Carbonium ions alkoxy

Adjacent atoms with one or more lone pairs of electrons strongly stabilize a carbonium ion. Alkoxy and dialkylamino groups are familiar examples of this effect. 

Apparently the alkoxy radical, R O , abstracts a hydrogen from the substrate, H, and the resulting radical, R" , is oxidized by Cu " (one-electron transfer) to form a carbonium ion that reacts with the carboxylate ion, RCO - The overall process is a chain reaction in which copper ion cycles between + 1 and +2 oxidation states. Suitable substrates include olefins, alcohols, mercaptans, ethers, dienes, sulfides, amines, amides, and various active methylene compounds (44). This reaction can also be used with tert-huty peroxycarbamates to introduce carbamoyloxy groups to these substrates (243). 

It will be observed that most syntheses yield pyrylium salts in which positions 2,4, and 6 are substituted. Since according to formulas Ib-lc these positions have a partial positive charge, it can readily be understood why electron-donating substituents (hydroxy, alkoxy, alkyl, or aryl) in these positions stabilize the pyrylium salts. Only three pyrylium salts which do not have substituents in either a-position have been reported and few unsubstituted in y or in one a-position they are less stable toward hydrolysis, and in the case of perchlorates they explode more easily, than 2,4,6-trisubstituted compounds. In fact, the former are secondary, the latter tertiary carbonium ions. This fact also explains why the parent compound (1) was prepared only in 1953. 

Cationic polymerisation involves carbonium ion active species and takes place with monomers which contain electron releasing substituents such as phenyl, vinyl, alkoxy and 1, 1 dialkyl. 

The substitution path b) (Eq. (94) ), is favored by the following experimental conditions low current density, grapliite as anode material, alkaline medium, water or water-pyridine as solvent, and admixture of foreign ions e.g., bicarbonate, sulfate, perchlorate, dihydrogen phosphate, Pb2+, Mn2+, Cu2+, Fe2+, Co2+. The carbonium ion path b) can furthermore be expected for carboxylates RR CHOO with a-substituents R such as alkyl, phenyl 198 hydroxy, halogen 1 amino, or alkoxy. These substituents facilitate oxidation of the intermediate alkyl radical R to the carbonium ion R+. Product formation occurs via carbonium ions and not, as is also conceivable, via mixed coupling of R with Nu ... 

The slow step is the formation of the resonance stabilized alkoxy-carbonium ion, R2C+—OR R2C=0+—R. Accordingly, the reaction is... 

The borohydride ion is a good trap for spontaneously generated carbonium ions (or ion pairs), such as those formed by secondary or tertiary benzylic or allylic halides in ionizing solvents, but would be expected to be inactive when the leaving group is alkoxy unless an acidic partner were also present. 

Further measurements with diazoacetophenones revealed enhanced rates forp-alkoxy substituents (Tsuno et al., 1959 Jordan, 1966). These were taken as indicating carbonium ion character in the transition state, but it is also possible that the effects are primarily on the reactants. 

The glycals react with water, alcohols, phenols, carboxylic acids, and certain bases, in the presence of an acidic catalyst, in the same way as does 2,3-dihydro-4/f-pyran (which gives, for example, 2-hydroxy-, 2-alkoxy-, and 2-aryloxy-tetrahydropyrans in high yield ). (Tetrahydro-pyranyl ethers are useful for protecting alcoholic groupings during reactions in basic media, and are readily hydrolyzed with acid,) The first step of these additions would be expected to be protonation at C-2, followed by attack of the nucleophilic reagent on the resonance-stabilized, C-1 carbonium ion. 

It has long been postulated that these facile reactions occur via formation of a cationic intermediate (upon acid-catalyzed loss of ROH) which can be formulated either as a a-bonded alkylcobalt carbonium ion or a cobalt(III)-olefin m complex. Recently, firm kinetic evidence has been obtained for the occurrence of an intermediate in the acid-catalyzed decomposition of 2-hydroxy- and 2-alkoxyethyl-cobaloximes [94]. Thus, while 2-hydroxyethylcobaloxime decomposes with strictly first-order kinetics in mildly acidic H2SO4/H2O mixtures, the alkoxy derivatives show a substantial lag followed by a first-order decay which is slower than that for the hydroxyethyl complex. In strongly acidic mixtures (//q < —5) all compounds show a rapid burst of absorbance change, followed by a slower first-order decay which is identical for all compounds whether measured spectrophotometrically or manometrically. These observations support the mechanism shown in Eqn. 56. 

Three-c(H)rdinate carbenium ion and five-coordinate carbonium ion intermediates satisfactorily account for many of the acid-catalyzed reactions of hydrocarbons at high temperatures. Yannoni et al. have characterized the structure and dynamics of several carbenium ions trapped in (noncatalytic) solids at low temperatures [32,94,95), but lifetimes of such ions on active surfaces at higher temperatures would preclude NMR observation in all but special cases. Maciel observed triphenyl carbenium ion on alumina 196). The alkyl-substituted cyclopentenyl ions discussed earlier are also special ions they are commonly observed products in conjunct polymerization reactions of olefins in acidic solutions. The five member ring cannot easily rearrange to an aromatic structure, and ions like I and II are apparently too hindered to be captured by the framework to form alkoxy species. 

An additional process which may occur on the phosphylated enzyme is a process termed ageing, which results in a covalently inhibited enzyme that is resistant to accelerated reactivation by therapeutic nucleophiles (see Section XII). Ageing results from PO—C bond cleavage in the alkoxy substituent as illustrated in equation 49 and occurs rapidly (within minutes) with phosphonofluoridates such as soman, whose highly branched alkoxy group produces a relatively stable secondary alkyl carbonium ion . The dealkylation is... 

The potential for dealkylation of the alkoxy group on the phosphonylated enzyme (known as ageing) is also dependent on the degree of branching of the side-chain this is related to the PO—R bond strength and the tendency towards the formation of a more stable secondary carbonium ion R. ... 

The reaction is specific acid-catalyzed for most acetals and ketals. This is consistent with the existence of a preequilibrium in which the ketal is protonated. A logical role for the proton is to assist in the departure of one of the alkoxy groups. In essence, this cleavage step is an SnI reaction with the ease of formation of the carbonium ion being greatly increased by the stabilization afforded by the remaining alkoxy substituent. 

Very stable carbonium ion (stabilized by both alkoxy function and aromaticity)... 

Chain polymerization in which the active center is a cation, usually carbonium ion, -C. Generally, the method is used to polymerize vinyl monomers carrying electron-releasing substituents (e.g., alkyl or alkoxy groups). The polymerization is initiated by an initiator and co-initiator, viz., BH3 + H2O. 

Reaction (6) appears to involve a dimeric aluminium species with bridging alkoxy and chlorine groups, which decomposes by liberation of a carbonium ion from the alkoxy bridge. 

The alkoxy ir -allylic derivatives underwent alkoxyl exchange very readily when treated with the appropriate alcohol containing a little mineral acid (10" M)(e.g. methoxy-ethoxy and vice versa). These reactions are believed to go via an intermediate carbonium ion stabilised by the delocalised electron system of the tt-allylic group. Methoxy tt-allylic complexes derived from 2,5-dimethylhexa-2,4-diene, cyclohepta-1,3-diene and cycloocta-1,3-diene on heating lost methanol irreversibly to give a8-unsaturated tr-allylic complexes e.g. cyclohepta-1,3-diene first gave a tt-methoxycycloheptenyl and then a TT -cycloheptadienyl complex. 

Alkoxy-l-methylpyridinium salts (XII-729) have been used to study the behavior of carbonium ions in solvolyses. They are easy to prepare and purify, and in solvolyses the leaving group does not undergo internal return. The solvolysis of 4-(cyclopropylcarbinyloxy)-l-methylpyridinium iodide (XII-729, r = ch2-c1) in 80% ethanol gives some cyclopropylcarbinol, cyclobutanol, and allylcarbinol but mainly the corresponding ethyl ethers (XII-730 R = cyclo-propylcarbinyl, cyclobutyl, allylcarbinyl R = CjHs) in a ratio of 3.6 1.5 1.0. 

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