Acid-Catalyzed Processes

The principal impetus behind the synthesis of thiols came from the need to produce synthetic mbber in the early 1940s. These mbbers, styrene—butadiene mbbers (SBRs), were produced by many companies at that time. Originally, 1-dodecanethiol was utilized, but the most important thiol became / fZ-dodecanethiol, which was made from propylene tetramer, using an acid-catalyzed process (54,55). 

In the acid-catalyzed process, the enol 6 reacts with the protonated carbonyl group of another aldehyde molecule 2 ... 

In general, the O-alkylation of benzoxepinones is accomplished via the anion. Alternatively, an acid-catalyzed process employing ortho esters may be used. For the acid-catalyzed formal O-alkylation of l-chloro-8-methoxydibenz[ft,/]oxepin-10(ll//)-ones with triethyl orthoformate rather drastic conditions are required (hot concentrated sulfuric acid) to give the 10-ethoxy derivative 12 in excellent yield.109... 

There are four acid-catalyzed processes which are entirely symmetric and reversible. Aal2 mechanism has never been observed. 

The synthetic aspects of this acid-catalyzed process have been studied extensively for many years, and they have been reviewed recently by Falbe (1970). The kinetic and thermodynamic details of the various reaction steps in the overall process have been investigated in this laboratory during the last few years with special emphasis on the carbonylation step. The present article reflects the state of affairs in this respect. 

For each catalyst, the mechanism for one direction is the exact reverse of the other, by the principle of microscopic reversibility. As expected from mechanisms in which the C—H bond is broken in the rate-determining step, substrates of the type RCD2COR show deuterium isotope effects (of 5) in both the basic- and the acid -catalyzed processes. 

According to this mechanism, it is the free amine, not the salt that reacts, even in acid solution and the active-hydrogen compound (in the acid-catalyzed process) reacts as the enol when that is possible. This latter step is similar to what happens in 12-4. There is kinetic evidence for the intermediacy of the iminium ion (13). 

Having seen that the excess acidity method works for second-order as well as for first-order acid-catalyzed processes, it is of interest to see whether it extends to reactions that are not acid catalyzed. The hydrolysis of acylimidazoles, equation (68), takes place in aqueous acids the substrate is protonated on the ring nitrogen in the pH range, and in acid media the reaction rate constants decrease steadily with increasing acidity.251,253... 

The reductive ether formation from keto epoxides is an acid-catalyzed process (Eqs. 234407 and 235408). 

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

Since the discovery of alkylation, the elucidation of its mechanism has attracted great interest. The early findings are associated with Schmerling (17-19), who successfully applied a carbenium ion mechanism with a set of consecutive and simultaneous reaction steps to describe the observed reaction kinetics. Later, most of the mechanistic information about sulfuric acid-catalyzed processes was provided by Albright. Much less information is available about hydrofluoric acid as catalyst. In the following, a consolidated view of the alkylation mechanism is presented. Similarities and dissimilarities between zeolites as representatives of solid acid alkylation catalysts and HF and H2S04 as liquid catalysts are highlighted. Experimental results are compared with quantum-chemical calculations of the individual reaction steps in various media. 

In the liquid acid-catalyzed processes, the hydrocarbon phase and the acid phase are only slightly soluble in each other in the two-phase stirred reactor, the hydrocarbon phase is dispersed as droplets in the continuous acid phase. The reaction takes place at or close to the interface between the hydrocarbon and the acid phase. The overall reaction rate depends on the area of the interface. Larger interfacial areas promote more rapid alkylation reactions and generally result in higher quality products. The alkene is transported through the hydrocarbon phase to the interface, and, upon contact with the acid, forms an acid-soluble ester, which slowly decomposes in the acid phase to give a solvated... 

The catalyst is reported to be a true solid acid without halogen ion addition. In the patent describing the process (239), a Pt/USY zeolite with an alumina binder is employed. It was claimed that the catalyst is rather insensitive to feed impurities and feedstock composition, so that feed pretreatment can be less stringent than in conventional liquid acid-catalyzed processes. The process is operated at temperatures of 323-363 K, so that the cooling requirements are less than those of lower temperature processes. The molar isobutane/alkene feed ratio is kept between 8 and 10. Alkene space velocities are not reported. Akzo claims that the alkylate quality is identical to or higher than that attained with the liquid acid-catalyzed processes. 

The catalyst is faujasite derived, with a high concentration of sufficiently strong Brpnsted acid sites and a minimized concentration of Lewis acid sites. It also contains a hydrogenation function. The process operates at temperatures of about 323-373 K with a molar isobutane/alkene ratio between 6 and 12 and a higher alkene space velocity than in the liquid acid-catalyzed processes. Preliminary details of the process concept have been described (240). 

Liquid acid-catalyzed processes are mature technologies, which are not expected to undergo dramatic changes in the near future. Solid acid-catalyzed alkylation now has been developed to a point where the technology can compete with the existing processes. Catalyst regeneration by hydrogen treatment is the method of choice in all the process developments. Some of the process developments eliminate most if not all the drawbacks of the liquid acid processes. The verdict about whether solid acid-catalyzed processes will be applied in the near future will be determined primarily by economic issues. 

Another example of a transannular cyclization that occurs in the solid state is provided by the epoxy alcohol 31. This compound is stable when dissolved in organic solvents and in 0.25N sulfuric acid. However, the crystals transform rapidly to 32. Although the process is accompanied by partial melting, it appears to be a true solid-state one. Interestingly, the reaction is slowed down appreciably when the dry crystals are covered with ether. Hydrogen bromide is eliminated in the reaction and it may be that an acid-catalyzed process is also occurring in the presence of solvent this process may be slowed down by the dissolution of the decomposition products in the solvent (77). 

This view has been challenged with more recent evidence indicating that AT-[(acyloxy)methyl] derivatives of both primary and secondary amides (8.170, Fig. 8.21) undergo decomposition by the same mechanisms, namely a) an acid-catalyzed process involving protonation followed by formation of an /V-acyliminium species (Fig. 8.21, Reaction a) b) a pH-independent heterolytic cleavage forming the same /V-acyliminium species (Fig. 8.21, Reaction b) and c) a base-catalyzed pathway, which for /V-[(acyloxy)methyl] derivatives of AT-methylamides is the normal mechanism (Fig. 8.21, Reaction c), but for AT-[(acyloxy)methyl] derivatives of primary amides involves substrate deprotonation followed by /V-acy limine formation (Fig. 8.21, Reaction d) [218],... 

The presence of acidic centers in the catalyst promotes acid-catalyzed processes in addition to the above reactions. Of course, their rates may be much higher than those of metal catalyzed ones. 

---