With Other Singly-bonded Electrophiles

Towards other singly-bonded electrophiles, the stannylamines behave as mild nucleophiles. The more reactive alkyl halides can be converted into the corresponding alkyl amines (equation 16-43 and 16-44)35, and acid anhydrides (equation 16-45)31 and acyl halides (equation 16-46)36 give the amides. 

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The catenation of nitrogen refers to the tendency of N atoms to be connected to each other, and is far lower than those of C and P. This is because the repulsion of lone pairs on adjacent N atoms weakens the N-N single bond, and the lone pairs can easily react with electrophilic species. The structures and examples of known compounds that contain chains and rings of N atoms are listed in Table 15.1.2. Note that none of these has a linear configuration of nitrogen atoms. 

Tris(trimethylsilyl)phosphine and its more reactive derivative lithium bis-(trimethylsilyl)phosphide-2tetrahydrofuran are very useful reagents for the preparation of compounds with a single or a multiple element phosphorus bond. They react readily with various element halides, with carboxylic acid chlorides, and with carboxylic esters, as well as with other organic electrophiles via a substitution of lithium and/or a cleavage of the weak polar Si-P bonds. 

Cyclopropcnones and heteroanalogs are readily converted to cyclopropenylium ions with electrophiles. On the other hand, nucleophiles attack the cyclopropenylium moiety accompanied by cleavage of the C-C single bond to afford ring-opened or ring-enlarged products. In some cases, nucleophilic substitution occurs at the alkene carbon. 

Synthesis and Characterisation of Phosphonium Ylides. - We start this section with a phosphanylidene-o -phosphorane (14) which, although not strictly an ylide, has a P-P bond which displays many of the properties of more conventional phosphonium ylides. A crystal structure of (14) revealed a P-P bond length of 2.084(2) A, which is similar to those found in other phos-phanylidene-o( -phosphoranes and is certainly shorter than a typical P-P single bond ca. 2.22 A), indicative of multiple bond character. Compound (14) undergoes reactions with electrophiles (Scheme 1) which demonstrate its nucleophilic... 

With the exception of hydrogenation, the addition reactions of alkenes presented in this text occur by an electrophilic addition mechanism. The electrophile (H+ or X+) attacks the electron-rich pi-bond of the double bond. The pi electrons are used to form a single bond between the carbon and attacking species the other carbon becomes a carbocation. The carbocation is then neutralized by halide ion or water the addition is complete. In bromination reactions, the bromine adds in a trans fashion. 

This reaction may not look like any reaction that you have seen before, but it is simply a 1,4-addition of an electrophile and a nucleophile to a conjugated diene. However, unlike the other 1,4-addition reactions you have seen—where the electrophile adds to the diene in the first step and the nucleophile adds to the carbocation in the second step—the Diels-Alder reaction is a concerted reaction The addition of the electrophile and the nucleophile occurs in a single step. The reaction initially looks odd because the electrophile and the nucleophile that add to the conjugated diene are the adjacent sp carbons of a double bond. As with other 1,4-addition reactions, the double bond in the product is between C-2 and C-3 of what was the conjugated diene. 

At the other extreme, metal carbenes that are electrophilic at carbon are called Fischer-type complexes, and they generally contain jt-donating heteroatom substituents [4], Fischer reported the first example in 1964 [5], In these cases, the metal-carbene interaction can be represented by three resonance structures, the first with a formal M=C double bond, the second with a M-C single bond and charge separation, and the third with additional multiple bond character between the carhon and the heteroatom substituent. 

HF and HSO3F (or other superacid) solutions (see subsequent discussion). Higher alkanes and isoalkanes gave yields of 5-10% and adamantane was nitrated in 30% yield. Data indicate that nitratioa (nitrolysis) of alkanes with nitronium salts proceeds in accordance with the generalized concept of electrophilic reactions of single bonds [77] involving two-electron, three-center bond carbocationic intermediates (transition states) as illustrated with case of the nitration of methane. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. The mechanism of the initiation of cationic polymerization and polycondensation has been extensively studied. Trivalent carbenium ions play the key role, not only in acid-catalyzed polymerization of alkenes, but also in poly condensation of arenes (n -bonded monomers), as well as in cationic polymerization of ethers, sulfides, and nitrogen compounds (nonbonded electron-pair donor monomers). Pentacoordinated carbonium ions, on the other hand, play the key role in the electrophilic reactions of a-bonds (single bonds), including the oligocondensation of alkanes and the cocondensation of alkanes and alkenes. 

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