Carbon-sulfur bond breaking

In the first reaction step, carbon-sulfur bond breaking occurs under very mild conditions (50 °C, 12 h). The / -sulfido /r3-alkylidyne cluster formed is then, in a second step, under slightly more severe conditions (80 °C, 7 days, (H2) = 7.2 atm) hydrogenated to form 54 and ethylbenzene in 84% yield. Also the very refractory... 

Carbon-sulfur bond breaking was observed while treating thiophene with Cp"Co(C2H4)2 at 70 °C in benzene. A cobalt metal center had inserted into the C-S bond and a second Cp Co unit had complexed to the cobalt, sulfur, and C-C double bond 330. Further reaction of the resulting compound with H2S produced a new butadiene dithiolate compound, in which a sulfur atom has been inserted into the metal-carbon bond 331. Reaction with dibenzothiophene produced an analogous dinuclear C-S insertion-reaction adduct (Equation (54)). ... 

The cleavage of a disulfide without the breaking of a carbon sulfur bond is possible over a palladium catalyst in aqueous acid at room temperature and atmospheric pressure. Good yields of the mercaptans have been obtained in this way 26" 178... 

In this last example, the flow starts with the electron-rich sulfur anion and forms a carbon-sulfur bond with the CH2 group. The pi bond breaks and forms a new pi bond. The flow finishes by breaking the carbon-oxygen pi bond and forming a new lone pair on the electronegative oxygen. Every time an arrow forms a bond to an atom that already has a complete octet, another bond to that atom must break, so the octet is not exceeded. 

In most cases direct evidence for actual devulcanization, i.e., breaking sulfur-sulfur and carbon-sulfur bonds without polymer chain scission, is lacking. However, in many instances the so-called devulcanization process increases the suitability for reuse (Fig. 2). 

The application of ultrasonic waves to the process of devulcanizing rubber is an attractive field of study. Most references indicate that rubber is vulcanized by ultrasound rather than devulcanized. Rubber devulcanization by using ultrasonic energy has been first discussed in Okuda and Hatano (1987). It was a batch process in which a vulcanized rubber was devulcanized at 50 kHz ultrasonic waves after treatment for 20 min under static conditions. The process claimed to break down carbon-sulfur bonds and sulfur-sulfur bonds, but not carbon-carbon bonds. The properties of the revulcanized rubber were found to be very similar to those of the original vulcanizates. 

The cyclic succession of reactions with self repair of the catalyst is illustrated for the catalytic reaction in which sulftu- is removed from thiophene with hydrogen. The successive reaction steps are shown for an Ni3S2 cluster in Fig. 8.1. Thiophene adsorbs on the vacant Ni site of the Ni3S2 complex. Hydrogen can dissociate on the 382 cluster and weaken the Ni-S bonds in the cluster. Hydrogen can subsequently add to the carbon-sulfur bond in thiophene, thus enhancing the C-S bond-breaking reaction. When the reaction is concluded, butadiene and H2S desorb and the 382 particle is restored... 

While breaking of the carbon-to-phosphorus bond is a nuisance in catalysis with organometallic complexes, the breaking of carbon-to-nitrogen and -to-sulfur bonds is a desired reaction in the oil industry. Hydrodenitrification (HDN) and hydrodesulfurisation (HDS) are carried out on a large scale in order to remove nitrogen and sulfur from the fuel feedstocks. 

Control of the multitude of pathways which feed molecules can take is the primary objective of aU catalyst and process developments. The work covered in this chapter focuses primarily on describing the approaches in material and catalysis development which have led to major advances in zeolite application in hydrocarbon conversion. The breaking and formation of carbon-carbon and carbon-hydrogen bonds constitute the majority of the chemical transformations involved here with the less prevalent, but very important, breaking of carbon bonds with sulfur, nitrogen and oxygen taking place in parallel. 

It has been proposed that sulfur-carbon bonds break at lower thermal stress than carbon-carbon bonds in sulfur-rich kerogens (24). Scission of sulfur-carbon bonds provides one explanation for the observed depletion of organosulfur compounds in the pyrolysates of high rank coals. The sulfur groups present in low rank coals are broken from the coal matrix to produce low molecular weight sulfur compounds which are not measured by the techniques used in this study. 

Data on the physical and chemical properties of PCDTs and PCTAs are scarce. Due to their structural similarity to PCDFs and PCDDs they are also supposed to possess some likeness in their physical and chemical properties. Sulfur and oxygen are both Group VI elements with two outer shell electrons available for covalent bonding. Structures of thiophene and furan with benzene carbon-sulfur (Cb-S) and carbon-oxygen bond (Cb-0), in PCDTs and PCDFs respectively, suggest similar chemical behavior. The bond dissociation energies (AH) show that less energy is required to break the Cb-S bond than the Cb-0 bond [17,36,37]. 

Colichman and Love [104] assumed a one-step, two-electron cleavage to the sulfide and a carbanion, but later studies pointed to a cleavage after the uptake of one electron [105,106]. In trialkylsulfonium salts a dissociative electron uptake probably takes place, whereas in aryl dialkylsulfonium salts the reduction may be a sulfur-carbon bond breaking concomitantly with the electron uptake [106] or the sulfaranyl radical, formed by addition of an electron to the aryl r-system, may have a certain lifetime [107]. In favor of the former possibility, it is argued that the values in CV are very sensitive to the electronegativity of the fragmenting radical. 

Insofar as the reduction potentials reflect the relative radical stabilities for the leaving radical, that is, as shown in Table 1, it is proposed that bond breaking is concomitant with electron transfer to the sulfonium cation. Thus, reduction potentials are a measure of the energy of the carbon-sulfur... 

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