Sulfur templates

Diaiyl sulfide templates have also been used to direct chlorinations. The selectivities indicate that the chlorine atom is bound to the sulfur, but the yields are not as good as those with aryl iodide templates. The problem is that the sulfur gets oxidized under the reaction conditions. As expected, a thiophene ring is more stable to oxidation and its sulfur atom can still bind chlorine in a radical relay process." The best sulfur template so far examined is the thioxanthone system (Scheme 20). Thus with 3 equiv. PhICh compound (18) undergoes directed C-9 chlorination in 100% conversion, affording a 71% yield of the A iO-alkene sifter base treatment, along with some polar products from excessive chlorination. The thioxanthone template can be recovered unchanged. 

The. AMBKR force field expects lone pairs to be added to all sulfur atom s an d computes th e in teraction s as if these Ion e pairs were atoms with a specific type just like any oth er atom. I h e templates automatically add the expected lone pairs to sulfur atoms when using the. AMHhIR force field. 

In the following examples, the metal exercises a template effect in the cyclization of 1-methylbilenes and can be easily removed by treatment with acids like trifluoroacetic acid or sulfuric acid. 

As an illustrative example of this method for electrochemical synthesis of sulfide compounds consisfed in utilizing a sulfur-modified mefal surface as a template for fhe elecfrodeposition of mefal sulfide films, Tacconi and Rajeshwar described fhe firsf aflempl fo obfain all-elecfrodeposifed indium sulfide tiiin films, by a dual batii procedure [95] (cf conventional deposition of indium chalcogenides). The... 

Synthesis. The first example of a stable, soluble pz peripherally substituted with a heteroatomic moiety involved appended thioether groups, M[pz(.V-Mc)8, as reported in 1980 by Schramm and Hoffman (2) (Scheme 9, 45-48). Mg[pz(5 -Me)s], 46, was prepared in a 60% yield by a magnesium templated macrocyclization of dinitrile, 45. Demetalation with sulfuric acid gave H2[pz(5 -Me)8], 47 (65%) and remetalation with the anhydrous acetate salts of copper and nickel gave 48 (80%) and 49 (65%). 

Hoff KG, Ta DT, Tap ley TL, et al. 2002. Hsc66 substrate specificity is directed toward a discrete region of the iron-sulfur cluster template protein IscU. J Biol Chem 277 27353-9. 

Several additional proteins identified as necessary for metal-sulfide cluster formation are present in bacteria and in eukaryotes, both in the cytosol and in mitochondria. They may serve as intermediate sulfur carriers, as scaffolds or templates for duster formation, or for insertion of intact Fe-S, Fe-S-Mo, or other types of clusters into proteins468 473f and into 2-selenouridine473g (see also p. 1617). Sulfurtransferases are also thought to be involved in insertion of sulfur atoms into organic molecules such as biotin, lipoic acid, or methanopterin.474... 

New chelate rings can be formed by the nucleophilic addition of alcohols to imine complexes. For example, the nickel(II) TAAB complex is susceptible to attack by bis-alkoxides (equation 31).127 It is not clear whether or not a kinetic template effect operates by prior coordination of the central oxygen or sulfur atom. However, such an effect is not necessary, as simple alkoxides undergo a similar addition reaction.128... 

The situation is even more clear cut in the formation of thioethers by alkylation of thiolate complexes. Such reactions have been described in Section 7.4.2.1.2, but in those examples new chelate rings were not formed. The reaction which led Busch to propose the kinetic template effect is a thiolate complex alkylation, which does indeed produce a new chelate ring (equation 40).110 162 163 This reaction is an example of the general type shown in equation (35) and it has been established clearly that the sulfur atoms remain coordinated to the metal ion throughout the... 

The first derivative of the 1,2,3-dithazepine template 70 was obtained as a minor product (<5% yield) of reaction between bicyclic oxime 67 and S2CI2 in the presence of Hiinig s base (Scheme 8) <1996JOC9178>. The structure of isolated dark-blue compound was confirmed by MS and NMR data. The compound 70 proved to be unstable and readily eliminated sulfur to yield 69. 

To further exemplify this methodology, let us take a typical example of the application of a template reaction as seen in the synthesis of a mixed N2S2 donor macrocyclic ligand 6.11. This compound is of interest to the co-ordination chemist as it possesses a potentially square-planar array of soft (sulfur) and harder (nitrogen) donor atoms. What sort of co-ordination chemistry is it likely to exhibit Will the hard or the soft characteristics dominate The most obvious route for the synthesis of 6.11 would involve the reaction of the dithiol 6.10 with l,2-bis(bromomethyl)benzene (Fig. 6-7). 

In contrast, use of metalloid elements, such as silicon, tin antimony or boron, which can form weak covalent bonds with oxygen, nitrogen or sulfur substituents during the course of the reaction, results in templated products that may be obtained metal-free by simple hydrolysis. These covalent template reactions (the M—X bond is essentially covalent in these cases) also have the advantage that the... 

To conclude this section, we note that an enormous amount of material on the use of the variant of template synthesis, examined in this section, for obtaining heterometallic clusters has been accumulated [499,500]. It is emphasized that the use of clusters as metal ligands is the most suitable approach in order to synthesize such compounds with programmed structure and composition. Especially, many such reactions have been carried out at the expense of sulfur atoms of ligand cluster structures, for instance those reported in Refs. 501 and 502. 

The radical relay process also works with other template types. Thus, the thioether unit in 92 directed chlorination of C-14 by S02C12 [165]. Also, the sulfur in the thiox-anthone template of 93 directed the radical relay process to C-9 [166]. The thiophene sulfur in 94 was able to direct chlorination to C-9 in all three attached steroids [167]. In all these cases, an intermediate is formed with a chlorine atom bonded to sulfur. 

The templates can be simply coordinated rather than attached. For example, complex 100 directed the radical relay chlorination to C-9, although the process was not as clean as with the attached templates [173]. We also used template-directed chlorina-tions to determine the conformations of flexible chains, just as we had previously with the benzophenone probes [174]. Also, by use of a set of tandem free radical chain reactions we could direct the formation of carbon-bromine and carbon-sulfur bonds, again with geometric control by the attached template [175]. 

Isothiazoles are reductively desulfurized by Raney nickel, and this method has been used to confirm structures.3,51 In his remarkable synthesis of colchicine, Woodward employed the isothiazole nucleus as a template on which the various rings were constructed, and finally removed the sulfur with Raney nickel to leave a nitrogen function in the correct position [Eq. (22)].e7,68... 

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