Thiolates, replacement

At room temperature CuCN is not soluble in most solvents. Therefore, the direct reaction of CuCN with PPh3 in DMF results in the formation of an amorphous precipitate that is not soluble in any solvent. But when 2,2 -bipyridine is introduced into the reaction system of CuCN and PPh3 in DMSO, a red solution is obtained. By slowly diffusing diethyl ether into this solution, red block crystals of the complex (CuCN)2(2,2-bipyridine)(PPh3) are obtained. In contrast, when pyridine-2-thiolate replaces 2,2 -bipyridine, the complex CuCN(PPh3)2 6 is formed instead. ... 

Various products were obtained from the reaction of sodium thiolate with the acetylene derivative HC CCMeHalCOgEt (Hal = Cl,Br), where the thiolate replaced the halogen, acted as a reducing agent or added across an acetylenic or ethylenic bond . 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Several iron sulfide nitrosyl compounds are known. These have stmctures that in some cases are formally related to the FeS clusters by replacement of thiolate by NO. The compounds include the anions [Fe2S2(NO)4] and [Fe4S2(NO)2] (Roussin s red and black salts, respectively) and the neutral compounds [Fe2S2(NO)4] and [Fe4S4(NO)4]. Roussin s black salt has found use as a NO releasing vasodilator. 

Replacement of bromine in ethyl bromofluoroacetate by thiolate anions occurs easily [44] (equation 40)... 

Replacement of iodine in (perfluoroalkyl)ethyl iodides predominates over the usual conversion to olefins when the reagent is very nucleophilic and weakly basic Soft nucleophiles like sodium thiocyanate and sodium thiolates react well in displacements [46, 47] (equation 42)... 

The SfjAr reaction between thiolates and monofluorobenzme and its derivatives requires high temperatures and polar aprotic solvents [19, 20, 21. Polyfluoroaro-matics show very little selectivity for fluoiine replacement [22, 2J] (equation 13). 

Fluorine replacement m perfluorobenzene can be achieved in good yields and high selectivity when heavy metal thiolates are used as nucleophiles [29, 30, 31, 32] (equations 19-21). 

The halides in mer-OsCl3(PR3)3 can be replaced by a thiolate ligand to give paramagnetic Os(SC6F5)3(PR3)2 an agostic Os—F—C interaction is believed to complete the coordination sphere of osmium [156]. 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

The Mo(III) d3-d3 complexes [CpMo(dithiolene)]2 are characterized by a single Mo—Mo bond, further stabilized by interaction with the n system of the dithiolene ligands. Indeed, the analogous complexes where the two dithiolene are replaced by four thiolate groups were found to oxidize more easily and salts of the cationic d3-d2 [CpMo(SMe)4MoCp]+ were even isolated and structurally and magnetically characterized [50]. 

Pyrimidine thioethers may also be synthesized via direct Pd-catalyzed C—S bond formation between halopyrimidines and thiolate anions. For very unreactive thiol nucleophiles such as 2-thiopyrimidine, both a strong base and a palladium catalyst are essential. Without a palladium catalyst or replacing f-BuONa with K2C03, the reaction failed to furnish the desired pyrimidine thioether [49]. 

Scheme 25 Example of thiolation (T) domains in type I NRPS. Nostocyclopeptide is shown as a representative nonribosomal peptide. Seven T domains are found in the seven modules, each depicted in a different color, responsible for the incorporation of the seven amino acid building block constituents of nostocyclopeptide. A reductase (R) domain replaces the more common TE domain for the formation of the unusual imine functionality.

The photoirradiation effect can be replaced by copper salt catalysis. The catalyzed reactions proceed rapidly and result in a high degree of transformation. Interestingly, ESR method reveals no organic paramagnetic particles in the course of the reaction between haloaryls and phenyl thiolates. The addition of oxidants (oxygen and DNB) or radical acceptors (di(tert-butyl)nitroxide) does not inhibit the substitution. These facts are understandable from Scheme 7.68 (Bowman et al. 1984, Liedholm 1984). 

The nature of the bridging thiolate ligands or the replacement of a terminal chloride ligand by water did not have much effect on the catalytic activity, complexes 105b-d and 106a,b being also operative in these transformations. In contrast, conventional monometallic ruthenium derivatives, as well as diruthenium complexes having no Ru-Ru bond, did not work at all. 

Malathion and parathion contain a P=S grouping, exemplifying a further carbonyl analogue, in which phosphorus replaces carbon, and sulfur replaces oxygen. Nevertheless, the same type of chemistry occurs, in which the serine hydroxyl of the insect s acetylcholinesterase attacks this P=S electrophile, followed by expulsion of the leaving group, here a thiolate. The esterified enzyme, however, is not hydrolysed back to the original form of the... 

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