Author Topic: Friedel-Crafts acylation of alkanes  (Read 1018 times)

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Kinetic

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Friedel-Crafts acylation of alkanes
« on: September 06, 2004, 01:09:00 AM »
This is a neat variation of the Friedel-Crafts acylation. The thread title is slightly misleading as the reaction is actually the Friedel-Crafts acylation of alkylsilanes, yielding dialkyl ketones.

Because of their slight polarity, each of the carbon atoms of alkylsilanes can be thought of as a weak nucleophile. When reacted with an acid chloride in the presence of AlCl3, they can attack the carbonyl in the same way an aromatic ring can do. Reacting phenylacetyl chloride with the readily available NMR standard tetramethylsilane will of course give phenylacetone.

Other tetraalkylsilanes are not commercially available (or are expensive) but they can be make from organometallics (dialkylzincs or Grignard reagents) and silicon tetrachloride. For examples of the synthesis of tetramethylsilane from dimethyl zinc and SiCl4, see Liebigs Ann. Chem. 136 (1865), 203-211 and Ann. Chim. (Paris) 19 (1870), 334-367 - authored by Friedel and Crafts themselves. For tetramethylsilane from methylmagnesium iodide and SiCl4 see J. Amer. Chem. Soc. 68 (1946), 481-484 or Chem. Ber. 44 (1911), 2640-2652. Yields are reported as being 40-60% for the latter two papers.

Synthetic Methods and Reactions; 40. A General Ketone Synthesis by the Friedel-Crafts Acylation of Alkylsilanes
George A. Olah, Tse-Lok Ho, G. K. Surya Prakash, B. G. Balaram Gupta
Synthesis
, 1977, 677-678




General Procedure for Ketone Synthesis from Tetraalkylsilanes and Acid Chlorides:

To an ice-cooled, stirred solution of the freshly distilled acid chloride (25 mmol) and tetraalkylsilane (25 mmol) in dichloromethane (75 ml) is added anhydrous aluminium chloride (3.5 g, ~25 mmol). The mixture is allowed to warm to room temperature and then heated under reflux for 18 h, quenched with dilute hydrochloric acid, extracted, and the residual oil is purified by derivitization into Girad T adducts and distilling the ketones obtained from the adducts. The ketones and their 2,4-DNPs give satisfactory analyses as well as physical and spectral properties identical with those reported.