Syntheses in Aqueous Media

The salt reacts rapidly with water precipitating some of the platinum as hydrated dioxide, most of it remaining in solution as hexafluoroplatinate(iv). This behaviour resembles the reductive interaction of the hexafluoro-rhenates(v)-osmates(v) and -iridates(v), with water to give the quadripositive fluorosalts. Since the latter cannot be synthesized in aqueous media, they are probably thermod5mamically unstable with respect to hydrolysis. If so, evidently the hexafluoroplatinate(v) ion must remain intact during reduction to the hexafluoroplatinate(iv) ion. 

The original intention of Leuchs and Fischer was to use NCAs in peptide synthesis, but the high reactivity of these reagents and their tendency to ohgomerize dampened the enthusiasm of most laboratories for these self-activated amino acids. Based on some successes in the early 1950s,workers at Merck developed conditions where controlled-sequence peptides could be synthesized in aqueous media.f l The crowning achievement involved the use of NCAs in the synthesis of ribonuclease A by the Merck group in 1969. 

Scheme 8.15 Some examples of MW-assisted nitrogen-containing heterocycles synthesized in aqueous media.

The molecular masses of poly(l) synthesized in aqueous media were typically high (M fs 106), and polydispersity indices (PDIs) were often lower (< 2.0) than the polydispersities of polymers produced by classical systems in organic solvents [27, 29, 30]. These low polydispersities have been attributed to the low occurrence of termination reactions during the polymerization and the relative inactivity of the propagating species toward the acyclic alkenes in the polymer, which suppress-... 

These reaction methods were applied to the preparation of PS containing PUFAs from squid skin lecithin (PC 44.2%, PE 29.4%, PS 3.3%, LPE 12.2%) and serine [35]. Lysophosphatidylglycerol was also synthesized from egg LPC and glycerol by PLD [36]. Toxic organic solvents should be avoided in the production of PLs for use as human food. Therefore, PG and PS have been synthesized in aqueous media, with 72% and 80% yields, respectively [37,38]. 

We have not attempted to carry out any direct electrochemical syntheses in aqueous media, but there are reports in the literature of work carried out some 40 years ago on the electrochemical oxidation of a number of metals in liquid ammonia [72,73], and since two of the metals in question were gallium and indium, this seemed a natural area for further investigation. The immediate conclusion, which we established by measuring Ep, is that indium is oxidized in liquid ammonia solutions of ammonium halides at -35 °C to the -I-II state, but unfortunately we were not able to isolate any compounds of this oxidation state from the resultant solution, though in one particular case we were able to show by Raman spectroscopy that species with the characteristic j (In-In) stretching mode were present in the solution [74]. When we attempted to work up these solutions both indium and I) halide derivatives of ammonia were obtained, and a mass balance, taking into account the quantity of material isolated and the quantity of indium dissolved, showed that the typical disproportionation reactions of indium +11 were indeed being reproduced under these conditions. We concluded that the overall stoichiometry is... 

Fig. 17.2 Transmission electron micrograph of polypyrrole particles of (a) 100 nm diameter, (b) 200 nm diameter, and (c) 300 nm diameter synthesized in aqueous media using a poly(vinyl alcohol) stabilizer, a poly(2-vinyl pyridine-co-Ai-butyl methacrylate) stabilizer, and a poly(ethylene oxide) stabilizer, respectively. Note the uniform spherical morphology. (Reproduced with permission from Ref. 28.1...

Fig. 17.6 Transmission electron micrograph of polypyrrole-silica nanocomposite particles synthesized in aqueous media using a commercial 20 nm silica sol. Note the unusual raspberry morphology. (Reproduced with permission from Ref. 64.)...

Burda, E., Hummel, W., and Groger, H. (2008) Modular chemoenzymatic one-pot syntheses in aqueous media combination of a palladium-catalyzed cross-coupling with an asymmetric biotransformation. Angew. Chem., Int. 

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