A Modified Clemmensen Reduction Procedure for Conversion of Aryl Ketones into Aryl Alkenes
G. A. Hiegel and Carney, John R.
Synthetic Communications, 26 (14), 2625-2531 (1996)
Here's a way to reduce aryl ketones (easily synthesized by way of the Friedel-Crafts acylation reaction) to internal aryl alkenes.
Experimental:
Reduction of 1-Phenyl-butanone. This procedure was used for all reactions reported in the table. In a round bottom flask were placed 10.11 g (0.155 mol) of Zn dust, 0.498 g (1.84 mmol) of HgCl2, 10 mL of H2O, 0.5 mL of conc HCl, and a stir bar. After 10 min of stirring, the liquid was decanted, and any lumps were broken up with a spatula. To the flask were added 20 mL of 88% formic acid, 6 mL of 95% ethanol, and 2.0 mL (13.78 mmol) of 1-phenyl-1-butanone. The mixture was refluxed and stirred for 1 h. The liquid was decanted into a separatory funnel, and the residue stirred with 20 mL of H2O for 5 min. The water was also added to the separatory funnel and the residue stirred with 20 mL of CH2Cl2 for 5 min. The CH2Cl2 was used to extract the aqueous phase. The residue was washed two more times with 20 mL each of CH2Cl2, and the CH2Cl2 was used to extract the aqueous phase each time. The combined CH2Cl2 solution was washed with 1 N NaOH until the wash was basic (60 mL and 30 mL) and with sat. NaCl solution (60 mL) and dried over MgSO4. After filtration and solvent removal, the residue was distilled using a Kugelrohr apparatus to give 1.46 g of product (80% yield based on the molecular weight of the alkene) (bath temp 113-121 C at 43 torr). The IR and NMR spectra were a good match for that of trans-1-phenyl-1-butene, and the NMR spectrum also showed peaks indicative of the presence of small amounts of cis-1-phenyl-1-butene, butylbenzene, and CH2Cl2. Analysis by GC (10% Carbowax 20M) showed significant peaks at 0.54 min (1.3%), 1.7 min (7.2%), 2.3 min (10.1%), 3.4 min (80.5%). The peaks at 0.54, 1.7, and 3.4 min correspond tothe retention times of CH3Cl2, butylbenzene, and trans-1-phenyl-1-butene, respectively. The peaks at 2.3 and 3.4 min were collected by preparative GC. The NMR of the peak at 2.3 min agreed eith that reported for cis-1-phenyl-1-butene, and the NMR of the peak at 3.4 min matched that of trans-1-phenyl-1-butene.
Here are the some of the more interesting segments of the article:
"Our modification of the Clemmensen procedure which gives the highest ratio of alkene to alkane and the highest isolated yields involves refluxing the ketone with amalgamated zinc in 88% formic acid and 95% ethanol for 1 h. Significant results are summarized in the Table. Acetic acid could be used in place of formic acid, but replacement of phosphoric acid, ammonium chloride, or anilinium chloride resulted in a lower ratio of alkene or a very slow reaction. Methanol as the cosolvent was about as effective as ethanol, but cylcohexane, benzene, t-butyl alcohol, dioxane, or no cosolvent gave a slightly lower ratio of alkene. Zinc amalgamated in the usual manner was used for extraction during the work-up was used for most experiments and gave the best results. When methylene chloride was used for extraction during the work-up instead of diethyl ether, the isolated yields were improved significantly.
"1-Phenyl-2-propanone and 2-octanone, both alkyl ketones, did not react appreciably under the mild conditions used for aromatic ketones. 1-Phenyl-1-ethanone was reduced to a 16:84 GC ratio of ehtyl benzene to styrene but a distilled yield of only 19% was obtained. A H NMR of the undistilled sample showed the presence of a cyclopropyl group which is believed to be formed from a zinc-carbene intermediate (Zn=C) reacting with the alkene.
"Rearrangement of the same zinc-carbene intermediate accounts for eh formation of the alkene. Under the milder Clemmensen reduction conditions used here, apparantly aryl ketones easily form the zinc-carbene intermediate, and rearrangement to the alken is normally the major reaction pathway."
Yeilds for valerophenone were 80.6% of the trans isomer and 10.1% of the cis isomer (80% overall yield)
I hope I interpreted this part right (in regard to the cis and trans thing):
Yields for propiophenone were 78.2% of the trans isomer and 13.5% of the cis isomer (80% overall yield)
There's more interesting stuff about the standard Clemmensen reduction but my hands are tired of typing.
That's all for now, folks!!!