Methanol as alkylating agent

The alkylation of different amines has been performed with methanol. Table 1 shows the results obtained with n-octylamine. 

With a primary amine such as n-octylamine, we observe at low conversion (8 %) high selectivity for the N-monoalkylated product whereas 90 % N,N-dimethylated amine are formed at high conversion (94 %). In reductive alkylation, it is even more difficult to obtain the monomethylated product selectively. For example, in the Eschweiler Clarke procedure [17], only the dimethylated amine is formed, even with an amine-to-formaldehyde ratio of 1. In the Hofmann-type reaction, a mixture of mono- and dimethylamine with the corresponding trimethylammonium salt is generally produced. 

With a primary aromatic amine such as aniline and under the same conditions as with n-octylamine, conversion and seleetivity evolve similarly-from N-methylated aniline at low conversion to N,N-dimethylated aniline at high conversion [18]. Selectivity is, moreover, better because 90% iV-monomethylaniline is produced at 45 % conversion and 85 % dimethylated product at total conversion. By-products resulting from C-alkylation of the aromatic ring are also formed at high conversion (Table 2), although they never exceed 10%. 

Among the different C-alkylated products, A,A-dimethyltoluidine is by far the most abundant. It can be concluded that aniline methylation on -Al203 follows a sequential reaction path of formation of A-methylaniline, then A,7V-dimethylani-line, then C-alkylated products. This is typical of acid catalysts [2]. a-Methylbenzylamine has also been tested (Table 3). 

Above 250 °C, deamination of the reagent is observed and styrene is formed. Below this temperature, however, it is possible to obtain A-methyl-a-methylben-zylamine with good selectivity (90% selectivity at 20% conversion, 220 °C). The selectivity towards the N,N-dimethylated product is lower at the same conversion than with the other primary amine already tested, i. e. the -oetylamine (38 % at 84 % conversion for a-benzylamine and 57 % at 60 % conversion for n-octyla-mine). The NH group is bound to a secondary carbon in a-methylbenzylamine, whereas it is linked to a primary carbon in -octylamine. This steric factor explains the different reaetivity. This relatively good selectivity for monomethyla-tion can be regarded as an advantage compared with reductive alkylation. 

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Alkylation is an important alteration that regularly employs toxic and hazardous reagents such as alkyl iodide (Johnstone and Rose, 1979) or dialkyl sulfate (Basak, et al., 1998). The use of imconventional reagents has been scarce due to the ruthless conditions required with dialkyIcarbonate or methanol as alkylating agents. 

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