Electrophilic addition steps

The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they act as electron-releasing groups promoting addition and also control the regiochemistry. A silyl or starmyl substituent strongly stabilizes carboca-tion character at the /3-catbon atom and thus directs the electrophile to the a-carbon. The reaction is then completed by the limination step, in which the carbon-sihcon or carbon-tin bond is broken. 

The reaction of lead tetraacetate with alkenes and arenes involve first an electrophilic addition step. The intermediate organolead derivative then can follow various pathways to yield oxidation products. 

The ability to promote j8 elimination and the electron donor capacity of the jS-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they promote addition and strongly control the regiochemistry. A silyl or stannyl substituent strongly stabilizes carbocation character at the j3-carbon atom and thus directs the electrophile to the a-carbon. Molecular orbital calculations indicate a stabilization of 38 kcal/mol, which is about the same as the value calculated for an a-methyl group. The reaction is then completed by the elimination step, in which the carbon-silicon or carbon-tin bond is broken. 

Both the regiochemistry of the electrophilic addition step and the elimination step are strongly directed by the MR3 substituent and are both more rapid and more selective than would be the case without the substituent. The electrophilic replacement of a trimethylsilyl group by acetyl is an example ... 

Write out two step-by-step mechanisms for the addition of HI to cyclohexene shown above (p. 489, margin). In the first, use a free proton as the electrophile. In the second, use undissociated HI in the electrophilic addition step. Make sure to include all necessary curved arrows to depict electron-pair movement. 

Addition of arylhydroxylamines to electrophilic allenes such as methyl propadienoate or l-methancsulfonyl-l,2-propadiene is another route to 0-vinyl derivatives[2]. The addition step is carried out by forming the salt of the hydroxylamine using NaH and the addition is catalysed with LiO CCFj. The intermediate adducts are cyclized by warming in formic acid. Yields are typically 80% or better. 

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

Under conditions of acid catalysis the nucleophilic addition step follows protonation of the carbonyl oxygen Protonation increases the carbocat ion character of a carbonyl group and makes it more electrophilic... 

Isoprene has sometimes been used as a starting matenal in the laboratory synthesis of ter penes In one such synthesis the first step is the electrophilic addition of 2 moles of hydrogen bromide to isoprene to give 1 3 dibromo 3 methylbutane... 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

Electrophile Addition. The addition of electrophilic (acidic) reagents HZ involves two steps the slow transfer of hydrogen ion from Z to the butylene to form a carbocation and, a rapid combination of the carbocation with the base Z. 

Both steps in this general mechanism are based on precedent. It is called electrophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the tt electrons of the double bond. Using the two tt electrons to fonn a bond to an electrophile generates a carbocation as a reactive intennediate normally this is the rate-detennining step. 

Write a structural formula for the compound formed on electrophilic addition of sulfuric acid to cyclohexene (step 1 in the two-step transformation shown in the preceding equation). 

Hydroboration-oxidation (Sections 6.11-6.13) This two-step sequence achieves hydration of alkenes in a stereospecific syn manner, with a regiose-lectivity opposite to Markovnikov s rule. An organoborane is formed by electrophilic addition of diborane to an alkene. Oxidation of the organoborane intermediate with hydrogen peroxide completes the process. Rearrangements do not occur. 

Electrophilic addition of HX to an alkene involves a two-step mechanism, the overall rate being given by the rate of the initial protonation step. Differences in protonation energies are usually explained by considering differences in carbocation stability, but the relief or buildup of strain can also be a factor. One of the following alkenes protonates much more easily than the other. 

There is some debate in the literature as to the actual mechanism of the Beirut reaction. It is not clear which of the electrophilic nitrogens of BFO is the site of nucleophilic attack or if the reactive species is the dinitroso compound 10. In the case of the unsubstituted benzofurazan oxide (R = H), the product is the same regardless of which nitrogen undergoes the initial condensation step. When R 7 H, the nucleophilic addition step determines the structure of the product and, in fact, isomeric mixtures of quinoxaline-1,4-dioxides are often observed. One report suggests that N-3 of the more stable tautomer is the site of nucleophilic attack in accord with observed reaction products. However, a later study concludes that the product distribution can be best rationalized by invoking the ortho-dinitrosobenzene form 10 as the reactive intermediate. 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

Mechanism of the electrophilic addition of HBr to 2-methyl-propene. The reaction occurs in two steps and involves a carbo-cation intermediate. 

Figure 6.8 Energy diagram for the two-step electrophilic addition of HBr to 2-methylpropene. The first step is slower than the second step.

How does the Hammond postulate apply to electrophilic addition reactions The formation of a catbocation by protonation of an alkene is an endergonic step. Thus, the transition state for alkene protonation structurally resembles the... 

Problem 6.18 What about the second step in the electrophilic addition of HCl to an alkene—the reaction of chloride ion with the carbocation intermediate Is this step exergonic or endergontc Does the transition state for this second step resemble the reactant (carbocation) or product (alkyl chloride) Make a rough drawing of what the transition-state structure might look like. 

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.)... 

Acid-catalyzed hydration of isolated double bonds is also uncommon in biological pathways. More frequently, biological hydrations require that the double bond be adjacent to a carbonyl group for reaction to proceed. Fumarate, for instance, is hydrated to give malate as one step in the citric acid cycle of food metabolism. Note that the requirement for an adjacent carbonyl group in the addition of water is the same as that we saw in Section 7.1 for the elimination of water. We ll see the reason for the requirement in Section 19.13, but might note for now that the reaction is not an electrophilic addition but instead occurs... 

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH2-) or a secondary radical (R2CH-). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed. 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Electrophilic addition of HCJ to a conjugated diene involves the formation of allylic carbocation intermediates. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then... 

When an alkene reacts with an electrophile, such as HC1, initial addition of H+ gives an intermediate cation and subsequent reaction with Cl" yields an addition product (Section 6.7). When an enol reacts with an electrophile, however, only the initial addition step is the same. Instead of reading with Cl- to give an addition product, the intermediate cation loses the -OH proton to give an cr-substituted carbonyl compound. The general mechanism is showm in Figure 22.3. 

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