Sulfur bond cleavages, transition metal

There are few reports of the transition metal complex-catalyzed isomerization of S-allyl sulfides and sulfones. This is clearly a consequence of the very strong coordinating ability of sulfur atoms and the resulting tendency for S-C (allyl) bond cleavage. In the case of a bulky substituent being present at the sulfur atom, the isomerization to 1-propenyl derivatives is successful (Eqs. 12.14 and 12.15) [20]. 

In another reaction of [Mo3S4] +, its aqua complex [Mo3S4(H20)9] + reacts with acetylene at room temperature in I m HCl to yield complex 61, according to Scheme 20. Here a bond is formed between a carbon atom within a hydrocarbon molecule and a sulfur atom of a metal-sulfide cluster. The adduct can be viewed as representing the transition state or intermediate presumed to be formed when an organosulfur molecule reacts with a sulfide vacancy of a heterogeneous catalyst (cf. Section 2.6.4) or a cluster. However, whereas the reaction depicted in Scheme 20 is the first step of a sulfurization process (C-S bond formation), the direction of the conversion would have to be reversed to make it the second step of a desulfurization process (C-S bond cleavage). 

Aromatic organosulfur compounds such as thiophenes, benzothiophenes and dibenzothiophenes are frequently contained in fossil oil and their sulfur atoms are generally difficult to remove in HDS process [106], In the industrial HDS process, Mo/Co/S or Ni/Mo/S heterogeneous catalysts supported on alumina are widely employed. In order to obtain ideas to develop more efficient catalysts as well as to shed some light on their mechanisms at a molecular level, transition metal complex-mediated cleavages of C-S bond are extensively studied. On the other hand, thiiranes and thietanes are frequently employed for preparation of transition metal sulfides, in which their C-S bonds are smoothly cleaved. In this section, the C-S bond cleavages of thiophene derivatives, thiiranes, thietanes, vinylic sulfides, allylic sulfides, thiols and dithioacetals are overviewed. 

Transition metal complexes usually favor insertion into the carbon-sulfur bond adjacent to C(alkenyl)-S bond (type a), but some complexes insert into the C(aryl)-S bond (type b) [117]. Ab initio calculations suggest that the most important controlling factor for determining selectivity in the C-S bond cleavage of benzothiophene is the thermodynamic stability of the resulting metal-carbon bond [118],... 

The dominant theme since the mid-1980s in the chemistry of thiophenes has undoubtedly been the application of transition metal catalysis. There are two distinct aspects of this work the first is the use of Pd or Ni as catalysts for bond formation. The other is the study of transition metal-mediated ring opening of thiophene by cleavage of S—C bonds. The former is discussed in Section 2.10.4.7. In addition, some mechanistically interesting results have also been reported during this period. These include the preparation of the first fully characterized, stable pentacoordinated sulfur compound with four C—S bonds (Section 2.10.3.1) and the first clear evidence for an organosulfur ate complex (Section 2.10.2.6.4). 

The reaction of alkyl aryl sulfides with diaryliodonium salts generated unsymmetrical diaryl sulfides in moderate to excellent yields through cleavage of the sulfur-carbon(sp ) bond and formation of a new sulfur-carbon(sp ) bond (Scheme 5.38) [60]. The synthesis was transition metal free and tolerated a host of different electron-withdrawing and donating groups. Another noteworthy aspect of the chemistry was that it was successful in an acidic environment. This renders the work complementary to the transition metal-catalyzed routes that are typically carried out under basic conditions. In related work, the... 

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