Electrophilic addition and

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Khaiasch proposed that hydrogen bromide can add to alkenes by two different mechanisms, both of which aie regiospecific. The first mechanism is electrophilic addition and follows Maikovnikov s rule. 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

With some alkenes, the initial p-halo nitroso compound is oxidized by the NOCl to a P-halo nitro compound. Many functional groups can be present without interference (e.g., COOH, COOR, CN, OR). The mechanism in most cases is probably simple electrophilic addition, and the addition is usually anti, though syn addition has been reported in some cases. Markovnikov s rule is followed, the positive NO going to the carbon that has more hydrogens. 

In the synthesis in Scheme 13.46, a stereoselective aldol addition was used to establish the configuration at C(2) and C(3) in Step A. The furan ring was then subjected to an electrophilic addition and solvolytic rearrangement in Step B. 

However, the reaction of ,/i-unsaturated enones, 1,2-allenylsilane and TiCl4 in CH2C12 afforded five-membered vinylic silanes 65 via the formation of 62, regiose-lective electrophilic addition and 1,2-shift of the trimethylsilany] group and cycliza-tion [34]. 

Substituent effects on the solvomercuration reaction differ markedly from those on many other electrophilic additions and these have been explained by assuming that the formation of the intermediate is often rate limiting in electrophilic additions whereas the reaction of the ionic intermediate with nucleophiles is rate limiting in solvomercuration147. In other words, the solvomercuration involves a fast pre-equilibrium formation of an intermediate, followed by rate-limiting attack of the nucleophile on this species. 

Several different methods of sidewall functionalisation, such as fluorination, radical addition, nucleophilic addition, electrophilic addition and cycloaddition, have been developed (Tasis et al., 2006). The sidewalls of vertically aligned CNTs have been functionalised with DNA using azide units as photoactive components. The azi-dothymidine reacted photochemically with sidewalls of CNTs utilising [2+1] cycloaddition. The oligonucleotides were grown in situ on the sidewalls of CNTs and the DNA-modified CNTs were obtained after the deprotection of the nucleic acid (Moghaddam et al., 2004). 

Fields et alf and Schmidt made closely parallel observations concerning polar cycloaddition of ethylenes substituted at the a-position by an electron-withdrawing group and having no substituent at the jS-position. In both cases the product observed was that to be expected if the electrophile had added to the j3-carbon atom. Since it is clear that the normal ground-state polarization of acrylonitrile (127) and methyl methacrylate (128) should tend to destabilize the cation produced by j8-addition, it was concluded that the orientation of polar cycloadditions could not be predicted by the rules of electrophilic addition and that this apparent anomaly pointed toward a more concerted type of cycloaddition reaction. 

The formation of cyclic terpenoids involves intramolecular electrophilic addition, and this can be exemplified by the following monoterpene structures, again with all reactions being enzyme controlled. 

A rule proposed by Ruzicka that monoterpenes and sesquiterpenes are derived by electrophilic additions and rearrangements from a common intermediate. 

Abstract Bismuth(III) salts are currently considered efficient and ecofriendly reagents and catalysts for the development of new applications in organic synthesis. The preparation of bismuth(III) triflate and its analogues is reviewed as well as some of their applications to the synthesis of bulk chemicals via electrophilic addition and cyclization reactions. The use of bismuth(III) salts in the development of new chemical processes involving steroids and terpenes as substrates is also discussed. 

The study of carbocations has now passed its centenary since the observation and assignment of the triphenylmethyl cation. Their existence as reactive intermediates in a number of important organic and biological reactions is well established. In some respects, the field is quite mature. Exhaustive studies of solvolysis and electrophilic addition and substitution reactions have been performed, and the role of carbocations, where they are intermediates, is delineated. The stable ion observations have provided important information about their structure, and the rapid rates of their intramolecular rearrangements. Modem computational methods, often in combination with stable ion experiments, provide details of the stmcture of the cations with reasonable precision. The controversial issue of nonclassical ions has more or less been resolved. A significant amount of reactivity data also now exists, in particular reactivity data for carbocations obtained using time-resolved methods under conditions where the cation is normally found as a reactive intermediate. Having said this, there is still an enormous amount of activity in the field. 

The author of this chapter has summarized his work in the area of electrophilic additions and application of transition and non-transition metals in organic chemistry in a personal account28. 

A range of methods has been developed for the protection of the carbonyl group in multifunctional aliphatic and alicyclic aldehydes and ketones. This has been necessary because in many multistage syntheses, modification of other functionalities (e.g. oxidation, reduction, hydrolysis, nucleophilic and electrophilic additions and displacements, etc.) requires a differing range of experimental conditions, and that protective group must be selected which is stable in the presence of the reaction medium. A further feature that should be noted is that... 

Specifically, hydroxyl radicals can oxidize organics by hydroxylation, hydrogen abstraction, electrophilic addition, and electron transfer, depending upon the nature of organic compounds. 

In 1,3-benzazoIes (benzoxazoles, benzothiazoies, benzoselenazoles) the heteroatom that is not nitrogen shows very low nucleophilicity and does not serve as a reaction center in electrophilic addition and oxidation-addition reactions. By contrast the tellurium atom in benzotellurazoles is quite susceptible to attack by electrophiles and oxidants. 

Both of the dehydroadamantane isomers undergo rapid electrophilic additions and may, in certain cases, provide useful syntheses of 2,4- and 1,3-disubstituted adamantanes. These reactions will be discussed in Section V.A. 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. 

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