Pressure control of enantiodifferentiating photoisomerization of cyclooctenes sensitized by chiral benzenepolycarboxylates. The origin of discontinuous pressure dependence of the optical yield

Abstract

Pressure effects on enantiodifferentiating geometrical photoisomerizations of (Z)-cyclooctene and (Z,Z)-cycloocta-1,5-diene sensitized by chiral benzene-1,2,4,5-tetracarboxylate were investigated over a pressure range of 0.1-750 MPa. Enantiomeric excesses (ee's) of the (E)- and (E.Z)-isomers obtained displayed discontinuous pressure dependencies, affording distinctly different differential activation volumes (delta delta V++) for each range, indicating alteration of the enantiodifferentiation mechanism. The switching of delta delta V++ occurred at essentially the same pressures of 200 and 400 MPa, which are shared by all the chiral sensitizers, irrespective of the chiral auxiliary employed. Circular dichroism spectral examinations at pressures of up to 400 MPa also revealed that the chiral sensitizers undergo discontinuous conformational changes at 200 MPa, which most likely lead to switching of the enantiodifferentiating sensitization mechanism in the exciplex intermediate.

Hydrostatic Pressure on Toroidal Interaction and Propeller Chirality of Hexaarylbenzenes: Explicit Solvent Effects on Differential Volumes in Methylcyclohexane and Hexane

“…Essentially, chiroptical methods such as CD spectra are more sensitive to conformational changes of the molecule. Nevertheless, the CD spectral investigation under pressure has been quite limited owing to the technical difficulties . To elucidate the nature of the propeller chirality and toroidal interaction of HABs, we first performed the pressure‐dependence study on the CD (as well as UV/Vis) spectra of H2 and H6 in methylcyclohexane (Figure ).…”

“…Nevertheless, the CD spectral investigation under pressure has been quite limited owing to the technical difficulties. [33,34] To elucidate the nature of the propeller chirality and toroidal interaction of HABs, we first performed the pressure-dependence study on the CD (as well as UV/Vis) spectra of H2 and H6 in methylcyclohexane ( Figure 2). Fortunately,t he observed spectral changes in the HABs upon applied pressure were substantially stronger than those for chiral biaryls( Figure S1 in the Supporting Information).…”

Supramolecular Photochirogenesis with Novel Cyclic Tetrasaccharide: Enantiodifferentiating Photoisomerization of (Z)‐Cyclooctene with Cyclic Nigerosylnigerose‐Based Sensitizers

“…In general, aromatic sensitizers multiply substituted by bulky chiral auxiliaries with the stereogenic center located closer to the chromophore are more advantageous in inducing high enantioselectivity, presumably as a result of the congested arrangement of chiral auxiliaries around the sensitizing aromatic moiety . External factors, such as temperature, solvent, and pressure, also affect the enantioselectivity …”

“…At the same time, weak and short-lived chiral interactions in the excited state are difficult in general to precisely manipulate. Nevertheless, foregoing studies have revealed the critical roles of chiral source used as auxiliary or supramolecular host [5][6][7][8][9][10] and maneuverable environmental variants, such as temperature, solvent, and pressure, [11][12][13][14][15][16] as well as irradiation wavelength. 17 Thus, seeking chiral sources suitable for a specific photochirogenic system is often the most crucial and demanding task, along with the subsequent optimization of environmental variants.…”

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

“…In a series of studies, we have demonstrated that various entropy-related factors, such as temperature, [6][7][8] pressure, [9][10][11][12] and solvation, [13][14][15] play vital roles in the essential enantiodifferentiating steps of various chiral photosensitizations to critically manipulate the relative stability and/or reactivity of the diastereomeric exciplex pair of chiral sensitizer with prochiral substrate. 2 In particular, the selective solvation to exciplex becomes a dominant factor determining product's enantiomeric excess (ee), when the exciplex is polarized or radical ionic in nature.…”