Carboxylation of alkenes

Unsaturated hydrocarbons (alkenes, dienes) react with carbon monoxide and a proton source (H20, alcohols, amines, acids) under strong acidic conditions to form carboxylic acids or carboxylic acid derivatives. Since a carbocationic mechanism is operative, not only alkenes but also other compounds that can serve as the carbocation source (alcohols, saturated hydrocarbons) can be carboxylated. Metal catalysts can also effect the carboxylation of alkenes, dienes, alkynes, and alcohols. 

The reaction is also called hydrocarboxylation. According to a later modification, the alkene first reacts with carbon monoxide in the presence of the acid to form an acyl cation, which then is hydrolyzed with water to give the carboxylic acid.97 The advantage of this two-step synthesis is that it requires only medium pressure (100 atm). Aqueous HF (85-95%) gave good results in the carboxylation of alkenes and cycloalkenes.98 Phosphoric acid is also effective in the carboxylation of terminal alkenes and isobutylene, but it causes substantial oligomerization as well.99 100 Neocarboxylic acids are manufactured industrially with this process (see Section 7.2.4). The addition may also be performed with formic acid as the source of CO (Koch-Haaf reaction).101 102 The mechanism involves carbocation formation via protonation of the alkene97 103 [Eq. (7.10)]. It then reacts with carbon monoxide... 

Neocarboxylic Acids. The acid-catalyzed carboxylation of alkenes with carbon monoxide in the presence of water is used commercially to produce neocarboxylic acids.82 Pivalic acid from isobutylene is the most important product. A number of... 

In combination with the incremental advances concerning reaction conditions in recent years, especially for low-pressure carbonylations, there is a trend toward increasing use of this chemistry to synthesize advanced building blocks. In this respect carboxylation of alkenes with an appropriate alcohol or amine function leads to the formation of lactones or lactams. Thus, cobalt, rhodium, or palladium chloride/copper chloride catalysts convert allyl and homoallyl alcohols or amines to the corresponding butyrolactones or butyrolactams, respectively [15]. 

Alkenoic acids. In the third step of a procedure for the carboxylation of alkenes the allylic selenides are treated with HjOj to initiate a [2,3]sigmatropic rearrangement and... 

Because of the low electrophilicity of trialkoxycarbenium ions, orthocarbonates cannot be used for the carboxylation of alkenes. This reaction can be achieved with dichloroacetals, however. Figure 14 shows that the formation of 1 1 products from 21 and ordinary alkenes requires addition rate control When 21 and isobutene are treated with ZnCl2 or catalytic amounts of BCI3, only the 2 1 products 23 are formed, since 22 ionizes to a greater extent than 21. With equimolar amounts of BCI3, the relative reactivities of reactants and products become controlled by the addition... 

Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

Most frequent are oxidations of alkenes that can be converted to a series of compounds such as epoxides, halohydnns and their esters, ozonides (1,2,4 tri-oxolanes), a-hydroxyketones, a-hydroxyketone fluorosulfonates, ot-diketones, and carboxylic acids and their denvatives... 

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

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