Entrainer Effect on Photochirogenesis in Near- and Supercritical Carbon Dioxide: Dramatic Enhancement of Enantioselectivity

Abstract

Diethyl ether added as an entrainer (cosolvent) to near- and supercritical CO2 significantly enhanced the enantioselectivity of photocyclization of 5,5-diphenyl-4-penten-1-ol sensitized by saccharide naphthalenedicarboxylate to give a cyclization product in enantiomeric excesses much larger than those obtained in conventional organic solvents, revealing the unique features of nc- and sc-CO2 as well as the critical role of entrainer clustering to the intervening diastereomeric exciplex pair.

Microenvironmental control of enantiodifferentiating photocyclization of 5‐hydroxy‐1,1‐diphenylpentene through selective solvation

“…In our previous study, we found that the fluorescence of sensitizer 3 was efficiently quenched upon addition of substrate 1 to give cyclization product 2 in good yield but failed to unambiguously prove the intervention of an exciplex intermediate in the photocyclization process by detecting the exciplex fluorescence. The lack of fluorescence is presumably due to the fast intramolecular nucleophilic attack of the terminal hydroxyl to the radical‐cationic olefin moiety of 1 , leading to cyclization product 2 (Scheme ).…”

“…The ee of product 2 was determined by chiral high‐performance liquid chromatography on a JASCOplus instrument equipped with a UV detector (detection wavelength: 220 nm) . The chiral high‐performance liquid chromatography analysis was run at 5 °C on a Daicel (Osaka, Japan) Chiralpak IB column (5 µm particles, 4.6 mm ϕ × 250 mm) eluted with a 98:1:1 mixture of n ‐hexane/isopropanol/1,2‐dichloroethane at a flow rate of 0.3 ml/min under the isocratic condition.…”

“…5‐Hydroxy‐1,1‐diphenylpentene ( 1 ) was prepared as reported previously . 5‐Methoxy‐1,1‐diphenylpentene ( 4 ) employed as a “dummy” substrate was prepared from 1 by the Williamson ether synthesis by using iodomethane and sodium hydride (Scheme ); 1 H NMR (CDCl 3 ): δ 1.72 (tt, J = 7.1 Hz, 2H), 2.18 (dt, J = 7.6 Hz, 2H), 3.29 (s, 3H), 3.36 (t, J = 6.6 Hz, 2H), 6.09 (t, J = 7.5 Hz, 1H), 7.17–7.38 (m, 10H); mass spectrometry (electron impact ionization) m/z (relative intensity) 252 (M + , 4), 236 (30), 207 (41), 166 (100),77 (28); high‐resolution mass spectrometry Calcd for C 18 H 20 O (M + ): 252.1514.…”

“…However, the detailed examination of selective solvation was not feasible in this system because of the presence of the alcohol reagent inevitably added to the solution. We therefore employed 5‐hydroxy‐1,1‐diphenylpentene ( 1 ), which is known to cyclize to 2‐(diphenylmethyl)tetrahydrofuran ( 2 ) upon photosensitization with electron‐accepting sensitizers, as a “self‐containing” substrate for the enantiodifferentiating photocyclization sensitized by bis(1,2;4,5‐di‐ O ‐isopropylidene‐α‐fructopyranosyl) 1,4‐naphthalenedicarboxylate ( 3 ) (Scheme ) in supercritical carbon dioxide (scCO 2 ) . Possessing a built‐in hydroxyl, this substrate allowed us to closely investigate the effects of diethyl ether added as an entrainer (cosolvent) to scCO 2 on the enantiodifferentiating photocyclization of 1 , revealing that the addition of ether significantly enhances the ee of cyclization product 2 as a result of the clustering of ether molecules around the exciplex intermediate in less polar scCO 2 .…”

“…We therefore employed 5-hydroxy-1, 1-diphenylpentene (1), which is known to cyclize to 2-(diphenylmethyl)tetrahydrofuran (2) upon photosensitization with electron-accepting sensitizers, 17,18 as a "self-containing" substrate for the enantiodifferentiating photocyclization sensitized by bis(1,2;4,5-di-O-isopropylidene-a-fructopyranosyl) 1,4-naphthalenedicarboxylate (3) (Scheme 1) in supercritical carbon dioxide (scCO 2 ). 16 Possessing a built-in hydroxyl, this substrate allowed us to closely investigate the effects of diethyl ether added as an entrainer (cosolvent) to scCO 2 on the enantiodifferentiating photocyclization of 1, revealing that the addition of ether significantly enhances the ee of cyclization product 2 as a result of the clustering of ether molecules around the exciplex intermediate in less polar scCO 2 . This intriguing result prompted us to expand the concept of the microenvironmental control in scCO 2 to the chiral photoreaction in conventional media exploiting the selective solvation of polar cosolvent in nonpolar solvent; the results of which will be reported in the following text.…”