On-demand synthesis

However, the minimization of the risk, both in terms of environment and process safety, could be equally reached by adopting an on-demand synthesis of phosgene and MIC [46]. This is the approach preferred industrially and evidences that the same goal could be reached by a different philosophy, other than the substitution of chemicals indicated by green chemistry principles. 

However, another reason why flash chemistry is needed is that our way of synthesizing compounds is currently changing. For example, combinatorial synthesis of chemical libraries has become very popular in academia and industry and on-site, on-demand synthesis is expected to be popular in the future. These new trends in chemical synthesis increase the demand for flash chemistry. 

For the rapid on-demand synthesis of 4,5-disubstimted oxazole libraries, a flow reaction system equipped with two columns containing PS-BEMP and QP-BZA (QuadraPure benzylamine, a primary amine functionalized resin), respectively, was developed by Ley ef fll. [71,72] (Scheme 6.18). This system is very valuable from the viewpoint of rapid access to 25 different oxazoles in yields between 83 and 99%, the ease with which the reactions can be scaled-up to 10 g and the ability to recycle the PS-BEMP containing columns between runs. 

Pascali and coworkers recently reported the concept of using microreactor technology (inner diameter 100 (tm, length 2.0 m, volume 15.6 p.1) to effect dose-on-demand synthesis of PET molecules [58]. Using the Advion system, they reacted ditosylates (55) with anhydrous fluoride in acetonitrile to prepare the [ F]tosylates (56) in situ before introduction of a neat solution of choline (57) to synthesize the [ F]fluorocholine derivatives (58) in 22-54% radiochemical yield within 15 min total processing time (Scheme 6.19). 

The cited reviews reveal that a number of research groups utilizing the continuous microfluidic approach only focus on stage 3, the synthetic step. However, it is also stated that a continuous setup performing all stages is required for an automated on-demand synthesis. The following examples concentrate on improvements of the continuous microfluidic approach. 

Coordination Complexes. The coordination and organometaHic chemistry of thorium is dominated by the extremely stable tetravalent ion. Except in a few cases where large and stericaHy demanding ligands are used, lower thorium oxidation states are generally unstable. An example is the isolation of a molecular Th(III) complex [107040-62-0] Th[Tj-C H2(Si(CH2)3)2]3 (25). Reports (26) on the synthesis of soluble Th(II) complexes, such as... 

New natural polymers based on synthesis from renewable resources, improved recyclability based on retrosynthesis to reusable precursors, and molecular suicide switches to initiate biodegradation on demand are the exciting areas in polymer science. In the area of biomolecular materials, new materials for implants with improved durability and biocompatibility, light-harvesting materials based on biomimicry of photosynthetic systems, and biosensors for analysis and artificial enzymes for bioremediation will present the breakthrough opportunities. Finally, in the field of electronics and photonics, the new challenges are molecular switches, transistors, and other electronic components molecular photoad-dressable memory devices and ferroelectrics and ferromagnets based on nonmetals. 

There appear to be considerable steric demands involved in formation of the Si-N bond in reaction (28). Support for the importance of steric constraints in dehydrocoupling comes from studies on the synthesis of oligosilazanes from PhSiHj and NH3. When reaction (29) is run at 60°C, NMR and elemental analysis... 

Synthesis Synthesized on demand derived from cholesterol Synthesized in advance derived from amino acids Synthesized in advance stored as part of thyroglobulin Synthesized in advance derived from tyrosine... 

Enzymes are proteins (gene products) synthesized by DNA transcription and messenger RNA (mRNA) translation. Many enzymes are described as being constitutive , meaning they are present at all times. Others are inducible , meaning that their synthesis can be increased on-demand when circumstances require. By increasing the concentration of certain enzymes, induction allows more substrate to undergo chemical reaction and the pathway accelerates. 

Following their synthesis, hormones may be secreted immediately, for example the steroids, or stored within the tissue and only released on demand . Proinsulin (pro = precursor) is synthesized as a single peptide chain and stored in vesicles within the... 

A similar approach was taken for the synthesis of 45 by Miyaura. " Shaughnessy and Booth synthesized the water-soluble alkylphosphine 46, and found it to provide very active palladium catalysts for the reaction of aryl bromides or chlorides with boronic acids. The more sterically demanding ligand 47 was shown to promote the reactions of aryl chlorides with better results than 46. Najera and co-workers recently reported on the synthesis of di(2-pyridyl)-methylamine-palladium dichloride complexes 48a and 48b, and their use in the coupling of a variety of electrophiles (aryl bromides or chlorides, allyl chlorides, acetates or carbonates) with alkyl- or arylboronic acids very low catalyst loadings at Palladium-oxime catalysts 8a and 8b) have also been developed. In conjunction with... 

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