Substrate studies organometallics reactions

Catalysts are heterogeneous sulfided nickel (or cobalt) molybdenum compounds on a y-alumina. The reaction has been extensively studied with substrates such as thiophene (Figure 2.40) as the model compound mainly with the aims of improving the catalyst performance. The mechanism on the molecular level has not been established. In recent years the reaction has also attracted the interest of organometallic chemists who have tried to contribute to the mechanism by studying the reactions of organometallic complexes with thiophene [41], Many possible co-ordination modes for thiophene have been described. 

The formation of chiral alcohols from carbonyl compounds has been fairly widely studied by reactions of aldehydes or ketones with organometallic reagents in the presence of chiral ligands. Mukaiyama et al. 1081 obtained excellent results (up to 94% e.e.) in at least stoichiometric addition of the chiral auxiliary to the carbonyl substrate and the organometallic reagent. 

The nature of the electrode plays a significant role in the direction and often the products of electrochemical processes, particularly reduction. Metals that can form relatively stable organometallics with the substrate under study often intervene directly to produce a product like that of direct organometallic reaction. The electro reduction of alkylmercury halides was studied on Pt, Hg and carbon electrodes. Whereas at Pt and carbon electrodes two-electron reduction was observed, at mercury-coated electrodes multistep reduction occurred and RHgHgX was observed213. 

Chromium produces some of the most interesting and varied chemistry of the transition elements. Chromium(O) and chromium(I) are stabilized in organometallics (Prob. 8). There have been extensive studies of the redox chemistry of Cr(II), Cr(III) and Cr(VI). Generally the Cr(IV) and Cr(V) oxidation states are unstable in solution (see below, however). These species play an important role in the mechanism of oxidation by Cr(VI) of inorganic and organic substrates and in certain oxidation reactions of Cr(II) and Cr(III). Examination of the substitution reactions of Cr(III) has provided important information on octahedral substitution (Chap. 4). 

For a decade or so [CoH(CN)5] was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic add (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rii(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4]. No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. 

The work described in this review shows that the organometallic chemistry of the heavier alkali metals has ceased to be an exotic backwater. A good deal of information about the interaction between metal cations and carbanionic fragments has been obtained. Much of it has come from studies on crystalline solids, and although it may be reasonable to expect that the species found in the solid are also present in solution, this has to be established experimentally in each case. Some evidence has been obtained from multinuclear or multidimensional NMR spectroscopy, but so far there have been few studies using solid-state NMR to link structures found in the solid with those in solution. Even when the dominant species in solution is established, still more work is required to determine which species react fastest with particular substrates. The active species in a reaction may be present only in low concentration. 

As most organometallic precursors, V(NEt2)4 pyrolysis involves a complicated mechanism highly dependent on the experimental conditions. For this reason, the CVD experiments were conducted at reduced pressure (Table 15.4) in order to improve the diffusivity of the species, reduce their interactions in the gas phase and disfavor subsequent reactions. Two CVD units (hot-wall and cold-wall) of the same geometry were used in this study. Since the reactions in the gas phase are likely to be different in these two types of reactors, we could use them to study the influence of the gas phase chemistry on the growth rate. The composition of the deposits was studied as a function of the substrate temperature under He gas and as a function of the nature of the carrier gas when H2 and NH3 were added in various amounts. 

The difficulties over the constitution of organomagnesium compounds mentioned above, prompted Abraham and Hill22 to study the kinetics of acidolysis of reactive organometallic compounds of well-defined constitution. Dialkylzincs are monomeric substances23 which may be purified by distillation, and should therefore be more suitable substrates. Acidolysis of di- -propylzinc by the weak acids p-toluidine and cyclohexylamine at 76 °C in solvent diisopropyl ether was shown to follow kinetics compatible with the two competitive consecutive second-order reactions (15) (R = Pr", R = p-tolyl or cyclohexyl) and (16) (R = Pr", R = p-tolyl or cyclohexyl),... 

Values of K, the equilibrium constant for reaction (4), are given in Table 2 and it can be seen that in the solvents commonly used for kinetic studies, values of K are around 104-107 l.mole-1. Since kinetic studies are usually carried out with a large excess of iodide ion, such values of K result in almost complete conversion of iodine into I3 , and at any time during a kinetic run [I2] [I3 ]. Hence for reaction (3) the decrease in the concentration of organometallic substrate RMX , denoted by R, will equal the decrease in [I3 ], and the velocity, v, of the iodinolysis may thus be expressed as... 

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