Enantiodifferentiating Z-E photoisomerization of cyclooctene sensitized by chiral polyalkyl benzenepolycarboxylates

Abstract

Highly efficient and unique enantiodifferentiating Z-E photoisomerization of cyclooctene (1) was achieved through singlet photosensitization with chiral polyalkyl benzenepolycarboxylates. Conversion dependence of optical yields and an attempted kinetic resolution of racemic (E)-isomer IE unequivocally showed that the enantiodifferentiating step is not the initial quenching of chiral sensitizer with enantiomeric IE but the rotational relaxation of planar 1Z to twisted singlet cyclooctene (1p) within the singlet exciplex with chiral sensitizer. Fluorescence quenching of some benzenetetracarboxylates with 1Z and IE provides further evidence for the intervention of an exciplex intermediate, showing a new emission at a longer wavelength, and also for the insignificant enantiodifferentiation in the quenching step, affording nearly diffusion-controlled quenching rate constants around 1010 s"1 for IE. This novel enantiodifferentiating'photosensitizing system via exciplex, in combination with low-temperature irradiation, not only affords the highest optical yields up to 53% but also exhibits unusual temperature-switching behavior of product chirality especially with the o-dicarboxylate sensitizers. The activation parameters obtained in the temperature-dependence study indicate that the temperature-switching phenomenon is attributed to the unequal activation entropies, or frequency factors, for the enantiodifferentiating relaxations of 1Z to (R)-and (S)-1 P within the exciplex, for which the dynamic structural changes in the relaxation process may be responsible.

Vital role of entropy in photochirogenesis

“…It has been demonstrated that this sensitized photoisomerization proceeds through a singlet exciplex 1 [Sens*•••1Z] [17] and that the enantiodifferentiation occurs upon rotational relaxation around the double bond of 1Z within the exciplex intermediate [8]. Thus, the relaxation process should be responsible for the temperature-switching behavior.…”

“…Equation 1 indicates that the logarithm of k S /k R is proportional to reciprocal temperature, and that the activation parameters (∆∆H ‡ S-R and ∆∆S ‡ S-R ) can be calculated from the slope and intercept of the ln(k S /k R )-versus-1/T plot. Hence, the enantiodifferentiating photoisomerization of 1Z sensitized by a variety of chiral sensitizers was performed at several temperatures to give 1E in varying ee's, and the ln(k S /k R ) value was plotted against the reciprocal temperature to give an excellent straight line for each sensitizer [6][7][8][9]. Typical absolute ∆∆H ‡ S-R and T∆∆S ‡ S-R values obtained for non-ortho sensitizers are <0.1 and <0.2 kcal•mol -1 , respectively, while those for ortho sensitizers are as high as 0.5-1.1 and 0.5-3.5 kcal•mol -1 , respectively.…”

“…The unique effect of ortho substitution was further investigated by comparing the enantiodifferentiating photosensitization behavior of hemichiral phthalate and terephthalate, in which one of the sensitizer's chiral moieties, (-)-menthyl in this case, was replaced by a methyl group [8].…”

“…Indeed, a great deal of effort has been devoted to the enantio-and diastereodifferentiating photochemical reactions, which afford good to excellent enantomeric excesses (ee's) and diastereomeric excesses (de's), using diverse chiral sensitizers and auxiliaries, respectively [2][3][4]. A more intriguing phenomenon, as revealed by the mechanistic investigation of asymmetric photosensitizations is the inversion of product chirality; induced by the alteration of environmental factors such as temperature [6][7][8][9][10][11][12][13], pressure [14], solvent [15], and substrate concentration [16]. The chirality switching behavior is not only of mechanistic interest but also of particular practical importance, since such a switching enables us to control the chirality and optical yield of the photoproduct simply by maneuvering these entropy-related external factors, affording both enantiomers without using the antipodal chiral sources.…”

Chiral Tetrafluorobenzobarrelenes as Highly Efficient Ligands for the Rhodium-catalyzed Asymmetric 1,4-Addition of Arylboronic Acids

“…1,4-Bis((À)-menthoxymethyl)benzene (2) 6 was prepared in 72% yield by the etheration of 1,4-bis(chloromethyl)benzene with (À)-menthol. The [4 + 2] cycloaddition of 2 with tetrafluorobenzyne according to a known procedure 5b,5c gave a 1 to 1 mixture of diastereomers, (R,R)-1 and (S,S)-1.…”