Dimerization reactions reaction modes

Figure 7.4. Enhanced dimer selectivity in the oligomerisation of compound A due to the biphasic reaction mode with an ionic liquid of high preferential solubility for A...

Figure 8.2 Reaction mechanism for ethanol oxidation on an Mo dimer/Si02 catalyst as an example of the reaction mode (a) in Figure 8.1...

To prevent the latter mentioned subsequent reactions, the bulky phos-phaalkyne Ph C P as well as tungsten alkoxides of reduced size as, e.g., [W2(ONp)6] were employed in these three-component reactions with no significant success [15]. The crucial steps for the side-product free synthesis of the phosphido complexes 18 are the introduction of a phosphaalkyne possessing a moderate steric bulkiness, which lies between those of f-BuC=P and Ph C P, and resulting from the P-NMR studies (cf. Eq. 5), a reaction temperature mode allowing the complete metathesis reaction to take place at very low temperatures over a long period of time until all the phosphaalkyne has been converted into the metathesis products (about 12 h) only then is the reaction mixture allowed to reach room temperature. We found that MesC=P meets these steric requirements, and the three-component-reaction between MesC=P, [W2(Of-Bu)6] and [M(CO)5(thf)] (M = Cr, W), carried out at -78 °C and warmed up to ambient temperature within 15 h, succeeded in the synthesis and isolation of the phosphido complex 18a,b (Scheme 2) [15]. Furthermore, if t-BuC=P is incorporated into these reactions, the steric requirements of the alkoxide dimer has to be slightly increased. Thus, f-BuC=P reacts with [W2(OPh )6] and [M(CO)5(thf)] (M=Cr, W) under the... 

The methylenecyclopropane derivative 3-SPh with its capto-dative substitution pattern has demonstrated essentially the same reaction mode and underwent dimerization to afford a mixture of E) and (Z)-17 (ratio 1.3 1) upon attempted cycloaddition of 3-SPh onto bicyclopropylidene [7h, 291 (Scheme 5). The assignment of these diastereomers was secured by an X-ray crystal structure analysis of E) and (Z)-16 [11c, 30] as well as E)-17 [29]. 

An excellent demonstration of the tunability of ionic liquids for catalysis is provided by an investigation of the dimerization of 1-butene (235). A Ni(cod)(hfacac) catalyst (Scheme 23) was evaluated for the selective dimerization of 1-butene after it was dissolved in various chloroaluminate ionic liquids. Earlier work on this reaction with the same catalyst in toluene led to the observations of low activity and difficult catalyst separation. In ionic liquids of varying acidity, little catalytic activity was found. However, a remarkable activity was achieved by adding a weak buffer base to an acidic ionic liquid. The reaction took place in a biphasic reaction mode with facile catalyst separation and catalyst recycling. A high selectivity to the dimer product was obtained because of a fast extraction of the Cg product from the ionic liquid phase, with the minimization of consecutive reaction to give trimers. Among a number of weak base buffers, a chinoline was chosen. The catalyst performance was compared with that in toluene. The catalyitc TOF at 90°C in toluene was... 

Metal complexes of heterocyclic compounds display reactivities changed greatly from those of the uncomplexed parent systems. All of the -electron system(s) of the parent heterocycle can be tied up in the complex formation, or part can be left to take part in alkenic reactions. The system may be greatly stabilized in the complex, so that reactions, on a heteroatom, for example, can be performed which the parent compound itself would not survive. Orbital energy levels may be split and symmetries changed, allowing hitherto forbidden reactions to occur. In short, a multitude of new reaction modes can be made possible by using complexes dimerization of azirines with a palladium catalyst serves as a typical example (Scheme 81). A variety of other insertion reactions, dimerizations, intramolecular cyclizations, and intermolecular addition reactions of azirines are promoted by transition metals. 

Although cycloadditions have frequently been observed in radical-cation chemistry, this reaction mode is apparently very rare in radical-anion chemistry because of the electron repulsion term. Few examples are known of Diels-Alder dimerizations [355], [2 -I- 2] cycloadditions [356], retro-[2 - - 2] cycloadditions [357], and cyclo-trimerizations [358]. Equally, little is known about electrocyclic reactions, despite their interesting stereochemical course [359]. 

A.i. Palladium-Catalyzed Linear Dimerization Reaction Modes... 

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