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. 2009;49(2):227-233.
doi: 10.1560/IJC.49.2.227.

Excited-State Proton Transfer in Chiral Environments: Photoracemization of BINOLs

Kyril M Solntsev  1 , Elizabeth-Ann Bartolo, George Pan, Gilles Muller, Shruthi Bommireddy, Dan Huppert, Laren M Tolbert
Affiliations

Excited-State Proton Transfer in Chiral Environments: Photoracemization of BINOLs

Kyril M Solntsev et al. Isr J Chem. 2009.

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.

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Figures

Fig. 1
Fig. 1
Emission spectra of S-(−)-BINOL in the presence of chiral and achiral proton acceptors, λex = 320 nm. (A) In MeOH/H2O 1/1 vol solutions. (B) In saturated aqueous solutions of α-D-methylglucoside. (C) in 0.11 M solution of S-(+)-2-aminobutanol in CH3CN. (D) In 0.47 M solution of S-(+)-2-butylamine in CH3CN.
Fig. 2
Fig. 2
Photolysis of S-(−)-BINOL and its ethers in toluene in the presence of 0.04 M 2-butylamine monitored by absorption, CD, and fluorescence spectroscopies. Color coding is the same for all graphs. For fluorescence spectra λex = 337 nm. (Color visible online.)
Scheme 1
Scheme 1
Photochemical processes in BINOL and MeBINOL. S is a proton acceptor.
Scheme 2
Scheme 2
Mechanism of photocyclization.
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