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 (C 2 vs C 4 ) with the C 2 -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 C 4 -site to generate an enamine adduct with no evidence for attachment at the more electrophilic C 2 -position. Equilibration between NaSMe and water also affords NaOH that displays a thermodynamic preference for C 2 -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 1 H/ 13 C NMR adduct signals, regioselectivity for various dye/nucleophile combinations, and suggests use of the Btz acceptor for direct thiolate detection.