Sulfur Participation

It was pointed out in the introduction (see p. 112) that the S group should be a powerful, intramolecular displacing-group, based on the driving force that was calculated for the substituted alkylthio moiety.  

The preparation of a number of anhydro thio sugars is based on this S neighboring-group participation. 

Creighton and Owen generated the mercaptide ion by treatment of an (S-acetyl group with base, in the conversions of the thiolacetates (266), (269), and (271) into their respective episulfides (267), (270), and (273). 

They also described the reaction of the diacetate (272) with base, to give (273), a reaction that emphasizes the powerful neighboring-group abihty of the thioalkoxide ion to eject acetate ion, a poor leaving-group. The conversion of the epoxide (268) into the episulfide (267) by thiourea was also reported.As was pointed out by Bordwell and Andersen, the 

The first example of a compound that contains an episulfide ring fused to a sugar ring was prepared by treatment of the imns-diaxial thiocyanate 

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Harger has studied the rearrangement of A-substituted N-phosphinoylhydroxylamines in the presence of base . He proposed a concerted mechanism based on the observed retention of the configuration at the phosphorous center during the transposition , and on studies with 0-labelled compounds . Similar cyclic transition states 572 were proposed in the base-induced rearrangement of A,0-bis(diphenylphosphinoyl)hydroxylamines (571) (equation 254). However, in the rearrangement of O-benzoyl-A-(diphenylphosphino-thiol)hydroxylamine where a transposition of O and S atoms occurs, the proposed cyclic transition state has sulfur participation . 

The question of aromaticity arises. Neither thiophenium salts nor thiophene sulfoxides are especially stable, making the classical reactivity test of electrophilic aromatic substitution difficult. The former dealkylate readily and the latter, at least for the case of thiophene sulfoxide, readily undergo self-dimerization (65CCC1158) (the bulky substituents of (57) impede this reaction). Aromaticity requires that the lone pair on sulfur participate in the aromatic sextet. If the lone pair, because of sp3 hybridization and improper symmetry, is not delocalized into the butadiene segment, the system will be antiaromatic. 

Strong transannular sulfur participation is seen in the solvolysis of the p-nitrobenzoate of tetrahydrothiophene-2-methanol (574) the rate is 5.4x 107 times that of the corresponding cyclohexyl derivative. The product is a mixture of (577) and (578), arising through the episulfonium ion (576) (74T2087). The same mixture is also obtained under identical conditions from (575). Even more dramatic is the acetolysis of 2-endo-ch oxo-l -thiabicyclo[2.2.1]heptane, which is accelerated by a factor of 4.7 x 109 compared with the exo-chloro compound. The endo-chloro derivative leads stereospecifically to the endo-product via the sulfonium intermediate (Scheme 239) (75JA2886). 

Table 13 Sulfur Participation in the Solvolysis of rhiacycloalkyl p-Nitrobenzoate Esters in 80% Aqueous Acetone...

Sulfur participates as a donor center in sulfide complexes (Muller and Diemann, p. 515), dithiolenes (Miiller-Westerhoff and Vance, p. 595), and other S-containing ligands (thiosulfates, thiourea, mono-, and dithioketones) (Livingstone, p. 633). Some other complex compounds with Se and Te as donor centers have been described (Berry, p. 661). 

In the rearrangement, the alcohol is protonated as before but no sulfur participation occurs. Instead, a ring expansion, also assisted by sulfur, produces a four-membered ring and hydrolysis of the a cation (an intermediate you have seen several times) gives a cyclobutanone. The difference between participation through space and C=S+ bond formation is not that great,... 

Sulfur participation in [3,3]-sigmatropic rearrangements allows for a broad variety of unique and synthetically useful transformations that often take place under mild conditions. We hope this article will encourage researchers to continue the development of new processes and applications within this field. 

On the other hand, there is the well-known fact that sulfur is an excellent carbanion stabilizer. But not for those reasons popularly ascribed to dir-pir backbonding of the carbanion lone pair into the vacant 3d orbital of sulfur, as suggested earlier. Recent molecular orbital calculations appear to challenge the validity of this model. These calculations, for example, predict the order of gas phase (no solvent effects to worry about) carbanion stabilization as sulfur > oxygen > carbon whether or not the 3d orbitals of sulfur participate in the calculations. Besides, these theoretical considerations surprisingly show that the C-S bond in the thiomethylene carbanion is longer than in the neutral thio-methane parent. Until recently, the prevailing mechanism of stabilization was centered around the classical concept of polarizability of the electron cloud around the sulfur nucleus. 

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