Carbonium elimination

The base [FeBr4] facilitates the elimination of a proton from the carbonium ion (I). 

C—O bonding and Cl—F fission of the intermediate cw-fluoro chlorate (29a), which in turn undergoes oxidation to the fluoro ketone (25) by a concerted elimination of chlorous acid. A similar transition state (30) approximating an allylic carbonium ion could be involved in the reaction of the dienol derivatives (6) with perchloryl fluoride, which would be expected to give rise to the c/5-adduct (30a). Reaction of the latter with water leads to product and chlorate ion. 

In contrast to the behavior of homoallylic alcohol (70a) when treated with methanesulfonyl chloride is pyridine, heating A -19-methanesulfonate (68b) in pyridine gives the 5)5,19-cyclo-6-ene (72). Vinylcyclopropane (72) is inert to the conditions used for converting vinylcyclopropane (73) to the A ° -B-homo-7)5-ol (70a). The latter results are only consistent with the existence of two discrete isomeric carbonium ion intermediates which give rise to isomeric elimination products. °... 

Acylation of norephedrine (56) with the acid chloride from benzoylglycolic acid leads to the amide (57), Reduction with lithium aluminum hydride serves both to reduce the amide to the amine and to remove the protecting group by reduction (58), Cyclization by means of sulfuric acid (probably via the benzylic carbonium ion) affords phenmetrazine (59), In a related process, alkylation of ephedrine itself (60) with ethylene oxide gives the diol, 61, (The secondary nature of the amine in 60 eliminates the complication of dialkylation and thus the need to go through the amide.) Cyclization as above affords phendimetra-zine (62), - Both these agents show activity related to the parent acyclic molecule that is, the agents are CNS stimulants... 

In the presence of strong acid, formic acid decomposes to water and carbon monoxide. In the process, reactive intermediates form which are capable of direct carboxylation of carbonium ions. Since many carbonium ions are readily generated by the reaction of alcohols with strong acid, the process of elimination and carboxylation can be conveniently carried out in a single flask. The carbonium ions generated are subject to the... 

Vinylsilanes react readily with a range of electrophiles to give products of substitution (1). The overall stereochemistry of such substitution will depend on a number of factors, including the stereochemistry of addition and subsequent elimination when 1,2-adducts are discrete species. However, the regiochemistry of substitution is normally unambiguous, the -effect ensuring that carbonium-ion development on attack by the electrophile will occur at the carbon terminus remote, i.e. /3, to silicon ... 

Aquilante and Volpi indicate (2) that propanium ions formed by proton transfer from H3 + are not collisionally stabilized at propane pressures as great as 0.3 mm. and that they decompose by elimination of hydrogen or a smaller saturated hydrocarbon to form an alkyl carbonium ion. Others (16, 19) have proposed one or the other of these fates for unstabilized propanium ions. Our observations can be rationalized within this framework by the following mechanisms ... 

Clearly the rate-determining step is the elimination of water from the protonated alcohol to form a carbonium ion, which loses a proton to give the readily oxidised olefin. 

These products are thought to result from attack of an intermediate p-nitro-phenyl radical on the solvent. Evidence for this intermediate was obtained by scavenging the radical with diphenylpicrylhydrazyl, halogens, and nitric oxide.(62) However, the presence of the p-nitrophenetole in the products formed in the presence of iodine, which effectively eliminates most other radical products, suggests that another mechanism involving a phenyl carbonium ion may be also operative ... 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

When oligoisobutenes are formed from gaseous isobutene at ambient temperature by BF3 and H20 the initial group is CH3, formed by addition of a proton to the monomer [8]. The predominant terminal groups are double bonds [8] formed by transfer reactions involving elimination of a proton from the growing carbonium ion ... 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

In the case of arylmethanols an OH-ion is eliminated, and a phenyl-carbonium ion is formed. Olah (1964) and Famum (1964) have published and discussed N.M.R. spectra of these ions. 

The catalytic homogeneous isomerization of olefins by protonic acids as well as by Lewis acids is well-known and there is little doubt that such isomerizations proceed through carbonium ion transition states or intermediates. Thus, strong acids isomerize 1-hexene to all possible hexene isomers, including cis and trans isomers where these are possible. The isomerization to 2-hexene may be written as a proton addition-elimination ... 

The anion presumably plays only a minor role, if any, especially in aqueous systems. Now the formation of 3-hexene may be explained in either of two ways. The intermediate carbonium ion, written in brackets, can undergo hydride migration to form a new carbonium ion, which can then collapse by proton loss to form the 3-hexene. Such a process does not require the intermediate formation of 2-hexene. The alternate explanation involves the discrete formation of 2-hexene followed by addition and elimination of a proton to give the desired 3-hexene. There is no question but that the hydride migration occurs, and with great... 

Although HCo(CO)4 is a strong acid in aqueous solution and is capable of protonating even weak bases like dimethylformamide, there is no evidence that it protonates olefins in hydrocarbon solvents to form carbonium ion intermediates which might then rearrange by conventional 1,2-hydride shifts followed by proton elimination ... 

The intermediate has a finite lifetime, but it is not free the less stable secondary carbonium ion is stabilized by specific interaction with two molecules of water. The same kinetic study on primary alcohols made by Dostrovsky and Klein (79) shows that oxygen exchange in dilute acid solution does not proceed by way of an ion, but by a concerted mechanism. For the same reason the elimination reaction has to be of a concerted nature and cannot proceed via an unsolvated carbonium ion. 

The equal distribution of at C(l) and C(3) in a-methylstyrene is best explained by a trans elimination reaction of the elements of water from 2-phenyl-l-propanol-l-C, followed by a rapid equilibration of a-methylstyrene produced through the formation of the highly stabilized tertiary carbonium ion ... 

In acetonitrile, carbonium ions combine with the solvent to form a nitrillium ion. The latter reacts with added water to form the N-substituted acetamide, often in good yield [5, 6, 7]. Thus electrochemical oxidation of alkanes in acetonitrile is a route for the introduction of an amino-substituent. Some carbonium ions are inefficiently quenched by acetonitrile and eliminate a proton to form an alkene. This alkene is readily oxidised at the anode potentials used and oxidation products contribute to electrode fouling. Pulsing of the anode potential to +0.3 V vs, see helps to... 

The most widely accepted mechanism (Whitmore, 13) for the polymerization of olefins involves the so-called carbonium ions. In accordance with this mechanism a carbonium ion (usually a tertiary ion) adds to the olefin to form a higher molecular weight carbonium ion which then yields the olefin polymer by elimination of, usually, a proton. With acid catalysts (e.g., sulfuric acid) the initial carbonium ion is formed by addition of the hydrogen ion from the acid to the extra electron pair in the double bond (the pi electrons) ... 

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