Activation of 2‐Cyclohexenone by BF₃ Coordination: Mechanistic Insights from Theory and Experiment

Abstract

Lewis acids have recently been recognized as catalysts enabling enantioselective photochemical transformations.M echanistic studies on these systems are however rare, either due to their absorption at wavelengths shorter than 260 nm, or due to the limitations of theoretical dynamic studies for larger complexes.I nt his work, we overcome these challenges and employs ub-30-fs transient absorption in the UV,incombination with ahighly accurate theoretical treatment on the XMS-CASPT2 level. We investigate 2-cyclohexenone and its complex to boron trifluoride and analyze the observed dynamics based on trajectory calculations including nonadiabatic coupling and intersystem crossing. This approach explains all ultrafast decaypathways observed in the complex. We show that the Lewis acid remains attached to the substrate in the triplet state,w hich in turn explains why chiral boronbased Lewis acids induce ah igh enantioselectivity in photocycloaddition reactions.

Enantioselective crossed intramolecular [2+2] photocycloaddition reactions mediated by a chiral chelating Lewis acid

“…Although the nπ* band of the cyclohex-2-enone is symmetry-forbidden, it leads to rapid population of the reactive T 1 state which has ππ* character. 23 The fact that the quantum yield (0.321 ± 0.034) is smaller than 1 can be accounted for by other decay pathways but also by an initial cyclization to an unproductive 1,4-diradical ( vide infra ). 12 More or less the same value (0.316 ± 0.011) was obtained if a sensitizer 24 was used ( λ = 382 nm) suggesting that sensitization leads with high quantum yield to the reactive T 1 state.…”

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] …”

Fluoreszenz‐detektierte Pump‐Probe‐Spektroskopie

“…Anwendungen in der physikalischen Chemie umfassen die elektronische Dynamik von Energie-und Ladungstransfer, [4] Dynamik in der Photokatalyse, [5] Solvatationsdynamik, [6] Struktur angeregter Zustände, [7] Mehrteilcheneffekte, [8] Photoproduktbildung wie z. B. bei Ringçffnung, [9] Photoreaktivität, [10] kohärente Bewegung von Kernwellenpaketen [11] oder Photoisomerisierung. [12] Tr otz ihres enormen Nutzens hat die PP-Spektroskopie ihre Einschränkungen.…”