Synthesis Of Partially Hydrogenated Intermediates Of Dimethyldibenzothiophene

Dibenzothiophene (DBT) and its derivatives that are present in oil have a low reactivity in HDS reactions, hydrodesulfuriztion (HDS) is the main desulfur method. Metal phosphides have shown to be very good HDS catalysts. To better understand the structure-activity relationships of this new family of HDS catalystsand design more efficient catalysts, the reaction mechanisms of the parent sulfur-containing molecules and their hydrogenated intermediates over transition-metal phosphides should be investigated. DBT,4,6-dimethyldi benzothiophene (4,6-DMDBT) and 4,6-dimethyldibenzothiophene (2,6-DMDBT) are important model structures in HDS. The HDS reaction network of DBT and its derivatives is complex. DBT may give biphenyl, cyclohexylbenzene and dicyclohexyl as well as the partially hydrogenated (tetrahydro, hexahydro and dodecahydro) derivatives of DBT. Here we have systematically investigated the synthesis of 4,6-DMDBT,2,6-DMDBT and partially hydrogenated intermediates of them, 1,2,3,4-tetrahydro-4,6-dimethyldibenzothiophene (TH-4,6-DMDBT), HH-4,6-DMDBT, TH- 2,6-DMDBT and HH-2,6-DMDBT.A practical co-synthesis of 4,6-DMDBT and TH-4,6-DMDBT is described. First 2-bromo-3-methyl-2-cyclohexen-l-one is coupled by vinylic substitution with 2-methyl- benzenethiol and the product is cyclized with the aid of polyphosphoric acid. The resulting dihydro-4,6-dimethyldibenzothiophene disproportionates into 4,6-DMDBT and TH-4,6-DMDBT. The latter molecule is a key intermediate in the hydrodesulfurization of 4,6-DMDBT.1,2,3,4-Tetrahydro-2,6-dimethyldibenzothiophene (TH-2,6-DMDBT) was synthesized by coupling of the mixture of 2-bromo-3-methyl-cyclohexanone and 2-bromo-5-methyl-cyclohexanone with 2-methylbenzenethiol, followed by cyclization of the product in the presence of polyphosphoric acid as well as separation by column chromatography and recrystallization.1,2,3,4,4a,9b-Hexahydro-2,6-dimethyldibenzothiophene (HH-2,6-DMDBT) was prepared by reduction of TH-2,6-DMDBT with Zn and trifluoroacetic acid at ambient temperature.The HDS reactions of 4,6-DMDBT, TH-4,6-DMDBT and TH-2,6-DMDBT on bulk Ni2P catalyst was testified based on the synthesis of them. The results show that bulk Ni2P catalyst possessed strong activity of dehydrogenation and methyl transfer on TH-DMDBTs. Because 2,6-DMDBT was tested when used TH-4,6-DMDBT as reactant, and 4,6-DMDBT was tested when used TH-2,6-DMDBT as reactant. Besides, a new dehydrogenation compound with m/z 212 was founded in the HDS of TH-4,6-DMDBT, which may be 3,6-DMDBT accouting for the transfer of the methyl from 4- to 3-position of DBT ring.