Alkylation with Miscellaneous Reagents

Ion Temperature (°C) Time (min) k jJkg ortho meta para ortho/para 

All data are the average of at least three parallel experiments. 

Dimethyl-chloronium and -bromonium ions give similar methylation results and are quite reactive even at temperatures as low as —50°C. The dimethyliodonium ion is less reactive and alkylates benzene and toluene in SO2CIF solution only when allowed to react (if necessary under pressure) at or above 0°C. 

The ethylation of toluene by diethylhalonium ions gives ethyltoluenes with ortho para isomer ratios between 0.60 and 0.96. The ortho para isomer ratios obtained for the alkylation of toluene in conventional Friedel-Crafts ethylations range from 1.17 to 1.84 (average 1.60). Such differences are considered to be due to the steric ortho effect caused by diethylhalonium ions, and are in accordance with the most probable displacement reaction on the bulky diethylhalonium ions by the aromatic substrate. This can be envisioned to proceed through an SN2-type transition state involving no free alkyl cations [Eq. (5.109)]. 

The alkylation data obtained from the reaction of dimethyl- and diethylhalonium ions provide evidence for direct alkylation of aromatics by dialkylhalonium ions. In addition, the data also indicate that dialkylhalonium ions are not necessarily involved as active alkylating agents in general Friedel-Crafts systems, although some of the reported anomalous alkylation results, particularly with alkyl iodides, could be attributed to reaction conditions favoring dialkylhalonium ion formation. 

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By reaction with alkyl halides, esters of sulfonic and carbonic acids, trimethy-loxonium tetrafluoroborate, diazo compounds, Mannich or Mitsunobu conditions or miscellaneous reagents. Methyl iodide is a versatile and effective methylating reagent that can be used at room temperature, at moderate temperatures in a tightly closed vessel or at high temperatures in an autoclave. 2,4-Dihydropyrazol-3-one 1 was methylated at N1 with sodium hydride in boiling 1,4-dioxane followed by methyl iodide at room temperature. l,2-Dihydropyrazol-3-one 2 was obtained in 59% yield (79AP853) (Scheme 1). 

The following section illustrates the use of phosgene equivalents for the synthesis of ureas, with particular emphasis on the advantages of completely phosgene-free synthetic methods based on the use of alkyl carbonates, CO, CO2, and other miscellaneous carbonylating reagents. 

Miscellaneous Reactions. Bromodimethylborane can also be used to convert dialkyl, aryl alkyl, and diaryl sulfoxides to the corresponding sulfides (eq 15). Typically, the sulfoxides are treated with 2.5 equiv of Me2BBr in dichloromethane at —23 °C for 30 min and at 0°C for 10 min. Bromine is produced in the reaction and must be removed in order to avoid possible side reactions. This is accomplished by saturating the solution with propene prior to introducing the reagent or by adding cyclohexene. Phosphine oxides and sulfones failed to react under the conditions used to deoxygenate sulfoxides. 

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