Nucleophilic Addition to Alkenes and Alkynes

Nucleophilic additions to alkenes and alkynes are also possible, but these reactions generally require that the substrate have substituents that can stabilize a carbanionic intermediate. Therefore, nucleophilic additions are most likely for compoimds with carbon-heteroatom multiple bonds, such as carbonyl compounds, imines, and cyano compounds. We may distinguish two main types of substituents that activate alkenes and alkynes for nucleophilic attack. The first type consists of those activating groups (labeled AG in equation 9.79) that can stabilize an adjacent carbanion by induction.  

In some cases, the addition is followed by elimination of one of the olefinic substituents, so the result of addition-elimination is nucleophilic vinylic substitution. Rappoport, Z. Adv. Phys. Org. Chem. 1969, 7,1. 

The synthesis of fluoroalkyl and chlorofluoroalkyl ethers occurs readily by base-catalyzed nucleophilic addition of alcohols to haloalkenes. An example is the s3mthesis of the alkyl 2,2-dichloro-l,l-difluoroethyl ethers, where R is methyl or 1° or 2° alkyl (equation 9.80). Nucleophilic addition can also occur with alkynes that are activated by the presence of groups such as trifluoromethyl. An example is the addition of methoxide to hexafluoro-2-but)me (87) to produce the methoxy ether with trans trifluoromethyl groups (88, equation 9.81). Note that the orientation of addition to the alkyne is anti, a pattern that is frequently— but not always—observed.  

The second type of activating group stabilizes the carbanion primarily through resonance, as illustrated in equation 9.82. 

For discussions of nucleophilic addition to alkynes, see Miller, S. I. Tanaka, R. in Thyagarajan, B. S., Ed. Selective Organic Transjbrmations, Vol. I Wiley-Intersdence New York, 1970 pp. 143-238 Dickstein, J. L Miller, S. I. in Patai, S., Ed. The Chemistry of the Carbon-Carbon Triple Bond, Part 2 Wiley-Interscience Chichester, England, 1978 pp. 843-911. 

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Hydrogen sulfide is a better nucleophile than water and will undergo nucleophilic addition to alkenes and alkynes in the presence of an acidic catalyst, e.g. sulfuric acid or aluminium chloride (Markownikoff addition). The reaction goes well with suitable alkenes and provides a preparative route to thiols (Scheme 9). 

In addition to alkenes and alkynes, allenes have attracted considerable interest due to their unique reactivity and multireaction sites. Therefore, transition-metal-catalyzed nucleophilic addition reaction of amines and imines to allenes has been extensively studied to prepare biologically important amines and nitrogen-heterocycles.31,31d... 

Addition reactions occur in compounds having n electrons in carbon-carbon double (alkenes) or triple bonds (alkynes) or carbon-oxygen double bonds (aldehydes and ketones). Addition reactions are of two types electrophilic addition to alkenes and alkynes, and nucleophilic addition to aldehydes and ketones. In an addition reaction, the product contains all of the elements of the two reacting species. 

Chapter 8 begins the treatment of organic reactions with a discussion of nucleophilic substitution reactions. Elimination reactions are treated separately in Chapter 9 to make each chapter more manageable. Chapter 10 discusses synthetic uses of substitution and elimination reactions and introduces retrosynthetic analysis. Although this chapter contains many reactions, students have learned to identify the electrophile, leaving group, and nucleophile or base from Chapters 8 and 9. so they do not have to rely as much on memorization. Chapter 11 covers electrophilic additions to alkenes and alkynes. The behavior of carbocations, presented in Chapter 8, is very useful here. An additional section on synthesis has been added to this chapter as well. 

The mechanism begins the same way as the Hg-mediated nucleophilic addition to alkenes. In the first step, an electrophilic 7r complex forms between the alkyne and Hg(II). Water attacks one of the C atoms of the ir complex in Markovnikov fashion to give a 2-hydroxy- l-alkenylmercuryfH) compound, an enol, which is pro-tonated to give a carbocation. Fragmentative loss of Hg(II) then occurs to give a metal-free enol, which tautomerizes to give the ketone product. 

Palladium salts also promote the addition of nucleophiles to alkenes and alkynes. The Pd-catalyzed additions of nucleophiles to alkynes, which is useful for intramolecular cyclizations such as the isomerization of 2-alkynylphenols to benzofurans, proceeds by exactly the same mechanism as does the Hg-catalyzed reaction. However, the Pd-catalyzed additions of nucleophiles to alkenes takes the course of substitution rather than addition because alkylpalladium complexes are unstable toward /3-hydride elimination. The Pd-catalyzed nucleophilic substitutions of alkenes are discussed later in this chapter (Section 6.3.6). 

Alkane-and arenesulfenyl chlorides add to alkenes and alkynes thus, the addition of 2,4-dinitrobezenesulfenyl chloride (61) may be used to prepare solid derivatives of alkenes, e.g. the adduct (62) (Scheme 37). The Markownikoff addition of a sulfenyl chloride to an alkene is stereospecifically trans the adduct may be sequentially treated with a Lewis acid and a nucleophilic reagent to achieve the introduction of a new carbon-carbon bond (64) via the episulfenium ion intermediate (63) (Scheme 38). The procedure can also be applied to the synthesis of unsaturated sulfides, e.g. (65) (Scheme 39). 

Recent Developments in Metal-Catalyzed Additions of Oxygen Nucleophiles to Alkenes and Alkynes... 

The metal-bound carbonyl ligand is readily subjected to the attack of not only carbanions but heteroatom nucleophiles such as alcohols and amines to form ligands useful for formation of compounds containing ester and amide functionalities. The ease with which the nucleophilic attack takes place at metal-coordinated alkenes and alkynes provides a basis for oxidation of these molecules in the presence of a transition metal complex catalyst [3,4a], as exemplified by the Wacker type alkene oxidation by the use of a Pd catalyst. Metal catalyzed addition of alcohols or amines to alkenes and alkynes also involve the analogous nucleophilic attack [4b-e]. The attack of carbanions and heteroatom nucleophiles... 

Electrophilic additions of Brs" to alkenes and alkynes have been carried out [48-50] both in [BMIM][Br] and in other ionic liquids bearing non-nucleophilic anions (Scheme 5.1-16). The reaction is always completely anti-stereospecific, independent of alkene or alkyne structure. It follows a second-order rate law, suggesting a concerted mechanism of the type reported for Brs" addition in aprotic molecular solvents, involving a product- and rate-determining nucleophilic attack by bromide on the alkene or alkyne-Br2 jt-complex initially formed. 

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... 

For reactions of A-acyliminium ions with alkenes and alkynes one has to distinguish between A-acyliminium ions locked in an s-trans conformation and those which (can) adopt an s-cis conformation. The former type reacts as a (nitrogen stabilized) carbocation with a C —C multiple bond. Although there are some exceptions, the intramolecular reaction of this type is regarded as an anti addition to the 7t-nucleophile, with (nearly) synchronous bond formation, the conformation of the transition state determining the product configuration. 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

In a, P-unsaturated carbonyl compounds and related electron-deficient alkenes and alkynes, there exist two electrophilic sites and both are prone to be attacked by nucleophiles. However, the conjugated site is considerably softer compared with the unconjugated site, based on the Frontier Molecular Orbital analysis.27 Consequently, softer nucleophiles predominantly react with a, (i-unsaturated carbonyl compounds through conjugate addition (or Michael addition). Water is a hard solvent. This property of water has two significant implications for conjugate addition reactions (1) Such reactions can tolerate water since the nucleophiles and the electrophiles are softer whereas water is hard and (2) water will not compete with nucleophiles significantly in such... 

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