Friedel-Crafts reactions defined

Various Brpnsted and Lewis acids can also be used in cationic polymerization of styrene.118 138 159 The molecular weight, however, is difficult to control. Of the usual transfer processes, transfer to the monomer is the most significant reaction. An additional difficulty, the occurrence of Friedel-Crafts reactions, arises if polymerization is carried out in aromatic solvents. As a result, cationic polymerization of styrene usually leads to ill-defined products and is mainly of academic interest.159... 

A Alkylation of silyl enol ether B, Diels-Alder reaction, C, Friedel-Crafts reaction. b As a rough estimate of the kinetics, f50, which is defined as the time necessary to obtain 50% of the final yield was used. 

The irregular centrotriindane 16 was functionalized by bromination to the stereoisomeric dibromides 121 [52] (Scheme 25). Subsequent hydrolysis or oxidation to the diketone 122 followed by reduction gave triindanediols 123 or 124 respectively, with defined stereochemistry. Two-fold Friedel-Crafts reaction of 123 in benzene yielded a C2-symmetrical triindane, 125, whereas the dibromides afforded mainly the diphenyltriindene 126. This compound represents one of... 

Types of Alkylated Compounds. Alkylation may be defined as the introduction of an alkyl radical by substitution or addition into an organic compound. We also include under this procedure the introduction of an aralkyl radical, such as benzyl, and those alkylations presented in discussions of the Friedel-Crafts reaction in previous editions of this book. Alkylation is of six general types, depending on the linkage effected ... 

Kim and co-workers have reported that the Friedel-Crafts reaction of aza-MBH adducts of A-tosylimine derivatives 93 with arenes catalyzed by sufuric acid provides a stereoselective methodology for the preparation of stereo-chemically defined trisubstituted olefins (Scheme 3.31). They have also investigated the Friedel-Crafts reaction by using chlorobenzene and toluene as substrates however, mixtures of ortho and para isomers were obtained. 

John Wilkes insightful review of the history of ionic liquids up to 2002, he showed that these neoteric compounds are not so new after all. In fact, their history dates back to the nineteenth century, where probably the first documented case of an ionic liquid was recorded when a red oil was observed to form during a Friedel-Crafts reaction. Work in the early twentieth century demonstrated that simple ammonium salts could be liquids below room temperature. For example, ethylammonium nitrate has a melting point of 12°C. Even though ionic liquids have been known for many years, it took a long time for them to become popular in the research community. As shown in Figure 1, very little publication activity on ionic liquids existed prior to 2000. From 1980 to 1989, one finds from a Web of Science search that only 35 articles appeared that used the term ionic liquid. Perhaps if we expand the search a bit and include those studies that used the term molten salt to mean ionic liquid as defined here, we may have retrieved a few more examples. Still, at one time in the recent past it was possible for a person to read every article published on the topic and still have time to eat, sleep, and carry out his or her own research. Everybody working in the field knew one another, and each of those researchers was familiar with the complete literature. What a difference a few years make ... 

Simple ways to catalyze nitrations and sulfonylations of aromatic compounds are of great interest since these reactions are carried out industrially on large scale. Clays and zeolites with defined pore structures and channels as acidic catalysts have been utilized in nitrations [109] and Friedel-Crafts sulfonylations [110]. 

The study of cocatalysis in cationic polymerizations is extremely complicated by the fact that Lewis acids can participate in a variety of ill defined reactions (91). Satchell (91, 92) regards the hydrogen exchange reaction between Bronsted acids and aromatics catalyzed by Lewis acids as a prototype for Friedel-Crafts catalysis. He postulates that cocatalytic efficiency is determined by the stability of the complex anion B -f HX + SnCl4 - BH SnCl4Xe. A simple enhancement of conventional acidity by B + HX -> BH Xe as proposed by Plesch (93) and Russel (94) is considered to be unimportant. The stability of the complex and its catalytic activity are determined by the electron density... 

A considerable amount of research has been concerned with the nature of the electrophiles that are involved in Friedel-Crafts acylation reactions. We will summarize the main points. Acyl halides and carboxylic acid anhydrides have been known, for many years, to form stable complexes with a variety of acid catalysts. A well-defined product is formed between acetyl fluoride and boron trifluoride at low temperatures. Analytical and conductivity data characterized the material as acetylium tetrafluoroborate, and this was further confirmed by IR measurements. In the system acetyl chloride-aluminum chloride the acetylium ion can be differentiated from the donor-acceptor complex involving the carbonyl group by means of their IR carbonyl stetching frequencies. A number of other acyl fluorides have been shown to form well-defined acylium salts by interaction with a number of metal fluorides. Acylium salts can also be prepared from acyl chlorides by means of metathetical reactions involving anhydrous salts such as silver hexafluoroantimonate. As well as characterization by means of IR spectroscopy, acylium salts have been studied in non-nucleophilic solvents by NMR spectroscopy. The NMR data for the ben-... 

Schemes aimed at the construction of elaborate two-dimensional -PAHs with boron at their center represent the cutting edge in synthetic methodology toward well-defined graphene-like materials with atomically precise 5-doping (Section 10.4.2.2). Yamaguchi and coworkers showed that oxidative (Friedel-Crafts/Scholl-type reactions) or radical-based cyclizations of triarylboranes with reactive pendant functionalities could be used to fuse the 5-aryl groups into planar arrays. The seminal synthesis of a planarized dibo-raanthracene (Scheme 10.7) is representative of the oxidative/acid-catalyzed approach [24]. Installation of pendant 2,6-di(2-propenyl)phenyl groups via...

Organosilicon compounds can be defined as chemical species having carbon-silicon bond(s). Since there are no organosilicon compounds in nature, all organosilicon compounds are artificial and therefore we can say that human wisdom built organosilicon compounds. Historically, organosilicon compounds have been known since Friedel and Crafts prepared tetraethylsilane by the reaction of diethylzinc and silicon tetrachloride in 1863. 

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