1- Methyl-1-cyclohexyl cations

Hyperconjugation has a profound effect on structure and stability of cyclohexyl cations. An elegant study combined theoretical results with experimental data to confirm that different hyperconjugative stabilization patterns lead to the formation of two equilibrating conformers of the 1-methyl-1-cyclohexyl cation where the carbocation p-orbital is oriented either pseudoaxiaUy or pseudoequatoriaUy. 

Other recent applications include, for instance, the methyl-cyclohexyl cation (9), which can adopt two different chair conformations characterized by CC and CH hyperconjugation, respectively, and the cyclopropylidenvinyl-cyclopropylmethyl cation (lO), the NMR spectrum of which could be assigned on the basis of highly accurate GIAO-CCSD(T) chemical shifts. 

The HF-SbFs system works well in the Gattermann-Koch formylatlon of arenes and the Koch carbonylation of alkanes [54]. For instance, biphenyl is diformylated in HF-SbFs-CO to afford 4,4 -diformylbiphenyl as a major isomer (Scheme 14.20). The carbonylation of alkanes with C5-C9 carbon atoms in the HF-SbFs-CO system affords mixtures of C3-C8 carboxylic acids after hydrolysis of the generated secondary carbenium ions [55]. Successive treatment of methylcyclopentane with CO in HF-SbF and with water produces cyclohexanecarboxylic acid as a major product (Scheme 14.21) [56]. It seems that a tertiary methylcyclopentyl cation readily isomerizes to the more stable cyclohexyl cation before being trapped by CO. Bicyclic a, -unsaturated ketones are functionahzed by HF-SbF or FSOsH-SbFs under a CO atmosphere to give saturated keto esters after methanolysis (Scheme 14.22) [57]. Alcohols with short carbon chains also react with CO in HF-SbFs to give the corresponding methyl esters [58]. y-Butyrolactones are carboxy-lated under the same conditions to afford 1,5-dicarboxyhc acids [59]. 

The presence of by-products such as 1-chloro-ethyl acetate 3 and a small amoimt of ethyl acetate, suggests that the initial step is an intermolecular hydride ion transfer from cyclohexane or methylcyclopentane towards acetyl chloride-aluminium chloride complex to give 1-methylcyclopentene. From cyclohexane, the cyclohexyl cation isomerizes to 1-methyl-l-cyclopentyl carbenium ion which loses a proton. Then, the 1-chloro-ethylate-aluminium chloride... 

Not all carbonium ion rearrangements can be adequately accounted for by 1,2 shifts. For example, the ring contraction of a cyclohexyl cation to a methyl-cyclopentyl cation is thermodynamically favorable, but would require a substantial activation energy if it proceeded through a primary cyclopentylmethyl cation. 

Enantioselective reduction of simple aliphatic ketones is one of the most challenging of the currently unresolved problems in this field. The Rh/4 complex catalyzed the hydrosilylation of 2-butanone with diphenylsilane at 0 °C, which after hydrolysis gave (S)-2-butanol in 56% ee (Scheme 3) [17]. 2-Octanone and 4-phenyl-2-butanone were reduced with diphenylsilane in the presence of the cationic Rh/EtTRAP-H at -50 °C and gave optical yields of 77% and 81%, respectively [12], The cationic Rh/(R,R)-t-Bu-MiniPHOS was also effective for the reduction of 4-phenyl-2-butanone with 1-naphthylphenylsilane at -20 °C, affording the R product in 80% ee [13]. 3-Methyl-2-butanone was reduced using the Rh/4 complex with 76% optical yield [17]. Hydrosilylation of cyclohexyl methyl ketone with the Rh/(R,S)-2 complex followed by hydrolysis afforded the R alcohol in 87% ee [8]. Highly enantioselective hydrosilylation of pinacolone with diphenylsilane at -20 °C was achieved by means of the Rh/4 complex and yielded the desired R product in 95% ee [17]. 

Selective hydrolysis of an amide. The analogous reagent a-chloro-N-cyclohexyl-acetaldonitrone was used in a total synthesis of vitamin B125 to effect selective hydrolysis of an amide function in the presence of six methyl ester groups. Thus treatment of the a-chloronitrone with silver tetrafluoroborate generates the electrophilic cation (1). This... 

Phenoxy acid herbicides, sulfonyl ureas, quaternary ammonium derivatives (quats), and aryloxy propanoic acids are the main classes of compounds subjected to capillary zone electrophoresis (CZE). Triazines are also separated using nonaqueous CZE, while low pfCa characterized chlorotriazines require an ion-pair-like solubilization using cationic surfactants (tetradecylammonium bromide, dodecyltrimethyl-ammonium bromide). Chiral selectors are added in CZE for obtaining enantioselectivity. Chiral selectors used for herbicide enantiomeric discrimination are vancomycin, y-cyclodextrin, ethyl carbonate -cyclo-dextrin, cyclohexyl-alkyl-)S-D-maltoside, sulpropyl ether a-cyclodextrin, and hexakis(2,3-di-0-methyl)-a-cyclodextrin. 

Carbocation Generation from Alkyl Halides. Alkyl chlorides, fluorides, and bromides are convenient and frequently used precursors for generation of alkyl cations in HSOsF-SbFs systems.It should be noted, however, that the HSOsF-SbFs system is less suitable than SbFs for the generation of alkyl, especially secondary alkyl, cations from the corresponding alkyl halides. Ionization of cyclohexyl chloride In Magic Acid Is accompanied by Isomerization, 3deldlng the 1-methyl- 1-cyclopentyl cation (eq 3). ... 

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