Simple and Efficient Microwave Assisted N-Alkylation of Isatin

We present herein the results of microwave promoted N-alkylations of isatin (1) with different alkyl, benzyl and functionalized alkyl halides. Reactions were carried out under different conditions, always employing methodologies compatible with MW assisted chemistry. Generation of isatin anion employing diverse bases and solvents or using the preformed isatin sodium salt was tested. The best results were achieved using K2CO3 or Cs2CO3 and a few drops of N,N-dimethylformamide or N-methyl-2-pyrrolidinone. These reactions present noteworthy advantages over those carried out employing conventional heating.


Introduction
N-alkylation of isatin (1, Scheme 1) reduces the lability of the isatin nucleus towards bases, while maintaining its typical reactivity. Thus, N-substituted isatins 2 have been frequently used as intermediates and synthetic precursors for the preparation of a wide variety of heterocyclic compounds [1,2]. In addition, properly functionalized N-alkyl isatins present different biological activities [1], and in recent years compounds showing potent cytotoxicity in vitro [3], antiviral activity [4] and potent and selective caspase inhibition [5] have been reported, among others.
As part of our ongoing investigations, we needed to use a series of isatinacetic acid derivatives. This fact led us to explore synthetic methods to obtain N-alkylisatins 2 (Scheme 1). Literature procedures include: a) direct synthesis from N-alkylanilines and b) N-alkylation of isatin. Direct synthesis involves tedious multistep processes which usually give N-alkylisatins in low to moderate overall yields [6]. N-Alkylation of isatin (1) is usually carried out generating the highly conjugated isatin anion (1 -) [7] with different bases, followed by treatment with appropriate alkylating agents, generally alkyl halides or sulphates (Scheme 1). These methods had been extensively reviewed [1,8] and include the use of bases such as NaOH, NaH, CaH 2 and K 2 CO 3 in different solvents. Synthesis of N-functionalized isatins using a parallel synthesis employing a polymer supported strong base for the deprotonation step has been recently reported [9]. Scheme 1. Synthetic route for N-substituted isatins 2.
Though some of the above mentioned methods provide good yields of N-alkylisatins, they generally present drawbacks related to: a) the base lability of the isatin nucleus [10], b) use of hazardous reagents such as metal hydrides, which require anhydrous solvents, c) use of aprotic organic solvents with high water solubility and high boiling points, leading to complex workups, d) use of carcinogenic solvents in some cases and e) side reactions due to the presence of keto-carbonyls (i.e. reductions when metallic hydrides are used, aldolisation when K 2 CO 3 in acetone is employed). Besides, reaction times are in general lenghty, with consequent formation of by-products, and hence low yields and difficulties in product isolation.
Our interest in this type of reactions prompted us to test the use of microwave (MW) irradiation as an alternative energy source. MW heating has gained popularity in the last decades as it remarkably accelerates a wide variety of reactions and minimizes thermal decomposition of the products. Since the initial work of Gedye [11] and Giguere [12], a rapidly increasing number of reports and reviews have been published demonstrating the importance of such methodology [13]. However, to the best of our knowledge, the potentialy of this method has not been exploited yet for the type of reactions of interest in this case [14].
We present herein results of MW assisted synthesis of N-alkylisatins 2 by N-alkylation of isatin (1) with different alkyl, benzyl and functionalized alkyl halides (Scheme 1, Table 1) and their comparison with those obtained under conventional heating. Reactions were carried out under different conditions, always employing methodologies compatible with MW assisted chemistry.

Results and Discussion
We initially examined reactions under "dry" conditions [16], irradiating the mixture of neat reactants, either generating isatin anion (1 -) in situ (Method A) or employing the pre-formed sodium isatin salt (Na + 1 -) (Method B). In all cases decomposition of reactants or recovery of unreacted starting material was observed. On the other hand, results improved notably when some drops of a polar aprotic solvent, enough to humidify the reaction mixture, are added, giving a polar mixture that is more prone to MW absorption [17]. This is a fundamental requirement in the case where the sodium salt of isatin or alkylating agents with high melting point (i.e.: N-methyl-N-phenylchloroacetamide) are used.
Using ethyl chloroacetate as alkylating agent, we optimized reaction conditions by testing several parameters such as different bases and solvents. The best results were obtained employing K 2 CO 3 or Cs 2 CO 3 in N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidinone (NMP). Results obtained with different alkyl halides are shown in Table 2. Employing low or medium power settings full conversions are achieved in a few minutes and moderate to high yields of compounds 2 are obtained. The use of NMP is specially important when poorly reactive halides are employed (see entry 19). We also observed that, in general, the use of Cs 2 CO 3 as the base facilitates the workup, but yields are lower in some cases (see entries 5 and 17).
In experiments carried out under conventional heating we encounter longer reaction times and lower yields. As an example, using K 2 CO 3 /DMF, the cinnamyl derivative 2e was obtained in high yield in two minutes, whereas under classical heating four hours were required (Entry 8). Furthermore, to reach satisfactory yields high amounts of solvent are required, thus making product isolation most difficult.

Entry
Reaction of isatin with phenacyl bromide, either under conventional heating or in the MW promoted reaction, leads to N-substituted derivative 2l in acceptable yields, although the MW procedure provided the best results (Entry 22). Variable amounts of epoxide 3 (Scheme 2), resulting from addition of the halometylketone anion (A) onto the isatin β-carbonyl and further cyclization were obtained as a side product [18].
MW promoted N-alkylation of isatin using the preformed sodium salt (Na + 1 -) requires higher power to complete the reactions, but the yields do not surpass 70% (entries 4, 6, 9, 15, 18 and 20). In the reaction with phenacyl bromide, the method facilitates work up and improves yields by minimizing epoxide formation (Entry 23). Absence of an excess of base which would make carbanion A formation difficult, accounts for such results (Scheme 2).
As an alternative, techniques combining MW irradiation with the use of isatin sodium salt supported on mineral surfaces under solvent free conditions were used (Method C), an eco-friendly methodology which had received attention in recent years [16]. Under such conditions, high powers were required. In order to avoid reactant and product decomposition, reactions were conducted with intermittent heating. This method was designed to avoid overheating of reactants, according to Varma et al., when a household microwave oven is employed. [19] However, yields did not exceed 62 %, and could not be improved using phase transfer catalysis (Entries 7, 16 and 21).

Scheme 2.
Probable mechanism for the synthesis of epoxide 3.

Conclusions
We have developed a simple and efficient MW assisted synthesis of N-alkylisatins 2 by Nalkylation of isatin (1) using a household oven. The procedure involves the use of K 2 CO 3 or Cs 2 CO 3 and a few drops of DMF or NMP, and is a general one for reactions with alkyl, benzyl and functionalized alkyl halides of different reactivity. The use of MW irradiation offers many advantages over conventional heating: it remarkably decreases reaction times, requires less solvent, thus facilitating reaction workups, and increases yields.

General
Melting points were taken on a Büchi capillary apparatus and are uncorrected. The 1 H-and 13 C-NMR spectra were recorded on a Bruker MSL 300 MHz spectrometer. DMSO-d 6 was used as the solvent, and the standard concentration of the samples was 20 mg/mL. Chemical shifts are reported in ppm (δ) relative to TMS as an internal standard. Splitting multiplicities are reported as singlet (s), broad signal (bs), doublet (d), double doublet (dd), triplet (t), double triplet (dt), quartet (q), and multiplet (m). MS (electron impact) were performed on a MS Shimadzu QP-1000 instrument at 70 eV. High resolution spectra were obtained with a model VG AutoSpec three sector (EBE) mass spectrometer (Waters, Milford, MA, USA) at a scan rate of 1 scan/4 sec, operating with variable magnetic field at 8000 resolving power (10% valley definition) using perfluorokerosene (PFK) as reference compound. TLC analyses were carried out on Silica gel 60 F 254 using chloroform:methanol (9:1) as solvent. Preparative thin layer separations (PLC) were carried out by centrifugally accelerated radial chromatography using a Chromatotron model 7924T. The rotors were coated with Silica Gel 60 PF 254 and the layer thickness was 2 mm. Chloroform and increasing percentages of methanol were used as eluent. Reagents, solvents and starting materials were purchased from standard sources and purified according to literature procedures. Reactions with reagents solid or high boiling point under MW irradiation were conducted in a domestic MW oven (BGH 16260) employing open vessels. Adaptation for reflux heating [20] was used when volatile alkylating agents were employed.
General procedure for synthesis of compounds 2 employing conventional heating A mixture of isatin (1, 147 mg, 1 mmol), potassium carbonate (1.82 mg, 1.3 mmol), the corresponding alkyl halide (1.1 mmol) and DMF (5 mL) was heated in an oil bath at appropriate temperature and monitored by TLC. When the reaction was completed, the reaction mixture was poured into ice-water. If the product crystallized, the resulting solid was filtered, washed with water and purified by recrystallization or by chromatographic methods. If not, the suspension was extracted with chloroform and the organic layer was washed with water, and then dried and concentrated in vacuo affording compounds 2. Details of the reactions (temperature, times and yields) are listed in Table 2. Using either higher temperatures or smaller amounts of solvent, the yields diminish.

General procedures for synthesis of compounds 2 employing MW irradiation Method A: Generating the isatin anion (1 -) in situ
Reaction conditions were selected using ethyl chloroacetate as alkylating agent. Na 2 CO 3 , K 2 CO 3, Cs 2 CO 3, CaH 2 , TEA, LiOH, NMM, NaOEt were tested as bases. The following polar aprotic solvents were evaluated: DMF, DMA, HMPT, MeCN, DMSO and NMP. The best results were obtained using K 2 CO 3 or Cs 2 CO 3 and a few drops of DMF or NMP. The following general procedure was employed: an intimate mixture of isatin (1, 147 mg, 1 mmol), the appropriate alkyl halide (1.1 mmol), base (1.3 mmol) and some drops of the corresponding solvent (giving a slurry at room temperature) was exposed to MW irradiation. The reaction mixture was cooled to room temperature and mixed thoroughly with ice-water. Compounds 2 were isolated following the procedure indicated above. Solvents, powers, times and yields are listed in Table 2 (entries 1- 3, 5, 8, 10-14, 17, 19 and 22).

Method B: Employing preformed isatin sodium salt
A solution of sodium (0.8 g) in absolute ethanol (16 mL) was added to isatin (6 g) suspended in absolute ethanol (24 mL), the mixture being well shaken to avoid caking. The violet-black isatin sodium salt (Na + 1 -) was collected, well washed with alcohol and finally with benzene until the washings were colourless and then dried. An intimate mixture of isatin sodium salt (Na + 1 -) (169 mg, 1 mmol), the appropriate alkyl halide (1.1 mmol) and some drops of the corresponding solvent was exposed to MW irradiation and the reaction products isolated as was indicated in the method A. Solvents, powers, times and yields are given in Table 2 (entries 4, 6, 9, 15, 18, 20 and 23).

Method C: Employing supported reagents
To a solution of isatin sodium salt (Na + 1 -, 169 mg, 1 mmol) in the minimum amount of water, neutral alumina (400 mg) was added. The mixture was evaporated with a rotary evaporator and the solid was dried 1 h at 110ºC. The corresponding alkyl halide (1.1 mmol) was adsorbed onto isatin on alumina and the mixture was irradiated by microwave in a Pyrex beaker (15 mL). After cooling at room temperature, the mixture was extracted with dichloromethane. The product was purified after evaporation of the solvent. Powers, times and yields are listed in Table 2 (Entries 7, 16 and 21).