Methallyl cation

For a long time it was believed that the trifluorovinyl group is not basic enough to stabilize an a-carbocationic center, and perfluorinated allylic cations were thought not to be stable [15,21,59]. However, in 1984 the formation of the F-methallyl cation 30 in the reaction of PFIB with SbF5 in S02C1F solvent was reported [74], followed by the F-allyl cation (1) [75] ... 

The absence of electronwithdrawing properties and small steric volume of hydrogen substituents are the reasons for the higher stability of cation 36. For 1-substituted analogs of F-allyl and F-methallyl cations (existing as a mixture of cis- and trans-isomers, except 43, X=H for which only czs-isomer was found in solution), stability correlates well with electronegativity of the substituent [63] ... 

The radical cation of frawi-hex-2-ene cleaves at an allylic bond to give a resonance-stabilized methallyl cation, m/z 55. 

Diels-Alder reactions are classified as [4 + 2] cycloadditions, and the reaction giving the cyclobutane would be a [2 + 2] cycloaddition. This classification is based on the number of electrons involved. Diels-Alder reactions are not the only [4 + 2] cycloadditions. Conjugated ions like allyl cations, allyl anions and pentadienyl cations are all capable of cycloadditions. Thus, an allyl cation can be a 2-electron component in a [4 + 2] cycloaddition, as in the reaction of the methallyl cation 6.2 derived from its iodide 6.1, with cyclo-pentadiene giving a seven-membered ring cation 6.3. The diene is the 4-electron component. The product eventually isolated is the alkene 6.4, as the result of the loss of the neighbouring proton, the usual fate of a tertiary cation. This cycloaddition is also called a [4 + 3] cycloaddition if you were to count the atoms, but this is a structural feature not an electronic feature. In this chapter it is the number of electrons that counts. 

TMSOTf, CH2CI2, rt, 2 b, 74-98% yield. Boc groups are not compatible with tbis method, since they are cleaved with this reagent. Electron-rich aromatics can also be problematic, because the methallyl cation can react to form a chro-mane. The addition of TESH might possibly prevent this side reaction. The f-Bu ester can be cleaved with this method. 

Treatment of p-methallyl iodide (4a X = I, Y = Me) with a silver salt gives a methallyl cation (5a Y = Me) which reacts with 1,3-dienes such as 1,3-butadiene, 1,3-cyclopentadiene or 1,3-cyclohexadiene to give the corresponding seven-membered ring compounds (20) and (21) and their isomers. However, cycloaddition of the parent allyl cation (5a) with 1,3-dienes generally produces satisfactory results. In fact, (5a Y = H) derived from allyl iodide (4a X = I, Y = H) and silver trifluoroacetate reacts with 1,3-cyclopentadiene to yield bicyclo[3.2.1]octa-2,6-diene (21 Y = H, n = 1) though in poor yield, along with... 

Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

The production of 1-alkenes from ethylene oligomerization was carried out with high selectivity in ionic liquids in the presence of a cationic nickel complex catalyst (ri -methallyl)-[bis(diphenylphosphino)methane-monoxide-K -P,0]nickel(II) hexafluoroantimonate, [(mall)-Ni(dppmo)]Sbp6 (240). The overall reaction rate of... 

Styrene undergoes polymerisation with the cationic organonickel(II) complex / 3-methallyl ( /4-cycloocta-l,5-diene)nickel hexafluorophosphate [(MeAll) (Co<7)/W]+[PF]6 combined in situ with tricyclohexylphosphine. The product of such polymerisation is a styrene oligomer with Mn = 1900, characterised by a... 

Sulfuric-fluorosulfonic acid mixtures give complete cyclization of allyl and methallyl esters and thiol esters to give the l,3-dioxolan-2-ylium and l,3-oxathiolan-2-ylium cations (72CRV357) as shown in equation (29). Such cations are frequently involved in the solvolysis of haloacyl derivatives. This neighboring group effect has important mechanistic and stereochemical consequences, and is frequently encountered when working with carbohydrates. 

PtMeL2] proceed in a Markownikov manner by electrophilic attack of Pf thus [Pt(A -2-methallyl)L2] is formed from allene and [PtMe-(acetone)L2], whereas the analogous 1,3-butadiene cation does not lead to a 7r-allylic derivative by Pt—Me insertion. The hydro cation, however, can react by either a Markownikov or an anti-Markownikov mechanism with either Pt+ or attack on the unsaturated ligand. This apparent versatility leads to the formation of Tr-allylic complexes from both allenes and 1,3-dienes with [PtHLg]. ... 

A precursor 11.89 for a palladium-TMM system has been designed, using acetate as a leaving group to form the cationic arm and silyl or stannyl groups to form the anionic arm (Scheme 11.30). The precursors may be formed in various ways. The most direct involves dilithiation of methallyl alcohol 11.88 and quenching with trimethylsilyl chloride, followed by acetylation of the alcohol. 

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