Effective homogeneity

To a mixture of 0.40 mol of neohexene ( commercially available) and 200 ml of dry diethyl ether 0.35 mol of bromine was added with cooling between -40 and -50°C. The diethyl ether and excess of neohexene were then completely removed by evaporation in a water-pump vacuum.In the second flask was placed a solution of 90 g of commercial KO-tert.-C9H9 (see Chapter IV, Exp. 4, note 2) in 250 ml of DMSO. The dibromo compound was added in five portions during 15 min from the dropping funnel after the addition of each portion the flask was swirled gently in order to effect homogenization. Much heat was evolved and part of the tert.-butylacetylene passed over. After the addition the flask was heated for 30 min in a bath at B0-100°C. 

An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

A number of catalysts are known to effect homogeneous hydrogenation of aromatic hydrocarbons, e.g., some oxidized rhodium complexes (/, p. 238), some rhodium 7r-complexes with phenyl carboxylates (/, p. 283), some Ziegler systems (/, p. 363), and Co2(CO)8 (/, p. 173). However, the catalysts in the first three systems are not well characterized, and the carbonyl systems require fairly severe hydroformylation conditions, although they are reasonably selective, possibly via radical pathways (Section II, C). 

It may not be that surprising that an effective homogeneous catalyst for the reaction shown by Eq. (4) has not been found it is difficult to imagine a facile mechanism by which oxalate anion or oxalic acid could be generated at a metal center. 

The samples to be distributed must be generally similar in matrix to the unknown samples that are routinely analysed (in respect of matrix composition and analyte concentration range). It is essential they are of acceptable homogeneity and stability. The bulk material prepared must be effectively homogeneous so that all laboratories will receive samples that do not differ significantly in analyte concentration. The co-ordinating laboratory should also show the bulk sample is sufficiently stable to ensure it will not undergo... 

The width of a band in the absorption spectrum of a chromophore located in a particular microenvironment is a result of two effects homogeneous and inhomogeneous broadening. Homogeneous broadening is due to the existence of a continuous set of vibrational sublevels in each electronic state. Inhomogeneous broadening results from the fluctuations of the structure of the solvation shell... 

Another effective, homogeneous method for the quantitation of kinase activity that requires neither antibodies nor solid phase is the use of a second adenosine triphosphate... 

We recall that our wave equation represents a long wave approximation to the behavior of a structured media (atomic lattice, periodically layered composite, bar of finite thickness), and does not contain information about the processes at small scales which are effectively homogenized out. When the model at the microlevel is nonlinear, one expects essential interaction between different scales which in turn complicates any universal homogenization procedure. In this case, the macro model is often formulated on the basis of some phenomenological constitutive hypotheses nonlinear elasticity with nonconvex energy is a theory of this type. 

Undoubtedly, the most notable feature of these new dendrimeric organometallic molecules is their ability to act successfully as effective homogeneous catalysts for the Kharasch addition reaction of polyhalogenoalkanes to olefinic C=C double bonds. Indeed, they show catalytic activity and clean regiospecific formation of 1 1 addition products in a similar way to that observed in the mononuclear compounds. Likewise, the nanoscopic size of these first examples of soluble dendritic catalysts allows the separation of such macromolecules from the solution of the products by ultrafiltration methods. 

The validity of the effective homogeneous medium approximation to describe heat transfer in the multichannel scale will be assessed following the derivation of the Eq. (38) for the computation of the effective conductivity. For this purpose a DPF segment of square shape containing 24 x 24 channels is considered. 

The square planar [NiX(bdppm)]+ complex was found to react with CO at room temperature and atmospheric pressure in EtOH—H20 solution according to equation (168). The nickel(O) carbonyl complex containing the protonated ligand Hbdppm reacts in turn with acids with evolution of H2 (equation 169). Thus the complex [NiX(bdppm)]+ acts as an effective homogeneous catalyst of the water-gas reaction.1373... 

TJased on several kinetic investigations on hydrogenations catalyzed by transition metal complexes conducted over the last few years, certain general requirements must be fulfilled if a complex is to form an effective homogeneous catalyst in solution (see Ref. 1). One condition is that the catalytically active complex must be coordinatively unsaturated another that M-H or M-C bonds must be present in the complex. 

A novel approach to electrode design using polymeric ligands was recently illustrated using the nickel—triphenylphosphine system.80 An effective homogeneous electroactive catalyst (Ni/PPh3) is solubilized with 2% crosslinked polystyrene phenylphosphine. The complexed polymer is then... 

The complex RhCl(ttp), where ttp = PhP(CH2CH2CH2-PPh2)2, in the presence of either triethylaluminum or diethylaluminum chloride, is an effective homogeneous catalyst for hydrogenation of 1-olefins and 1-octyne. The rates of hydrogenation of substituted olefins are considerably slower than for terminal olefins. H-l and P-31 NMR spectra were used to identify several different chemical species [including RhH(ttp)] in these catalyti-cally active solutions. The observed rate of hydrogenation of 1-octene to n-octane at 20 0.3°C and under a constant H2 pressure of 750 torr is 6.4 x min 9... 

The compounds Rh6(CO)16 and Re2(CO)10 are also effective homogeneous catalysts for autoxidating cyclic alcohols to dicarboxylic acids. Solvent effect data for cyclohexanol are shown in Table IV. Again low yields are found in benzene solvent, and considerably higher conversions in cyclohexane. The yields of carboxylic acids obtained from both cyclic and acyclic alcohols are shown in Table V. It is apparent that the acid yields are small for acyclic alcohols. There is no difference in catalytic activity whether the compound Rh6(CO)16 or Re2(CO)10 is used and low yields are obtained from both primary and secondary alcohols. 

The di-copper-substituted y-Keggin silicotungstate [y-H2SiWio036Cu2( j,-l, 1-N3)2]4- could act as an effective homogeneous catalyst for the oxidative homocoupling of various kinds of alkynes, including aromatic, aliphatic and heteroatom-containing ones (6.8) [108] ... 

Soil samples with high clay contents they cannot be effectively homogenized and have a high variability of contaminant distribution... 

However, Cu11 (as the acetate), Pt11 (as ethylene chloride), and Co0 (as the carbonyl), which do not have this electron distribution, are also effective homogeneous catalysts for activation of hydrogen. It does not appear possible at present, therefore, to set forth a simple generalization of electronic qualifications for the known active catalysts, much less to predict new systems. Of course what should be considered is the electronic structure of the metal in the catalytically active complex, rather than the electron distribution of the ion alone as shown above. If a system is known to be active, it is possible to write metal complexes with possible electronic structures which are in accord with the observations. Such post hoc structures each appear special and without generalization in any detail. 

We have mentioned in Section II.B.2 studies of the oxidation of olefins by molecular oxygen in the presence of low-valent Group VIII metal complexes, with the expectation of effecting homogeneous, nonradical oxidation processes. However, these reactions were shown to involve the usual free radical chain autoxidation, and no direct transfer of oxygen from a metal-dioxygen complex to an olefin was demonstrated. 

Chiral hydrogenation catalysts. Rhodium and ruthenium phosphines are effective homogeneous catalysts for hydrogenation. Chiral ligands can be attached to accomplish asymmetric induction, the creation of a new asymmetric carbon as mostly one enantiomer. 

The bis(rj-cyclopentadienyl)titanium(III) aryl complexes form interesting dinitrogen derivatives (Section III,B) the 1-methyl, 17-allyl complex (Tj-C5H5)2Ti(i73-C4H7) is the precursor to a particularly effective homogeneous olefin hydrogenation catalyst (Section VI). Attempts to prepare bis(i7-cyclopentadienyl)titanium(III) monoalkyl complexes were unsuccessful (40). 

In 1965 Wilkinson invented the rhodium-tris(triphenylphosphine) catalyst as a hydrogenation catalyst [60]. It still forms the basis for many of the chiral hydrogenations performed today. The most effective homogeneous hydrogenation catalysts are complexes consisting of a central metal ion, one or more (chiral) ligands and anions which are able to activate molecular hydrogen and to add the two H atoms to an acceptor substrate. Experience has shown that low-valent Ru,... 

The reaction in Fig. 4.19 is catalyzed by compounds of high-valent, early transition metals such as Mo(VI), W(VI), V(V) and Ti(IV). Molybdenum compounds are particularly effective homogeneous catalysts and are used in the ARCO process in combination with TBHP or EBHP. In the Shell process, on the other hand, a heterogeneous Ti(IV)/Si02 catalyst is used with EBHP in a continuous,... 

A considerable improvement in effective homogeneity across the sample is often achieved by spinning the sample tube about its axis. If the field gradient across the sample in a direction transverse to the spinning axis is given by AB,... 

The effective homogeneous direct correlations Cay(r p) are in turn interpolated over the... 

---