Photoredox Catalysis: The Need to Elucidate the Photochemical Mechanism

Abstract

electron transfer ·photocatalysis ·pyrenes ·triplettriplet annihilation ·tris(bipyridine)ruthenium (II) During the last decade photocatalysis in organic synthesis has become ab looming field of research. [1][2][3] In most cases, photocatalytic reactions involve electron transfer processes that yield highly reducing or oxidizing intermediates.In search of highly reducing species generated upon visible light excitation, arecent Communication published in this journal [4] reports the use of av isible-light absorbing photocatalyst, namely [Ru(bpy) 3 ] 2+ ,toproduce radical anions of UV-absorbing polycyclic aromatic hydrocarbons,s uch as pyrene,w hich in turn were able to drive the reductive activation of chemical bonds for carbon-carbon and carbonheteroatom bond formation.Thep roposed photochemical mechanism mimics that of the natural photosynthesis:l ight absorption, followed by energy transfer and electron transfer steps,asdescribed by the following scheme (the relevant electronic excited states participating in the reactions are shown in parentheses).½RuðbpyÞ 3 2þ ðS 1 Þ!½ RuðbpyÞ 3 2þ ðT 1 Þ ð2Þ ½RuðbpyÞ 3 2þ ðT 1 ÞþPy !½ RuðbpyÞ 3 2þ þ PyðT 1 Þ ð3ÞUpon light excitation of [Ru(bpy) 3 ] 2+ ,t he lowest triplet excited state of the Ru II complex (T 1 ), which is populated quantitatively by the S 1 !T 1 intersystem crossing (processes 1a nd 2), sensitizes the population of the triplet state of pyrene,P y(T 1 )( by triplet-triplet energy transfer, process 3). Py(T 1 )i ss ubsequently reduced by N,N-diisopropylethylamine (DIPEA, reaction 4), resulting in formation of astrongly reducing pyrene radical anion (Py À ), which is able to drive the reduction of organic substrates.H owever,n o experimental evidence supporting the formation of the radical anion of pyrene was reported.We would like to note that, based on the thermodynamic data, the pyrene radical anion is highly unlikely to form as proposed in the scheme (Figure 1, black lines). To af irst approximation, that is,n eglecting the electrostatic work term, [5] the free energy change of the photoinduced outersphere electron transfer reaction can be obtained [6] from the redox potentials of the ground state couples and the oneelectron potential corresponding to the zero-zero excitation energy,according to the values reported and discussed in the original paper: [4] E[Py(T 1 )/Py À ] = E[Py/Py À ] + E 00 [Py/Py(T 1 )] = À2.1 + 2.0 = À0.1 V E[DIPEA + /DIPEA] =+0.9 Vvs. SCE [7] In the case of reaction (4):Therefore,r eaction (4) is highly endoergonic and its occurrence ( Figure 1) is extremely unlikely.Indeed, even the electron transfer quenching of [Ru(bpy) 3 ] 2+ (T 1 )b yDIPEA, which is much less endoergonic (ca. 0.2 eV), was reported to be extremely slow. [4] Apossible alternative,thermodynamically allowed mechanism (Figure 1), underpinning formation of the pyrene radical anion and subsequent reduction of organic substrates, could involve energy transfer from the photoexcited Ru II complex to pyrene [reaction (3)],f ollowed by triplet-triplet annihilation of two ...

Mechanisms of photoredox catalysts: the role of optical spectroscopy

“…However, the mechanistic proposals mostly rely on fluorescence quenching studies without a backup of in-depth excited state kinetic studies or real-time elucidation of catalytic intermediates. 6,26,27,28 Therefore, a detailed understanding of the mechanisms involved is essential for further advancement in the field. 29,30,31 Further, light-driven reactions occurs through short-lived transient species, and principles of thermal reactions are not necessarily valid in photochemical reactions.…”

Polyconjugated Materials for Printed (Opto)Electronics: Introducing Sustainability

“…Alongside palladium-mediated cross-coupling reactions, photoredox direct arylation protocols are gaining increasing attention, particularly when they are compatible with direct sunlight irradiation as the activation method. 152 153 154 155 156 157 158 159 Photoredox arylation protocols require the same reagents as palladium mediated arylation: an aryl halide (mostly bromides) and an activated, generally electron-rich, arene. The role of the palladium catalyst is, in this case, played by a photoredox mediator (Cat), which is a molecule that – once excited in the corresponding first excited state (Cat*) – can convert the arylhalide (Ar-X) into the corresponding neutral radical (Ar • ) plus one equivalent of halogenide (X – ) (Scheme 10 ).…”

Recent advances in the application of [2 + 2] cycloaddition in the chemical synthesis of cyclobutane-containing natural products

“…The electron transfer strategy can also utilize the excited state transition metal complexes as electron donors to transfer their own single electrons to the LUMO orbitals of the electron-deficient olefin to produce an active radical anion. This electron-rich species can readily undergo nucleophilic attack on another electron-deficient olefinic substrate and subsequently loses a single electron, completing the catalytic cycle to form a carbon–carbon bond [ 102 – 107 ] (Scheme 20 ).

Scheme 20 Mechanism of the [2 + 2] cycloaddition mediated by SET

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