Dibenzothiophene, formation

Pyrolysis of the phosphorodichloridothioate (59) at 550 °C gives mainly dibenzothiophen and a smaller amount of the cyclic phosphonochlorido-thioate (60). Thermal decomposition of di-t-butyl peroxide in triethyl phosphate gives rise to diethyl methyl phosphate in a reaction which may be interpreted as resulting from attack of methyl radical on the phosphoryl oxygen. An extension of this mechanism accounts for the formation of (61) from tri-isopropyl phosphate under the same conditions. 

In contrast with the relatively facile thermal rearrangement of sulfinates to sulfones discussed in the preceding section, the reverse process is relatively, rarely encountered and is usually observed only at elevated temperatures. One of the first thermal sulfone to sulfinate isomerizations has been invoked by Fields and Meyerson to occur during the pyrolysis of dibenzothiophene S, S-dioxide (26) to dibenzofuran, through elimination of sulfur monoxide from the sultine intermediate 27 (equation 27). More recently, the flash vapor-phase pyrolysis of various 2,5-dialkyl and diaryl thiophene-S, S-dioxides has also been shown to involve SO extrusion and formation of the corresponding furans in good yields . 

The fragmentation of dibenzothiophene sulfoxide resembles that of dibenzothiophene rather than that of the sulfone. This is due to primary loss of 0 to give the dibenzothiophene ion, which is the strongest feature of the spectrum. Much of the breakdown which follows is due to that of the dibenzothiophene ion. There are, however, some aspects of sulfone behavior, notably the formation of the dibenzofuran ion (14) by the loss of sulfur from the rearranged molecular ion (13). ... 

Treatment of dibenzothiophene with diphenylsilane under reflux for 6 days gave starting material (84%) and tetraphenylsilane (16%). In related heterocycles, such as thianthrene, low yield replacement of sulfur by diphenylsilicon occurs, and in this case the formation of tetraphenylsilane may be indicative of the intermediacy of such an insertion product which then undergoes carbon-carbon bond fission. ... 

The overall effect of ring formation, e.g., in going from diphenyl sulfide to dibenzothiophene, is a reduction in the reactivity of positions ortho and para to the heteroatom, the reduction usually being greatest for the ortho position. 

The mechanism of formation of the sulfoxide (21a) from dibenzothiophene with chlorine in acetic acid has been studied. Sodium acetate showed a strong accelerating effect and the results suggested the formation of a dibenzothiophene-chlorine adduct which then decomposed giving the sulfoxide. 

An alternative synthesis of 4-nitrodibenzothiophene involves heating 2-amino-2 -nitrodiphenyl sulfide in a sealed tube at 190° (20%). The reaction probably proceeds via homolytic cleavage of the derived diazonium ion which could have been formed from nitrous acid liberated during the formation of phenothiazines, which were also detected. Similarly, 2-methyl-4-nitrodibenzothiophene is formed from 2-amino-2 -nitro-4 -methyldiphenyl sulfide (10%), and in this case the intermediacy of the diazonium ion was further indicated in that the same material was obtained by pyrolysis of the separately prepared diazonium salt of the sulfide. Although yields are poor in this reaction, it nevertheless represents the only route to substituted dibenzothiophenes containing a nitro substituent in the 4-position and as such is worthy of further attention. 

The formation of 4-lithiodibenzothiophene from dibenzothiophene and butyllithium has been dealt with in an earlier review however, several references to its chemistry have since appeared ... 

Molecular ions obtained from thianthrenes are normally the base peak in their mass spectra. The principal fragmentation involves loss of sulfur (87PS377), and this is interpreted as formation of a dibenzothiophen radical cation (16). Further loss of sulfur then occurs. CSH is lost from both the dibenzothiophen fragment ion and from the molecular ion species such as 17, from the parent ion, are proposed (74JHC287). The mass spectroscopic fragmentation pattern of fluorothianthrenes is comparable (720MS373). 

Benzyne generated from 2-carboxybenzenediazonium chloride reacted with sulfur monochloiide to give dibenzothiophene 13 (8-10%) and thiantherene 14 (26-35%) (1989SUL83). A mechanism involving the addition of sulfur mono-chloride to benzyne with the formation of betaine 15 followed by the elimination of SCI2 to afford benzothiirene 16 and a further reaction with another benzyne molecule or dimerization to thianthrene 14 is given in Scheme 8. 

Similar results were achieved over a Rh/alumina monolith catalyst " using catalytic POX for the reforming of a simulated JP-8 military feed containing 500 ppm of sulfur (as benzothiophene or dibenzothiophene). Stable performance for over 500 h with complete conversion of the hydrocarbons to syngas at 1,050°C, 0.5 s contact time, and LHSV of about 0.5 h was reported. At this high temperature, carbon formation was not reported and the sulfur exited as hydrogen sulfide. 

In describing catalytic activities and selectivities and the inhibition phenomenon, we will use a common format, where possible, which is based on a common reaction pathway scheme as outlined in Scheme 1. In contrast to the simple one- and two-ring sulfur species from which direct sulfur extrusion is rather facile, in the HDS of multiring aromatic sulfur compounds such as dibenzothiophene derivatives, the observed products are often produced via more than one reaction pathway. We will not discuss the pathways that are specific for thiophene and benzothiophene as this is well represented in the literature (7, 5, 8, 9) and, in any event, they are not pertinent to the reaction pathways involved in deep HDS processes whereby all of the highly reactive sulfur compounds have already been completely converted. 

Most researchers have found pseudo-first-order behavior for the various steps, and so it is possible to match theoretical curves with data to obtain the best rate constant values. Unfortunately, in most instances, too few data points were obtained to generate a unique theoretical fit. It is absolutely imperative that data be obtained for at least four conversion levels that are well spaced in the conversion matrix and extend to over 95% conversion. The partially hydrogenated dibenzothiophene intermediates are most often never detected as their desulfurization rates are extremely high (fcD, and kn2). The cyclohexylbenzenes and bicyclohexyls can arise from two different routes, and the concentrations of their precursors (biphenyl and cyclohexyl-biphenyl, respectively) pass through maximum values that can easily be calculated from the relative values of the formation and conversion rate constants. However, unique values for these relative rates can only be predicted if data are available well prior to and well beyond the times of maximum concentrations for these intermediates, because minor experimental errors can confuse curve-fitting optimization. 

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