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Excited‐State Proton Transfer in Chiral Environments: Photoracemization of BINOLs
Author(s) -
Solntsev Kyril M.,
Bartolo ElizabethAnn,
Pan George,
Muller Gilles,
Bommireddy Shruthi,
Huppert Dan,
Tolbert Laren M.
Publication year - 2009
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1560/ijc.49.2.227
Subject(s) - chemistry , photochemistry , racemization , proton , excited state , circular dichroism , fluorescence , acceptor , stereochemistry , physics , quantum mechanics , nuclear physics , condensed matter physics
We have studied excited‐state proton transfer (ESPT) from chiral proton donors to chiral and achiral acceptors. The key role of the exergonicity of the reaction and the transition‐state position along the reaction coordinate for the existence of an enantiomeric effect was established. This effect was observed for “super” photoacids (Δ G ≪ 0) and vanished for endergonic reactions (Δ G > 0) where a “late” transition state similar to planar achiral binaphtholate anion occurs. As a result, photoracemization was observed, as confirmed by circular dichroism spectroscopy. The photoracemization effects were studied for several chiral photoacids (BINOLs and their ethers) and proton acceptors (amines, aminoalcohols, and water) using UV‐vis, steady‐state fluorescence, and time‐resolved fluorescence spectroscopies. The nature of the solvent and the proton acceptor, as well as the chemical structure of the BINOL, played a pivotal role in the photochemical reactivity of the system. Two proposed pathways competed for photoracemization: excited‐state inter‐ and intra‐molecular proton transfer, the former being more effective. Irradiation of the dimethoxy BINOL derivative, which lacks an acidic proton and cannot undergo ESPT, produced no appreciable reaction or racemization.