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Discussions on the Synthesis of
Allylbenzene/Propenylbenzene from Cinnamyl Alcohol

Posted at The Hive Forums

HTML by Rhodium

Rev drone:

Cinnamyl alcohol is 3-phenyl-2-propen-1-ol. Available by the metric ton, its a cornerstone of the perfume and flavor industry. Its also a geometric isomer of phenylacetone's enols, 1-phenyl-1-propen-2-ol and 3-phenyl-1-propen-2-ol. Enols equilibriate with their ketones, and in the case of P-2-P, where the ketone is favored in the equilibrium greatly over the enol, the end result is everybody's second favorite ketone.

So, how to do it? Using whatever transition metal organometallic catalytic black magic, or whatever else might do the job, how would one isomerize cinnamyl alcohol to the ketone, or one of its derivatives? 10 points for one-pot-shot methods, 7-9 points for mutlipe-step isomerizations, 4-6 points for reactions that do it cheaply and quickly with other methods. 1-3 for elaboray byzantine, impractical method that at least would get it done.

Rhodium:

Ok, I'll go for the byzantine route first:

  1. Oxidize cinnamyl alcohol to benzoic acid with KMnO4 in acid solution.
  2. Reduce the benzoic acid to benzyl alcohol with LAH.
  3. Oxidize the alcohol to benzaldehyde with pyridine chlorochromate.
  4. Condense benzaldehyde with nitroethane to give phenyl-2-nitropropene.
  5. The above is reduced with Fe/HCl to give P2P.

There is NO doubt this would work. There was no 'minimum yield' clause, right?

Acme:

IMHO the one-pot one-shot is a fantasy in this case. Here is a way, hopefully for some runner-up points.

Cinnamyl -> Allyl benzene

6M HCl, 1M NaI 70°C electrochemical redn Acta. Chem. Scand. Ser. B 38(5), 387 (1984)

Rhodium:

Couldn't the double bond be catalytically hydrogenated to form 3-phenyl-1-propanol, which in turn is dehydrated to allylbenzene with KHSO4, if one favors hydrogenations over electrolytic reductions?

Oxidation of cinnamyl alcohol with a mild oxidant would give the corresponding aldehyde, which then is reacted with tosylhydrazide. The resulting Cinnamaldehyde Tosylhydrazone can be reduced by NaBH4 to Allylbenzene in rather good yield.

Psychokitty:

This appears optional. According to the review, commercial Zn dust worked just fine when used in procedure A.

Commercial zinc dust (16 g., 325 mesh) was activated by stirring for 3-4 minutes with 100 ml of 2% hydrochloric acid. The zinc was immediately filtered under suction, washed to neutrality with water, and then washed with 50 ml of ethanol, 100 ml of acetone, and diethyl ether. The resulting powder was dried at 90°C under vacuum (10 minutes) and was used within 10 hours of preparation.

Procedure A:

Cinnamyl alcohol (1.30 mm) was dissolved in 75 ml of dry ether saturated with hydrogen chloride at 0°C. Activated zinc dust (5.0 g; 0.076 mol) was slowly added to the cooled mixture with vigorous stirring at a rate such that the temperature maintained below 5°C. The reaction was exothermic and considerable hydrogen evolution occurred. The reaction mixture was stirred for 1 hour at 0°C and then filtered. The filtrate was shaken with 500 ml of ice water and then washed to neutrality with aqueous sodium carbonate. The aqueous washings were dried over sodium sulfate and evaporated under vacuum. Chromotography of the residual oil over silica gel (Mallinckrodt, 100 mesh, 25 g) using benzene as eluant afforded 60% yield of allylbenzene (4 parts) and propenylbenzene (1 part).

The above ratios seems a little steep for the average bee with the volume of solvent too great. A more practical method that is on a preparative scale can be found in Tetrahedron 27, 5081 (1971).

Rev Drone:

I know nobody likes it, but how about going from cinnamyl alcohol to allyl benzene using LAH? Yeah, its overkill, and yeah its water sensitivity is a pain-in-the-ass, but you know it'll get the job done. The work-up should be particularily easy, as far as LAH reduction work-ups go around here. A little esterification with mesyl chloride wouldn't hurt things either (but isn't necessary unless you're really worried about maximizing your yield.)

Ref's:

Also, you could go this route:

cinnamyl alcohol + HI/RP -> propenyl benzene

( "" -> phenylacetone)

Ref = Chem.Ber. 11, 671 (1878)

Just to add onto Rhodium's idea of cinnamyl alcohol -> 3-phenyl-1-pronanol -> allyl benzene:

aryl propenyl alcohol + H2 + (Cat.) -> 3-aryl-1-propanol

There are actually dozens more ref's on this, but I'm getting tired of listing them, so I think you get the eneral idea... Anyways, the next question is: what about the ref's for the dehydration? The "cinnamyl alcohol + -> allyl benzene" step? Well,

3-Phenyl-1-Propanol --[Reagent]--> Allyl benzene