Inversion at sulfur

Tricoordmate sulfur compounds are chiral when sulfur bears three different sub stituents The rate of pyramidal inversion at sulfur is rather slow The most common compounds m which sulfur is a chirality center are sulfoxides such as... 

Both thermal- and acid-induced equilibrations of 3,3-disubstituted thietane oxides were very slow 10 s ) . The results suggest that thietane oxides are similar to the various acyclic sulfoxides with respect to the rates of thermally induced pyramidal inversion at sulfur , and that this inversion process, therefore, does not interfere significantly in the above exchange/racemization studies. 

The participation of sulfoxy groups can be used to control the stereochemistry in acyclic systems. In the reaction shown below, the internal sulfoxide captures the bromonium ion and then undergoes inversion at sulfur in the hydrolytic step. 

Inversion at sulfur (also selenium and, occasionally, tellurium) coordinated to Pt(II), and indeed to a number of other transition metal centers, e.g., W(0), Re(I), Rh(III), and especially Pt(IV), has been studied extensively over several decades (246). Observation of the kinetics of such processes is often complicated by concurrent changes elsewhere in the complex, for example by hindered rotation about C-S in the square-planar complexes [(XS)2Pt(p-SX)2Pt(SX)2]2 with X = C6F4H, CgF5, C6F4(4-CF3). Resolution of the observed kinetics here indicated barriers (AG ) of between 54 and 59kJmol-1 for inversion at coordinated sulfur, between 40 and 60 kJ mol-1 for the hindered rotation (247). 

Between 236 and 237, the two other sulfurane intermediates that may be responsible for inversion at sulfur, 237 appears to be the more probable candidate for two reasons. The first is that the ethoxy and hydroxy groups occupy the proper apical positions in accord with their Idgh apicophilicities, and the lone electron pair is placed equatorially, consistent with all the available structural data on sulfuranes. Second, the four-membered ring in a stable cyclic difluorosulfurane 238 was found by Denney et al. (287) to occupy a diequatorial position. 

Bromination of (-)-(5 )-benzyl methyl sulfoxide 34 with bromine in pyridine affords a mixture of two regioisomers a-bromomethyl benzyl sulfoxide 328 and a-bromobenzyl methyl sulfoxide 329, in a molar ratio of 3 2 (325). Oxidation of thp latter gives the corresponding sulfone (-)-(S)-330, the absolute configuration of which was determined by X-ray analysis. In this context, it is interesting to point out that the formation of the sulfoxide 328 is accompanied by retention at sulfur, whereas 329 is formed with inversion at sulfur. 

C7-C3 1.3989 Barrier to inversion at sulfur 234 kJ mol-1 Gross atomic charges C5-C4-C3 carbon plane-S1 121.6 10.6... 

New possibilities were reported by Kagan [529], who observed that vinyl sulfoxides could be activated for the Diels-Alder reaction by alkylation into vinyl sulfonium salts. A number of cycloadditions of these salts occurred then under mild conditions and with high stereoselectivities. The results could be particularly striking, as in the case of the low-temperature (-78°C) reaction of ethoxy p-tolyl vinyl sulfonium tetrafluoroborate (2) with cyclopentadiene. Only the endo isomer (3) was formed with a de. higher than 99%. and isolated in a 62% yield after transformation of the sulfonium adduct into a sulfoxide (with inversion at sulfur) by sodium hydroxide. 

Related SR clusters Os3(/z-SR)2(CO)10, synthesized by oxidative addition of R2S2 to Os3(CO)u(MeCN) exist as a single isomer with nonequivalent SR ligands when first formed. This isomer requires heating to 95°C to give an isomer with equivalent SR groups, which implies that there are much higher barriers to inversion at sulfur than is normal. These isomers are believed to be 15 and 16, respectively (93). 

Sulfur-lithium exchange is easier and has much greater potential (much of it still unrealised) when the sulfur is at the sulfoxide oxidation level. It has long been known that organolithiums, like Grignard reagents, will attack a sulfoxide, displacing with inversion at sulfur the substituent best able to support an anion. The reaction has been commonly used to form sulfoxides with defined stereochemistry 152157... 

Grignard reagent displaces menthol with inversion at sulfur (SN2) to give sulfoxide as a single enantiomer... 

Dynamic behavior in solution is revealed by variable temperature NMR measurements and is primarily due to inversion at sulfur. The trans-anti and trans-syn isomers are the major species in the solution of the nickel complex. In solutions of Pd and Pt complexes, the cis-anti isomers are also detectable. The cis-anti isomer of the Pt complex has been isolated and is more stable than that of the Pd analogue (64). Hypothetically, a fourth isomer, the cis-syn isomer, may also exist. However, this species may not be stable even at temperatures as low as —50 °C. Inspection of the crystal structure of the trans-anti isomer... 

Preparative Methods prepared from 2-bromo-2-cyclopenten-l-one in three steps (eq 1), involving a key reaction of the vinyl-lithium and (5)-menthyl p-toluenesulfinate which proceeds with complete inversion at sulfur. ... 

Enantiomerically pure sulfonium salts can undergo racemization, though the process is not usually rapid. Three processes have been implicated in racemization these are (1) reversible dissociation (Scheme 5.3a), similar to that suggested for the racemization of tetraalkylammonium salts (Section 5.6) (2) pyramidal inversion at sulfur, analogous to nitrogen inversion in amines and (3) reversible dissociation into a carboca-tion and a sulfide (Scheme 5.3b) (see Andersen10). 

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