Olefins complexes with Bronsted acids

In the alkylation of arene systems with olefins, the above chemistry proceeds via a carbenium ion mechanism. Alkylation of an aromatic compound with an olefin occurs by the interaction of a Bronsted acid site of any of the catalysts with a participating olefin, creating a carbenium ion, via protonation of the double bond, and thus a polarized complex is formed, as shown below ... 

The majority of catalytic carbonylations employ palladium catalysts and the water soluble complex, Pd(tppts)3, is easily prepared by reduction of PdCl2/tppts with CO in water at room temperature [39]. Hence, this complex is readily generated in situ, under carbonylation conditions. It was shown to catalyze the car-bonylation of alcohols [40, 41] and olefins [42-44], in the presence of a Bronsted acid cocatalyst (Fig. 7.10). 

The oldest method of alkylation with ethylene is the liquid phase reaction using anhydrous aluminum chloride as the catalyst. This reaction is a form of the classic Friedel-Crafts reaction and was discovered in 1879 by Balsohn. Most Lewis and Bronsted acids are known to be active for olefin alkylations. Alkylation by H2SO1, and H3PO1, was first shown by Ipatieff, et al, in 1936 who extended the reaction to isoparaffins. For the liquid phase alkylation of benzene with ethylene, however, aluminum chloride is preferred over the other acids, although a co-catalyst or promoter is usually needed to obtain efficient alkylation. AICI3 when dissolved in benzene containing some HCl forms a complex which can be simply described as ... 

Ene reactions involve the addition of a compound bearing a double bond (enophile) to an olefin possessing an allylic hydrogen atom (ene). They can be thermally activated, but, as the enophile, like the dienophile in the Diels-Alder reaction, should be electron deficient, complexation with a Lewis acid increases the reaction rate thus allowing to carry out the reaction under milder conditions. On the other hand Bronsted acids can also catalyze the reaction through protonation of the carbonyl group and rearrangement to form a more stable carbonium ion. 

The step of cationic initiation can be subdivided into two separate reactions. The first one consists of formation of ionic species and the second one of reactions of these ionic species with the olefins, a cationization process. This reaction, termed priming by Kennedy and Marechal, is a process of ion formation in a non-nucleophilic media through (1) dissociation of protonic acids to form protons and counterions, (2) reactions of Lewis acids with Bronsted acids, (3) dissociation of dimeric Lewis acids, (4) complexation of Lewis acids with water or with alkyl halides or with ethers, and so on. These reactions may take place through a series of complicated steps. The second reaction, the cationization of the olefins, may also include several intermediate steps that will eventually lead to propagating species. 

The polymerization mechanism of a-olefins is often defined as anionic-coordinated. The term anionic , if intended to denote an asymmetric electronic distribution on the M—bond with a higher electronic density on P than on M, seems justified (see hydrolysis with tritiated Bronsted acids. Section 4). The term coordinated is used to stress the assumption that, like other reactions catalyzed by complexes of transition metals, propylene should coordinate at the transition metal before insertion into the M —P bond. Polymerization would then take... 

---