones 6.335 and 6.338, respectively. Thus the reaction is stereospecific, at least in a nonpolar solvent. Evidently the allyl cation is not formed, otherwise the two chlorides... 

This first product is a tertiary allylic alcohol so it will lose water under the acidic work-up conditions and readdition of water to the other end of the allyl cation gives an alcohol that might cyclize to the final product. 

In this case, only one compound is formed because attack at either end of the allylic cation gives the same product. But when the allylic cation is unsymmetrical this can be a nuisance as a mixture of products may be formed. It doesn t matter which of these two butenols you treat with HBr, you get the same delocalized allylic cation. 

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 ... 

In the present instance, protonation of the C1-C2 double bond gives a carbo-cation that can react further to give the 1,2 adduct 3-chloro-3-methylcyclohexene and the 1,4 adduct 3-chloro-L-methylcyclohexene. Protonation of the C3-C4 double bond gives a symmetrical carbocation, whose two resonance forms are equivalent. Thus, the 1,2 adduct and the 1,4 adduct have the same structure 6-chloro-l-methyl-cyclohexene. Of the two possible modes of protonation, the first is more likely because it yields a tertiary allylic cation rather than a secondary allylic cation. 

Tetraene 4 (Scheme 1.3), when treated with 40 mol % of triflic acid in methylene chloride at -23 °C for 1 h, gives the adducts 5 and 6 in a 1 1 ratio as the main reaction products. The formation of these adducts has been justified [21] by a stepwise mechanism that requires an initial reversible protonation of 4 to produce the allyl cation 7, which then cyclizes to 8 and 9 in a non-reversible process. Deprotonation of 8 and 9 gives 5 and 6, respectively. 

Other examples that involve intermediate allyl cations are illustrated in Scheme 1.4. The cationic palladium(II) complex [Pd(dppp)(PhCN)2](BF4)2 coordinates the carbonyl oxygen of benzaldehyde and the activated carbonyl carbon attacks the isoprene, forming the allyl cation 10 which then cyclizes to give the 4-methyl-6-phenyl-5,6-dihydro-2H-pyran [22]. 2-Oxopropyl acrylate 11, in the presence of trimethylsilyltrifluoromethane sulfonate (TMSOTf) and methoxytrimethylsilane (MeOSMT), generates the cation 11a which is an efficient dienophile that reacts easily with the cyclohexadiene to give the Diels-Alder adduct in good yield [23]. 

It should be noted that the kinetics were first-order over at least three half-lives (with the exception of the dicyclopropylcarbonium ion), but the reaction products were not well defined in some cases— probably due to relatively fast consecutive reactions of the unsatmated oxocarbonium ions formed. In the case of the oxocarbonium ions formed from the allyl cations a novel quantitative eyclization to give cyclopentenone derivatives was observed (Hogeveen and Gaasbeek, 1970) ... 

As a result, protonation both in solution143 and gas phase144 occurs at a terminal carbon to give the 2-propenyl cation, not the allylic cation. 

Scheme 10.1 gives some representative examples of laboratory syntheses involving polyene cyclization. The cyclization in Entry 1 is done in anhydrous formic acid and involves the formation of a symmetric tertiary allylic carbocation. The cyclization forms a six-membered ring by attack at the terminal carbon of the vinyl group. The bicyclic cation is captured as the formate ester. Entry 2 also involves initiation by a symmetric allylic cation. In this case, the triene unit cyclizes to a tricyclic ring system. Entry 3 results in the formation of the steroidal skeleton with termination by capture of the alkynyl group and formation of a ketone. The cyclization in Entry 4 is initiated by epoxide opening. 

Removal of an electron from an allyl radical gives the allyl cation => the electron is removed from the nonbonding tz molecular orbital. 

The reaction of carbenes with alcohols can proceed by various pathways, which are most readily distinguished if the divalent carbon is conjugated to a tt system (Scheme 5). Both the ylide mechanism (a) and concerted O-H insertion (b) introduce the alkoxy group at the originally divalent site. On the other hand, carbene protonation (c) gives rise to allylic cations, which will accept nucleophiles at C-l and C-3 to give mixtures of isomeric ethers. In the case of R1 = R2, deuterated alcohols will afford mixtures of isotopomers. 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

Protonation of tetrakis(trimethylsilyl)allene 33 with HSO3F/ SbF5 (1 1) gives the 1,1,3,3-tetrakis(trimethylsilyl)-l-propen-2-yl cation 34. The isomeric allyl cation 35 is not formed (12, 44). 

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