Cs 2 CO 3 / [ bmim ] Br as an Efficient , Green , and Reusable Catalytic System for the Synthesis of N-Alkyl Derivatives of Phthalimide underMild Conditions

Aza-conjugate addition of phthalimide to 𝛼,𝛽-unsaturated esters efficiently achieves in the presence of catalytic amount of Cs2CO3 and ionic liquid 1-butyl-3-methylimidazolium bromide ([bmim]Br) under mild reaction conditions (70∘C) to afford N-alkyl phthalimides in high yields and relatively short reaction times.


Introduction
Over the past decade, room temperature ionic liquids have been emerged as a new class of stable and inert solvents.They exhibit high thermal stability, high polarity due to their ionic nature and a great ability to solubilize polar and nonpolar organic compounds.To date, various organic reactions have been carried out and investigated in ionic liquids, such as conjugate addition of sulfonamides [1,2], azide ion [3], amines and N-heterocycles [4,5], indoles [6], thiocyanide ion [7], active methylenes [8,9], and thiols [10] to electrophilic alkenes, and other carbon-carbon, carbonnitrogen, carbon-oxygen as well as carbon-sulfur bonds formation [11,12].
N-alkyl derivatives of phthalimide have attracted much interest due to their potential use as antipsychotic [13], anti-inflammatory [14], hypolipidemic [15], agents and receptors [16], and so on.Moreover, these compounds are very useful intermediates in organic synthesis as they can be easily converted to primary amines (Gabriel synthesis) [17].Therefore, there is a great deal of interest in the synthesis of this class of compounds.The azaconjugate addition of phthalimide to electrophilic alkenes can be used as a useful synthetic route toward N-alkylated phthalimides [18][19][20][21].Several catalysts have been applied to achieve this transformation such as Na/EtOH [18], AlMe 2 Cl [19], ZnO [20], and 1,4-diazabicyclo[2,2,2]octane [21].However, these reported methods are associated with one or more of the following drawbacks: (i) moderate yield, (ii) relatively long reaction time, (iii) harsh conditions, (iv) the use of more reactive phthalimide salts instead of phthalimide, (v) difficult experimental procedure, and (vi) the necessity of stoichiometric amount of catalyst.Moreover, the aza-conjugate addition reaction of phthalimide has been scarcely studied.Therefore, it seems highly desirable to find an efficient new protocol for this reaction.
Considering the above subjects and also in continuation of our previous studies on green organic synthesis [20,21,[29][30][31][32], we report here an efficient, green, and simple method for the preparation of N-alkyl phthalimides via aza-conjugate addition of phthalimide to α,β-unsaturated esters in the presence of catalytic amount of Cs 2 CO 3 in [bmim]Br at 70 • C (Scheme 1).This present method has none of the above disadvantages at all.

Results and Discussion
We have found previously Cs 2 CO 3 acts as an efficient basic reagent for N3-alkylation of N1-substituted pyrimidines [22], and N-arylation of nucleobases [23].Moreover, this base has been frequently applied for alkylation and arylation reactions [24][25][26][27][28].These subjects encouraged us to use Cs 2 CO 3 as catalyst for N-alkylation of phthalimide via aza-conjugate addition reaction.Therefore, firstly we used different amounts of Cs 2 CO 3 to accomplish azaconjugate addition of phthalimide (2 mmol) to ethyl acrylate (2.4 mmol) in [bmim]Br (1 g) at range of 25-100 o C to provide compound 1a (Scheme 1).The results showed that the reaction proceeded efficiently in the presence of 20 mol% of Cs 2 CO 3 at 70 o C and the product was obtained in 98% yield after 90 minutes.We also examined the reaction in the presence of sodium and potassium carbonate in which the product was produced in 59 and 82% within 240 and 180 minutes, respectively.Thus, we selected Cs 2 CO 3 as catalyst for our reaction.
To compare the efficiency of ionic liquid versus the conventional solvents, we examined the reaction between phthalimide (2 mmol) and ethyl acrylate (2.4 mmol) using Cs 2 CO 3 (20 mol%) in some conventional solvents (10 mL) at 70 o C (Table 1).As it can be seen from Table 1, higher yield and shorter reaction time were obtained in [bmim]Br.The reaction was also tested in solvent-free conditions; however, these conditions were not efficient (Table 1).Therefore, ionic liquid is an essential factor to promote our reaction.
After optimization of the reaction conditions, we reacted phthalimide with different α,β-unsaturated esters.The results are summarized in Table 2.As Table 2 indicates, all reactions proceeded efficiently and the N-alkyl phthalimides were produced in excellent yields and relatively short reaction times.
The interesting behavior of [bmim]Br/Cs 2 CO 3 system lies in the fact that it can be reused after simple washing with Et 2 O, rendering this process more economical.The yields of compound 1a (model compound) in the 2nd and 3rd uses of the [bmim]Br/Cs 2 CO 3 were almost as high as in the first use (see Table 3).

Experimental
All chemicals were purchased from Merck, (Germany) or Fluka, (Switzerland) Chemical Companies.The 1 H NMR (250 MHz) and 13 C NMR (62.5 MHz) were run on a Bruker Avance DPX-250, FT-NMR spectrometer.Mass spectra were recorded on a Shimadzu GC MS-QP 1000 EX apparatus.Melting points were recorded on a Büchi B-545 apparatus in open capillary tubes.

General Procedure for the Synthesis of N-Alkyl Phthalimides
To a well-ground mixture of phthalimide (0.294 g, 2 mmol) and Cs 2 CO 3 (0.130 g, 0.4 mmol) in a 10 mL round-bottomed flask connected to a reflux condenser [bmim]Br (1 g) and α,β-unsaturated ester (2.4 mmol) were added.The resulting mixture was stirred in an oil bath (70 o C) for the times reported in Table 1.Afterward, the reaction mixture was cooled to room temperature and was extracted with Et 2 O (3 × 50 mL).The organic extracts were then combined.After removal of the solvent, the crude product was purified by short column chromatography on silica gel eluted with EtOAc/n-hexane (1/3).After isolation of the product and evaporation of the remaining Et 2 O in ionic liquid, the ionic liquid containing the catalyst (Cs 2 CO 3 /[bmim]Br) was used for the next run under identical reaction conditions.

Conclusions
In summary, we have developed a new method for the synthesis of N-alkyl phthalimides as biologically interesting compounds via aza-conjugate addition reaction.This new strategy has the advantage of high yield, short reaction time, mild conditions, ease of product isolation, potential for recycling of the catalytic system, and compliance with the green chemistry protocols.

Table 1 :
Comparing the reaction of phthalimide with ethyl acrylate in conventional solvents versus [bmim]Br.
a Isolated yield.

Table 2 :
Synthesis of N-alkyl phthalimides using Cs 2 CO 3 /[bmim]Br catalytic system.All compounds are known and their structures were identified by comparison of their melting points and spectral data with those in the authentic samples.
a b Isolated yield.

Table 3 :
Aza-conjugate addition of phthalimide to ethyl acrylate in the presence of recycled Cs 2 CO 3 /[bmim]Br.
a Isolated yield.