Carbanions oxygen-stabilized

Dipole-Stabilization is a term coined by Beak to describe the situation that results when a carbanion is stabilized by an adjacent dipole. Such a situation arises when, for example, an amide is deprotonated a to nitrogen. The chemistry of these systems has been reviewed, - so only a few pertinent points will be made here. Firstly, metalation occurs syn to the carbonyl oxygen, and when the system is cyclic, the equatorial proton is removed selectively, and the electrophile attacks equatorially, as shown in equation (5), 24, 25 Thus, in contrast to nitrosamines, amide anions give the less stable equatorial product. 4,i25... 

Enantioselective deprotonations a to the oxygen or nitrogen atoms of carbamates 4.1 and 4.2 can be performed with s-BuLi in the presence of sparteine 2.5. The pro (5)-hydrogen atom is selectively removed, and lithiated carbanions 4.3 are formed. These carbanions are stabilized by dipolar interactions [982] and coordination to sparteine [564, 983-987], Such species are stable up to -30°C, and react with electrophiles at -78°C with retention of configuration [984-987]-... 

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

Protonation of the enolate ion is chiefly at the oxygen, which is more negative than the carbon, but this produces the enol, which tautomerizes. So, although the net result of the reaction is addition to a carbon-carbon double bond, the mechanism is 1,4 nucleophilic addition to the C=C—C=0 (or similar) system and is thus very similar to the mechanism of addition to carbon-oxygen double and similar bonds (see Chapter 16). When Z is CN or a C=0 group, it is also possible for Y to attack at this carbon, and this reaction sometimes competes. When it happens, it is called 1,2 addition. 1,4 Addition to these substrates is also known as conjugate addition. The Y ion almost never attacks at the 3 position, since the resulting carbanion would have no resonance stabilization " ... 

The intramolecular addition of carbon nucleophiles to alkenes has received comparatively little attention relative to heterocyclization reactions. The first examples of Pd-catalyzed oxidative carbocyclization reactions were described by Backvall and coworkers [164-166]. Conjugaled dienes with appended al-lyl silane and stabilized carbanion nucleophiles undergo 1,4-carbochlorination (Eq. 36) and carboacetoxylation (Eq. 37), respectively. The former reaction employs BQ as the stoichiometric oxidant, whereas the latter uses O2. The authors do not describe efforts to use molecular oxygen in the reaction with allyl silanes however, BQ was cited as being imsuccessful in the reaction with stabihzed car-banions. Benzoquinone is known to activate Ti-allyl-Pd intermediates toward nucleophilic attack (see below. Sect. 4.4). In the absence of BQ, -hydride eUm-ination occurs to form diene 43 in competition with attack of acetate on the intermediate jr-allyl-Pd" species to form the 1,4-addition product 44. 

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