Carbonium ions, carboxylation

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). 

Carbonylation, or the Koch reaction, can be represented by the same equation as for hydrocarboxylation. The catalyst is H2SO4. A mixture of C-19 dicarboxyhc acids results due to extensive isomerization of the double bond. Methyl-branched isomers are formed by rearrangement of the intermediate carbonium ions. Reaction of oleic acid with carbon monoxide at 4.6 MPa (45 atm) using 97% sulfuric acid gives an 83% yield of the C-19 dicarboxyhc acid (82). Further optimization of the reaction has been reported along with physical data of the various C-19 dibasic acids produced. The mixture of C-19 acids was found to contain approximately 25% secondary carboxyl and 75% tertiary carboxyl groups. As expected, the tertiary carboxyl was found to be very difficult to esterify (80,83). 

Kinetic data on acetate displacement from C-3 using a number of sulfur and nitrogen nucleophiles in aqueous solution at near neutral pH demonstrate that the reaction proceeds by an 5 1 mechanism (B-72MI51004). The intermediate in this reaction is depicted as a dipolar allylic carbonium ion (9) with significant charge delocalization. Of particular significance in this regard is the observation that the free carboxylate at C-4 is required since... 

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... 

The reaction between carbonium ions and carbon monoxide affording oxocarbonium ions (acyl cations) is a key step in the well-known Koch reaction for making carboxylic acids from alkenes, carbon monoxide and water ... 

As mentioned in the Introduction, rearrangements of the intermediate alkyl cation in the Koch synthesis may compete with the carbonylation. Under the kinetically controlled conditions prevailing in the Koch synthesis of carboxylic acids, the rearrangements occur only from a less stable to a more stable carbonium ion, e.g. from a secondary to a tertiary ion. The reverse rearrangements—from a more stable to a less stable... 

First, the rates of carbonylation of secondary and tertiary alkyl carbonium ions can now be compared quantitatively with the known rates of competing intramolecular rearrangements of these ions. The product distribution in the Koch synthesis of carboxylic acids depends, amongst other things, on these relative rates. 

A further effect which has been known for many years is that of anions, which are specifically adsorbed at high anodic potentials on platinum, on the products of the oxidation of carboxylate ions. For example, carbonium ion-derived products can be obtained in the presence of such specific adsorption and this demands a complete change in reaction route (Fioshin and Avrutskaya, 1967 Glasstone and Hickling, 1934). 

From the above discussion it follows that the probability of carbonium ion formation during decomposition of RTIX2 compounds by a Type 5 process is low when X is carboxylate, but significantly higher when X is nitrate, sulfate, perchlorate, or fluoroborate. The important role played by the anion of the metal salt in oxymetallation has in fact been recognized only very recently for both oxymercuration 11, 12) and oxythallation (92). The... 

Similar results have been obtained with substituted triphenylcarbinols in sulfuric acid.174 Some unsaturated hydrocarbons form carbonium ions by simple addition of a proton, as do some carboxylic acids.175-177 But other carboxylic acids behave like the triarylcarbinols, the most prominent example being mesitoic acid. 

For a given carbonium ion the order of degree of ionization appears to be hydroxide, alkoxides, and carboxylates < cyanide < thiocyanate < ferrocyanide < azide < chloride < bromide < sulfate and perchlorate. 

R was varied. Since the mechanism for the methyl ester is certainly A-1 and since intramolecular general acid catalysis should give a different transition state structure and therefore a different p value, it was concluded that the mechanism was A-1 in both cases. The rate enhancement provided by the carboxyl group substituent was ascribed to electrostatic catalysis whereby a proton is stabilized on the acetal oxygen, thus lowering the dissociation constant of the conjugate acid. Complete protonation of methoxymethoxybenzoic acids might be required because of the unstable carbonium ion intermediate. 

The monoanionic species is most reactive, but its associated rate constant for intramolecular general acid catalysis is only 65 times greater than that for the unionized species. Most of the large rate enhancement in comparison with the dimethyl ester is due to participation by one carboxyl group, as is the case with the unionized acetal [77]. If the carboxylate anion of the monoanionic species is electrostatically stabilizing the incipient carbonium ion in the reaction [8], its effect on the rate must be small. 

Reaction of the carbonium ion with water could be reduced if overlap occurred with the carboxylate anion of aspartic acid-52 either during or after the glycoside-cleavage step. Since the carboxylate anion would be held adjacent to the carbonium ion in the active site, equilibrium should be far to the side of the acylal. Reaction of acylal with H2O would then very probably be ratedetermining in the forward direction. Evidence has been obtained that the solvent is directly involved in the hydrolysis of the cyclic acylal 2-(p-nitrophenoxy)phthalide where steric factors are similar... 

While the rates of alkaline and pH-independent hydrolysis of [93] and [94] are similar, hydronium ion catalysis is 2500 times less favourable in the case of the cyclic acylal. The large rate difference in comparison with the open chain analogue may reflect the fact that, if a carbonium ion intermediate with a carboxyl group held rigidly adjacent to the carbonium carbon atom were formed in an A-1 reaction, rapid reclosure of the ring would result from recapture of the carbonium carbon by the carboxyl group [equation (55)]. A greatly diminished rate of hydrolysis would result. Perhaps there is a... 

The enone function undergoes very fast intermolecular radical addition reactions and will trap the intermediate from oxidation of the carboxylate group. This step leads to a radical centre adjacent to the electron withdrawing carbonyl function of (he original enone and this centre is not readily oxidised to the carbonium ion. Dimerization to a diketone is thus the final stage in the reaction [108]. 

Substituents stabilising a carbonium ion influence the course of the anodic oxidation of carboxylic acids by promoting fast oxidation of the radical intermediate to the carbonium ion. Subsequent chemical steps are those expected of this ionic intermediate and the overall process is termed the non-Kolbe reaction. Reaction at... 

A carbonium ion rearrangement, triggered by anodic oxidation of a carboxylic acid, has been used as one stage in another synthesis of muscone [118]. 

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