Sulfur, pentacoordinated

Oae found that for both base- and acid-catalyzed hydrolysis of phenyl benzenesul-fonate, there was no incorporation of 0 from solvent into the sulfonate ester after partial hydrolysis. This was interpreted as ruling out a stepwise mechanism, but in fact it could be stepwise with slow pseudorotation. In fact this nonexchange can be explained by Westheimer s rules for pseudorotation, assuming the same rules apply to pentacoordinate sulfur. For the acid-catalyzed reaction, the likely intermediate would be 8 for which pseudorotation would be disfavored because it would put a carbon at an apical position. Further protonation to the cationic intermediate is unlikely even in lOM HCl (the medium for Oae s experiments) because of the high acidity of this species a Branch and Calvin calculation (See Appendix), supplemented by allowance for the effect of the phenyl groups (taken as the difference in between sulfuric acid and benzenesulfonic acid ), leads to a pA, of -7 for the first pisTa of this cation about -2 for the second p/sTa. and about 3 for the third Thus, protonation by aqueous HCl to give the neutral intermediate is likely but further protonation to give cation 9 would be very unlikely. 

Although there have been few reports on the chemistry of transient silanethiones,113 some stable examples have been successfully isolated by thermodynamic stabilization. In 1989, Corriu et al. reported the first synthesis of an isolable silanethione 31a (mp 170-171°C) by the reactions of the pentacoordinated functionalized silane 30a with elemental sulfur or carbon disulfide (Scheme 8).17... 

Corriu et al. also described an alternative synthetic method for internally coordinated silanethiones starting from the pentacoordinated diaminosilanes.28 As shown in Scheme 9, the pentacoordinated diaminosilanes 32 are allowed to react with sulfur-containing heterocumulenes such as carbon disulfide or phenyl isothiocyanate to give the corresponding insertion products 33, which undergo thermal decomposition to produce the corresponding silanethiones 31, 34, and 35.28... 

Typical values at 4.2 K. Isomer shifts are taken relative to the standard value for Fe metal at 298 K. b Fe—(O, N) hexacoordinate or pentacoordinate sites. c Fe-S tetrahedral sulfur ligation. 

The literature on these structures is sparse, and generally the articles report preparations of several of them in similar ways. These compounds are constituted by oxygen, sulfur, or nitrogen adducts at the silicon atom, thus providing pentacoordinated silicon compounds or structural analogs (Scheme 33). 

Tetracoordinate sulfur compounds containing a lone pair of electrons at sulfur possess a more or less distorted trigonal-bipyramidal structure, in common with the vast majority of other pentacoordinated molecules of the main group elements (189,191,199). A common name, sulfurane, is generally accepted for this type of compound. In principle, sulfuranes are chiral. However, both the number of optically active isomers and their optical stability depend on the nature of substituents bonded to the central sulfur atom, the apicophilicity of the substituents, and the energy required for permutational isomerization processes. In this context it is interesting to note that acyclic sulfuranes with four different ligands should exist in 20 isomeric forms. 

Since the first reaction undoubtedly proceeds with inversion of configuration at sulfur, and since additional experiments demonstrated that the formation of chlorosulfurane 177 from sulfoxide 180 takes place with retention at sulfur, (5>chirality was assigned to (+>177. As for the designation of absolute configuration, Martin and Balthazor (195) proposed a system of nomenclature for optically active pentacoordinate species. 

We hope that this review of chiral sulfur compounds will be useful to chemists interested in various aspects of chemistry and stereochemistry. The facts and problems discussed provide numerous possibilities for the study of additional stereochemical phenomena at sulfur. As a consequence of the extent of recent research on the application of oiganosulfur compounds in synthesis, further developments in the field of sulfur stereochemistry and especially in the area of asymmetric synthesis may be expected. Looking to the future, it may be said that the static and dynamic stereochemistry of tetra- and pentacoordinate trigonal-bipyramidal sulfur compounds will be and should be the subject of further studies. Similarly, more investigations will be needed to clarify the complex nature of nucleophilic substitution at tri- and tetracoordinate sulfur. Finally, we note that this chapter was intended to be illustrative, not exhaustive therefore, we apologize to the authors whose important work could not be included. 

The reaction is initiated by the attack of the organolithium at sulfur to produce the pentacoordinate sulfur species. This seems to be the prime reaction of all electron-deficient sulfur compounds with organolithiums (Eq. 45). 

The intermediacy of cr-sulfurane, with a pentacoordinate sulfur, has been invoked to explain the action of alkyl- and aryl-lithium on S-aryldibenzothiophenium ions (Scheme 26) (73JA5288, 71JA6077, 74CPB2020). 

Recent use of phosphine ligands in the areas of catalysis,3,4 pentacoordination,5,6 and oxidative-addition reactions7 has prompted investigations into variations of donor basicity, chelate chain length, and mixed sets of donor atoms. Thus, relatively accessible synthetic routes to chelating aliphatic phosphines are needed. Removal of sulfur from a diphosphine disulfide8 and subsequent preparation of NaPR2 in liquid ammonia provides a useful laboratory route to aliphatic phosphines. For example,... 

The pentacoordinated indium(m) compound 13 has a quasi-square-pyramidal environment rather than trigonal bipyramidal with a short indium-axial iodine bond distance (2.713 A) as compared to those in equatorial positions (2.780 A) <1998EJI203>. An example of pseudooctahedral coordination around an indium(m) center can be given by the [(bipy)InCl(SC 0 Ph)2] structure in which the indium center is coordinated to a planar bipyridine ligand, a chloride ion, and two thiobenzoate anions <2002MI467>. Of the latter, one thiobenzoate ligand binds in a bidentate fashion, while the other is bonded mainly through the sulfur atom. 

The reaction of Ph2P(CH2)2SnCl3 with nitrogen monoxide in acetone or sulfur in dichloromethane results in the formation of pentacoordinated chelates, which can form dimeric hexacoordinated structures in the solid state (Equation 7) <2002JOM25>. 

Fu H, Xu JH, Wang RJ et al (2003) Synthesis, crystal structure, and diastereomeric transfer of pentacoordinated phosphoranes containing valine or iso-leucine residue. Phosphorus Sulfur Silicon Relat Elem 178 1963-1971... 

Pentacoordinate centers are mainly exemplified by phosphorus, whereas some pentacoordinate sulfur derivatives exist, and a few other elements can be envisaged [36], In the case of phosphorus derivatives (phosphoranes), the trigonal-bipyramidal arrangement is the low-energy geometry, while interconversion of isomers occurs by pseudorotation through the tetragonal-pyramidal transition state [23,36]. 

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