Heteroatom-induced stabilization

As hyperconjugation can be related to cationic stabilization by neighboring lone pairs, relationships between these types of effects must be noted. As shown in Figure 5.9, heteroatom-induced stabilization is a stronger effect than hyperconjugation. 

FIGURE 5.11 Proposed mechanisms of participation of N heteroatoms in electrochemical processes on carbon surfaces (a) possible pseudo-Faradaic reaction of the pyridinic group in aqueous medium [95] (b) N-induced stabilization of carbene-like edge carbon atoms, sites responsible for the development of positive charge in acidic solution. 

Deprotonation of ethers is another route to the a-alkoxy anions, but this pathway is often precluded by a kinetic barrier. Unless the a-carbon is benzylic [175], surmounting this barrier usually requires conditions that are not favorable to the survival of the anion [164]. Notable exceptions are the hindered aryl esters studied by Beak [176], Figure 3.13a, and the carbamates studied by Hoppe [177], shown in Figure 3.13b. In both cases, ec-butyllithium is required for deprotonation, and the carbonyls which direct the metalation by a complex-induced proximity effect [178] must be shielded from the base by large alkyl groups. Once formed, the organo-lithiums are chelated and stabilized by the heteroatom-induced dipole [179]. 

The reaction with sulfides occurs efficiently only when the resulting carbon-centered radicals are further stabilized by a a-heteroatom. Indeed, (TMSfsSiH can induce the efficient radical chain monoreduction of 1,3-dithiolane, 1,3-dithiane, 1,3-oxathiolane, 1,3-oxathiolanone, and 1,3-thiazolidine derivatives. Three examples are outlined in Reaction (12). The reaction of benzothiazole sulfenamide with (TMS)3SiH, initiated by the decomposition of AIBN at 76 °C, is an efficient chain process producing the corresponding dialkylamine quantitatively. However, the mechanism of this chain reaction is complex as it is also an example of a degenerate-branched chain process. 

If two different external electrophiles are to be added to the dimetallic species, diastere-oselectivity can only be achieved if the two carbon-metal bonds are properly discriminated against the reaction with a first electrophile. Moreover, the resulting monometallic species has to exhibit significant configurational stability. Coordination by a heteroatom, which turned out to be essential for achieving substrate-induced diastereoselection, also nicely served these purposes. 

The formation of 286 was rationalized by considering that the initially generated ally lie 1,1-dimetallic species 287, which was presumably less stabilized in the absence of appropriately located heteroatoms and/or silyl groups, reacted with another molecule of allenymetal to produce a new ally lie dimetallic species 288. This process could in principle happen again and induce polymerization but instead, compound 288 underwent a type ... 

Carbanions which bear an a-heteroatom, which is bonded to a functional group capable of inducing a partial positive charge on that heteroatom (64), are both of theoretical interest and of synthetic importance (Seebach et al., 1983). Such have been termed dipole stabilized carbanions (Beak and Reitz,... 

Heteroatom-stabilized Carbanion Equivalents Tabk 2 lTon(III) Chloride Induced Cyclizations of Vinylsilanes... 

The nitrogen-containing conductive polymers, polypolypyrrole (PPY) and polyaniline (PAN) in particular, have been of great interest because of their controllable electrical conductivity, environmental stability and interesting redox properties associated with the chain heteroatoms, More importantly, the century-old [57,58] aniline family of polymers has been found to exhibit solution [59,60] and counter-ion induced [61-63] processability. Furthermore, the electrical properties of the aniline polymers can be substantially improved through secondary doping [64]. The aniline polymers have the general formula [(-B-NH—B—NH—), (-B—N=Q=N-)i.,.]v, in which B and Q denote the... 

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