Hypervalent sulfur

However, on account of the particularly short C-S distances, multiple bonds (probably of the d-si-p-si type) between hypervalent sulfur and carbon occur in a number of species. 

Hexacoordinate, hypervalent sulfur fluorides have an octahedral geometry that is symmetrical for SF6, which appears as a sharp singlet at +56 ppm, but which has magnetically nonequivalent (axial and equatorial, ab4 system) fluorines for compounds of the structure R-SF5. Compounds with the general structure R-SF4-X can exist as cis- and trans-isomers, the former having three types of fluorine, and the latter only one (Scheme 7.17). 

Crystalline trithia dication 151 contains a 3-ccntcrA-clcctron bond with a central hypervalent sulfur atom. It was shown to be sufficiently stable to be isolated and studied spectroscopically. One important feature is its boat-boat conformation resulting from the formation of the transannular bond between the three sulfur atoms <1988JA1280>. 

The next debate in the literature was whether these molecules have C2v or Cs symmetry. The nuclear motion of a C2v symmetric structure would be described by a single-well potential (see Figure 10). The alternative is a rapid interconversion of two valence tautomers, each of Cs symmetry. This would occur via the C2v structure as transition state (see Figure 11). In this case the motion of the central sulfur would be described by a double-well potential, and dioxathiapentalene and trithiapentalene would be misnomers for (3//-l,2-oxathiol-3-ylidene)acetaldehyde 180 and (3/7-1,2-dithiol-3-ylidene)thioacetaldehyde 181. One advantage of C2v symmetry is aromatic stabilization from the 1071 electrons <2001CRV1247>. The alternative Cs symmetry has the advantage of avoiding a hypervalent sulfur. 

Heterocycles with Hypervalent Sulfur or Selenium at the 5 5 Ring Junction 847... 

Although most of the fundamental studies of cycl[3.2.2]azines were reported in CHEC(1984) (see Section 12.16.6.3 for leading references), there is continuing interest - synthetic, spectroscopic, and theoretical - in these and their benzo- and dibenzo-fused analogues, all of which may be considered as bridged analogues of [10]-, [14]-, and [18]-annulenes, respectively. The same level of theoretical interest continues to apply to those />m -fused systems with a hypervalent sulfur or selenium at the 5 5 ring junction (Section 12.16.6.6). 

Transformation of the thiadiazolopyrimidine compound 138 to the fused dithiazole 140 also follows a fairly complicated pathway <2004JHC99>. When the derivative 138 is treated with carbon disulfide, a cyclization reminiscent of 1,3-dipolar cyclization takes place with the reagent acting as a dipolarophile to give a />OT-fused tricyclic intermediate containing a hypervalent sulfur atom 139. This intermediate can undergo isothiocyanate elimination to furnish 140. It is interesting to note that the sulfur atom of the thione moiety in the product is derived from carbon disulfide. 

The condensation of 5-amino-3-methyl-l,2,4-thiadiazole (118) with aliphatic or aromatic nitriles yields 1 1 adducts, which are, according to their H NMR spectra, equilibrium mixtures of (119) and (120) (Scheme 28) <82AHC(32)285>. These adducts are produced by a bond switch at the n-hypervalent sulfur in (121). X-ray analysis of the adduct formed from the reaction of (118) with chloroacetonitrile showed the adduct to exist as (122) in the crystals <81AX(B)185>. Further examples of this type of bond switch at rc-hypervalent sulfur are observed in the reaction of 5-imino-1,2,4-thiadiazolines with various electrophilic reagents (Section 4.08.6.1). 

Halogen abstraction from hypervalent sulfur halides has also been reported for the imidazol-2-ylidenes (IV) (Scheme 8.22). This reaction gives a nice example of the synthetic utihty of V-heterocyclic carbenes. Indeed, this adduct is the first structurally characterized derivative featuring the chlorosulfite ion (SO2CI ). ... 

The 1,2,4-thiadiazolidine (369) and the 1,2,4-dithiazolidine (370) are interconvertible in the presence of electrophilic nitriles and give the l,2,4-thiadiazoline-5-ones (371) as products (Scheme 61) (91JOC3268). It is suggested that the reaction goes by a consecutive cycloaddition - elimination mechanism via hypervalent sulfur intermediates in which the nitrile approaches in the plane of the heterocycle. 

The reversible formation of a monoanionic trigonal bipyrimidal mtennediate with hypervalent sulfur (325) has been supported in the alkaline hydrolysis of the /J-sultam (326).297 A second deprotonation by hydroxide takes place to give (327) before the... 

The reactions of thiadiazolopyrimidines 58 (R = Me, allyl) with ethoxycarbonyl isothiocyanate and CS2 gave heterocycle 59 via thermal decomposition of 1 1 cycloadducts, which have a hypervalent sulfur (Equation 10). The mechanistic and reactivity features of these reactions were described <2004JHC99>. 

Reaction kinetics for the interaction of 5-alkyliminothiatriazoles 52 or 58 with heterocumulenes, nitriles, ketones, imines, or other dipolarophiles a=b show that the decomposition of the thiatriazole is bimolecular, and new heterocyclic five-membered rings 71 are formed (Scheme 15). The term masked 1,3-dipolar cycloaddition was used by L abbe and co-workers for this type of reaction <1978JOC4951>, the thioimidate function being the masked 1,3-dipole. The reaction is thought to involve a thiapentalenic intermediate 70 with hypervalent sulfur. The product 71 is itself a masked dipole and often further reactions take place. 

Reduction of dinitrobenzothiadiazoles 280 with iron dust in acetic acid gave diamines 281 (Scheme 39). The reaction of diamines 281 with selenium dioxide gave [l,2,5]selenadiazo[4,5-c]-2,l,3-benzothiazole derivatives containing a hypervalent sulfur atom 82 and 83 in 40% and 82% yields, respectively <1997T10169>. 

This alkaline hydrolysis shows a rate term that is second order in hydroxide ion concentration, which is indicative of a stepwise mechanism involving a TBPI with a hypervalent sulfur atom. Reversible attack of the hydroxide ion on a /3-sultam generates a monoanionic TBPI-, which requires deprotonation by a second hydroxide ion before the intermediate can collapse to products. 

Reaction of a stable carbene with SCI2 results in the hypervalent sulfur compound 41 (Scheme 22).50 The sulfur-NHC bond length of 1.732(3) A is significantly longer than that of a typical C=S double bond (ca. 1.60 A) indicating a highly polarised compound. 

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