Acceptor Influence on Thiolate Sensing by Hemicyanine Dyes

Promoting selective interactions between a nucleophile and electrophilic dye in complex environments is a central goal in nucleophilic chemosensor development. Commonly employed dyes are hemicyanines containing either the N-methylbenzothiazolium (Btz) or the N-methyl-3,3-dimethylindolium (Ind) acceptors. The dyes are related to α,β-unsaturated carbonyls and contain two sites of reactivity (C₂ vs C₄) with the C₂-site directly attached to the quaternary nitrogen possessing greater electrophilicity. We demonstrate the regioselectivity between reactions of sodium thiomethoxide (NaSMe) with two electrophilic hemicyanine dyes bearing Btz (1) or Ind (2) in dipolar aprotic solvent-water mixtures. Adduct complexation was followed by NMR spectroscopy, and structures were optimized in the gas phase to estimate relative adduct stability. The key results include finding a preference for thiolate attachment at the C₄-site to generate an enamine adduct with no evidence for attachment at the more electrophilic C₂-position. Equilibration between NaSMe and water also affords NaOH that displays a thermodynamic preference for C₂-attachment. Dye 1 containing the Btz moiety exhibits greater selectivity for the thiolate addition, with dye 2 being more reactive toward adventitious water to generate OH-adducts. Our data affords diagnostic ¹H/¹³C NMR adduct signals, regioselectivity for various dye/nucleophile combinations, and suggests use of the Btz acceptor for direct thiolate detection.