Alkenes nucleophilicity

The strict geometrical requirements for elimination can be put to further use, as illustrated by elegant procedures for the geometrical isomerization of alkenes. Trimethylsilyl potassium (10) and phenyldimethylsilyl lithium (11) both effect smooth conversion of oxiranes into alkenes, nucleophilic ring opening being followed by bond rotation and spontaneous syn fi-elimination ... 

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

A major limitation of these alkylation reactions has been the regiospecificity and/or need for directing groups of the nucleophile. MacMillan has overcome this and expanded the scope of the reaction to include alkene nucleophiles by using trifluoroborate salts (Scheme 18) [87]. This approach enables alkylation of the 2-position of indoles, complimenting the 3-selective alkylation shown in Scheme 16. One equivalent of hydrogen fluoride was found to be necessary in the reaction in order to sequester the boron trifluoride generated. 

In contrast, the a-peroxy lactones, also members of the dioxetane family, display a higher reactivity toward nucleophiles, in view of the inherent polarization of the peroxide bond by the carbonyl functionality. Consequently, the nucleophilic attack is expected to take place at the more sterically hindered but more electrophilic alkoxy-type oxygen atom of the peroxide bond. A recent detailed study of the oxidation of various di-, tri-and tetrasubstituted alkenes 6 with dimethyl a-peroxy lactone (7) revealed, however, much complexity, as illustrated in Scheme 7 for R = CH3, since cycloaddition (8), ene-reaction (9 and 10) and epoxidation (11) products were observed. In the presence of methanol, additionally the trapping products 12 and 13 were obtained, at the expense of the polyester 14. The preferred reaction mode is a sensitive function of the steric demand imposed by the attacking alkene nucleophile. 

Nucleophilic Additions to Alkenes. Nucleophilic additions to alkenes (Scheme 3.9) are mechanistically very closely related to an ElcB process. In fact, the addition process simply involves a reversal of the steps in response to an equilibrium constant that favors the addition product over the alkene. A notable example is the Michael addition of an enolate to an alkene bearing a strong electron-withdrawing group (EWG). 

Keywords Fluorinated alkenes nucleophilic attack fluoride ion oligomerisation rearrangements displacement carbanions. 

Although the data shown in Fig. 3 clearly indicate that this reaction proceeds via a chain mechanism, it is not clear how this reaction is initiated. Possible mechanisms that must be considered include exciton-based schemes involving surface localized holes that facilitate the attack of the alkene nucleophile (such as proposed for the white light alkylation of porous silicon [23]). Flowever, given the low efficiency of the initiation process it is difficult to completely rule out the role of photogenerated radicals from impurities in solution, even when high purity reactants are used. 

Until recently, the intramolecular cyclization procedure had been used only to synthesize fused heterocyclic structures. The first report of a bridged product, in 1978, involved only a minor amount (9%) of an azabicyclo[3.3.1]nonane derivative obtained from the reaction of ds-carveol with acetonitrile and BFj-EtiO. However, several effective examples are now known. These all involve reaction of the nitri-lium intermediate with an internal alkenic nucleophile to yield a 1-azacyclohexene ring and a new carbe-nium ion which undergoes conventional, but stereospecific, Ritter reaction fiom the least hindered face. Such reactions are typified by formation of the multicyclic structures (64 equation 38) 5<) and (65 equation 39), 5i considerable potential in the synthesis of complex nitrogen heterocyclic systems... 

We shall discuss the conversion of 35 into 36 later in the chapter. Elimination and epoxidation stereoselectively produces the epoxide 37 on the outside (ejto-face) of the folded alkene. Nucleophilic opening with PhSe gives the iran.v-diaxial product (chapter 21). 

The similar step involving alkyne ligand coordinated with Pt(II) center and external alkene nucleophile was believed to induce formation of vinyl-platinum bond and carbonium ion center as shown in 14 and 15 of Scheme 8.37 [59], The... 

Oxidative addition of Arl is followed by insertion of the alkene, p-elim-ination gives a new alkene, nucleophilic attack on which by the N lone pair is followed by loss of MeOH to aromatize the system. 

At least three characteristic features can make a molecule or an ion electrophilic. The most common one is a region of low electron density, reflected by a partial or full positive charge. The full or partial positive charge aids reaction with an electron-rich region of a nucleophile through electrostatic attraction. A second characteristic is the lack of an octet on an atom. The electrophiles listed below fit one or both of these criteria. We examine some of their reactions with alkene nucleophiles in this chapter. 

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