Swamping catalyst effect

The most selective 4-substitution is obtained in the Friedel-Crafts isopropylation of 2-acetylthiophene, which under certain conditions gives as much as 99% of this isomer and 1% of the 5-isomer. An--other case of selective 4-substitution is the bromination of 2-thienyl alkyl ketones using the swamping catalyst effect (i.e., brominating in the presence of excess AlCb without solvent), which yields 43-63% of apparently isomer-free 4-bromo-2-thienyl alkyl ketones. Gold-farb et al. also have applied this method to the chloromethylation of... 

The rate of the second reaction is quite low as long as there is enough free sulfonyl chloride to react with additional amounts of aluminum chloride. Once this is no longer true, further additions of catalyst enormously increase the reaction rate (27). The recently discovered swamping catalyst effect in the halogenation of aromatic donor species (35) probably exhibits analogous kinetic behavior. The basic rate expression found by Olivier consisted of only one term for reaction when a relatively small amount of aluminum chloride was present ... 

With bromine and excess aluminium chloride, 2-acetylfuran was converted into a mixture of 2-acetyl-4,5-dibromofuran (major product) and about equal quantities of the 4- and 5-bromo derivatives. The swamping catalyst effect is operating here. Coordination of the catalyst with the carbonyl function makes the substituent more electronegative, and in the presence of a large excess of Lewis acid catalyst, positions ortho and para to the substituent are deactivated more than the me/a-position [68AG(E)519 82AHC(30)167]. In terms of a 2-acylfuran, this means that... 

Electron-withdrawing groups can overcome thiophene s high a-selectiv-ity, but the requirement to use a Lewis acid catalyst with these less reactive substrates can complicate matters, since these catalysts may complex with substituent and modify its effect. When there is an excess of Lewis acid catalyst (swamping catalyst effect), or if a substituent complexes very strongly with such a catalyst, a higher proportion of/3-substitu-... 

Sulfur trioxide-Dioxane, 1126-1127 Sulfiir trioxide-Pyridine, 1127-1128 Sulfur trioxide-Trimethylamine, 1128 Sulfuryl chloride, 196,1106,1128-1131 Swamping catalyst effect, 32,33 Synthesis gas, 224 Syringic aldehyde, 430,1047... 

Pearson27 was to manipulate orientation in various ways to obtain any isomer desired. An example cited on 1, 32-33 is the meru-bromination of acetophenone, described by Pearson as a swamping catalyst effect. In the bromination of a phenol, para substitution ordinarily predominates over ortho substitution, but considerable increase in the proportion of ortho isomer can be achieved by operating at — 70° in the presence of a strongly basic aliphatic amine.28 The best procedure was to add bromine to a cold solution of f-butylamine in toluene, cool to about — 7(P, and add a phenol dropwise over a short period of time. By this procedure, phenol was converted by 1 equivalent of bromine into 2-bromophenol in 60% yield and by 2 equivalents of bromine into 2,6-dibromophenol in 87% yield. Tertiary amines such as DABCO and triethylamine serve also for enhanced o-bromination of phenols. Chlorination under the same conditions gave a mixture of o- and p-chlorophenols in the ratio 2 1. 

Change of orientation by complex formation—Swamping catalyst effect s. 17, 600... 

Swamping catalyst effect 14, 592 17,600 Sydnone imines, N-acyl-... 

J-cardenolides 17, 781 Catalysis, cationic 16, 279 Catalyst s. a. Swamping catalyst effect Catalysts (s. a. Promoters)... 

Aluminum chloride hydrogen chloride Change of orientation by complex formation Swamping catalyst effect Direct m-halogenation of anilines... 

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