Introduction
This topic describes a very easy two-step route to the Modafinil (Provigil, Alertec, Modavigil) precursor diphenylmethanethioacetamide from benzhydrol (diphenylmethanol) in 90% yield and with 95% purity. A 200 g batch is made in a 2000 ml vessel using water as reaction medium and ethyl acetate for recrystallization of the product. Diphenylmethylbromide is prepared in situ from benzhydrol and react it with thiourea in a one-pot reaction to form the corresponding isothiouronium salt. The crude salt is then reacted with chloroacetamide (by generating the thiolate cation in situ), and after filtration and washing, diphenylmethylthioacetamide is isolated in excellent yield and good purity. After oxidation of the thioacetamide with hydrogen peroxide, followed by recrystallization, the overall yield of Modafinil is 67% from the benzhydrol.
Boiling Point: 559.1 ± 50.0 °C/760 mm Hg
Melting Point: 164 - 166 °C
Molecular Weight: 273.35 g/mol
Density: 1.3±0.1 g/ml (20 °C)
Index of Refraction: 1.646
CAS Number: 68693-11-8
Equipment and glassware:
- Three necked round bottom flask 0.5, 1 and 2 L (1 and 2 will be enough);
- Distillation apparatus;
- Retort stand and clamp for securing apparatus;
- Laboratory grade thermometer (0 °C to 200 °C) with flask adapter;
- Drip funnel 0.5 L;
- Water bath;
- Heater;
- Magnetic stirrer;
- Laboratory scale (0.1 - 200 g is suitable);
- Reflux condenser;
- Water-jet aspirator;
- Buchner flask and funnel (or Shott filter);
- Beakers 1 L x2; 200 ml x2; 100 ml x2;
- Vacuum desiccator;
- Measuring cylinders 100 and 1000 ml;
- Glass rod.
Reagents:
- Diphenylmethanol 130 g, 0.7 mol;
- Thiourea 65 g, 0.85 mol;
- Hydrobromide HBr 48% aq. 130 g, 1.61 mol;
- Distilled water 3 L;
- Sodium hydroxide [aqueous solution NaOH 46 %] 98 ml, 1.68 mol
- Chloroacetamide 80 g, 0.84 mol;
- Glacial acetic acid, 610 ml;
- Hydrogen peroxide (H2O2) 5.8% solution 500 g;
- Sodium metabisulfite (Na2S2O5) 18.3 g;
- Ethyl acetate ~2 L;
- Methanol ~2 L.
Preparation of isothiouronium Salt (IV)
Diphenylmethanol (130 g, 0.7 mol) and thiourea (65 g, 0.85 mol) are added in 0.5 l three necked round bottom flasks with thermometer and reflux condenser charging with distilled water (325 ml). The mixture is heated to 95 °C. (an emulsion is obtained) and 48% HBr (130 g, 1.61 moles, 2.3 equivalents) is then added gradually during 0.5 hour via drip funnel. The acid should be added until the reaction mixture turns completely clear. The mixture is heated under reflux (106-107 °C) for 0.5 hour and cooled to 80-85 °C. At this temperature, the mixture is seeded with several crystals of the product and the mixture is stirred at that temperature for 0.5 hour and then cooled to 25 °C. The colorless crystals are collected by filtration, washed with water (200 ml) yielding about 240 g of wet crude isothiouronium salt.
Preparation of diphenylmethylthioacetamide
A 2 L three necked round bottom flasks with thermometer and reflux condenser was charged with diphenylmethylisothiouronium bromide crude wet obtained (240 g) and water (700 ml). The suspension was heated to 60 °C and 46% aqueous NaOH solution (98 ml, 1.68 mol, 2.4 eq.) was added. The reaction mixture was heated to 85 °C and stirred until all the solid was dissolved. Then, it was cooled to 60 °C and chloroacetamide (80 g, 0.84 mol, 1.2 eq.) was added in five portions an hour at 60-70 °C during one hour via drip funnel. The suspension is stirred at 70 °C for 4-5 hours. The mixture was filtered while warm and the cake was washed with hot water (250 ml). Diphenylmethylthioacetamide crude wet is obtained 220 g, yield: 95% from diphenylmethanol. 20 g of the product was recrystallized twice from ethyl acetate, dried in vacuo to give 15 g of pure title compound.
Preparation of Modafinil
A 1.0 L three necked round bottom flasks was charged with diphenylmethylthioacetamide crude wet (220 g) obtained above and glacial acetic acid (610 ml). The mixture was heated to 40 °C and stirred until full dissolution is achieved. 5.8% H2O2 solution (500 g, 1.2 eq) was added dropwise through drip funnel during 0.5 hours at 40-45 °C. The reaction mixture was stirred at 40-45 °C for 4 hours. Then sodium metabisulfite (18.3 g) in 610 ml water was added in order to quench the unreacted H2O2 and the suspension was stirred for 0.5 hours. Then the reaction mixture was cooled to 15 °C and filtered. The cake was washed with water (610 ml) and dried on air to obtain crude wet Modafinil (205 g). Re-slurry in refluxing ethyl acetate with reflux condenser, followed by recrystallization from methanol:water (4:1) solution afforded pure Modafinil [125 g, HPLC assay: 99.9%, HPLC purity: 99.9%, yield: 67% (from diphenylmethanol)].
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