Alkene electrophilic

Characteristically, the reagents that react with the aromatic ring of benzene and its der ivatives are electrophiles. We aheady have some experience with electrophihc reagents, par ticulariy with respect to how they react with alkenes. Electrophilic reagents add to alkenes. 

Limonene, a fragrant hydrocarbon found in lemons and oranges, is bio-synthesized from geranyl diphosphate by the following pathway. Add curvec arrows to show the mechanism of each step. Which step involves an alkene electrophilic addition (The ion 0P2064- is the diphosphate ion, and "Base is an unspecified base in the enzyme that catalyzes the reaction.)... 

The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkable example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alkene electrophilic reactions. Subsequently, strong evidence supporting the mechanism came from the work of George Olah, who prepared and studied stable... 

Practically everything we ve said in previous chapters has been stated without any proof. We said in Section 6.8, for instance, that Markovnikov s rule is followed in alkene electrophilic addition reactions and that treatment of 1-butene with HC1 yields 2-chJorobutane rather than 1-chlorobutane. Similarly, we said in Section 11.7 that Zaitsev s rule is followed in elimination reactions and that treatment of 2-chlorobutane with NaOH yields 2-butene rather than 1-butene. But how do we know that these statements are correct The answer to these and many thousands of similar questions is that the structures of the reaction products have been determined experimentally. 

When fluorine is passed through a mixture of methanol in acetonitrile at -45 °C or propionitrile at -75 °C a solution of methyl hypofluorite, stabilised by the solvent, is produced [200]. While methyl hypofluorite can be isolated, its chemistry has been investigated by treating various substrates with freshly prepared solutions rather than with isolated material. Unlike the hypofluorites which have been discussed so far, methyl hypofluorite behaves as a synthon for the rare methoxylium species, MeO+ , and adds to various alkenes electrophilically in accordance with Markovnikov s Rule [201]. In the reaction between methyl hypofluorite and indene, almost exclusive trans addition occurred. 

Fluoromethanol (4) and symmetrical bis(fluoromethyl) ethers (5) are obtained from paraformaldehyde in hydrogen fluoride.205,209 Dissociation of fluoromethanol creates the electrophilic species [HOCH2 + ]F, which then attacks the 7r-bond of an alkene (electrophilic addition).209... 

The major focus in this chapter will be on synthesis, with emphasis placed on more recent applications, particularly those where regiochemistry and stereochemistry are precisely controlled. The reader is referred to the earlier reviews for full mechanistic information and details of historic interest. Electrophilic addition of X—Y to an alkene, where X is the electrophile, gives products with functionality Y (3 to the heteroatom X. Further transformations of X and/or Y provide the basis for diverse synthetic applications. These transformations include replacement of Y by hydrogen, elimination to form a ir-bond (either including the carbon bonded to X or (3 to that carbon so that X is now in an allylic position), and nucleophilic or radical substitution. Representative examples of these synthetic methods will be given below. This chapter will include examples of heterocycles formed in one-pot reactions where the the initial alkene-electrophile adduct contains an electrophilic group that can react further. Examples of heterocycles formed in several steps from alkene-electrophile adducts will also be considered. Cases in which activation by an external electrophile directly results in addition of an internal heteroatom nucleophile are treated in Chapter 1.9 of this volume. 

The reactions of a conjugate diene reflect the fact that a conjugated diene should be viewed as a functional group in its own right, rather than as two separate alkenes. Electrophilic... 

Step 2, in which the double bond electrons add to the carbocation, is an alkene electrophilic addition. 

Steps 1 and 2 are alkene electrophilic additions, and steps 3 and 4 involve carbocation rearrangements. 

Well nee more details about alkene electrophilic addition reactions ahortly, but fia the present wc can imugine the readioa as takii place by Che pathway shown in Figure 5.4, Hie reaction begins when the alkene donates a pair of electrons from its OC bond to HBr to form a new C-H bond... 

At infrequent intervals, different chemists have juxtaposed properties of alkynes and alkenes. The purpose, of course, was to enrich their understanding of both families In so doing, a number of misconceptions of alkyne vs. alkene reactivity were clarified. The major conclusion that evolved was simple nucleophiles react faster with alkynes than with alkenes electrophiles (including radicals ) react slower with alkynes than with alkenes. Since new kinds of data are available, we believe that the comparisons are especially illuminating now. 

Most reactions of carbonyl groups occur by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, am carbonyl condensation. These mechanisms have many variations, just a alkene electrophilic addition reactions and 8 2 reactions do, but the varia tions are much easier to learn when the fundamental features of the mechanisms are understood. Let s see what the four mechanisms are and what kinds of chemistry carbonyl groups undergo. 

The alcohol is nucleophilic, so the alkene must be made electrophilic. The HgCl2 first coordinates to the alkene to form a 77 complex. The coordination of the Hg(II) makes the alkene electrophilic. (In principle one could draw a metal-lacyclopropane resonance structure for the 77 complex, but in this resonance structure, Hg would be in the (IV) oxidation state, and Hg(IV) is a high-energy oxidation state.) The coordination weakens the C=C 77 bond and makes the carbon atoms electrophilic. The nucleophile then attacks one of the carbon atoms of the 77 complex, and the electrons from the C=C 77 bond move to form a a bond between Hg(II) and the other C to give a stable, isolable organomercury compound. 

It should be noted that alkene electrophiles do not always react on a ring carbon atom (Section ILC.l.d). The ester 152 reacts with diethyl ethoxymethylenemalonate at 120 °C to give the N-substitution product 153 (78%) (Eq. (22)). At a higher temperature (150 °C), the amide 154 (14%) was also formed by competing 1,2-addition of the amine to the diester (95JHC1283). 

The obvious limitation of this reaction so far is in the electrophile as we have suggested naturally electrophilic enones as ideal partners for aryl or vinyl palladium o-complexes. In fact, complexation with palladium makes all alkenes electrophilic and nucleophilic addition can in principle occur to a simple alkene while it and the nucleophile are both bound to the palladium atom. Such reactions are known for aryl halides. Even ethylene itself does satisfactory Heck reactions and its reaction with the bromopyridine 132 is the basis for a large scale process leading to a drug.21... 

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