Carbonation single-step process route

The current two-step industrial route for the synthesis of methanol, from coal or methane to synthesis gas and then from synthesis gas to methanol, has certain drawbacks. The economic viability of the whole process depends on the first step, which is highly endothermic. Thus a substantial amount of the carbon source is burned to provide the heat for the reaction. It would be highly desirable, therefore, to replace this technology with a technically simpler, single-step process. This could be the direct partial oxidation of methane to methanol, allowing an excellent way to utilize the vast natural-gas resources. Although various catalysts, some with reasonable selectivity, have been found to catalyze this reaction (see Sections 9.1.1 and 9.6.1), the very low methane conversion does not make this process economically feasible at present. 

The most direct route towards functionalized aliphatic polyesters is based on the functionalization of polyester chains. This approach is a very appealing because a wide range of functionalized aliphatic polyesters could then be made available from a single precursor. This approach was implemented by Vert and coworkers using a two-step process. Eirst, PCL was metallated by lithium diisopropylamide with formation of a poly(enolate). Second, the poly(enolate) was reacted with an electrophile such as naphthoyl chloride [101], benzylchloroformate [101] acetophenone [101], benzaldehyde [101], carbon dioxide [102] tritiated water [103], ot-bromoacetoxy-co-methoxy-poly(ethylene oxide) [104], or iodine [105] (Fig. 26). The implementation of this strategy is, however, difficult because of a severe competition between chain metallation and chain degradation. Moreover, the content of functionalization is quite low (<30%), even under optimized conditions. 

It is important for the discussion below to distinguish between direct and indirect process routes. Direct carbonation is the simplest approach to carbonate production (or mineral carbonation see Section 14.4) and the principal approach is that a suitable feedstock-for example, serpentine or a Ca/Mg-rich solid residue-is carbonated in a single process step. For an aqueous process this means that both the extraction of metals from the feedstock and the subsequent reaction with the dissolved C02 to form carbonates takes place in the same reactor. 

Thorough mechanistic studies have established that dehydration over acidic oxides follows two major routes. A single-step, concerted E2 mechanism, usually results in alkenes with Saytzeff orientation (more substituted alkene isomers, 1) (Scheme 1). The El mechanism, in turn, is a two-step process which starts with the removal of the OH group. Because carbocationic intermediates are involved they eventually give rise to a mixture of isomeric alkenes (1-4). A third route of lesser significance (ElcB mechanism), initiated by the removal of a proton from the P carbon, occurs characteristically on basic oxides. In this route the Hofmann orientation (formation of the less substituted alkene, 2) usually prevails. 

This chemistry has provided one of the most direct routes to the indolo[2,3-a]carbazole alkaloid ring system (Scheme 11), a common functionality of several biologically active molecules such as the potent antitumor agent rebeccamycin and arcyriaflavin A. The indolo[2,3-fl]carbazole derivative has been obtained in satisfactory yield through the reaction of l,4-di((9-trifluoroacetamidophenyl)-l,3-butadiyne with 3,4-dibromomaleimide in the presence of tetrakis(triphenylphosphine)palladium(0) and potassium carbonate. The proposed mechanism of this polyannulation process, which generates four new bonds in a single step, is outlined in Scheme 12. 

This was followed shortly by a stereo- and enantiocontrolled synthesis of (—)-chimonanthine (154) and calycanthine (150) as well as a second route to meso-chimonanthine (152). The central step in this synthesis features the use of a double Heck cyclization to create vicinal quaternary carbon centers in high yields and with complete stereocontrol 124). The synthesis commenced with a double alkylation of the lithium dienolate of dimethyl succinate 194 and tartrate-derived diiodide 195 to give a diastereomeric mixture of the saturated diesters. Subsequent oxidation of the diesters, followed in succession by aminolysis, A-benzylation, removal of the benzyl ethers, and silylation, provided the cyclization substrate 197, which on Heck cyclization yielded a single product, 198, a pentacyclic bisoxindole, subsequently shown to have the meso relationship of the two oxindole groups. Further manipulations of 198 led eventually to the diazide derivative 199, which can be processed to we.so-chimonanthine (152), following the procedure established in the preceding synthesis (Scheme 14). 

Up to this point, the focus has been on improving the yield of the catalytic reaction to reduce CO2 emission. However it is important to consider entirely new process chemistry that might reduce the number of steps, lower the temperature, and as a result, also lower CO2 production. An excellent illustration of this point involves the production of methyl methacrylate (MMH). Current commercial catalytic routes use C4 feedstocks and involve two high-temperature gas-phase catalytic steps followed by esterification. The first two steps occur above 350°C with an overall yield of about 75%. The main by-product is carbon dioxide. A new process to methyl methacrylate is under development by Asahi Chemical.This process combines the second and third steps into a single oxidative esterification step... 

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