Excited-state proton transfer in chiral environments: Photoracemization of BINOLs1

Kyril Solntsev*, Elizabeth Ann Bartolo, George Pan, Gilles Muller, Shruthi Bommireddy, Dan Huppert, Laren Tolbert

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

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 (ΔG >0) and vanished for endergonic reactions (ΔG < 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.

Original languageEnglish
Pages (from-to)227-233
Number of pages7
JournalIsrael Journal of Chemistry
Volume49
Issue number2
DOIs
StatePublished - 1 Dec 2009

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