Mixed aggregate

The downward curvature observed in this and other systems could be easily explained in terms of a mixed aggregate between the catalyst and the nucleophile. A hydrogen-bond donation to the amide catalyst would render the amine a better nucleophile, up to a value of saturation , after which increasing amounts of catalysts should have no further effect. The results in Table 15 can be easily explained in the same terms, where K measures the equilibrium of the association between the amine and the catalyst. 

If the dimer mechanism interpretation is correct, addition of a HBA co-solvent, e.g. dimethyl sulphoxide (DMSO) (/> value = 0.76)178, in catalytic amounts should increase the reaction rate by forming a mixed aggregate RNH2 OS(CH3)2 (B DMSO), equation 35, where the amine acts now as a HBD, and therefore this mixed aggregation should increase its nucleophilicity. DMSO has been shown to increase the nitrogen electron density of primary and secondary amines161. 

Studies of the amine concentration rate dependence show that the reactions are strictly third-order in amine for DMSO <2%. For DMSO constants >10% the reactions show the classical behaviour usually found in base-catalysed SjvAr180. The specific solvent effects observed for small additions of the HBD co-solvent are consistent with the formation of the mixed aggregate, and a linear correlation was found between kA and [DMSO], shown by equation 36, which expresses that the third-order term is more affected by the small additions of DMSO than the fourth-order term. Equation 36 is valid for [DMSO] <2% (0.282 M). 

Decomposition of this equilibrium mixture with catalytic amounts of CuOTf affords a mixture of all three possible biaryls. The formation of the unsymmetrical biaryl 2-Me2NCH2C6H4C6H4Me-4 can only be explained by the occurrence of aggregated copper species in which both the C6H4CH2NMe2-2 and the C( H4Me-4 groups are bound to the same copper core [77]. It was furthermore observed that the ratio of the formed biatyls is not statistical, which points to significant differences in the thermodynamic stabilities of the various mixed aggregates present in solution. 

Starting compounds Cation exchange Anion exchange Mixed aggregates Transmetalated compounds... 

In the same group, Feustel and G. Muller have observed an interesting reaction between two equivalents of the tris(phosphinomethyl)methanol 52 and five equivalents of n-butyl-lithium . Formally, the elimination of lithium oxide leads to the formation of a mixed aggregate of the lithium aUtoxide 53 and the dilithiated trimethylenemethane dianion 54 in the solid state (Scheme 19, Figure 15). The mechanism of the formal elimination of lithium oxide, which could also be part of a further mixed aggregate, has not been cleared up yet. 

For systems exhibiting positive deviations from ideality of mixing, aggregate patches of different composition may form on the surface. If methods (e.g., spectroscopic) could be developed to detect the average size of these distincly different patches, this would give insight into the monolayer formation process. 

The formation of mixed aggregates on solid surfaces (admicelles) has only in the last decade been addressed in systematic fashion. The surface has only been scratched in this important topic. 

The second problem results from the fact that the azaenolates exist as aggregates or mixed aggregates in ethereal solvents. Cryoscopic measurements established that the azaenolate generated from 4,5-dihydro-4-methoxymethyl-2-methyl-5-phenyloxazole and butyllithium in tetrahydrofuran is a dimer18. When the metalation was performed with butyllithium or tert-butyl-lithium, an aging effect was observed. Thus, the enantiomeric excess on alkylation drops from 43% to 28% and 11 % when the azaenolates generated at — 78 °C were allowed to warm to... 

The situation becomes even more complex when other lithium salts are present in the solution. For example, when a lithium halide such as LiBr is added to solutions of alkyllithiums, mixed aggregates are formed where a bromide anion takes the... 

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