Lewis basic sites

In a Lewis-acid catalysed Diels-Alder reaction, the first step is coordination of the catalyst to a Lewis-basic site of the reactant. In a typical catalysed Diels-Alder reaction, the carbonyl oxygen of the dienophile coordinates to the Lewis acid. The most common solvents for these processes are inert apolar liquids such as dichloromethane or benzene. Protic solvents, and water in particular, are avoided because of their strong interactions wifti the catalyst and the reacting system. Interestingly, for other catalysed reactions such as hydroformylations the same solvents do not give problems. This paradox is a result of the difference in hardness of the reactants and the catalyst involved... 

The initial investigation focused on the use of threitol-derived auxiliaries with various substituent groups on the dioxolane ring (Table 3.3). However, it became evident that the oxygen atoms in the substituents had a detrimental effect on selectivity. Comparison of the diastereoselectivities for the ketals 69-71, which contain Lewis basic sites in the substituents at the 1 and 2 positions, with those from simpler diol derived ketals 72-74 demonstrates the conflicting effects of numerous coordination sites. The simpler, diol-derived ketals provide superior results compared to the threitol derived ketals. The highest diastereoselectivity is observed in the case of the 1,2-diphenyl ethane-1,2-diol derived ketal 74. 

It has also been possible to develop macrocyclic structures for the recognition of anions7 and Lewis basic sites. There are relatively few of these, however, and their numbers reflect the generally more difficult task of arranging Lewis acids in a convergent sense vs. arranging Lewis bases in such a manner. Indeed, the special problems posed by converging carboxylic acids led our own research, and this is an account of our recent adventures in this area. 

The method suggests using 20 mol% of the ligand which, due to the presence of both Lewis-acidic and Lewis-basic sites, is capable of activating both the ketone and the nucleophile. A linear correlation between the ee of the catalyst... 

A species with at least one Lewis basic site which can participate in complex formation. 

In most reported cases, the covalently bound chiral auxiliary has been attached to the dienophile via an acyl linkage, but there are also many examples known in which the auxiliary has been attached to the diene via an acyl, alkyl or heteroatom linkage, the first example of the latter being Trost s diene147. Lewis acids are often added to the reaction mixtures when the chiral auxiliary attached to the dienophile contains an additional Lewis basic site. This is not only to enhance the reaction rate, but especially to enhance the diastereofacial selectivity by complexing to the dienophile in a bidentate fashion. This makes the dienophile more conformationally rigid. 

The reasons for the ewrfo-selectivity of Diels-Alder reactions are only useful for the reactions of dienophiles bearing substituents with lone pairs without a Lewis basic site no secondary orbital interactions are possible. But even in reactions of pure hydrocarbons the ewrfo-selectivity is observed, requiring alternative explanations. For example, the ewrfo-preference of the reactions of cyclopropene with substituted butadienes have been rationalized on the basis of a special type of secondary orbital interactions70. Apart from secondary orbital interactions which are probably the most important reason for the selec-tivities of Diels-Alder reactions, recent literature also advocates other interpretations. 

Lewis acid catalysis enormously enriches the scope of Diels-Alder reactions, but it is limited to reagents containing Lewis basic sites, i.e. functional groups with lone pairs such as carbonyl, amino, ether or nitro close to the reaction centre. As we have seen in the discussion about the FMO aspects of Lewis acids, the major reason for catalysis is the reduction of the HOMO-LUMO gap. In case of Diels-Alder reactions with normal electron demand, it follows that the coordination of the Lewis acid lowers the LUMO energy of the dienophile. Such interactions are only possible if there is a spatial proximity or an electronic conjugation between the coordinated Lewis basic site and the reaction centre. Fortunately, in nearly every Diels-Alder reaction one of the reagents, mostly the dienophile, meets this requirement. 

Besides metal containing Lewis acids, non-metal additives have also found application in catalysis. These studies are quite pertinent to the development of artificial enzymelike catalysts. As there is a large number of Lewis basic sites in living systems able to be involved in hydrogen bonds, the analysis of the catalytic activity of hydrogen bonding additives would give some indication as to the existence of Diels-Alder reactions... 

Axially chiral phosphoric acid 3 was chosen as a potential catalyst due to its unique characteristics (Fig. 2). (1) The phosphorus atom and its optically active ligand form a seven-membered ring which prevents free rotation around the P-0 bond and therefore fixes the conformation of Brpnsted acid 3. This structural feature cannot be found in analogous carboxylic or sulfonic acids. (2) Phosphate 3 with the appropriate acid ity should activate potential substrates via protonation and hence increase their electrophilicity. Subsequent attack of a nucleophile and related processes could result in the formation of enantioenriched products via steren-chemical communication between the cationic protonated substrate and the chiral phosphate anion. (3) Since the phosphoryl oxygen atom of Brpnsted acid 3 provides an additional Lewis basic site, chiral BINOL phosphate 3 might act as bifunctional catalyst. 

Concerning the nature of Lewis basic sites, little work has been done to establish general rules and models, except for alkaline earth metal oxides and zeolites. With respect to the former, i.e., the nature of oxygen Lewis basic sites on alkaline earth metal oxide catalysts, a charge-density model predicts that the strength of the sites decreases in the order > OH > H2O > H30. ... 

An important class of alkali and alkaline earth metal amides are Mulvey s inverse crown complexes (also discussed in Chapter 2, dealing with sodium and potassium amides), in which cationic homo- or heterometallic macrocycles are hosts to anionic guest moieties.The term inverse crown indicates that the Lewis acidic/Lewis basic sites are reversed or exchanged in comparison to conventional crown ether complexes. Scheme 3.9 illustrates the range of recently published alkali and alkaline earth metal amide inverse crown complexes (for related Zn species see Chapter 7 on group 12 amides). 

Figure 3. Three basic strategies for the incorporation of multiply bonded metal-metal guest species into vanadyl and zirconium phosphate host layers, (a) The direct intercalation of solvated M—— M cores into the native layered phosphate host structure, (b) Incorporation of M—— M complexes with ancillary ligands containing a Lewis basic site, (c) Coordination of M—— M cores with ligands provided from modified phosphate layers.

In a broad sense, an acid site can be defined as a site on which a base is chemically adsorbed. Conversely, a basic site is a site on which an acid is chemically adsorbed. Specifically, a Bronsted acid site has a propensity to give a proton, and a Bronsted base has the tendency to receive a proton. Additionally, a Lewis acid site is capable of taking an electron pair and a Lewis basic site is capable of providing an electron pair. These processes can be studied by following the color modifications of indicators, and by using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and calorimetry of adsorption of the probe molecules (see Chapter 4). 

In the additions to 45 it was noted, through variations of the stoichiometric loading of Lewis acid, that amounts of InCl3 less than 2 equiv resulted in lower diastereoselectivity. From this it may be inferred that the Lewis acid, aside from its usual chelation by the /V-acylhydrazone. may interact with another Lewis basic site (e.g., the ester). 

As we deal with oxide surfaces, the existence of protonic (Br nsted) acid sites, of Lewis acidic and Lewis basic sites, and of reducing and oxidizing sites is to be... 

Consider whether any of the reactants is a strong enough electrophile to react without being activated. If not, consider how one of the reactants might be converted to a strong electrophile by protonation of a Lewis basic site (or complexation with a Lewis acid). 

Bipyridine is bidentate it has two Lewis basic sites that can coordinate a cation or other Lewis acid. 14-Ane-4 is tetradentate, by virtue of the four Lewis basic electron pairs on the nitrogen atoms. The acetone-furan cyclic tetramer is also tetradentate but the Lewis basic donors in this case are oxygen atoms. The pyridocrown shown on the right-hand side of Figure 1 is hexadentate by virtue of one nitrogen and five oxygen atoms. All four of these structures are heterocycles. Bipyridine is actually two pyridine heterocycles that are linked. The furan-acetone tetramer and crown are both true heteromacrocycles but both contain heterocyclic units within them. These are sometimes called subcyclic units. 

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