Metal solvent effect

Keywords Dilithiovinylsilanes / Lithium Hydride Elimination / Reductive Metalation / Solvent Effect / Vinyllithium Dimerization... 

YETI is a force held designed for the accurate representation of nonbonded interactions. It is most often used for modeling interactions between biomolecules and small substrate molecules. It is not designed for molecular geometry optimization so researchers often optimize the molecular geometry with some other force held, such as AMBER, then use YETI to model the docking process. Recent additions to YETI are support for metals and solvent effects. 

Aromatic steroids are virtually insoluble in liquid ammonia and a cosolvent must be added to solubilize them or reduction will not occur. Ether, ethylene glycol dimethyl ether, dioxane and tetrahydrofuran have been used and, of these, tetrahydrofuran is the preferred solvent. Although dioxane is often a better solvent for steroids at room temperature, it freezes at 12° and its solvent effectiveness in ammonia is diminished. Tetrahydrofuran is infinitely miscible with liquid ammonia, but the addition of lithium to a 1 1 mixture causes the separation of two liquid phases, one blue and one colorless, together with the separation of a lithium-ammonia bronze phase. Thus tetrahydrofuran and lithium depress the solubilities of each other in ammonia. A tetrahydrofuran-ammonia mixture containing much over 50 % of tetrahydrofuran does not become blue when lithium is added. In general, a 1 1 ratio of ammonia to organic solvents represents a reasonable compromise between maximum solubility of steroid and dissolution of the metal with ionization. 

The donor-acceptor approach to solvent effects on the rates of redox reactions between different metal complexes, R. Schmid, Rev. Inorg. Chem., 1979,1,117-131 (48). 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

Remarkable solvent effects on the selective bond cleavage are observed in the reductive elimination of cis-stilbene episulfone by complex metal hydrides. When diethyl ether or [bis(2-methoxyethyl)]ether is used as the solvent, dibenzyl sulfone is formed along with cis-stilbene. However, no dibenzyl sulfone is produced when cis-stilbene episulfone is treated with lithium aluminum hydride in tetrahydrofuran at room temperature (equation 42). Elimination of phenylsulfonyl group by tri-n-butyltin hydride proceeds by a radical chain mechanism (equations 43 and 44). 

Solvent effects on the rate of the decarbonylation of MeCOMn(CO)5 were examined by Calderazzo and Cotton (50) and are presented in part in Table IV. In general they are very small, and no regular trends can be discerned. This virtual lack of dependence of the rate on the nature of the solvent and very little correlation between the rate and the dielectric constant of the solvent are typical of substitution reactions of metal carbonyls (J). In the light of the foregoing, a qualitative observation that CpFe(CO)2-COMe decarbonylates much more readily on treatment at reflux in nonpolar heptane or cyclohexane than in polar dioxane is somewhat intriguing 219). 

In the general case, an incoming nucleophile would be expected to be favoured by (i) a high basicity consistent with ( ) a high polarizability, and the metal complex to favour its approach if (in) it contains electron-acceptive, or B class ligands. An interpretation of the available data may be essayed on these lines. The infrared data upon Ni(CO)4 are consistent with a weakening of the C-O bond , and it would be of interest to examine the solvent effect upon the Ni-C bond. 

The activity of transition metal catalysts depends on both the metal and the ligands. In addition, solvent effects, etc. can play a role. Table 3.10 shows examples of transition- metal catalysts with the reactions for which they are active (Farkas, 1986). 

Kitchens, C.L., McLeod, M.C. and Roberts, C.B. (2003) Solvent effects on the growth and steric stabilization of copper metallic nanoparticles in AOT reverse micelle systems. Journal of Physical Chemistry B, 107 (41), 11331-11338. 

Heterobimetallic clusters (Figure 58,125 and 126) with solvent-dependent structures were also obtained upon mixing alkali metal tert-butoxides and -trimethylsiloxides in THF, TMEDA, and toluene.184 The common occurrence of heterocubes shows that there is a strong driving force for the formation of heterocubic structures in organozinc alkoxides. Solvent effects are important, however, as demonstrated by the formation of seeo-diheterocubic compounds in TMEDA. 

We have reported the first example of a ring-opening metathesis polymerization in C02 [144,145]. In this work, bicyclo[2.2.1]hept-2-ene (norbornene) was polymerized in C02 and C02/methanol mixtures using a Ru(H20)6(tos)2 initiator (see Scheme 6). These reactions were carried out at 65 °C and pressure was varied from 60 to 345 bar they resulted in poly(norbornene) with similar conversions and molecular weights as those obtained in other solvent systems. JH NMR spectroscopy of the poly(norbornene) showed that the product from a polymerization in pure methanol had the same structure as the product from the polymerization in pure C02. More interestingly, it was shown that the cis/trans ratio of the polymer microstructure can be controlled by the addition of a methanol cosolvent to the polymerization medium (see Fig. 12). The poly(norbornene) prepared in pure methanol or in methanol/C02 mixtures had a very high trans-vinylene content, while the polymer prepared in pure C02 had very high ds-vinylene content. These results can be explained by the solvent effects on relative populations of the two different possible metal... 

As of now no details of the synthesis of optically active tritiated compounds produced under microwave-enhanced conditions have been published. Another area of considerable interest would be the study of solvent effects on the hydrogenation of aromatic compounds using noble-metal catalysts as considerable data on the thermal reactions is available [52]. Comparison between the microwave and thermal results could then provide useful information on the role of the solvent, not readily available by other means. 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

The best way to take advantage of the organic solvent effect without simultaneously diluting the sample is by employing solvent extraction. By this method the element to be analyzed can actually be concentrated and a solution of the element is obtained in essentially pure organic solvent. One of the most commonly used systems involves formation of the metal chelate with ammonium 1-pyrro-lidinecarbodithioate (APDC) and then extracting this into methylisobutyl ketone (MIBK). APDC chelates of many elements form and extract into MIBK from acid solution. 

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