Catalyst-free synthesis processes

Life-cycle assessment of multistep mfinamide synthesis from isolated reactions in a batch to a continuous microreactor network is reported [148]. A continuous solvent- and catalyst-free flow process utilizing relatively inexpensive and green dipolarophile, ( )-methyl 3-methoxyacrylate, was shown. Here, benzyl chloride, instead of very reactive benzyl bromide, was employed for azide formation, which was produced utilizing benzyl alcohol and hydrogen chloride. 

High pressure processes P > 150 atm) are catalyzed by copper chromite catalysts. The most widely used process, however, is the low pressure methanol process that is conducted at 503—523 K, 5—10 MPa (50—100 atm), space velocities of 20, 000-60,000 h , and H2-to-CO ratios of 3. The reaction is catalyzed by a copper—zinc oxide catalyst using promoters such as alumina (31,32). This catalyst is more easily poisoned than the older copper chromite catalysts and requites the use of sulfiir-free synthesis gas. 

The ultimate greening of fine chemical synthesis is the replacement of multistep syntheses by the integration of several atom-efficient catalytic steps. For example. Figure 9.9 shows the new Rhodia, salt-free caprolactam process involving three catalytic steps. The last step involves cyclization in the vapor phase over an alumina catalyst in more than 99% conversion and more than 99.5% selectivity. 

Table V shows the salient features of several Fischer-Tropsch processes. Two of these—the powdered catalyst-oil slurry and the granular catalyst-hot gas recycle—have not been developed to a satisfactory level of operability. They are included to indicate the progress that has been made in process development. Such progress has been quite marked in increase of space-time yield (kilograms of C3+ per cubic meter of reaction space per hour) and concomitant simplification of reactor design. The increase in specific yield (grams of C3+ per cubic meter of inert-free synthesis gas) has been less striking, as only one operable process—the granular catalyst-internally cooled (by oil circulation) process—has exceeded the best specific yield of the Ruhrchemie cobalt catalyst, end-gas recycle process. The importance of a high specific yield when coal is used as raw material for synthesis-gas production is shown by the estimate that 60 to 70% of the total cost of the product is the cost of purified synthesis gas.

Block copolymers of propylene with ethylene have been produced in commercial polymerization processes using heterogeneous Ziegler-Natta catalysts. In all processes the block copolymers are produced in small concentrations, and the major products are homopolymers. Well-defined block copolymers free of homopolymer impurities can be prepared with catalysts exhibiting a living polymerization character. In this section we deal with the synthesis of well-defined block copolymers using the living polypropylene which has been prepared with soluble vanadium-based catalysts. 

The raw synthesis gases from partial oxidation of heavy hydrocarbons and coal differ mainly in two aspects from that produced from light hydrocarbons by steam reforming. First, depending on the feedstock composition, the gas may contain a rather high amount of sulfur compounds (mainly H2S with smaller quantities of COS) second, the CO content is much higher, in some cases in excess of 50%. The sulfur compounds (Section 4.3.1.4) can be removed ahead of the shift conversion to give a sulfur-free gas suitable for the classical iron HTS catalyst. In another process variant the sulfur compounds are removed after shift conversion at lower concentration because of dilution by C02. The standard iron catalyst can tolerate only a limited amount of sulfur compounds. With a sulfur concentration in the feed >100 ppm sulfur will be stored as iron sulfide (Eq. 87) ... 

A new strategy has been proposed for the one-step synthesis of block copolymers, based on living/controlled free-radical process. It involves the use of an asymmetric difimctional initiator that is able to start simultaneous polymerization of two comonomers by different polymerization chemistries in such a way that this initiator remains attached to each type of the growing chain (Mecerreyes et al., 1998). The implementation of one-step synthesis is not simple, however. The two catalysts must be tolerant to each other as also to the two comonomers and the reaction temperature must be closely controlled. Living radical polymerization and ROP by coordination and insertion can meet these requirements. 

A catalyst-free protocol has been described by Adib for the synthesis in good to excellent yields of 3-aminoimidazo[l,2-o]pyridines and 5-aminoimidazo[2,l-6] [l,3]thiazoles via three-component reactions between 2-aminopyridines or 2-ami-nothiazoles, aldehydes and isocyanides in water (Scheme 1.10) [14]. Presumably, this process involves the initial formation of an imine, which then reacts with the isonitrile in a formal [4+2] cycloaddition. 

Dabiri has developed a catalyst-free three-component synthesis of a series of stmc-turally diverse 5-substituted tetrazoles 24 and 25 in good yields under mild conditions from caiboityl compounds (including benzaldehydes, isatin and ninhydrin), malononitrile and sodium azide in water [19], This process can be assumed to proceed through a domino Knoevenagel condensation/l,3-dipolar cycloaddition sequence (Scheme 1.13). 

While the largest tonnages by far are based on the high-pressure process and the Ziegler process, the newest method, via metallocene catalysis, offers great promise for controlled properties. Note that only the high-pressure process is based on free-radical synthesis. The reader is referred to Brydson (1) and Benedikt and Goodall (2) for catalyst and synthesis details. 

The development of a Heck reaction in aqueous media, enabling a one-pot process with both reaction steps (Heck reaction and biotransformation) running in aqueous media, was also reported by Cacchi and coworkers [65]. This type of Heck reaction is based on the use of a phosphine-free perfluoro-tagged palladium nanoparticle. After detailed catalyst characterization and process development, Cacchi and coworkers also succeeded in combining this Heck reaction efficiently with the subsequent asymmetric ketone reduction toward a one-pot process. A representative example is shown in Scheme 19.25. The enzymatic process turned out to be very compatible with the Heck reaction, and the desired allylic alcohol products were obtained in yields of up to 92% and with excellent enantioselectivities of >99% ee in aU cases. This two-step one-pot process was, for example, successfully applied for the synthesis of (k)-(—)-rhododendrol ((J )-81) in 90% yield and with excellent (>99%) ee (Scheme 19.25) [65]. 

Dou XY, He LN, Yang ZZ, Wang JL (2010) Catalyst-free process for the synthesis of 5-aryl-2-oxazolidinones via cycloaddition reaction of aziridines and carbon dioxide. S5mlett... 

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