Electrophilic addition to unsaturated carbon

The characteristic reaction of alkenes is electrophilic addition, in which the carbon-carbon jr bond is replaced by two a bonds. 

The rr bond of an alkene results from overlapping of p orbitals and provides regions of increased electron density above and below the plane of the molecule. These electrons are less tightly bound than those in the a bonds, so are more polarizable and can interact with a positively charged electrophilic reagent. This forms the first part of an electrophilic addition, in which the electrons are used to form a a bond with the electrophile and leave the other carbon of the double bond electron deficient, i.e. it becomes a carbocation. This carbocation is then rapidly captured by a nucleophile, which donates its 

The carbocation is an intermediate in the reaction sequence and corresponds to a minimum in the energy profile (see Section 5.4). Its formation 

Essentials of Organic Chemistry Paul M Dewick 2006 John Wiley Sons, Ltd 

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The complementary approach, activation of unsaturated hydrocarbons toward electrophilic attack by complexation with electron-rich metal fragments, has seen limited investigation. Although there are certainly opportunities in this area which have not been exploited, the electrophilic reactions present a more complex problem relative to nucleophilic addition. For example, consider the nucleophilic versus electrophilic addition to a terminal carbon of a saturated 18-electron metal-diene complex. Nucleophilic addition generates a stable 18-electron saturated ir-allyl complex. In contrast, electrophilic addition at carbon results in removal of two valence electrons from the metal and formation of an unstable ir-allyl unsaturated 16-electron complex (Scheme 1). 

This approach to the thiophene ring seems most direct and involves (1) intramolecular nucleophilic addition of thiol, thiolate, and dithiocarboxylate sulfurs and, in a rare case, sulfide sulfur to sp and sp carbons (2) electrophilic attack of sulfenium and sulfonium ions and their equivalents on unsaturated carbon-carbon bonds (3) addition of thiyl radicals to unsaturated carbon-carbon bonds (4) addition of vinyl and aryl radicals to the sulfur atom of sulfides and (5) electrophilic attack of a carbocation on the sulfur atom of sulfides. 

Electrophilic addition to a,/3-unsaturated carbonyl compounds is analogous to electrophilic addition to isolated double bonds, except that the electrophile adds to the carbonyl ojqrgen, the most basic atom in the molecule. After that, the nucleophile adds to the p carbon, and the resulting intermediate enol tautomerizes to the more stable carbonyl compound. These reactions may also be considered as the electrophilic counterparts of the nucleophilic Michael and 1,4-addition reactions discussed in Chapter 3, Section 3.C. 

Addition of halogens to unsaturated carbon-carbon bonds is a classical example of electrophilic addition. Addition to alkynes generally leads to 1,2-dihaloalkenes. Various mechanisms of addition have been discussed in the earlier review by Schmid. No major changes in the mechanistic picture have been published since. The actual addition might be preceded by complex formation. Ault applied the matrix isolation technique to study complex formation between GIF and acetylene, propyne and 2-butyne and also between CI2 and acetylene and propyne. The data seem to support a T-shaped structure. 

The addition of nucleophiles such as carbon nucleophiles to electrophiles such as unsaturated carbon-heteroatom bonds is an important reaction in organic chemistry, allowing carbon-carbon bond formation. Although these reactions are usually performed in organic or eventually in organic/aqueous two-phase system, it was recently shown that they could be performed in water alone or in a monophasic water/organic system. This development has also allowed reactions to proceed under less drastic conditions. 

The reaction of ozone with allyl alcohol is expected to involve, as for other unsaturated alcohols and olefins, electrophilic addition at the carbon-carbon double bond leading to the formation of a trioxide followed by its unimolecular decomposition into two carbonyls and two biradicals (Criegee intermediates) ... 

When the addition is initiated by attack of the jr-electrons in an unsaturated bond on an electrophile to form a carbocation the reaction is an electrophilic addition, a very common class of reactions for alkenes. The reaction is governed by Markovnikov s rule, which states that in addition of HX to a substituted alkene, the H will form a bond to the carbon of the alkene carrying the greater number of hydrogen atoms. 

The previous sections dealt with reactions in which the new carbon-carbon bond is formed by addition of the nucleophile to a carbonyl group. Another important method for alkylation of carbon nucleophiles involves addition to an electrophilic multiple bond. The electrophilic reaction partner is typically an a,(3-unsaturated ketone, aldehyde, or ester, but other electron-withdrawing substituents such as nitro, cyano, or sulfonyl also activate carbon-carbon double and triple bonds to nucleophilic attack. The reaction is called conjugate addition or the Michael reaction. 

The platinum(0) complex [Pt(PhNO)(PPh3)2] reacts with C02 to afford the metallacyclic nitroso species [Pt 0N(Ph)C(0)0 (PPh3)2] (60), the first example of insertion of C02 into a Pt—N bond.186 Other unsaturated carbon compounds such as CS2 and electrophilic alkenes and alkynes react similarly. The diradical peril uoro-/V,/V -dimethylethane-l,2-bis(amino-oxyl) reacts readily by oxidative addition to the platinum(0) precursor Pt(PPh3)4 to afford the corresponding platinum(lI)-nitroso complex containing a seven-membered chelate ring (61). The resulting complex is stable in air for several days at room temperature.187... 

The addition reactions of nucleophilic and electrophilic reagents to optically active a, /3-unsaturated sulfoxides have also been found to proceed in an asymmetric way. Addition of piperidine to chiral (i )-cis-propenyl p-tolyl sulfoxide 309 affords a 87 13 mixture of diastereomeric sulfoxides 310 (318). The configuration at the 3-carbon atom of the predominant diastereomer (i yS )-310 was determined by means of chemical correlation starting from optically... 

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