Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

This paper reports a room temperature visible light photocatalyzed method for the C–H amination of arenes and heteroarenes. A key enabling advance in this work is the design of N-acyloxyphthalimides as precursors to nitrogen-based radical intermediates for these transformations. A broad substrate scope is presented, including the selective meta-amination of pyridine derivatives. A radical aromatic substitution mechanism is proposed.

http://pubs.acs.org/doi/pdf/10.1021/ja501906x

N-acyloxyphthalimides as nitrogen radical precursors in the visible light photocatalyzed room temperature C-H amination of arenes and heteroarenes

Photokatalyse mit sichtbarem Licht: Welche Bedeutung hat sie für die organische Synthese?

During the past decade, visible light photocatalysis has become a powerful synthetic platform for promoting challenging bond constructions under mild reaction conditions. These photocatalytic systems rely on harnessing visible light energy for synthetic purposes through the generation of reactive but controllable free radical species. Recent progress in the area of visible light photocatalysis has established it as an enabling catalytic strategy for the mild and selective generation of nitrogen-centered radicals. The application of visible light for photocatalytic activation of amides, hydrazones, and imides represents a valuable approach for facilitating the formation of nitrogen-centered radicals. Within the span of only a couple of years, significant progress has been made for expediting the generation of amidyl, hydrazonyl, and imidyl radicals from a variety of precursors. This Perspective highlights the recent advances in visible light-mediated generation of these radicals. A particular emphasis is p...

Photochemical Generation of Nitrogen-Centered Amidyl, Hydrazonyl, and Imidyl Radicals: Methodology Developments and Catalytic Applications

Four different 1,8-naphthalimide derivatives were examined in phosphorescent organic light emitting diodes (OLEDs), i.e., 1,8-naphthalimide, N-phenyl-1,8-naphthalimide, N-2,6-dibromophenyl-1,8-naphthalimide (niBr), and bis-N,N-1,8-naphthalimide. Photoluminescence from all four naphthalimides have violet-blue fluorescence and phosphorescent bands between 550 and 650 nm (visible at 77 K). While all four compounds gave good glassy films when doped with a phosphorescent dopant, only the niBr films remained glassy for extended periods. OLED studies focused on niBr, with two different architectures. One OLED structure (type 1) had the niBr layer as a doped luminescent layer and an undoped niBr layer to act as a hole-blocking layer. The alternate structure (type 2) utilizes a doped CBP layer as the luminescent layer and the niBr layer is used as a hole-blocking layer only (CBP = 4,4'-N,N'-dicarbazolylbiphenyl). Type 1 and 2 OLEDs were prepared with green, yellow, and red emissive phosphorescent dopants (Irppy, btIr, and btpIr, respectively). The dopants were organometallic Ir complexes, previously shown to give highly efficient OLEDs. Of the three dopants, the btpIr-based OLEDs showed the best device performance in both structures (peak efficiencies for type 2: 3.2% and 2.3 lum/W at 6.3 V; type 1: 1.7% and 1.3 lm/W at 6.1 V). The green and yellow dopants gave very similar performance in both type 1 and 2 devices (peak efficiencies are 0.2-0.3%), which were significantly poorer than the btpIr-based OLEDs. The emission spectrum of the btIr- and btpIr-based devices (type 1 and 2) are the same as the solution photoluminescence spectrum of the dopant alone, while the Irppy device gives a broad red emission line (lambda(max) = 640 nm). The red Irppy.niBr emission line is assigned to an Irppy.niBr exciplex. The type 2 Irppy-based device gave a voltage-dependent spectrum, with the red emission observed at low bias (4-8 V), switching over to strong green emission as the bias was raised. All other devices showed bias-independent spectra. Estimates of HOMO, LUMO, and excited-state energies (dopant, niBr, and exciplex) were used to explain the observed spectral properties of these devices. btpIr-based devices emit efficiently from isolated dopant states (external efficiencies = 3.2 %, 2.3 lum/W). Irppy-based devices emit only from exciplex states, with low efficiency (external efficiency = 0.3%). btIr.niBr films have very similar energies for the dopant, exciplex, and niBr triplet states, such that relaxation can go through any of these states, leading to low device efficiency (external efficiency = 0.4%). High device efficiency is achieved only when dopant emission is the dominant pathway for relaxation, since exciplex and niBr triplet states give either weak or no electroluminescence.

J. C. Day

Papers

The chemistry of 1,8-naphthalenedicarboximidyl and phthalimidyl radicals

Imidyl Radicals. The Chemistries of 1,8-Naphthalenedicarboximidyl and Phthalimidyl Radicals.