Synthesis of Phenyl-2-Propanone via Phenyl-2-Propanol


The preparation of phenylacetone is possible from 1-phenyl-2-propanol. This alcohol can't be preparated from benzylmagnesium chloride and acetaldehyde, as people with no real knowledge in metallorganic chemistry might think, as benzylmagnesium chloride rearranges in many reactions and in this case, also 1-(2-methylphenyl)-ethanol is formed (Tiffeneau-rearrangement of benzylmagnesium halogenides).

There are two ways:

Allylbenzene from bromobenzene, Oxymercuration of allylbenzene

Bromobenzene is reacted with excess magnesium in dry THF to form the GRIGNARD reagent, then the solution is decanted from unreacted magnesium (this is washed with extra THF), 1mol-% CuI is added, and 1,25 equivalents allylbromide (calculated on the amount of bromobenzene) is added carefully with external cooling. Workup with ammonium chloride solution (extraction, drying, dist. in vacuum) will give allylbenzene, yield 80 % (calc. bromobenzene) (Hershberg, 1932). Without cooling, isomerisation to 1-phenylpropene will occur.

The allylbenzene is the oxymercurated in methanol/water with mercury(II)-acetate to yield 1-phenyl-3-acetatomercuriopropanol, which is reduced (in the original solution) with sodium boranate (in an alkaline medium).

Grignard reaction of bromobenzene, coupling with propylene oxide

The usual GRIGNARD reagent is preparated from bromobenzene and magnesium, and then 1 mol-% of CuI is added. 1,2 equivalents of propylene oxide (methyloxirane) is added with caution. After some days of stirring, the usual workup is done (ammonium chloride solution, mineral acid might dehydrate the alcohol). Extraction, drying (sodium sulfate), and vacuum dist. will yield about 70% of the 1-phenyl-2-propanol.

The experimental details can be found (for serious researchers) in standard chemical journals.

The use of CuI is discussed in Norbert Schultze, "Metallorganische Chemie", Springer-Verlag (1995 or 1996).

The high yield oxidation of 1-phenyl-2-propanol is possible with pyridinium dichromate (Collins reagent) in dichloromethane, a 4-6-fold excess of the oxidizing agent is recommended. The slurry of chromate salts are simply filtered of after the reaction, and the solution is washed with water, dried, concentrated, and the residue distilled.

The Collins reaction is described in Tetrahedron Letters, and the given information is copied therefrom.