Pentamethylbenzene synthesis

Yield:464-06-2 16.7%

Reaction Conditions:

with isopropyl alcohol;indium (III) iodide at 200; for 2 - 3 h;

Steps:

39a; 39c
Mixtures of InI₃ and methanol, in molar ratios varying from 1 :2 to 1 :4, along with a initiator (typically 2.5 mol% /-propanol) were heated in a closed vessel at 200 ⁰C. Approximately two hours are required for complete conversion of methanol/DME to hydrocarbons and water. Increasing the relative amount of methanol inhibits reaction: at a molar ratio of 1 :5 only traces of triptane form under the above conditions. However, more than 5 equivalents of methanol per In can be converted as follows: 1 -2 equivalents of methanol per In are added and the reaction is carried out as described, the reaction mixture is cooled and all volatiles removed in vacuo. A fresh charge of methanol is then added, and the cycle repeated. Using this protocol, activity for converting methanol to thptane appears to be sustained indefinitely. Analysis of the dried residue after a reaction cycle by powder-pattern XRD shows that InI₃ is the major species present.[0069] Reactions can be carried out at temperatures as low as 160 ⁰C, although longer reaction times (about 8 h) are required to achieve complete conversion; no reaction is observed at 140 ⁰C. If DME is used as a feedstock the reaction proceeds more rapidly and at still lower temperatures: complete conversion is seen after 4 h at 160 ⁰C, and substantial formation of thptane is observed after 24 h at 120 ⁰C; no reaction was found at 100 ⁰C. For comparison, ZnI₂ is inactive below 180 ⁰C with methanol and 140 ⁰C with DME.[0070] After cooling to room temperature, the reaction mixture contained two liquid phases (an upper organic layer and a lower aqueous layer) and a significant amount of solid. The organic layer was analyzed using a variety of techniques including GC, GC/MS, ¹H and ¹³C NMR spectroscopy. A typical GC trace is shown in Figure 3. The largest peak in the GC trace is thptane; several other alkanes are present in significant quantities. The main arene peaks observed are pentamethylbenzene (PMB) and hexamethylbenzene (HMB). No methanol or dimethyl ether is observed in the organic layer.[0071] Typical yields (determined by comparison of peak heights to that of an added internal standard, having previously calibrated response factors) are around 15% for triptane and 3% for HMB, based on total carbon in the feed (methanol plus initiator). As with ZnI₂, several factors must be controlled in order to obtain reproducible results. These include ensuring that the entire reaction vessel is heated so that there was no temperature gradient, only comparing results from vessels with the same headspace, and using reagents of the same purity.[0072] Selected samples were subjected to PIANO (paraffin, iso-paraffin, arenes, naphthene, olefin) analysis, a standard refinery GC routine, which revealed that a large number of components were present. Selected results (including all major peaks) of the PIANO analysis are summarized in Table 5; results for an analogous reaction with ZnI₂ are included for purposes of comparison. The two major classes of compounds present are iso-paraffins and arenes, with a negligible amount of olefins.Table 5: PIANO analysis results.Compound or Class Weight%, InI₃³ Weight%, ZnI₂³ n-Paraffins 0.6 1.3 lsoparaffins 58.7 45.0 Arenes 23.3 10.7 Naphthenes 4.6 5.2 Olefins 0.4 14.2/-butane 2.8 2.62-methylbutane 9.1 2.92-methylpentane 2.3 0.4 3-methylpentane 1.6 0.3 2,3-dimethylbutane 5.3 1.8 total C₆ isoparaffins 9.1 2.52,3-dimethylpentane 2.4 0.7 2,4-dimethylpentane 1.5 0.4 Triptane 26.6 24.9 total C₇ isoparaffins 30.7 26.2 Triptene 5.6 total C₈ isoparaffins 4.3 3.81 ,2,3,5- 1.7 0.5 tetramethylbenzene 1 ,2,4,5- 1.2 0.3 tetramethylbenzene Pentamethylbenzene 13.1 0.6 Hexamethylbenzene 5.5 3.4As fraction of product organic layer.[0073] Other indium halides are much less effective at generating triptane, as shown in Table 6: use of InBr₃ or InCI₃ as sole catalyst gives small amounts of or no triptane respectively, while even partially replacing InI₃ with either InBr₃ or InCI₃ reduces the yield of triptane.Table 6: Effect of halide on triptane yield³Molar % InI₃ Molar % InBr₃ Molar % InCI₃ Triptane Yield (%)100 0 0 16.780 20 0 10.560 40 0 4.960 0 40 3.9 0 100 0 1.50 0 100 0 ^aAII reactions were performed using the standard reactions conditions (as described in Sections 39. a. and 39.c of Example 39) and with /-propanol added as an initiator. The combined molar ratio of MeOH : lnX3 (X = I, Br, or Cl) was held fixed at 3:1.; All reactions were performed in thick-walled pressure tubes equipped with Teflon stopcocks (Ace Glassware), rated up to 10 bar. The procedure for reactions involving Inl3 is based on the procedure reported earlier for ZnI₂. In a typical experiment, the tube was equipped with a stir bar and charged with indium iodide (2.05 g, 4.1 mmol), methanol (0.5 ml_, 12.4 mmol) and 'PrOH (50 μl_) as an initiator. (The indium iodide was generally weighed out in a glove box due to its hygroscopic nature; however the reactions were carried out in air). The pressure tube was placed in a preheated oil bath behind a blast shield and stirred at 200 ⁰C for the desired period of time, usually 2-3 hours. After heating, the tube was removed from the bath and allowed to cool to room temperature. The stopcock was removed and chloroform (1.0 ml_), containing a known amount of cyclohexane as an internal standard, was pipetted into the reaction mixture followed by water (0.5 ml_). The stopcock was replaced, the mixture was shaken vigorously and the organic layer separated. A small aliquot was diluted with acetone or tetradecane for GC analysis. In cases of samples to be used for NMR analysis, deuterated chloroform was used for the extraction.[0086] In reactions involving dimethyl ether, all ingredients except DME were loaded into the tube. The tube was then degassed using three consecutive freeze-pump-thaw cycles and frozen in liquid nitrogen. The desired amount of DME was condensed into tube, which was allowed to warm to room temperature and then heated as usual.

References:

CALIFORNIA INSTITUTE OF TECHNOLOGY;BP CHEMICALS LIMITED WO2008/24896, 2008, A2 Location in patent:Page/Page column 19-20; 25-26

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