This review summarizes recent advances in microflow photochemical technologies and transformations. The portfolio of reactions comprises homogeneous and heterogeneous types, among them photoadditions, photorearrangements, photoreductions, photodecarboxylations, photooxygenations and photochlorinations. While microflow photochemistry is most commonly employed as a micro-scale synthesis tool, scale-up and technical production processes have already been developed.

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Recent Advances in Microflow Photochemistry

The hydrogen abstraction (HA) reaction by the triplet of α-naphthoflavone (1) has been investigated experimentally by the use of laser flash photolysis (LFP) and theoretically with density functional theory (DFT) and atoms in molecules (AIM). The triplet excited state of 1, in acetonitrile, has an absorption maximum at 430 nm and lifetime of 10 μs. The quenching rate constants for the triplet of 1 with 1,4-cyclohexadiene, substituted phenols and amines were determined. The low reactivity of this ketone with respect to HA from 1,4-cyclohexadiene is in accord with a π,π* excited state. HA from phenols in acetonitrile is proposed to occur in a diffusion controlled reaction from free phenol based upon the determination of the Abraham βH2 value for acetonitrile and correction of the quenching rate constants for hydrogen bonding of the phenols to acetonitrile. A molecular orbital analysis of the triplet (SOMO and SOMO-1) of 1 reveals contributions from the carbonyl oxygen atom, but principally from the α-carbon and the associated π-bond network, consistent with a π,π* excited state. From a thermodynamic point of view, the triplet HA from phenol to oxygen of the carbonyl group is 17 kcal mol−1 less demanding than the transfer to the α-carbon, consistent with the acidic nature of the phenolic hydrogen atom. DFT and AIM analysis of the hydrogen abstraction reaction reveals that the transition state (TS) is pseudo-symmetrically polarized and that HA in the hydrogen bonded exciplex occurs in a concerted manner but not necessarily by simultaneous electron and proton transfer.

A laser flash photolysis and theoretical study of hydrogen abstraction from phenols by triplet α-naphthoflavone

Photochemical reduction of a stable enone, flavone (1), was accomplished by excimer laser photolysis using NaBH4 in ethanol, which formed flavanone (2) as a primary product in contrast to the formation of dimers that predominate in conventional photolysis. Flavanone (2) underwent successive thermal reduction with NaBH4 leading to flavanol (3) and photolysis to ethyl salicylate (4), in which the formation of 4 was completely suppressed when a single laser shot was used for the photolysis. The photolysis was conducted using quartz batch reactors having different optical paths and a quartz micro-channel reactor, in combination with KrF and XeCl excimer laser irradiation. This showed a considerable acceleration of the reaction with the micro-channel reactor, and the selectivity of the photoproducts 2, 3, and 4 depended on the optical path of the reactors, the laser wavelengths, the concentration of NaBH4, and the laser repetition rate. An optimal irradiation condition - application of sufficient time interval between the XeCl laser pulses, a low concentration of 1, and high concentration of NaBH4 - showed a considerable increase in the selectivity of 3 by eliminating the formation of 2 and 4.

Photochemical reduction of flavone with NaBH4 in batch and micro-channel reactors using excimer lasers

Spin-forbidden transitions in flavone.

Laser photolysis and pulse radiolysis studies on silybin in ethanol solutions

By means of nanosecond laser photolysis and steady-state photolysis at room temperature, it is concluded as follows: (1) Both the lowest excited triplet states of flavanone and flavone with mixed n£ ∗ -££ ∗ character abstract a hydrogen atom from ethanol generating the solute and solvent radicals. (2) The radical species of flavanone is undoubtedly its ketyl yielding the pinacol-type isomers and the solvent adduct. (3) The ketyl radical of flavone is indistinguishable from the enol-benzyl-type radical but the photoproducts formed from the latter radical (or from the corresponding keto-benzyl-type radical) are obtained.

A comparison of the photochemistry of flavanone with that of flavone originating from their lowest excited triplet states in ethanol

The lowest excited triplet state of chromone with mixed nπ*-ππ* character abstracts a hydrogen atom from the solvent. Although the ketyl-type radical thus generated is indistinguishable from the β-enol-type radical, this radical (or its keto form, i.e., the β-keto-type radical) gives rise to formation of the β-keto-type solvent adduct(s).

Tadashi Shimizu

Papers

A comparison of the photochemistry of flavanone with that of flavone originating from their lowest excited triplet states in ethanol

Hydrogen-Atom Abstraction from the Solvent by the Lowest Excited Triplet State of Chromone Yielding the β-Keto-Type Solvent Adduct(s)