Principal properties Lewis acids

The principal factors affecting solvent-ion interactions can be classified as ion-dipole, Lewis acid-base, hydrogen-bonding, solvent structural, and steric. The solvent obviously plays a major part in these interactions. Therefore, to interpret trends in conductance data, bulk solvent properties such as viscosity and dielectric constant should be considered. Table 1 lists selected physical properties for a number of organic solvents. 

In the preceding section we discussed the use of co-ordinated hydroxide as an intramolecular nucleophile. It could also act as a nucleophile to an external electrophile. Over the past few decades, there has been considerable interest in the nucleophilic properties of metal-bound hydroxide ligands. One of the principal reasons for this relates to the widespread occurrence of Lewis acidic metals at the active site of hydrolytic enzymes. There has been a lively discussion over the past thirty years on the relative merits of mechanisms involving nucleophilic attack by metal-co-ordinated hydroxide upon a substrate or attack by external hydroxide upon metal-co-ordinated substrate. As we have shown above, both of these mechanisms are possible with non-labile model systems. 

Selection of Lewis acids by their principal properties for synthetic exploration has been reported for syntheses of benzamides [59], of silyloxydienes [33], and of indoles [60]. 

To clarify whether certain combinations of catalysts and solvents would afford a general selectivity, the study described here was untertaken [80]. Five dissymmetric methylene ketones, twelve Lewis acid catalysts, and ten solvents were selected to afford a fairly uniform spread in the principal properties score plots, see Fig. 12. 

In the introduction to this chapter, the idea of the principal properties was illustrated by a small set of 28 Lewis acis. However, to be of practical use, the set of potential test candidates must be much larger. A set af 116 Lewis acids characterized by 20 descriptors is summarized in Appendix 15A, Table 15A.5. The following descriptors are included in the data set ... 

The number of combinations of possible carbonyl substrates, substituted phenylhydrazines, add catalysts, and solvents is overwhelmingly large. The present study was limited to include dissymmetric ketone substrates with a and a methylene groups, phenylhydrazine, Lewis acids, and common solvents. The selection of test systems was based on the principal properties of the ketone, the Lewis acids, and the solvents. The selection was made to achieve approximately uniform distributions of the selected items in the score plots, see Fig.17.6. 

Introduction. The principal uses of CUSO4 in organic synthesis stem from the chemical properties of Cu +. Firstly Cu + functions as a Lewis acid towards electron donor functions and can thus promote a variety of acid-catalyzed processes. Secondly Cu + reacts readily with diazo compounds to give carbenoid intermediates useful in subsequent addition reactions. Thirdly Cu + functions as an effective redox catalyst in several mixed oxidizing systems. 

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