External Reversal of Chirality Transfer in Photoswitches

Abstract

The transfer of stereoinformation is at the heart of asymmetric reactions. By incorporating the natural monoterpene l‐menthone into the backbone of a diarylethene, we achieved efficient chirality transfer upon photocyclization, resulting in the preferred formation of one major closed isomer in a diastereomeric ratio (d.r.) of 85:15. More significantly, we were able to completely reverse the diastereomeric outcome of the ring closure simply by altering the chemical environment or the irradiation conditions. As a result, we could selectively accumulate the less favored minor closed isomer, with remarkable d.r. values of >99:1 and 74:26, respectively. Computations revealed that a stability inversion after photocyclization is the basis for the observed unprecedented control over diastereoselectivity.

Proton‐Gated Ring‐Closure of a Negative Photochromic Azulene‐Based Diarylethene

“…The closed isomer c ‐ATE‐H + undergoes thermal ring‐opening reaction with a half‐life of around 7 min at room temperature (Figure c), similar to reported proton‐responsive DAE‐type photoswitches . Notably, the reaction did not lead to the exact same absorption spectrum of o ‐ATE‐H + , suggesting the thermal back reaction was accompanied with some decomposition of ATE (Figure c and S2).…”

“…Synthesis of o ‐ATE was accomplished by an initial Pd‐catalyzed cross‐coupling between cyclohexanone and 3‐bromo‐2‐methyl‐5‐[4‐(trifluoromethyl)phenyl]thiophene followed by conversion into the corresponding enol triflate. Subsequently, the guaiazulenyl moiety was introduced via Suzuki coupling involving 2‐guaiazulenylboronic acid pinacol ester (see Scheme S1).…”

Protonenvermittelter Ringschluss eines negativ photochromen, Azulen‐basierten Diarylethens

“…Die Synthese von o ‐ATE erfolgte zuerst durch eine Pd‐katalysierte Kreuzkupplung zwischen Cyclohexanon und 3‐Brom‐2‐methyl‐5‐[4‐(trifluormethyl)phenyl]thiophen, gefolgt von einer Umwandlung in das entsprechende Enoltriflat. Anschließend wurde die Guaiazulenylgruppe über eine Suzuki‐Kupplung mit 2‐Guaiazulenylboronsäurepinakolester eingeführt (siehe Schema S1) .…”

“…Analog zu bekannten protonensensitiven DAE‐Photoschaltern durchläuft das geschlossene Isomer c ‐ATE‐H + eine thermische Ringöffnung mit einer Halbwertszeit von etwa 7 Minuten bei Raumtemperatur (Abbildung c) . Es anzumerken, dass die Reaktion nicht zu dem exakt gleichen Absorptionsspektrum von o ‐ATE‐H + führte, was darauf hindeutet, dass die thermische Rückreaktion mit einer gewissen Zersetzung von ATE einherging (Abbildung c und S2).…”

Click chemistry towards thermally reversible photochromic 4,5-bisthiazolyl-1,2,3-triazoles

“…They are known to show thermally irreversible photochromism. However, some of them are thermally reversible [4]: (1) when the aromatic stabilization energy of the aromatic rings is large [5]; (2) when the substituent groups on the ring-closing carbon atoms are large [6]; (3) when the substituent groups on the ring-closing carbon atoms are strongly electron-withdrawing [7]; or (4) when the dialkylamino group on the side chain [8] or a carbon atom of the conjugation system in a strained closed form [9] are protonated.…”