Asymmetric Photolysis of 2-Phenylcycloalkanones with Circularly Polarized Light: A Kinetic Model for Magnetic Field Effects

Abstract

Magnetic field effects (MFE) on asymmetric photolysis involving a biradical intermediate have been investigated on a kinetic model where an intersystem crossing (ISC) of the intermediate is taken into account. Changes in the enantiomeric excesses (ee) of chiral substances with circularly polarized light (CPL) irradiation have been simulated, and the necessary conditions for observing the MFE were obtained. The asymmetric photolysis of racemic 2-phenylcycloalkanones (2-PCAs) with CPL has been carried out in both the presence and absence of a magnetic field. Since the anisotropy g factors of 2-PCAs are considerably large, the CPL-induced ee are achieved to a few percent after 90% decomposition though the MFE are not observed. The photolysis mechanism of 2-PCAs in an air-saturated solution has been also clarified. Triplet acyl–benzyl biradicals, formed via photochemical α-cleavage of 2-PCAs, react with O2 dissolved in the solution and result in the formation of acetophenone and alkenoic acids. The bimolecular reactions are diffusion-controlled and the rates are comparable to those of ISC to the singlet biradicals for all 2-PCAs. The recombination yields of the biradicals are sufficiently large. However, the biradical ISC rate shows little magnetic field dependence, which explains the absence of the MFE in this asymmetric photolysis.

Photochemistry of large ring 2-phenylcycloalkanone in the presence of molecular oxygen

“…We estimated the recombination yield of 2PCDD in the same manner as reported in our previous paper [18]. This estimation is based on the measurement of time evolution of 2PCDD enantiomers starting from the one optically pure enantiomer.…”

“…In the presence of molecular oxygen, 2PCAs afford several products through different processes. We have recently reported that the triplet acyl-benzyl biradicals (C 6 -C 8 ) react with molecular oxygen to produce benzoylalkanoic acids, which can be further excited by the UV light, and then subsequent intramolecular hydrogen abstraction (Norrish Type Ⅱ) occurs to produce acetophenone and the corresponding alkenoic acids [18]. Turro et al have observed enhanced 13 C CIDNP signals at carbonyl carbon on the products in 3 the photolysis of 2-phenylcyclododecanone (2PCDD) in aerated solution, in which the presence of an oxidized peroxyester or peracid species has been suggested [6].…”

“…Time profiles of the product formation have been examined on the basis of product identification and quantification on gas chromatography/mass spectrometry (GC/MS). Yield of recombination from the singlet acyl-benzyl biradical ( 1 BR) to the parent 2PCDD has also been determined by measuring time evolution of enantiomer concentration of 2PCDD starting from one optically pure 2PCDD enantiomer [18].…”

Is Circularly Polarized Light an Effective Reagent For Asymmetric Synthesis?

“…enantiomeric excesses of substrate and/or product [19][20][21][22]. Michl and coworkers have photolyzed 1-azidoadamantane with circularly polarized light in a low temperature matrix and obtained a chiral cyclic imine having 2% optical purity [23].…”

Photoreaction of a racemate proceeding through radical or ion pairs. The effect of circularly polarized light on the enantiomeric excess of recovered reactant

“…9 -11 This equation cannot be solved exactly except in special circumstances although numerical methods can be used instead. 11,12 If A is small, however, a situation which usually can be attained by adjusting the concentration of the substrate, (1À10 ÀA )/A c ln10 and a constant. Equation 2 thus becomes…”