Sulfur insertions

Sulfur insertion into thiols RSH to give symmetrical trisulfides in high purity is easily accomplished by diimidazolylsulfide (reaction conditions hexane, 0-25 °C, 0.5-2 h).[34]... 

Diimidazolyldisulfide, dibenzimidazolylsulfide and -disulfide, ditriazolylsulfide and ditriazolyldisulfide as well as the corresponding dibenzo derivatives have also been used for sulfur insertions 151... 

A more complex example of sulfur insertion is the formation of a bithiopyran (4) from an acyclic precursor (5) when treated with a mixture of hydrogen sulfide and hydrogen chloride.7... 

This review will not discuss the large body of literature concerning thiourea-induced sulfur insertions giving thiiranes from epoxides5 and from haloalcohols.6... 

The formation of this complex may arise from sulfur insertion into the M-C bond of the corresponding thiocarboxamido complex (183). 

Since the first report of the sulfur-addition reaction to 1,1-dithio complexes (156), a number of structural studies have verified the originally proposed ring expansion of the MS2C ring following this reaction. In addition, mechanistic studies on the sulfur insertion reaction and electronic spectral studies of the perthiocarboxylate Ni(II) complexes have been reported. 

The question of whether sulfur insertion occurs at the C-S or the M- S bond was raised following the first isolation of the perthio-l,l-dithiolate complexes (156). Early studies of the sulfur-addition and sulfur-abstraction reactions using 33S were reported for Ni(CS3)2" (Eq. 54). It was concluded that the added sulfur was inserted into the C -S bond (156). 

On the other hand, when di-terf-butyltetrachlorodigermane (71) is reacted with lithium sulfide at —78°C, the corresponding bis-nor-adamantane 72 is formed along with pen-tasulfide (64a) (Scheme 19). Presumably, due to the long Ge—Ge bond distance, 72 is less strained than the silicon analogues 69 and 70, and thus sulfur insertion is less favored. 

Preparative Methods the title compound can be prepared by reaction of (R)-2-[l-(dimethylamino)ethyl]phenyllithium with elemental sulfur (eq 1). A solution of pure (R)-2-[l-(dimethyl-amino)ethyl]phenyllithium in THF is slowly added at —50°C to a suspension of a stoichiometric amount of freshly sublimed sulfur. The solution is warmed to room temperature and quenched with an equimolar amount of a 10 M aqueous HCl solution. All volatiles are evaporated at reduced pressure and the residue is sublimed at 120 °C in vacuo (0.1 mmHg). The nitrogen-functionalized derivatives (R)-2-[l-(l-pyrrolidinyl) ethyljbenzenethiol and (R)-2-[l-(l-piperidinyl)ethyl]benzen-ethioP may be prepared in a similar way. It should be noted that reaction with MesSiCl instead of HCl after the sulfur insertion reaction affords the corresponding trimethylsilyl thio ether, which also is a valuable catalyst precursor. ... 

The deuterium isotope effect on the rate of the insertion process was studied using perdeuteropropane, 1,1,1,3,3,3-propane-de and 2,2-pro-pane-d2- No change in the rate of insertion, upon isotopic substitution, was found i.e., kn ll)/ku, (D) — 1.0, within experimental error. This is very significant in view of the fact that the activation energy for sulfur insertion appears to be at least as large as that for methylene insertion, and in the latter case, an H/D isotope effect of ca. 1.5-2.0 has been reported. ... 

The formation of the thione complex 211 involves the coupling of two thioformaldehyde units, addition to the carbyne carbon of 210 to form a thiocarbene ligand, sulfur insertion into the metal-carbene bond, and... 

Synthesis via Sulfur Insertion into Metal-Carbon Bonds... 

Sulfur reacts with hexamethylsilacyclopropane in tetrahydrofuran (THF) to give 2,3-dithia-l-silacyclopentane in 38% yield <79JOM(164)305>. Exposure of rranj-1,1-di-r-butyl-2,3-dimethyl-silacyclopropane (47) to elemental sulfur at room temperature for 3-6 h gave trans-1,1 -di-i-butyl-1,2-silathietane (48) and rra s-3,3-di-r-butyl-4,5-dimethyl-l,2-dithia-3-silacyclopentane (49) in 60% and 19% yield respectively (Equation (10)). Identical results were obtained with the m-silacyclo-propane, suggesting a stepwise radical mechanism <900M2205>. The presence of r-butyl substituents on silicon was a key factor in this reaction, since sulfur insertion cannot be stopped at a single atom when there are small substituents on silicon <79JOM(164)305>. 

In 1994, it was found that the rate and selectivity of both one- and two-atom sulfur insertions could be dramatically affected by the addition of salts and crown ethers. Thus, reaction of trans-silacyclopropane (47) with sulfur in the presence of 10% potassium fluoride and 10% 18-crown-6 gave thiasiletane (48) exclusively, in quantitative yield, in under 1 min. By contrast, treatment of (47) with sulfur in the presence of 10% tetrabutylammonium chloride for 3 min yielded only silacyclopentane (49) (Scheme 9) <940M37i5>. 

A transamination reaction, using S-adenosylmethionine as the amine donor, gives 143. Urea formation, followed by sulfur insertion into the C-IH and the C-4H bonds gives biotin. 

The sulfur insertion occurs with scrambling of stereochemistry at C-1 and with retention at C-4 [135,136]. The protons at C-2 and at C-3 are not lost during the reaction. 1-Mercaptodethiobiotin may be an intermediate [137,138]. In the reconstituted system, sulfur from cysteine or SAM was not incorporated into biotin [132] suggesting that the iron sulfur cluster is the sulfur source. 

This reaction is analogous to the sulfur insertion reaction involved in biotin biosynthesis, and LipA and BioB show significant sequence similarity [145,146]. 8-Mercapto-octanoic acid and 6-mercapto-octanoic acid are intermediates... 

The sulfur insertion into the C-6H bond occurs with inversion of stereochemistry [ 149]. Cysteine is the sulfur source [150]. Lipoic acid synthesis in a defined cell free system has not yet been accomplished. 

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