Sulfur elimination

In the presence of sulfide or sulfhydryl anions, the quinonemethide is attacked and a benzyl thiol formed. The P-aryl ether linkage to the next phenylpropane unit is broken down as a result of neighboring-group attack by the sulfur, eliminating the aryloxy group which becomes reactive phenolate ion (eq. 2). If sulfide is not present, a principal reaction is the formation of the stable aryl enol ether, ArCH=CHOAr. A smaller amount of this product also forms in the presence of sulfhydryl anion. 

In the presence of ZnO, the dump temperature is a parameter of paramount importance in mixing of a silica compound containing a couphng agent with a polysulfidic moiety The sulfur eliminated from the coupling agent causes premature scorch in the presence of ZnO. This problem is partially solved when ZnO is added in the productive mixing step on the two-roU mill. 

The reaction of tetrahydro-l,3-thiazine-2-thione and diethyl 2-chloro-malonate in the presence of triethylamine in boiling methylene chloride for 1.5 hr gave tetrahydro-1,3-thiazin-2-ylidenemalonate (508) in 33% yield via 507 through Eschenmoser sulfur elimination, together with traces of the mesoionic derivative (509) [77JCS(P 1) 1107]. In a similar reaction, diethyl 2-bromomalonate afforded the mesoionic compound (509) in 80% yield. Tetrahydro-l,3-thiazin-2-ylidenemalonate (508) was also obtained in 42% yield from 509 by irradiation in the presence of tributylphosphine in ethanol for 15 hr under argon [77JCS(P1)1107]. 

Reduction of dibenzothiophene with sodium in liquid ammonia has been shown to be sensitive to the experimental methods employed however, the major product is usually 1,4-dihydrodibenzothiophene. 27 -28i The electrochemical reduction of dibenzothiophene in ethylene-diamine-lithium chloride solution has been shown to proceed via stepwise reduction of the aromatic nucleus followed by sulfur elimination. In contrast to the reduction of dibenzothiophene with sodium in liquid ammonia, lithium in ethylenediamine, or calcium hexamine in ether, electrolytic reduction produced no detectable thiophenol intermediates. Reduction of dibenzothiophene with calcium hexamine furnished o-cyclohexylthiophenol as the major product (77%). Polaro-graphic reduction of dibenzothiophene 5,5-dioxide has shown a four-electron transfer to occur corresponding to reduction of the sulfone group and a further site. ... 

In some cases involving dicyano-substituted thiocarbonyl ylides of type 9, ring closure to a thiirane and spontaneous desulfurization results in the formation of dicyano alkenes of type 68 (19,20). As a rule, the presence of electron-withdrawing substituents facilitates sulfur elimination. On the other hand, with alkyl and aryl substituents, desulfurization requires elevated temperature or the use of phosphanes (42,99,105,109,124-127). 

In an attempt to correlate sulfur distribution with elemental composition, the mechanism of sulfur elimination was assumed to involve two processes ... 

The acid-induced cyclization of unsaturated thioacetals (19) gives anti-Markovnikov products (20), apparently involving sulfur elimination and readdition.37... 

Extending the reaction times to 3 hours gave a considerably different distribution of products. Not only was the benzothio-phene consumed, but the major product was toluene. These results indicate that the overall reaction of benzothiophene with molten hydroxide involves a ring opening and elimination of a one-carbon fragment to form o-thiocresol (Equation 1), followed by a slower sulfur elimination to form toluene (Equation 2). 

Thiol Decomposition. As explained previously, the elimination of sulfur from benzothiophene occurs stepwise after the aromatic thiol (o-thiocresol) has formed and not in a concerted fashion from the thiophenic ring system. Extrapolation of this implies that thio-phenic sulfur in coal is eliminated by conversion to an aromatic thiol that subsequently undergoes desulfurization. Since the aromatic thiol is the apparent organosulfur species that undergoes desulfurization, it is of interest to understand the chemistry involving the elimination of sulfur from aromatic thiols. Thio-phenol was used as a model compound to examine reactions, primarily the thermal decomposition reactions that might lead to sulfur elimination. In the experiments with caustic and benzothiophene, the intermediate (o-thiocresol) most likely exists in the salt... 

Heteroatom Elimination. Sulfur elimination from hydrorefined filtrate varied from 42 wt % at 650°F to 98 wt % under the most severe conditions at 750°F. The data presented in Figure 4 shows a very rapid initial decrease in sulfur content probably related to an initially rapid catalytic conversion of sulfur compounds particularly susceptible to catalytic reaction but thermally stable. Such compounds could be di-... 

The oxidation reactions of carbon and sulfur on hydroprocessing catalysts seem to be kinetically controlled by oxygen diffusion inside the catalyst porosity. Figure 3 shows the carbon and sulfur removal for Cat C which contains a very high amount of nickel and molybdenum, and an appreciable load of carbon. It is clear that the sulfur elimination occurs at higher temperatures than for the other catalysts and is simultaneous to carbon combustion. A tentative explanation of this phenomenon would be that the diffusion of oxygen in the microporosity is limited by coke deposit which needs to be at least partly removed to allow complete sulfur oxidation. 

Upon heating, the bisthio derivatives underwent sulfur elimination and subsequent ring opening to the corresponding cycloheptatriene derivatives. 

Thiochalcones reacted with 2,3-diphenylcyclopropenone to give 2,3,5-triphenylthiophene (37%) and 2,3,4,6-tetraphenylphenol (19%) by arylketene/sulfur elimination from the intermediate 2,3,5,7-tetraphenylthiepin-4(5//)-one. Similarly, 2-(arylmethylene)-3,4-dihydronaph-thalene-l(2//)-thione yielded only 4,5-dihydro-2,3-diphenylnaphtho[l,2-/ ]thiophene (52-59%). "... 

Attempts to copolymerize monomer I with sulfur resulted in formation of just the poly(arylene polysulfide) since all the sulfur separated out on cooling to room temperature. This is apparently because when eight or more sulfur atoms appear in sequence in a polymeric chain, the chain is unstable, and some of the sulfur eliminates to form the more stable eight-membered rings instead. 

The coupling reaction is referred to commonly as the sulfide-contraction or sulfur-extrusion reaction. However, this reaction is distinct from other sulfur-eliminating, carbon-carbon bond-forming... 

The importance of choosing the proper benzyne precursor for a successful cycloaddition with thiophene was mentioned in CHEC-I. The most suitable precursor was found to be diphenyl-iodonium-2-carboxylate. This reaction has been critically reexamined <88JOC5799,89JOC2i42>. Several mono- and di-substituted thiophenes have been used as substrates in this study, and minor products identified. The major pathway appears to be the [4 + 2] cycloaddition, leading to naphthalene after sulfur elimination. The [2 + 2] pathway is also operative, especially with 3-halothiophenes. A [3 + 2] cycloaddition, followed by elimination of acetylene, leads to the observed small amounts of benzo[6]thiophene. 

Of key importance is the lead to sulfur ratio of the concentrate, since low levels (corresponding with low lead grades) will markedly restrict the lead throughput of the sulfur elimination stage of the smelting process (see Chapter 4, Table 4.2). 

The use of oxygen enrichment of sinter plant air feed has been evaluated as a means of increasing machine capacity in terms of the sulfur elimination rate. It is debatable whether there is any benefit, since excessive bed temperatures must be avoided, requiring a reduction in the fuel content of sinter feed and hence, lead sulfide feed. However, even if there were some benefit, oxygen is used in considerable excess and inefficiently for sulfur combustion. On this basis alone enrichment is generally not economically justified. 

Meeting the requirements of high sulfur elimination, low lead slags and minimal volatilisation in a direct smelting operation utilising the roast reaction alone in a single stage is relatively impractical, and involves unacceptable compromises between these requirements. 

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