Dimerisation Reactions

An isotopic effect (H or D) has been demonstrated when starting from 2-methyl-4-phenylthiazole or from 2-methyl-4-phenyl-5-D-thiazole (224) in the dimerisation reaction. 

Now retain this favorite reaction but abandon all assumptions. If one even assumes the solid catalyzed dimerisation reaction of ... 

Schrock has proposed that the reverse reaction occurs during some catalytic alkene dimerisation reactions (20) but, in studies of decomposition of alkyl substituted platinacyclobutanes, no... 

It is remarkable that in the same year, 1934, two independent approaches, those of Stoll et al. and of Kuhn, led to the definition of two quantities which are conceptually quite similar and can be practically identical in many actual cases. In either case the intramolecular reaction is compared to a corresponding intermolecular process. This is the dimerisation reaction of the bifunctional reactant in the definition of the cyclisation constant C in the case of the effective concentration Crff Winter must be determined with the aid of an inter-molecular model reaction, the choice of which is not always obvious and can possibly lead to conceptual as well as experimental difficulties. It is also worth noting that although these early workers established a firm basis for interpretation of physical as well as of preparative aspects of intramolecular reactions, no extensive use of quantities C and Qff appears to have been made in the chemical literature over more than three decades after their definition. This is in spite of the enormous development of studies in the field of... 

The particular enhancement of a radical dimerisation reaction, Eqs. (45a) and (45b), is observed on hydrophobic metals like Hg, Pb, and Sn at potentials close to their respective point of zero charge where electrosorption is strongest and diminishes at more negative and positive potential. If the reduction potential of the oxo-compound, which in accordance with the equilibrium reaction (45b) is pH dependent (60 mV per pH unit), is shifted by more than 100 mV away from the point of zero charge, then the ketyl radicals are no longer adsorbed strongly enough and they are displaced from the... 

The observed greater thermal stability of SNAP (49) over SNAC (50) has been investigated by ab initio calculations and DSC and TGA measurements.65 Results indicate a two-step procedure for decomposition with the enhanced thermal stability of SNAP (49) directly related to the steric interaction in the dimerisation reaction leading to disulfide formation. The methyl groups were found to have no substantial effect on the N—S bond strength.65 Ab initio calculations have also been used to study the mechanism of decomposition of AcONCb. Results indicated that the most thermo-... 

Many other sulfur compounds exhibit dimerisation reactions of this type in the presence of metal ions and oxygen. For example, dithiocarbamates are readily oxidised to the disulfides in the presence of copper salts (Fig. 9-13). Very often, attempts to form complexes with dithiocarbamate ligands are complicated by the parallel oxidation process. 

However, in batch mode all dimerisation reaction (with and without added IL) were found to stop at a maximum MA conversion of about 80%. By adding fresh feedstock (and by some other experiments) we could reveal that the reaction suffers at this conversion from a product inhibition effect. 

However, an undesired product A2 is also formed by the second-order dimerisation reaction ... 

Disubstituted furans were provided in high yield through a novel oxidative cyclisation-dimerisation reaction between two different allenes as shown below <02AG(E)1775>. 

Photodimerisation of Alkenes. When alkenes are irradiated, they often dimerise to give cyclobutanes. In a dimerisation reaction, the set of orbitals on the left of each pair in Fig. 8.1 and the set on the right will have identical... 

Dimerisation Reactions with other Metal-Catalysts... 

In the classical oxidative alkyne-dimerisation reaction large quantities of solvent are often required which makes alternative procedures attractive. With 20 mol% CuCl and 20 mol% TMEDA, the dimerisation product of phenylacetylene, viz. 1,4-diphenyl-1,3-butadiene, was obtained at ambient temperature in [C4Ciim][PF6] in 95% y ield.1 2 Products were isolated by extraction of the ionic liquid phase with toluene and reuse of the catalyst was possible, but addition of further catalyst was necessary every two runs to keep reaction rates more or less constant. 

Analysis of the kinetics yielded Arrhenius parameters for the reversible cyclic dimerisation reaction involving tetrafluoroethylene and perfluorocyclobutane (see Table 1) and for the decomposition of perfluoroisobutene (A — 1.1 x 10 see E = 82.7 kcal.mole ). The dimerisation reaction has also been studied by Lacher et and the reverse process by Gray and Pntchard, and by Butler . These reactions appear to be elementary however. Gray and Pritchard have argued that the reverse (dissociation) reaction is not a simple unimolecular process. 

Rarely, some other forms of decomposition may be encountered. These include hydration (found in some alkaloids of ergot), polymerisation (which can affect solutions of the antibiotic ampicillin) and dimerisation reactions (which can be seen as a result of free radical attack on morphine). While these methods of decomposition are important and should be borne in mind, the majority of chemical deterioration can be explained by consideration of the few mechanisms outlined above. 

Small amounts of the molecule Sn2Se2(g) were found in the mass spectrometric investigations in the temperature range 792 to 979 K in [64COL/DRO] and in the range 550 to 693 K in [79KUL/ZLO]. The values reported for the dimerisation reaction... 

The authors also evaluated the enthalpy of the dimerisation reaction 2SnSe(g) Sn2Sc2(g) to be A,// (298.15 K) = -(194.6 + 22.0) kJ-mol in a third law analysis of measurements made in the temperature range 936 to 991 K.. No evaluation was made according to the second law because of the short temperature range. The third law evaluation employed Gibbs energy functions calculated from parameters estimated by comparison with the isoelectronic molecules As4(g) and Sb4(g). In the absence of better values of the molecular parameters, no recalculation was made by the review. 

A dimerisation reaction of N-methylanabasine (17) has been reported and a modification of the useful dealkylation reaction for tertiary amines has been used in a partial degradation (17) -> (18). The harsh conditions necessary for the von Braun reaction of heterocyclic benzamides with phosphorous penta-bromide has generally precluded its application to structural determination. It has now been found that carbonyl bromide is a useful reagent for this reaction... 

This is a base-catalysed dimerisation reaction for all aldehydes and ketones with a-hydrogens. 

Isocyanates give two types of dimerisation reactions formation of uretidinediones and of carbodiimides [1, 3, 12, 13, 15, 23-25] ... 

These dimerisation reactions of terminal alkynes have been further extended to the catalytic cyclisation of a,co-diynes. For example, treatment of 1,15-hexa-decadiyne with 10 mol% of 7a affords the endo-msLCTOcydic product, (Z)-l-cyclohexadecen-3-yne with complete stereoselectivity (Equation 5). This novel cyclisation is of particular utility, because synthetic routes to endo-cyc ic (Z)-l-en-3-ynes are extremely limited. A related palladium-catalysed cyclisation of a,co-diynes to give the corresponding exo-cyc c l-en-3-ynes has been reported by Trost and co-workers. 

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