Photoracemization of Blestriarene C and Its Analogs

Abstract

Two analogs of blestriarene C (4,4'-dimethoxy-1,1'-biphenanthrene-2,2',7,7'-tetraol) bearing no 7,7'-dihydroxy (3) and 4,4'-dimethoxy groups were prepared. Unlike blestriarene C (1), compounds and , as well as 1,1'-biphenanthrene-2,2'-diol (5), do not racemize under fluorescent lamp illumination. Cyclic voltammetry analysis reveals that compound has a lower half-wave potential (E(1/2)) than compounds , suggesting that a redox cycle is involved in the racemization. Compound racemizes by absorbing UV light corresponding to the (1) L(b) band. During the reaction, no side products are observed. The racemization is significantly inhibited under nitrogen. Based on these observations, we propose a feasible mechanism for the easy racemization of compound , which is mediated by a cation radical generated in situ by a reversible photo-induced oxygen oxidation.

Chirality and Excited State Proton Transfer: From Sensing to Asymmetric Synthesis

“…Subsequent experiments revealed that the isomerization rate was strongly dependent on the presence of oxygen. Additionally, the trend in isomerization rate ( 55 > 56 > 57 > 58 > 59 in Figure ) correlated with the redox potential of the blestriarene C derivative (i. e. more easily oxidized=faster isomerization) . These observations lead the authors to propose the isomerization mechanism shown in Figure .…”

“…Given the favorable driving force for the electron transfer and that 55 is easily oxidized in ambient conditions (i. e. O 2 +light), the mechanism is feasible. But, as noted by the authors, because the reaction still proceeds in the absence of oxygen “[photoredox] is not the sole mechanism operating in the photoracemization.” Given the structural similarity to BINOL, an additional ESPT based photoisomerization mechanism is likely occurring. Nevertheless, these experiments make it clear that both the presence of oxygen as well as the redox potential of the chiral molecule are important considerations for this type of photoisomerization reaction.…”

Photoracemization‐Based Viedma Ripening of a BINOL Derivative

“…The application of UV light to molecules having sufficient absorbance in the UV region can lead to racemization by bond breakage or by excitingt he molecular orbitals, allowing for rearrangements that are forbidden in the ground state. [18] Up to now,afew successful examples of resolution through dynamic crystallization of prochiral molecules racemizing under UV light starting from completely achiral conditions have been reported. [19,20] Here, we aim to extend the application of Viedma ripening to molecules racemizing by photocatalysis, and focus the attention on chiral BINOL derivatives.…”

“…Photocatalysis is a powerful tool to promote racemization of photosensitive organic compounds. The application of UV light to molecules having sufficient absorbance in the UV region can lead to racemization by bond breakage or by exciting the molecular orbitals, allowing for rearrangements that are forbidden in the ground state . Up to now, a few successful examples of resolution through dynamic crystallization of prochiral molecules racemizing under UV light starting from completely achiral conditions have been reported .…”

Enabling Racemization of Axially Chiral Subphthalocyanine‐Tetracyanobutadiene‐Aniline Enantiomers by Triplet State Photogeneration

“…Among the many light-activated processes,p hotoracemization-the isomerization of one enantiomer into its mirrorimage counterpart using light-stands out as ap articularly interesting one.T his photoinduced process can take place through various mechanisms such as,f or example,e xcitedstate proton transfer (ESPT), [7] photo-induced oxidation [8] or pyramidal inversion, [9] or bond breaking/formation. [10] Although many chiral molecules have been investigated in the context of photoracemization, no studies have been reported, to the best of our knowledge,o ns terically congested, axially chiral buta-1,3-diene derivatives,a nd in particular,o no ne of the most interesting members of this large family,that is tetracyanobuta-1,3-diene (TCBD).…”