Halide bridgehead tertiary

Bridgehead tertiary halides were chosen as the experimental model because rear-side attack is not possible, and therefore, if displacement is involved, there would be a decrease in the amount of alkylated products. The results in Table II are consistent... 

All are tertiary halides so that attack by the S mode would not be expected to occur on (16) or (17) any more than it did on (8) (cf. p. 82). Sn2 attack from the back on the carbon atom carrying Br would in any case be prevented in (16) and (17) both sterically by their cagelike structure, and also by the impossibility of forcing their fairly rigid framework through transition states with the required planar distribution of bonds to the bridgehead carbon atom (cf. p. 84). Solvolysis via rate-limiting formation of the ion pair (SN1), as happens with (8) is... 

Halides that do not undergo SN2 reactions readily (tertiary, cyclopropyl and bridgehead halides) react by silver-assisted substitution in the presence of silver salts89. Tertiary halides have also been reduced using the in situ generation of cyanoborohydride reagents90. Since primary and secondary halides are apparently unaffected by this reagent, a selective reaction has been developed. 

As mentioned above, quaternary ammonium salts derived from cinchona alkaloids have occupied the central position as efficient PTCs in various organic transformations, especially in the asymmetric a-substitution reaction of carbonyl derivatives. A cinchona alkaloidal quaternary ammonium salt, which acts as a PTC in various organic reactions, is prepared by a simple and easy chemical transformation of the bridgehead tertiary nitrogen with a variety of active halides, mainly arylmethyl halides. Other moieties of cinchona alkaloids (the 9-hydroxy, the 6 -methoxy, or the 10,11-vinyl) are occasionally modified for the enhancement of both chemical and optical yields (Figure 6.4). 

The S l reaction was first discovered and developed for nitroalkane anions, but it is applicable to several other types of nucleophiles. The S l reaction is applicable to various aryl and tertiary alkyl halides and has also been extended to other leaving groups. The reaction has found a number of synthetic applications, especially in substitution of aryl and bridgehead alkyl halides that are resistant to other substitution mechanisms. 

Photostimulated SRN1 reactions of bridgehead halides [119, 120] and of tertiary chlorides [122] has been examined and considered as a possible route to nucleophilic substitution of these normally unreactive substrates. In the case of the reaction of 1-iodoadamantane with ketone enolates, the photosubstitution product yield was improved in the presence of 18-crown-6 [103], This is probably related to the ion pairing effect already discussed in a preceding section. 

Tertiary alkyllithiums are prepared in hydrocarbons from the appropriate alkyl chlorides with Li dispersions. Once the reaction is initiated, secondary reactions between the reagent and its precursor halide may cause low yields. Slow rates of addition of the alkyl chloride minimize these secondary reactions . Polycyclic and bridgehead reagents are less susceptable to such bimolecular reactions with their precursor chlorides. 

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