Cycloalkyl ions

Fragmentation of ethyl and higher alkyl ethers of cycloalkanols with > 3 C atoms Alkene elimination to yield the protonated cycloallcanol (m/z 72, 86, 100,...) and heterolytic cleavage of the C-0 bond to give dominating cycloalkyl ions (m/z 69, 83,...). 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

C. The Fluorosvlphonic Acid-Antimony Pentafluoride and Hydrogen Fluoride-Antimony Pentafluoride Solvent Systems Attempts to generate simple alkyl-, arylalkyl-, and cycloalkyl-carbonium ions in sulphuric acid or oleum solution generally result in... 

The utility of SCFs for PTC was demonstrated for several model organic reactions - the nucleophilic displacement of benzyl chloride with bromide ion (26) and cyanide ion (27), which were chosen as model reversible and irreversible Sn2 reactions. The next two reactions reported were the alkylation and cycloalkylation of phenylacetonitrile (28,29). Catalyst solubility in the SCF was very limited, yet the rate of reaction increased linearly with the amount of catalyst present. Figure 5 shows data for the cyanide displacement of benzyl bromide, and the data followed pseudo-first order, irreversible kinetics. The catalyst amounts ranged from 0.06 (solubility limit) to 10% of the limiting reactant, benzyl chloride. 

Stabilization conferred by aromatic hyperconjugation resolves a puzzle concerning the relative stabilities of arenonium ions. As judged by rates of solvolysis reactions, normally a phenyl group is more effective than vinyl in stabilizing a carbocation center.166 This difference is moderated for cycloalkyl substrates, so that benzoannelation has little effect, for example, on the rate of hydrolysis of 3-chlorocyclohexene (Cagney H, Kudavalli JS, More O Ferrall... 

Tertiary cycloalkyl cations, such as the 1-methylcyclopent-l-yl cation 28, show high stability in strong acid solutions. This ion can be obtained from a variety of precursors (Figure 3.7).143,144 It is noteworthy to mention that not only cyclopentyl- but also cyclohexyl-type precursors give 1 -methylcyclopent-1 -yl cation 28. This indicates that the cyclopentyl cation has higher stability, which causes isomerization of the secondary cyclohexyl cation to the tertiary methylcyclopentyl ion. 

C NMR spectroscopic and theoretical studies (DFT, ab initio, IGLO) of a series of cycloacylium ions were performed by Prakash et al.577 The study showed that the cycloalkyl groups have little effect on the shift of the carbocationic carbon. Furthermore, charge calculations showed that the delocalization into the cycloalkyl group is greater than in the protonated carboxylic acid (carboxonium ion), where two oxygen atoms participate in delocalization. 

Various oligomers of fluorinated alkenes and cycloalkenes have been prepared by fluoride ion induced oligomerisation of various monomers (Sect. 5.3), and the chemistry of these systems provides some unique reactions. The oligomers of special interest here may be described as of types (95) or (96) (Scheme 59), i. e. systems with either four (95) or three (96) perfluoro-alkyl or -cycloalkyl groups attached to the double bond, whereas systems with two perfluoroalkyl groups attached, i. e. (97) and (98), have a chemistry more similar to fluorinated alkenes that may be derived from other sources. 

In addition to chlorosarin and chlorosoman (see Section 2.3), there are two families of key precursors placed on the Schedule 1 list. DF (see Table 1) is the best known example of the Schedule 1.B.9 chemicals. The molecular ion, at mlz 100, gives the base peak in its El mass spectrum (4). The fragmentation is somewhat complicated, but gives abundant peaks for the loss of CH30 (m/z 69) and HF (m/z 80). The trivalent alkyl/cycloalkyl 2-dialkylaminoethyl alkylphosphonites (Schedule 1.B.10), which are related to the VX family of compounds, have QL (see Table 1) as their main representative because... 

The rearrangement of [13] bears a close resemblance to the transannular reactions observed in medium sized rings that have been reviewed by Prelog and Traynham (1963) and Cope et al. (1966). Recently Sorensen and coworkers have studied medium sized cycloalkyl cations under stable ion conditions in non-nucleophilic media and demonstrated that their structures are ji.-hydrido-bridged. The bonding situation in these ions contrasts sharply with that in the ions described above and rather corresponds to that of transition states (or intermediates) for intramolecular hydride transfer in these ions. 

Of course, the traditional problem of the lack of precise knowledge of the heats of solvation for the passage of these ions into solution, makes the above criteria of stability less valuable to the condensed-phase chemist. A major breakthrough in this classical impasse has been achieved by Arnett and coworkers " who have recently carried out calorimetric measurements leading to reliable values of the enthalpy of ionisation of various alkyl, cycloalkyl and aiyl halides in solution. These determinations owe their validity to the use of superacid conditions and the NMR verification that the ions expected were in fact formed in those media without Ihe occurrence of secondary reactions. One of the most important conclusion of these studies is that on the whole the relative stabUities of carbenium ions are the same in the gas pha% and in the solvents used, i.e., electrostatic solvation effects do not alter the order of stability. The importance of this new experimental approach is quite obvious and one can except in the near firture considerable advances in the field of the thermodynamics of reactive carbenium ions in solution through the attmnment of a precise knowledge of AG° values for their formation in various media. 

Cycloalkyl esters for the side-chain protection of aspartic acid in SPPS have been developed to increase resistance to aspartimide formation. Based on mechanistic studies of this side reaction, these protection groups should fulfill the following criteria provide steric hindrance to intramolecular aminolytic attack of the ester by the amide nitrogen in acidic and basic media, provide increased stability toward repetitive TFA treatments but quantitative cleavage by HE, as well as stabilization of the carbenium ion produced by cleavage of the protecting group to prevent recapture by the peptide. The secondary cycloalkyl esters are more acid stable and more sterically hindered if compared to the primary benzyl esters. In Scheme 7, different cycloalkyl esters are shown. 

The electrolysis products of different carboxylates have been compared with the ionization potentials of the intermediate radicals. From this it appeared that alkyl radicals with gas-phase ionization potentials smaller than 8 eV mainly lead to carbenium ions. Accordingly, a-substituents such as carboxy, cyano or hydrogen support the radical pathway, whilst alkyl, cycloalkyl, chloro, bromo, amino, alkoxy, hydroxy, acyloxy or aryl more or less favor the route to carbenium ions. Besides electronic effects, the oxidation seems also to be influenced by steric factors. Bulky substituents diminish the extent of coupling. The main experimental factors that affect the yield in the Kolbe electrolysis are the current density, the pH of the electrolyte, ionic additives, the solvent and the anode material. 

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