Abstract Benzothiazolium salts are key intermediates in the synthesis of cyanine dyes and related functional materials, yet their preparation is often complicated by alkyl scrambling arising from equilibrium with benzothiazole-2-thione species. Unexpected salt mixtures observed during N-alkylation are rationalized through halide- or heteroatom-mediated cleavage of S-alkyl groups, regenerating reactive alkyl halides and thione intermediates. Herein, we report a practical and scalable strategy that enables complete control over N- and S-alkyl group differentiation. The method proceeds via initial S-alkylation, catalytic rearrangement to the corresponding thione, and mild recrystallization of the desired benzothiazolium salt. This approach allows gram-scale access to structurally diverse dialkylbenzothiazolium salts, which are subsequently employed in the efficient synthesis of thiazole orange derivatives under low-temperature conditions that suppress salt degradation. The insights and methodology presented provide a general solution to long-standing challenges in benzothiazolium salt synthesis and cyanine dye preparation.
Almekinder et al. (Wed,) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: