1-Adamantyl cation, formation

Azoadamantane exposed to 2 mol equivalents of T CIO4 at room temperature rapidly and quantitatively evolved nitrogen, and thianthrene and products derived from the adamantyl cation were obtained. Equations (38)-(40) (AA, azo-adamantane Ad, adamantane) make clear why 2 mol equivalents of the radical oxidant are required (85JA2561). The comparable interaction of T with phenylazotriphenylmethane and di-ter/-butyl diazene, using a 2 1 ratio of radical cation to substrate, also leads to the formation of thianthrene and nitrogen (85PS111). 

The AcBr-2AlX3 (X = Cl, Br) complexes display high activity in the alkylation of adamantane with alkanes to form poly alkylated adamantanes (Cn < C < C33) and bisadamantylalkanes (C23 < C < C50)119 [Eq. (5.70)]. The suggested pathway includes the 1-adamantyl cation and alkyl cations generated by hydride removal by the superacidic complexes. The 1-adamantyl cation then alkylates alkenes equilibrating with the alkyl cations. Various transformations may follow, resulting in the formation of additional products. 

The rearrangements of several twistane derivatives to adamantyl cations under the same conditions, on the other hand, appear to involve reversible, random carbonium ion formation, at least to a limited extent. Rearrangement of 2-twistanol-2-d (38) occurs with considerable intermolecular hydrogen scrambling (Eq. (15)) 40T Similar intermolecular rearrangements are observed when a 50 50 mixture of 1-adamantanol and l-adamantanol-3,5,7-d3 in S02 is treated with SbFs 40). 

Treatment of adamantane with aluminum halide catalysts in the presence of nonane or 2,3,5-trimethylhexane results in the formation of a mixture of methylated adamantanes 354f Apparently, the 1-adamantyl cation generated by Lewis acid catalysts can initiate intermolecular alkyl shifts (presumably via attack on olefinic intermediates). 

As described in the case of the 1-adamantyl cation, protosolvation of carbon-carbon and carbon-hydrogen sigma bonds can lead to cationic 2e-3c bonding and the formation of carbonium ion centers. The role of this interaction in the chemistry of superelectrophiles is typihed by protosolvation of the tert-butyl cation (1) in superacid to provide the dicationic... 

The important field of ionic liquids, in most cases 1,3-dimidazolium salts (143) has been studied by IR and Raman spectroscopies supported by B3LYP calculations. Additionally, calculations of the heteroaromaticity (HOMA index) showed that cation formation causes a decrease of the aromaticity of the imidazole ring. However, when the R groups at positions 1,3 are benzyl or adamantyl, the aromatic nature of the heterocyclic moiety increases. Moreover, the electron distribution performed using the GAPT method indicated the positive charge delocalization in the imidazolium ring [143],... 

This dichotomy is quite evident in the reaction of cydooctane with nitronium trifluoroacetate. The products obtained were cyclooctyl triflu-oroacetate, cyclooctyl nitrate and nitrocydooctane. Conversion of adaman-tane to 1-fluoroadamantane in 95% 3ueld on reaction with nitronium tetrafluoroborate in pyridine polyhydrogen fluoride indicates that formation of adamantyl cation by formal hydride abstraction is a significant alternative to the nitration-protodenitration pathway. 

The face selectivity in stericaUy unbiased systems, exhibited in 2-adamantyl cation addition and elimination reactions, among others, has been reviewed, " and so have the elecuonic factors governing the diastereofacial selectivity of many reactions, including the nucleophilic capture of 5-substituted adamantyl cations. The standard enthalpy of formation of the 1-adamantyl cation (99) in the gas phase has been determined experimentally to be 162.0 2.0 kcalmoP, and the ab initio calculations on this species have been re-assessed. It has been found that the inuamolecular hydride transfer occurring in the cation derived from (198) is competitive with intermolec-ular processes, and that the rates of the hydride transfers and the reaction products observed were dependent upon R. The direct synthesis of stable adamantylide-neadamantane bromonium salts is reported the anions used contained or Mo, and the resulting salts proved to be more stable than the Brs salts. The synthesis, reactions, and properties of some 2,8-didehydronoradamantane derivatives are reported. " ... 

The apocamphyl case is an extreme one, and bridgehead carbonium ions in other systems are more accessible. Inclusion of more atoms in the bridge gives a more flexible molecule and allows carbonium ion formation to proceed with a somewhat lower activation energy. Thus, the relative solvolysis rates of the bridgehead bromides 1-bromoadamantane, l-bromobicyclo[2.2.2]octane, and 1-bromobicyclo[2.2.1]heptane in 80% ethanol at 25°C are 1,10", and Under the same conditions, the rate of solvolysis of tert-butyl bromide is 1000 times that of 1-bromoadamantane. The 1-adamantyl cation is sufficiently stable to be generated in antimony pentafluoride in concentrations sufficient for observation by NMR. ... 

The tertiary-secondary 1,2-H shift O itlO is not rate-determining in the interconversion of 5 and 6, but may become so in a conformationally fixed system. It has been found for the interconversion of tertiary and secondary adamantyloxocarbonium ions that <10" sec at 70°C (Hogeveen and Roobeek, 1971a) as compared with k= 1-5 x 10 sec at 20°C for the reaction 5 6. The absence of interconversion between tertiary and secondary adamantyloxocarbonium ions is due to the circumstance that 1,2-H shifts do not occur in the tertiary adamantyl ion as a result of the effect of orbital orientation (Brouwer and Hogeveen, 1970 Schleyer etal., 1970). That the secondary adamantyloxocarbonium ion can lose CO is demonstrated by the reaction with isopropyl cation in SbFs—SO2CIF solution at 0°C with formation... 

Another approach to a donor adduct of the methylene phosphenium cation is the addition of a phosphonium cation to the phosphaalkyne. The reaction of the protic cation [HPPhal + lCFaSOa] with CjoHuCP produced a white powder which was identified as the P-phosphonio-substituted phosphaalkene [74]. Alternatively to the elimination reaction the phosphaalkynes were protonated. C-protonation of adamantylphosphaacetylene and ferf-butylphosphaacetylene occurred in superacid media under formation of phosphavinyl cations. From these spirocyclic betaines by reaction of l-Ad-C=P (Ad = adamantyl) withB(OTf)3 a phosphavinyl cation could be detected [75]. 

The fact that NHCs form stable compounds with beryllium, one of the hardest Lewis acids known and without p-electrons to back donate, shows the nu-cleophilicity of these ligands. Reaction of l,3-dimethylimidazolin-2-ylidene with polymeric BeCl2 results in the formation of the neutral 2 1 adduct 23 or the cationic 3 1 adduct 24. The first NHC-alkaline earth metal complex to be isolated was the 1 1 adduct 25 with MgEt2- Whereas l,3-dimesitylimidazolin-2-ylidene results in the formation of a dimeric compound, the application of sterically more demanding l,3-(l-adamantyl)imidazolin-2-ylidene gives a monomeric adduct. ... 

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