Acid-catalyzed oligomerization

It seems certain that acid-catalyzed true oligomerization occurs via a carbocatio-nic mechanism as first suggested by Whitmore.5 Thus, the formation of diisobutylenes by dimerization of isobutylene in the presence of acidic catalysts occurs according to the following equation  

Friedel-Crafts catalysts and hydrogen halides, protic acids, silica-alumina-type catalysts, or other protic catalysts are effective. 

Carbocation 1 can undergo hydride and methyl shifts to yield 2,3,4-trimethyl-pentenes  

Trimers may be formed through the addition of a tert-butyl carbocation to 2 or via addition of carbocation 1 to isobutylene. The trimer fraction mainly consists of 4-methylene-2,2,6,6-tetramethylheptane and 2,2,4,6,6-pentamethyl-3-heptene. 

Since isobutylene is a very reactive olefin, its oligomerization can be promoted by almost any electrophilic catalyst. More recently fluorinated alumina,6 cation-exchange resins,7 benzylsulfonic acid siloxane,8 pentasil zeolites,9 and perfluori-nated resinosulfonic acids10 were studied. Some of these catalysts may bring about improved oligomerization. 

Indole forms a dimer and a trimer under various acidic conditions. The dimer is formed by electrophilic attack on indole by the 3-protonated species. The structure of the trimer, first proposed by G. F. Smith [15], was proven by synthesis [16]. Trimer results from acid-catalyzed opening of the indoline ring in the dimer, followed by electrophilic attack on a second indole. The dimerization-trimerization process is evidently reversible in 0.5 M H2SO4. A composition of approximately 1 0.4 0.3 is reached from the indole, the dimer or the trimer [17]. An isomeric trimer 5-C can be 

Other 3-substituted indoles such as indole-3-acetic acid give 2,2 -dimers [21]. The dimer has trans stereochemistry at the indoline ring, as was determined by X-ray crystallography on a derived lactam [22]. 

Other compounds that give high yields of 2,2 -dimers are the methyl esters of indole-3-acetate, indole-3-butanoate, and the A -acetyl derivative of trypt-amine [23]. For the corresponding tryptophan derivative, the dimerization is stereoselective giving two dimers, each having trans orientation in the indoline ring [24]. 

A mixture of dimers and trimers is formed when A-pivaloylindole is exposed to AICI3 in CH2CI2. These products are the result of substitution in the carbocyclic ring [25]. The electrophile might be the AICI3 complex, or conceivably the proto-nated species if any water is present. 

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If one heats acetone and pyrrole in the presence of catalytic amounts of acid, so-called acetone pyrrole is formed in over 80%i yield. This colorless, macrocyclic compound contains four pyrrole units which are connected by dimethylmethylene bridges, ft is formed by electrophilic-a-substitution of pyrrole by acetone, acid-catalyzed oligomerization, and spontaneous. 

Fig. 5.17. Acid-catalyzed oligomerization of imipenem (5.46) starting with an intermolecular attack by the carboxy group on the ji-lactam bond [123]...

Isobutylene is present as 20-30% of the C4 fraction from the naphtha cracking process. A number of different upgrading reactions with isobutylene have been carried out industrially (with and without prior separation from the C4 fraction). One of these includes the acid-catalyzed oligomerization... 

Both InBr3 and InCl3 mediate the conjugate addition of pyrroles and indoles to enones <2001S2165, 2002JOC3700>, without problems of acid-catalyzed oligomerization, e.g., Scheme 44. Cerium(III) chloride heptahydrate and sodium... 

An example of acid-catalyzed oligomerization by intermolecular dehydration between an amino and a hydroxy group in a monomer is the usual industrial preparation of piperazine by heating ethanolamine and ammonium chloride at 250° in glass-lined vessels (for other methods of preparing piperazine, see Section III, A, 3). 

Many features of reactions of pyrrole, furan, thiophene and their derivatives bearing electron-releasing substituents with electrophiles are governed by their ready formation and the high stability of corresponding 2H-hetarenium ions, mainly a-C-protonation products like A (E = H). The reversible formation of such cr-complexes (Scheme 2, reaction 1) competes with acid-catalyzed oligomerization (reaction 2), which is practically irreversible and starts from the attack of an hetarenium cation on the neutral five-membered heterocycle analogously to the first step of electrophilic substitution. 

In an attempt to characterize an H-source (of acid-catalyzed oligomerization), a homogeneous system was studied to facilitate NMR detection. 

Protonation and Acid-Catalyzed Oligomerization 2.1 Equilibrium Protonation... 

Similar acid-catalyzed oligomerization of methyl e<-D-glucopyranoside lead to the methyl glycosides of isomaltose, gentiobiose, cellobiose,... 

The polycondensation of Phla requires its acetylation. In contrast to fully aromatic hydroxy acids, the acetylation of Phla is affected by acid catalyzed oligomerization, which renders the isolation of the pure monomer more cumbersome. Silylation of the crude reaction mixture allows an efficient purification and isolation of the silylated monomer (1) by distillation in vacuum (eqs. 1,2). Furthermore, the trimethylsilyl ester can be used as monomer without saponification. As revealed quite recently (12,13) the silyl ester group is the only ester group which undergoes polycondensation with acetylated phenols even in die absence of transesterification catalysts. 

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