Reactive intermediates Carbocations

Chapter 10 considers the role of reactive intermediates—carbocations, carbenes, and radicals—in synthesis. The carbocation reactions covered include the carbonyl-ene reaction, polyolefin cyclization, and carbocation rearrangements. In the carbene section, addition (cyclopropanation) and insertion reactions are emphasized. Recent development of catalysts that provide both selectivity and enantioselectivity are discussed, and both intermolecular and intramolecular (cyclization) addition reactions of radicals are dealt with. The use of atom transfer steps and tandem sequences in synthesis is also illustrated. 

Three-bonded carbon atoms are most likely reactive intermediates carbocations in reactions involving strong electrophiles, carbanions in reactions involving strong nucleophiles, and free radicals in radical reactions. If you draw condensed formulas... 

As we have just seen the rate determining intermediate m the reaction of tert butyl alco hoi with hydrogen chloride is the carbocation (CH3)3C Convincing evidence from a variety of sources tells us that carbocations can exist but are relatively unstable When carbocations are involved m chemical reactions it is as reactive intermediates formed slowly m one step and consumed rapidly m the next one... 

Both steps m this general mechanism are based on precedent It is called elec trophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the rr electrons of the double bond Using the two rr electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate normally this IS the rate determining step... 

Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates m chemical reactions The third type of conjugated system that we will examine conjugated dienes, consists of stable molecules... 

The rate of addition depends on the concentration of both the butylene and the reagent HZ. The addition requires an acidic reagent and the orientation of the addition is regioselective (Markovnikov). The relative reactivities of the isomers are related to the relative stabiUty of the intermediate carbocation and are isobutylene 1 — butene > 2 — butenes. Addition to the 1-butene is less hindered than to the 2-butenes. For hydrogen bromide addition, the preferred orientation of the addition can be altered from Markovnikov to anti-Markovnikov by the presence of peroxides involving a free-radical mechanism. 

Mechanistic Transform. A transform involving a sequence of reactive intermediates such as carbocations or carbon radicals which are generated in a stepwise mechanistic manner and which lead finally to stable predecessor structure(s). 

Carbocations as reactive intermediates play an essential role in organic reactions and have been thoroughly researched 102, l0J). The individual quality of the cationic polymerization results from the reproduction of the cationic reactive intermediate in every propagation step during the addition of monomers. 

A bridged carbocation with a two-electron, three-centre bond was proposed as early as 1939 (Nevell et al., 1939) for the 2-norbornyl cation [lO ] as a reactive intermediate in the solvolysis of 2-norbornyl system (see also Winstein and Trifan, 1949). It has now been isolated as the SbFe salt and the bridged structure is accounted for using solid-state nmr studies... 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

Carbocations are ubiquitous reactive intermediates, with a formal charge +1 on car-bon.i The recognition of their existence can be traced to the early 1900s, when carbocations were first the subject of much doubt, but then later were accepted as unstable, nonisolable species. Research efforts during the first 50 years after they... 

A large number of DNA-alkylating agents are known and we will not attempt a comprehensive survey here. A number of excellent reviews provide an overview of this area. " Here we will review DNA alkylation by three types of reactive intermediates that are important in medicinal chemistry and toxicology— episulfonium ions, aziridinium ions, and carbocations. 

Reactions Involving Carbocations, Carbenes, and Radicals as Reactive Intermediates... 

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