Carbocations with alkynes

As the reactions of carbocations with alkynes have higher activation enthalpies and less negative activation entropies than analogous reactions with alkenes, the relative reactivities of styrene and phenylacetylene have been found to strongly depend on temperature (Table 8) [213],... 

For reactions of A-acyliminium ions with alkenes and alkynes one has to distinguish between A-acyliminium ions locked in an s-trans conformation and those which (can) adopt an s-cis conformation. The former type reacts as a (nitrogen stabilized) carbocation with a C —C multiple bond. Although there are some exceptions, the intramolecular reaction of this type is regarded as an anti addition to the 7t-nucleophile, with (nearly) synchronous bond formation, the conformation of the transition state determining the product configuration. 

The reaction is highly dependent on the alkyne used. When structurally simple 3-hydroxy alkynes such as propargyl alcohol are used, acceptable yields of the addition products are obtained, whereas reactions with alkynes prone to carbocation formation and rearrangement, such as 1-ethynyl-cyclohexanol, give a complex mixture of products.68,73... 

Because of the availability of their Ti-electrons, alkenes and alkynes are sensitive to electrophilic addition. It is an intriguing question why, with certain reagents and under certain conditions, alkenes are more reactive than alkynes, whereas in other cases the reverse is true. This question has been addressed by Modena and coworkers. They consider protons and carbocations as reagents that react with alkenes at similar or lower rates than with alkynes. Addition of these reagents gives rise to open carbocations in a ratedetermining step. 

The IR spectrum of the pentachlorocyclopentadi-enyl cation C5CI5+ (31) has been measured in an SbFs matrix, and this cation is determined to have D h symmetry by DFT calculations. Substituted cyclopentadienyl cations in the gas-phase undergo carbon skeletal rearrangements and loss of alkyne units that can be explained by the formation of carbocations with a square pyramidal structure (Figure 6). /) This strucTure was initially estimated to be more stable than planar structures, but later calculations confirm the latter are more stable. Cyclopentadienyl cations with several strongly electron-donating substituents have been obtained as stable salts. ... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

A plausible mechanism for this new alkyne aza-Prins cyclization is outlined in Scheme 27. Thus, reaction of the homopropargyl tosyl amine with an aldehyde promoted by ferric halide generates the W-sulfonyl iminium ion. This intermediate evolves to the corresponding piperidine, via the vinyl carbocation. Ah initio theoretical calculations support the proposed mechanism. 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

The cyclization of alkenols can also occur through an interesting relay mechanism (Equation (125)). Under AuC13 catalysis, the alkyne of an enynol is first activated to promote a cationic G-G bond-forming cyclization with the alkene. The G-O bond-forming ring closure then occurs via capture of the carbocation by the alcohol.451... 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

Carbo-oxylation (carbohydroxylation and carboalkoxylation), whereby an organic group and an oxy (hydroxyl or alkoxy) group add across a carbon-carbon double bond or a triple bond, is an important transformation in organic synthesis. We found that the reaction of a cation pool with an alkene or alkyne followed by the trapping of the resulting carbocation by water led to the... 

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. 

Sterically hindered alkenes can be hydrogenated at —50°C using triflic acid and a hydride donor.483 In addition to Et3SiH, transition-metal carbonyl hydrides such as HM(CO)3Cp (M = W, Mo, Os) and HMn(CO)5 (M = Mn, Re) are suitable hydride donors. Alkenes that form tertiary carbocation on protonation are hydrogenated in high yields (90-100%), whereas yields for styrenes are lower (50-60%). Alkynes are converted to the corresponding saturated hydrocarbon by using HW(CO)3Cp in combination with triflic acid.484... 

Chapter 8 begins the treatment of organic reactions with a discussion of nucleophilic substitution reactions. Elimination reactions are treated separately in Chapter 9 to make each chapter more manageable. Chapter 10 discusses synthetic uses of substitution and elimination reactions and introduces retrosynthetic analysis. Although this chapter contains many reactions, students have learned to identify the electrophile, leaving group, and nucleophile or base from Chapters 8 and 9. so they do not have to rely as much on memorization. Chapter 11 covers electrophilic additions to alkenes and alkynes. The behavior of carbocations, presented in Chapter 8, is very useful here. An additional section on synthesis has been added to this chapter as well. 

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