Sulfur substituents, stabilization carbanions

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... 

Sulfur compounds are useful as nucleophilic acyl equivalents. The most common reagents of this type are 1,3-dithianes, which on lithiation provide a nucleophilic acyl equivalent. In dithianes an umpolung is achieved on the basis of the carbanion-stabilizing ability of the sulfur substituents. The lithio derivative is a reactive nucleophile toward alkyl halides and carbonyl compounds. 11... 

Quite a number of other electron withdrawing groups containing multiple heteroatomic bonds, such as the ester carbonyl, nitrile, carboxamide, etc., can likewise stabilize carbanions. The lithium salt of tert-butyl acetate 32 is an example of an enolate anion sufficiently stable as a salt to be used as a shelf reagent. Substituents containing easily polarizable atoms, such as sulfur or selenium, are also capable of stabilizing an adjacent anionic center. 

These transformations are of special importance since various sulfur derivatives are widely utilized as synthetically useful precursors, owing to the ability of sulfur substituents to stabilize both carbanionic and carbocationic adjacent centers. 

The C(2) equatorial hydrogen is selectively removed when 1,3-dithianes are deprotonated. Furthermore, if the resulting carbanion is protonated, there is a strong preference for equatorial protonation, even though it leads to the less stable axial orientation for the 2-substituent. Discuss the relevance of these observations to the structure of the sulfur-stabilized carbanion in MO terminology. 

The a-ethylthiosulfoxides can be converted to the corresponding carbonyl compounds by hydrolysis catalyzed by mercuric ion. In both of these systems an umpolung is achieved as the result of the carbanion-stabilizing ability of the sulfur substituents. The same sulfur substituents provide access to the carbonyl group by the hydrolysis mechanism. 

To broaden our understanding of the chemical behavior of these novel monomers, it would be appropriate to try anionic polymerizations of captodative monomers. Inasmuch as sulfur is able to stabilize adjacent carbanions, a-alkylthioacrylates and -acrylonitriles should respond well to anionic initiators. Cationic polymerization of certain captodative monomers may also be of interest as alkylthio- and cyano-substituents can stabilize a cationic propagating center. 

The interaction between thiophenes and oxocarbenoids, in general, leads to sulfur ylides 42 as the initial products. Depending on the substituents R R, R and the temperature, these ylides can be isolated (R, R carbanion-stabilizing substituents), rearrange to a 1,4-oxathiocin (2,5-dichlorothiophene and ethyl 2-diazo-3-oxobutanoate ° ) or to 6-acyl-2-thiabicyclo[3.1.0]hex-3-enes, 2-acyl-2//-thiopyrans or 2-acylmethylthiophenes. Theoretical calculations and experimental results suggest intermediate 43 as the common precursor for the latter three products. 

The steric effect on regioselectivity shows clearly in the series in Eq. (31), Table 4. Comparing similar anion type, except for size, entries 1, 3, and 7 show that ortho substitution is very signiflcant with a primary carbanion but essentially absent with a tertiary cyano-stabilized anion. It is striking that as the size of the alkyl substituent on the arene increases, not only is ortho substitution disfavored, but meta is as well compare entries 3-6 and compare 7-9. With the very large CH(t-Bu)2 group (entry 9), only para substitution is observed. Regioselectivity is also dependent on the electronic nature of the nucleophile. Most remarkably, addition of the primary sulfur-stabilized anion shows nearly equal... 

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