Solvent mediators absence

Solvent-mediated electron-transfer reactions involves the transient transfer of an electron to the solvent acting as a bridge between the donor and the acceptor [1]. Clusters offer the possibility to split this sequential process, the first step of which is the transfer on an electron from the donor to the solvent. In the absence of an acceptor, this step also occurs in solution, but cluster experiments allow a more direct characterization as follows. 

Schweizer and collaborators have elaborated an extensive mode-coupling model of polymer dynamics [52-54]. The model does not make obvious assumptions about the nature of polymer motion or the presence or absence of particular long-lived dynamic structures, e.g., tubes it yields a set of generalized Langevin equations and associated memory functions. Somewhat realistic assumptions are made for the equilibrium structure of the solutions. Extensive calculations were made of the molecular weight dependences for probe diffusion in melts, often leading by calculation rather than assumption to power-law behaviors for various transport coefficients. However, as presented in the papers noted here, the model is applicable to melts rather than solutions Momentum variables have been completely suppressed, so there are no hydrodynamic interactions. Readers should recall that hydrodynamic interactions usually refer to interactions that are solvent-mediated. 

Several grafted calcium ISEs have been reported (73,76,77). In one instance membranes were synthesized by cross-linking the styrene-b-butadiene-b-styrene (SBS) triblock elastomer with triallylphosphate, followed by alkaline hydrolysis of the resultant covalently bound trialkyl phosphate to give a pendant dialkyl phosphate. Good calcium electrodes with lifetimes greater than 6 months were obtained, but the limited selectivity for magnesium and sodium could relate to the absence of a solvent mediator (76). 

Thiophenes of type 31 (X-Y = CH) were generated via Lawesson s reagent-mediated cyclization of 1,4-dicarbonyl compounds 30 under microwave irradiation in the absence of solvent [37]. The reaction was carried by mixing the two solid reagents in a glass tube inserted inside a household microwave apparatus and irradiating until the evolution of H2S ceased. An interesting application of this method is the preparation of liquid crystals and other ferro- and antiferroelectric material such as compound 33 (Scheme 10). 

Supported metal carbonyl clusters are alternatively formed from mononuclear metal complexes by surface-mediated synthesis [5,13] examples are [HIr4(CO)ii] formed from Ir(CO)2(acac) on MgO and Rh CCOlie formed from Rh(CO)2(acac) on y-Al203 [5,12,13]. These syntheses are carried out in the presence of gas-phase CO and in the absence of solvents. Synthesis of metal carbonyl clusters on oxide supports apparently often involves hydroxyl groups or water on the support surface analogous chemistry occurs in solution [ 14]. A synthesis from a mononuclear metal complex precursor is usually characterized by a yield less than that attained as a result of simple adsorption of a preformed metal cluster, and consequently the latter precursors are preferred when the goal is a high yield of the cluster on the support an exception is made when the clusters do not fit into the pores of the support (e.g., a zeolite), and a smaller precursor is needed. 

A somewhat related process, the cobalt-mediated synthesis of symmetrical benzo-phenones from aryl iodides and dicobalt octacarbonyl, is shown in Scheme 6.49 [100]. Here, dicobalt octacarbonyl is used as a combined Ar-I bond activator and carbon monoxide source. Employing acetonitrile as solvent, a variety of aryl iodides with different steric and electronic properties underwent the carbonylative coupling in excellent yields. Remarkably, in several cases, microwave irradiation for just 6 s was sufficient to achieve full conversion An inert atmosphere, a base or other additives were all unnecessary. No conversion occurred in the absence of heating, regardless of the reaction time. However, equally high yields could be achieved by heating the reaction mixture in an oil bath for 2 min. 

It is also essential that competing radical pathways are excluded. The radical intermediates should therefore be relatively persistent. This is the case here, because tertiary radicals are relatively slowly trapped by hydrogen atom donors, e.g., THF, which is usually applied as solvent in titanocene-mediated or -catalyzed reactions, or a second equivalent of Cp2TiCl. Flowever, in the absence of other pathways this reduction, which was followed by a -hydride elimination, was observed [75,76]. Our results with 10 are summarized in Table 5. 

Notably, when surface-mediated syntheses are carried out in the absence of solvent, reactions must occur on the surface of the inorganic oxide. However, when surface-mediated syntheses are carried out in the presence of a solvent the reactions leading to the product can really occur in solution, influenced by the surface. For example, in the synthesis of [Pti5(CO)3o] ", by using MgO and methanol as reaction medium, the surface of MgO behaves only as a solid base, working as a slurry. 

On the other hand, the regioselectivity is not seriously affected by the choice of reaction solvent. Even when THF is used in the reaction with crotyl alcohol, the 2-isoxazoline-5-methanol regioisomer is the only product produced (Scheme 11.31). The reaction rate is decreased in THF, but is still much faster than that in the absence of magnesium ions. Kanemasa et al. (130) concluded that the magnesium-mediated nitrile oxide cycloadditions to allylic alcohols compete with the thermal... 

A novel microwave-mediated three-component coupling of a-acyl bromides, pyridine and internal alkynes was carried out in the absence of a solvent on activated basic alumina to provide a collection of indolizines (Scheme 3.5)7. It was proposed that the reaction proceeded via in situ generation of a dipole from an N-acyl pyridinium salt, followed by a [3+2] cycloaddition reaction. A dedicated laboratory microwave system was... 

Realizing that protonation steps are important, reductions of 69 were performed in diglyme-0.5% H20. Like in DMF, in the absence of catalyst the potential was —3.10 V and the simple alcohol 71 was the only product (86%). The DMP+ mediated reduction, however, was different in this solvent. It took place at the same potential —2.70 V (SCE), but the reactant was consumed after only 1 F mol-1 and the correspoonding pinacol 72 was the only product (60% after repeated recrystallizations). 

The silver(I)-mediated ring opening of halocyclopropanes has been used to construct complex frameworks through the inter- and intramolecular trapping of cationic intermediates with heteronucleophiles. An obvious extension of this work is the involvement of carbon-based nucleophiles to form new carbon-carbon bonds. In 1996, Kostikov and coworkers reported the participation of aromatic solvents in the capture of halocyclopropane-derived allyl cations even in the absence of silver(I).30 However, this early example of intermolecular attack by a carbon nucleophile is one of very few such reports. In the same year, Gassman et al. reported cationic cyclizations of gem-dibromocyclopropanes tethered to remote diene moieties (Scheme 4.16).31... 

Peng et a/.[45] reported the nitration of toluene with liquid nitrogen dioxide and oxygen in the presence of a variety of zeolites, and using toluene as reactant and solvent. The reactions were performed at room temperature over 22 h. In the absence of catalyst the reaction was highly unselective giving a mixture of MNTs, dinitrotoluenes, phenylnitromethane and benzaldehyde. In the presence of zeolites, HZSM-5 and HBeta, the selectivity to the p-nitrotoluene was enhanced. In contrast with the results reported by Smith et a/.,[14] HZSM-5 zeolite exhibited better selectivity than Beta zeolite (57 % and 46 %, respectively), which can be attributed to the absence of mediation of the chlorinated solvent molecules, as well as to the higher Si/Al of the HZSM-5 sample (Si/Al =1000). It was found that zeolites... 

---