The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Bronsted acids, organocatalysts, enzymes, and transition metal complexes.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00018

Dual Catalysis Strategies in Photochemical Synthesis

For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757

Physical mechanisms of generation and deactivation of singlet oxygen.

The past decade of organic chemistry may be charcterized as a period of violent development of new synthetic methods. This was accompained by a systematization of the analysis of synthetic problems (synthetic strategy). The planning of the synthesis of an organic target molecule is greatly facilitated by distinguishing between reagents X(C)n … with normal reactivity (acceptor properties at C1,3,5…, donor properties at X, C2,4…) and with reactivity umpolung (acceptor properties at X, C2,4…, donor properties at C1,3,5…). In this context, reactivity umpolung turned out to be useful as a heuristic principle, as a classification scheme, and as an aid for locating so-called strategic bonds (synthon, transform, and antithesis according to E. J. Corey). There are six principal methods of umpolung: 1 2n-oxidation, heteroatom exchange and modification, homologation and its reversal, the cyclopropane “trick”, use of acetylenes, and redox reactions; under certain circumstances none of these techniques is necessary in cases where direct umpolung is possible. Throughout the article, normal reactivity is indicated by green print; reactivity umpolung by red print.

Methods of Reactivity Umpolung

The behavior of 1,n-enynes in the presence of transition metals.

Transition Metal-Catalyzed Carbocyclizations in Organic Synthesis.

Pyrazino-TCNQ (1a) prepared from 5, 8-diiodoquinoxaline (4a) is, like TCNQ itself, a strong electron acceptor and gives a stable anion radical salt as well as highly conductive charge-transfer crystals with donors. Substituted derivatives 1b-i were similarly prepared from 3,6-diiodo- 1,2-phenylenediamine (5) as a common intermediate, and bulky substituents such as the phenyl or pyridyl groups have very little effect on either the redox properties or planar geometry of 1a. Neutral radicals 3d-g derived from pyridyl-substituted derivatives 1d-g, respectively, are open-shell donor-[pi]-acceptor systems with high electrical amphotericity designed as a new motif for single-component organic conductors. The powder conductivity of 3f was as high as 3.2 [times] 10[sup [minus]5] S cm[sup [minus]1]. 29 refs., 4 figs., 2 tabs.

Single-component organic conductors based on neutral radicals containing the pyrazino-TCNQ skeleton

Fluorescence quenching rate constants k q by molecular oxygen 3 O 2 ( 3 Σ g - ) were measured anthracenecarbonitriles (AC) in acetonitrile and benzene and for five acridinium ions in acetonitrile. The k q 's for acridinium ions decrease with an increase in the free enthalpy change ΔG of full electron transfer from the first excited singlet fluorescer 1 M * to 3 O 2 in acetonitrile and become smaller than one-hundredth of the diffusion-controlled limit, k diff

New aspects of fluorescence quenching by molecular oxygen

Irradiation of the electron donor-acceptor (EDA) complexes of 2,2-diaryl-1-methylenecyclopropanes, 1,1,2,2-tetraarylcyclopropanes, 2,3-diaryloxiranes, or 2,3-diarylaziridines with tetracyanoethylene (TCNE) under aerated conditions involved oxygenation and/or the (3 + 2) cycloaddition with TCNE. The same oxygenation took place in the dark when the EDA complexes of 2,2-diaryl-1-methylenecyclopropanes or 1,1,2,2-tetraarylcyclopropanes with TCNE were simply stirred in oxygen-saturated solvents. Oxygenation occurred much more efficiently as the solvent polarity and the electron-donative nature of donor substrates increased. 1,2,4,5-Tetramethoxybenzene (TMB) used as a quencher efficiently suppressed oxygenation but not the (3 + 2) cycloaddition at all. Oxygenation occurs through the solvent-separated radical cations diffused from the photogenerated geminate radical ion pairs, whereas the cage coupling of the radical ion pairs involves the (3 + 2) cycloaddition with TCNE.

Simultaneous capture of two distinct radical ion intermediates generated from the EDA complexes of three-membered compounds with TCNE by photoexcitation and in the dark

Heavy-atom effects on the free radical yield {Phi}{sub R} and the triplet yield {Phi}{sub T} of the fluorescence quenching were studied in acetonitrile by using 9,10-dicyanoanthracene as the electron-accepting fluorescer and a series of para-halogenated anisole (I), aniline (II), and N,N-dimethylaniline (III) as the electron-donating quenchers. {Phi}{sub T} increases as the atomic number of the halogen substituent increases for all the systems, whereas {Phi}{sub R} decreases for the system I and does not change for the systems II and III. These heavy-atom effects are interpreted in terms of the spin-orbit coupling between the singlet exciplex and the locally excited triplet state for the system I, and in terms of the spin-orbit coupling between the geminate radical pair state with singlet spin and the locally excited triplet state for the systems II and III.

Tsutomu Miyashi

Papers

Single-component organic conductors based on neutral radicals containing the pyrazino-TCNQ skeleton

New aspects of fluorescence quenching by molecular oxygen

Simultaneous capture of two distinct radical ion intermediates generated from the EDA complexes of three-membered compounds with TCNE by photoexcitation and in the dark

Heavy-atom effects on the excited singlet state electron-transfer reaction

Free anthalpy dependence of free-radical yield of photoinduced electron transfer in acetonitrile