Sulfur substituents, stabilization

CF3 compounds like (23) in which the sulfur substituent stabilizes the carbocation, /3-allylation and -cyanation is possible. Scheme 10. Normally, / -substitution of CF3 compounds is problematic because of competing elimination [34]. 

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... 

Additional types of k C-k E chelating ylide complexes merit mentioning here, in addition to the species already presented, and some of them are shown in Scheme 24. The first is formed by ylides containing a pyridine functionality as substituent of the ylidic carbon. This type of ligand has allowed the S3mthesis of many loose clusters (82) of Cu, Ag, and Au which show weak metal( / °)-metal( / °) interactions. These facts have prompted the definition of a new phenomenon numismophilicity) to account for these weak interactions, uniquely shared by the three coinage metals [164]. Nitrogen and sulfur keto-stabilized ylides have been reacted with Pt and... 

Since sulfenylhalides exhibit high anti-selectivity in addition reactions to alky-nes 202) and since a bulky substituent at the sulfur should stabilize the trans-cyclo-alkene relative to the cis-alkene, trichloromethylsulfenylchloride was used as an electrophile. 

Properties such as conductivity of doped polymers and magnetism of high-spin structures are bound to the stability of radical and/or ion sites. Various attempts to stabilize doped materials on a molecular basis have been considered, e.g. via the introduction of polarizable sulfur substituents, or by methods of processing such as immobilization in films or isolation in matrices. 

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 stabilization of an anion with an a-Si or a sulfur substituent is well known and this can be seen readily in the metallation reactions of the thiasilinane 14 which undergoes sequential metallations to give first the trimethyl derivative 15 followed by the tetramethyl derivative 16 (Scheme 3) <2006JGU103>. Similar reactions occur for the 1,4-thiasilinane isomer of 14 to give a product in which both the carbons a to the sulfoxide group have been monomethylated to afford a pair of stereoisomers <2006JGU103>. 

A single diquinone, BE-45985X (28), has been isolated previously from nature [42]. Diquinones usually have a higher oxidation potential than simple quinones, so that rapid reduction by substrate components occurs. The stability, especially in the case of 27b, is certainly due to the sulfur substituent, which lowers the oxidation potential. The new celastramycins exhibited moderate antibacterial activity against Escherichia coli. Staphylococcus aureus, and Bacillus subtilis. 

The pK of 1,3-dithiane is 31 (in cyclohexylamine). There are several factors that can contribute to the anion-stabilizing effect of sulfur substituents. Inductive effects can contribute but cannot be the dominant factor since oxygen substituents... 

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