Lewis acids alkene acetylations, acetyl chloride

Several modifications of the Simmons-Smith procedure have been developed in which an electrophile or Lewis acid is included. Inclusion of acetyl chloride accelerates the reaction and permits the use of dibromomethane.174 Titanium tetrachloride has similar effects in the reactions of unfunctionalized alkenes.175 Reactivity can be enhanced by inclusion of a small amount of trimethylsilyl chloride.176 The Simmons-Smith reaction has also been found to be sensitive to the purity of the zinc used. Electrolytically prepared zinc is much more reactive than zinc prepared by metallurgic smelting, and this has been traced to small amounts of lead in the latter material. 

AICI3 is a moisture-sensitive and strong Lewis acid. It is a first choice for Friedel-Crafts-type reactions, which provide numerous important transformations in laboratory and industry. It can also be applied to the transformation of alkenes to ketones via alkylaluminum halides.303 Hydrozirconation of an olefin and subsequent transmetalation from zirconium to aluminum gives the corresponding alkylaluminum dichloride, and the subsequent acetylation by acetyl chloride affords the corresponding ketone in high yield (Scheme 66). 

Other methods used to improve the cyclopropanation in Simmons Smith reactions are ultrasonic cavitation and the use of catalytic amounts of titanium(IV) chloride to promote the reaction. A much better method is to use 1 mol% of acetyl chloride (based on zinc) and dibromo-methane in the presence of zinc dust and copper(I) chloride in diethyl ether. This system not only strongly accelerates alkene cyclopropanation, but also causes no special problems with Lewis acid sensitive substrates. Acetyl chloride works as a promoter by reacting with... 

When terf-pentyl alcohol was reacted with acetic anhydride and perchloric or tetrafluoroboric acids, the reaction proceeded via the more stable trisubstituted alkene (21) under thermodynamic control and the reaction product was mainly a 2,3,4,6-tetramethylpyrylium salt (25). However, with terf-pentyl chloride, acetyl chloride and aluminum chloride or antimony pentachloride, the diacetylation occurred under kinetic control the alkene equilibration proceeds very fast with these Lewis acids so that the product (26) is derived from the more reaetive but less stable disubstituted alkene (22) 63,64 formulas below, the intermediate monoaeetylation products... 

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