Phosphorus-based catalysts isomerization

The nature of the Lewis acid and the residence time in the reactor drastically affect the ratio exo/endo of these terminal unsaturations. AICI3-based initiation leads to about 10% of vinylidenes with respect to total unsaturations, whereas the use of BF3-based catalysts, allowing a shorter contact time between the acid and the polymer and thus limiting isomerization, increases this exo structure to 70% or more [25,29]. The vinylidene double bond is much more reactive than the endo one, and favors considerably the furt her terminal functionalization of the oligomers for example, maleination, epoxidation, ozonolysis, reaction with phosphorus pentasul-fide, etc. (Fig. 2). 

Noyori s BINAP catalysts deserve special attention because their chirality is based on the bulkiness of the naphthalene groups, rather than on carbon or phosphorus asymmetric centers (Figure 3.28, inset) [77]. One of the many examples of asymmetric catalysis using BINAP is the synthesis of (—)-menthol, an important additive for flavors, fragrances, and pharmaceuticals. Starting from myrcene, the process is carried out by Takasago International on a multi-ton scale. The key step is the isomerization of geranyldiethylamine to (R)-citronellal enamine [78], which is then hydrolyzed to (R)-citronellal with nearly 99% ee. 

Reactions of an alcohol with acetic anhydride are greatly accelerated by acid catalysts, such as sulfuric acid, zinc chloride, phosphorus pentoxide, ferric chloride, etc. However, such catalysts cannot be used with sensitive alcohols, such as linalool, which are isomerized or otherwise affected by them. The effective acid strength of a catalyst in an anhydrous medium is the predominant factor in determining its activity. The acidities of mineral acids in glacial acetic acid correspond remarkably well with their activities as acetylation catalysts. Sulfuric and perchloric acids in acetic acid solution have been termed superacid solutions because of thdr exceptional strength as compared with the strength of other acids. Most anhydrides react more rapidly with an alcohol in the presence of a base. The base can be sodium hydroxide, the sodium salt of the acid, or a tertiary amine, which can also be the solvent for the reaction. ... 

In recent years, numerous attempts have been made to correlate the geometry of catalyst/substrate intermediates with the l/b ratio of product aldehydes derived from the hydroformylation of 1-olefins [38]. Conclusions are mainly based on spectroscopic measurements or chemical calculations [39]. Especially, Tolman s cone angle (9) [40] and the natural bite angle ( ) [38], respectively, are used to estimate the space-filling properties of a ligand (see Chapter 2). These studies mainly refer to phosphorus-modified rhodium catalysts and 1-olefins as substrates, but they can also contribute to a better understanding of the isomerization - hydroformylation [41]... 

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