Microwave heating and its application in organic chemistry for the reactions in 'dry'media is currently developed successfully, and in the past few years there has been a tremendous interest in this area. Remarkable decreases in reaction times, and, in some cases, cleaner reactions and better yields have been reported with microwave irradiation. Likewise water polar organic compounds can be heated through the dipole rotation. On the other hand, hydrocarbons that do not have dipole moment adsorb microwave energy poorly and can not be heated under such conditions. Since teflon, polystyrene, and glass are nearly transparent to microwaves, they can be used for the manufacture of rection vessels.
Synthesis without solvents under microwave irradiation offers several advantages. The absence of solvent reduce risk of hazardous explosions when the reaction takes place in a closed vessel in a oven. Moreover, solvents like aprotonic dipolar solvents with high boiling points are expensive and difficult to remove from the reaction mixtures. During microwave induction of reactions under dry conditions, the reactants adsorbed on the surface of alumina, silica gel, clay, and others absorb the microwaves whereas the support does not, nor does it restrict their transmission. Consequently, such supported reagents efficiently induce reactions under safe and simple conditions with domestic microwave ovens instead of specialized commercial microwave systems that require sealed Teflon bombs.
The Gabriel synthesis for converting halides to primary amines is based on the reaction of halide with potassium phthalimide. The reaction is usually rather slow but can be conventionally speeded by the use of aprotic solvent such as DMF.
In the present paper, we would like to report the remarkable fast synthesis method of N-alkylphthalimides via alkylation of phthalimide in 'dry' media under microwave irradiation. The syntheses were carried out by simply mixing of phthalimide with 25\% excess of an alkyl halide and a catalytic amount of tetrabutylammonium bromide (TBAB). The mixtures were adsorbed on potassium carbonate and irradiated in an open Erlenmeyer flask in a domestic microwave oven for 4-10~min.
The reaction of phthalimide with alkyl halides
Since the shape and size of the reaction vessel are important factors for the heating of dielectrics in a microwave oven, the preferred reaction vessel is a tall beaker with a loose cover of much larger capacity than the volume of the reaction mixture. A large Erlenmeyer flask with a funnel as a loose top cap can be used in place of the beaker. Since superheating of liquids is common under microwave irradiation, the strategy of the reactions is to keep the reaction temperature substantially bellow the boiling point of each compound use for the reaction.
After the reaction, the work-up procedure is reduced to a treatment with an appropriate solvent (e.g., THF) and recrystallization. All the products gave satisfactory IR, $^1$~H~-~NMR, and MS data. Melting points of all the compounds are in good agreement with literature data.
In conclusion, we have been developing a simple method for the N-alkylation of phthalimide that occurs under mild conditions, using inexpensive reagents and a household microwave oven as the irradiation source. Moreover, our synthesis method of N-alkylphthalimides is superior to other methods, since, as the starting material, we use phthalimide instead of its potassium salt, which makes the synthesis procedure very simple and convenient. The experiments have not been optimized yet.