Sulfur atom, insertion

Pathway III of Fig. 26 has been demonstrated for thiophene and benzo-thiophene with Ir complexes (4) and for all thiophenes, including dibenzo-thiophene, with Rh complexes (94, 95). These oxidative additions appear to be influenced by substituents present on the carbon atoms adjacent to the sulfur atom. Insertion between sulfur and the unsubstituted carbon is highly preferred. For 2-methylthiophene the exclusive product is the 1-5 bond insertion product, whereas for 3-methylthiophene, no preference for insertion was observed (1-2 and 1-5 bond insertion products were equal). In competitive studies, thiophene was found to be about twice as reactive as 2,5-dimethylthiophene. This behavior is similar to that observed for relative reaction rates of substituted thiophenes observed with conventional HDS catalysts. Thus steric limitations can occur, even with monomeric, homogeneous catalysts. 

Sulfur atom inserts into the Sb—Si bond in Ph2SbSiMe3 to form Ph2SbSSiMe3 (equation 105 ). 

Mercaptans can be reacted with elemental sulfur at moderate temperatures (50-120°C) in the presence of alkaline catalysts, such as trialkylamines, to produce polysulfides. The first step in the reaction sequence is always disulfide formation (with liberation of H2S). The subsequent insertion of additional sulfur atoms occurs readily. The number of sulfur atoms inserted will depend on the molar ratio of sulfur to mercaptan employed, according to the stoichiometry of the following equations ... 

The methodology of heteroatom-directed lithiation has been applied to the synthesis of a variety of organochalcogen compounds, including unstable, low-valent compounds (Section 15.6). ° The Se N interactions in the diselenide 15.4 activate the Se-Se bond toward insertion of a sulfur atom or a methylene group to give the derivatives 15.21a,... 

Introduction of a sulfur atom into saturated hydrocarbons leads to a considerable decrease, by an order of magnitude, in the yield of radicals [G(radicals) for hydrocarbons 420]. This was explained as due to the existence of low-lying d-orbitals at the sulfur atom. This explanation agrees with the observation that this decrease is smaller in the case of insertion of a SO group, as there are four p electrons of the S atoms which may be excited at the d-orbital for sulfide and only two p electrons in the case of sulfoxides. Additional proof is that for sulfones, where there is no available p electron, the yield of the radicals [G(radicals) = 3.7 for tetramethylene sulfone] is about the same as for hydrocarbons. 

Scheme 19 Synthesis by the insertion reaction of sulfur atoms into the M-C and M-M bond...

In addition to halogen bonded complexes or ionic salts, it is also possible for sulfur and selenium electron donors to form complexes in which the electron donor atom inserts into the X2 bond, giving a hypervalent donor atom with a T-shaped geometry. It has been recently reported [147] that for dibromine and selenium, this type of complex is favored over halogen bonded complexes. While no purely halogen bonded complex is reported for dibromine, there is one complex (IRABEI) in which one selenium atom of each of several selenanthrene molecules in the asymmetric unit does insert into a Br2 bond, but for one of the molecules, the other selenium atom forms a halogen bond with a Br2 molecule to form a simple adduct (A). 

Regular pyrimidines are less effective ligands for Ni11 ions. They may use, inter alia, their C=0 donor to yield monodentate coordination.1835 Insertion of a sulfur atom into a pyrimidine moiety increases considerably its binding ability.1836 Thiolation of uridine at C(2) or C(4) results in formation of a quite effective S,N3 four-membered chelate in the complexes with Ni11. Thiolation of purine at C(6) increases the stability constant by 3.5 orders of magnitude. 

Hampden-Smith and co-workers have prepared [Zn(SEt)Et]10 (Figure 30) by the insertion of sulfur into the Zn—C bond of diethylzinc. 7 Although this decameric thiolate possesses an arrangement of zinc and sulfur atoms similar to that found in wurtzite, pyrolysis of the material at 250 °C led to predominantly cubic ZnS. Cubic ZnS was also formed when the precursor is used in a spray CVD process. 

The S02 molecule has unshared pairs of electrons on both the sulfur and oxygen atoms. As a result, it forms numerous complexes with transitions metals in which it is known to attach in several ways. These include bonding through the sulfur atom, through an oxygen atom, by both oxygen atoms, and various bridging schemes. In most cases, the complexes involve soft metals in low oxidation states. Another important reaction of sulfur dioxide is known as the insertion reaction, in which it is placed... 

Conformation and stability of thiopeptides (formed by replacing the amide oxygen atom with a sp2 sulfur atom) have been investigated by calculations. It has been confirmed that insertion of a thioamide linkage into a peptide structure is not conformationally neutral. It is predicted to be more rigid than peptide and produces substantial changes in peptide structure, primarily in the residues on the C-terminal side of the thioamide.78 80... 

In the diagram shown in Figure 6.9, a sulfur atom has been inserted at the cluster s center in agreement with current thinking.24 The M center is attached to protein side chains at only two locations, cysteine 275 (at Fel) and histidine 442 (at Mo), and is unusual in having three-coordinate irons rather than the normal four-coordinate tetrahedral iron coordination sphere.29b The molybdenum forms... 

A one-pot synthesis of 3,3-disubstituted indolines was achieved by taking advantage of a sequential carbopalladation of allene, nucleophile attack, intramolecular insertion of an olefm and termination with NaBPh4 (Scheme 16.6) [10]. First, a Pd(0) species reacts with iodothiophene selectively to afford ArPdl, probably because the oxidative addition step is facilitated by coordination with the adjacent sulfur atom. Second, the ArPdl adds to allene, giving a Jt-allylpalladium complex, which is captured by a 2-iodoaniline derivative to afford an isolable allylic compound. Under more severe conditions, the oxidative addition of iodide to Pd(0) followed by the insertion of an internal olefm takes place to give an alkylpalladium complex, which is transmetallated with NaBPh4 to release the product. 

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