Photochemically Induced Ring Opening of Spirocyclopropyl Oxindoles: Evidence for a Triplet 1,3‐Diradical Intermediate and Deracemization by a Chiral Sensitizer

Abstract

The photochemical deracemization of spiro[cyclopropane-1,3'-indolin]-2'-ones (spirocyclopropyl oxindoles) was studied. The corresponding 2,2-dichloro compound is configurationally labile upon direct irradiation at l = 350 nm and upon irradiation at l = 405 nm in the presence of achiral thioxanthen-9-one as the sensitizer.T he triplet 1,3-diradical intermediate generated in the latter reaction was detected by transient absorption spectroscopya nd its lifetime determined (t = 22 ms). Using ac hiral thioxanthone or xanthone,w ith al actam hydrogen bonding site as ap hotosensitizer,a llowed the deracemization of differently substituted chiral spirocyclopropyl oxindoles with yields of 65-98 %and in 50-85 %ee(17 examples). Three mechanistic contributions were identified to co-act favorably for high enantioselectivity:t he difference in binding constants to the chiral thioxanthone,t he smaller molecular distance in the complex of the minor enantiomer, and the lifetime of the intermediate 1,3-diradical.

Enantioselective, Visible Light Mediated Aza Paternò–Büchi Reactions of Quinoxalinones

“…If operational, it would be difficult to explain why the enantioselectivity is so high with a catalyst loading of only 10 mol %. In addition, the intramolecular energy transfer in complexes of 2 with the quinoxalinone should be preferred [13c,g, 19] . An alternative reaction pathway via single electron transfer from quinoxalinone is thermodynamically disfavored.…”

Photochemically Induced Ring Opening of Spirocyclopropyl Oxindoles: Evidence for a Triplet 1,3‐Diradical Intermediate and Deracemization by a Chiral Sensitizer

“…[9,10,12] Thea bsolute configuration of compound 8a was determined by anomalous X-ray diffraction and the assignment for the other cyclopropanes was based on analogy. [27] Thes econd substrate class which we investigated carried ad ialkyl-substituted carbon atom in the cyclopropane ring and optimization was performed with 2,2-diethyl-spiro[cyclopropane-1,3'-indoline]-2'-one (rac-9a). Although compound 9a has as imilar triplet energy (E T = 298 kJ mol À1 ,7 7K, EtOH) as 8a,a na ttempted deracemization with thioxanthone 1 failed.…”

Fluorescence‐Detected Pump–Probe Spectroscopy

“…[3] There,o ne pulse called pump is used to excite the sample into an electronically and/or vibrationally excited state,w hich is then probed by observing the change in transmission of asecond pulse after awell-defined time delay. PP ("transient absorption") spectroscopy is av ery robust technique,c apable of following the photo-induced dynamics across timescales.A pplications in physical chemistry include electronic dynamics of energy and charge transfer, [4] dynamics in photo-catalysis, [5] solvation dynamics, [6] structure of the excited states, [7] multi-particle effects, [8] photo-product formation such as ring opening, [9] photo-reactivity, [10] coherent motion of nuclear wave-packets, [11] or photo-isomerization. [12] Despite its tremendous utility,P Ps pectroscopy has its limitations.B ecause signal changes are detected against the bright background of the probe pulse,a ccurate detection is difficult in samples producing weak signals,s uch as highly dilute solutions.T his has also been am ajor limitation of transient absorption microscopy.…”

“…PP (“transient absorption”) spectroscopy is a very robust technique, capable of following the photo‐induced dynamics across timescales. Applications in physical chemistry include electronic dynamics of energy and charge transfer, [4] dynamics in photo‐catalysis, [5] solvation dynamics, [6] structure of the excited states, [7] multi‐particle effects, [8] photo‐product formation such as ring opening, [9] photo‐reactivity, [10] coherent motion of nuclear wave‐packets, [11] or photo‐isomerization. [12] …”