Issue in Honor of Prof. Richard Bartsch ARKIVOC 2010 (vii) 19-37
Results and Discussion
Synthesis of N-alkyl benzothiazolium iodide salts
A systematic strategy to prepare benzothiazolium chlorides, bromides, and iodides is limited by the reactivity of the alkyl halides used as alkylating agent which usually increases in the order RCl < RBr < RI. We first attempted several N-alkylation reactions of benzothiazole with alkyl chlorides in a pressure vial at temperatures of 50 - 180 oC, but none of the Nalkylbenzothiazolium chlorides attempted could be isolated in satisfactory yields. By contrast N- alkylation of benzothiazole was successful using several different alkyl iodides under the optimized and relatively convenient conditions reported here. While we did not employ alkyl bromides in these reactions, we do believe based on our preliminary results that the use of alkyl iodides was dictated by their much higher reactivity than their bromide or chloride analogs.
Although many literature methods are available for the synthesis of N-alkyl benzothiazolium iodides they require long reaction times,18,19 use of excess alkyl halide20 and the use of a solvent in order to obtain close to 100% yields.21 Our initial attempts to synthesize N-methylbenzo- thiazolium iodide using solvent-free conditions in the reaction of benzothiazole with iodomethane at different temperatures were successful, but the yield of the product was not comparable with the previously reported method.22 We then performed the reaction under ultrasonic irradiation at room temperature with equal mole ratios of reactants and without solvent resulting in 99.8% yield after 1 h (Table 1). Unfortunately, using these conditions with longer chain alkyl iodides gave unsatisfactory product yields (average c.a. 20%). For these syntheses, conventional heating and stirring method was used and optimization of the reaction conditions led to good yields without use of solvents and with equal mole ratios of the reactants.
Thirteen N-alkylbenzothiazolium iodide salts were obtained using temperatures from 30 to 200 oC, depending on the alkyl halide chain length, for 2 h to 9 h; (Scheme 1; Table 1). The reaction protocol was similar to that reported by Hor et al.,22 for the solvent-free synthesis of five examples of N-alkylbenzothiazolium halides (including 2a, 3a, and 5a reported here), with the difference that the reaction times reported by Hor et al. were from overnight to 24 h with temperatures between room temperature and 80 oC.
In general, the yields varied between 75 and 99%. Each salt was analyzed by 1H and 13C NMR, FTIR, HRMS, and thermal analyses (DSC and TGA). Three of the iodide salts (12a, 13a, 14a) were successfully crystallized and single crystal X-ray diffraction analysis was carried out.
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