Addition of other electrophiles

As with addition of other electrophiles, halogenation of conjugated dienes can give 1,2- or 1,4-addition products. When molecular bromine is used as the brominating agent in chlorinated hydrocarbon solvent, the 1,4-addition product dominates by 7 1 in the case of butadiene. ... 

The formation of the bridged intermediate has been represented as an SN2-like displacement of the leaving group from the sulfenyl sulphur of 85116, or alternatively, as reported in equation 90 in agreement with the addition of other electrophiles to alkenes, it has been proposed that the reaction involves the initial formation of jr-complex 86 in a rapid equilibrium with the reagents76. 

Titanium tetrachloride is a useful Lewis acid for the addition of other electrophiles to allylsilane these electrophiles include acid anhydrides (Eq. 85) [229], acyl halides (Eq. 86) [2,3,159,161,230], and SNl-active halides such as rert-alkyl halides (Eq. 87) [165] and a-halosulfides [2,3,159,161]. The allylation of the substrate shown in Eq. (88) was accompanied by stereoselective opening of the cyclobutane ring to give an open-chain triene with defined olefin geometry [231]. A benzylic selenonium... 

Two major aspects are involved in the additions of electrophiles to bicyclobutane, namely the product distribution and the stereochemistry of the reaction. In the following. section we will address acid-catalyzed additions to bicyclobutane. This will be followed by a discussion of the addition of other electrophiles such as halogens, and will be concluded by a general discussion of the mechanism of electrophilic additions to bicyclobutanes. 

Fahey and Lee (reference 283) found the addition of HCl to 3-hexyne in acetic acid to be termolecular, first order in the alkyne and second order in HCl. The reaction showed primarily anti addition at 25°C, although there was more syn addition at higher temperatures. Similar considerations apply in the addition of other electrophiles to alkynes (reference 275). 

The reactivity of most singlet carbenes with alkenes is dominated by the electrophilic character of the carbene (the empty p orbital). Thus, the more electron rich the alkene, the faster the carbene addition. Increasing alkyl group substitution on alkenes increases the rate of addition. This trend parallels the reactivity for the addition of other electrophiles with alkenes, such as acids, Xj, and borane. Dialkylcarbenes are less selective than dihalocarbenes, whereas carbenes with neighboring O or N atoms are resonance stabilized (see margin) and are highly selective. This trend tracks the reactivity-selectivity principle (see Chapter 7), where the more stable carbenes are the more selective. 

The same size effects appear to prevent the addition of other electrophiles to cluster metal atoms. The attack of other nucleophiles normally leads to their attachment to the ligand sphere at the high electron density point. Thus for instance the reaction of FeCo2(/i3-S)(CO)9 with the electrophile Cr (CO)5 which attacks at the /13-sulfur ligand may be explained. 

Replacement of the olefin in the complexes [Rh(/r-pz )(C2Fl4)2]2 (pz =pyrazolate (pz), 3-methylpyrazolate (Mepz), 3,5-dimethylpyrazolate (Mc2pz)) by / /Z-butyl isocyanide gave [Rh(/r-pz )(CN Bu)2]2 51. These complexes oxidatively added Mel, as a consequence of the metal basicity induced by the isocyanide ligands. The mechanisms of the oxidative addition of other electrophiles of the type RCFI2CI were also studied. 

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