Tehshik P. Yoon

Efficient visible light photocatalysis of [2+2] enone cycloadditions.

We report that Ru(bipy)3Cl2 can serve as a visible light photocatalyst for [2+2] enone cycloadditions. A variety of aryl enones participate readily in the reaction, and the diastereoselectivity in the formation of the cyclobutane products is excellent. We propose a mechanism in which a photogenerated Ru(bipy)3+ complex promotes one-electron reduction of the enone substrate, which undergoes subsequent radical anion cycloaddition. The efficiency of this process is extremely high, which allows rapid, high-yielding [2+2] cyclizations to be conducted using incident sunlight as the only source of irradiation.

Dual Catalysis Strategies in Photochemical Synthesis

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 Brønsted acids, organocatalysts, enzymes, and transition metal complexes.

Crossed Intermolecular [2+2] Cycloadditions of Acyclic Enones via Visible Light Photocatalysis

Efficient [2+2] heterodimerizations of dissimilar acyclic enones can be accomplished upon visible light irradiation in the presence of a ruthenium(II) photocatalyst. Similar cycloadditions under standard UV photolysis conditions are inefficient and unselective. Nevertheless, a diverse range of unsymmetrical tri- and tetrasubstituted cyclobutane structures can be produced in good yields and excellent diastereoselectivities using this new method. The reaction is promoted by any visible light source, and efficient, gram-scale cycloadditions can be conducted upon irradiating with ambient sunlight.

[2+2] Cycloadditions by Oxidative Visible Light Photocatalysis

Photochemical reactions are remarkable for their ability to easily assemble cyclobutanes and other strained ring systems that are difficult to construct using other conventional synthetic methods. We have previously shown that Ru(bpy)(3)(2+) is an efficient photocatalyst that promotes the [2+2] cycloadditions of electron-deficient olefins with visible light. Here, we show that Ru(bpy)(3)(2+) is also an effective photocatalyst for the [2+2] cycloaddition of electron-rich olefins. This transformation is enabled by the versatile photoelectrochemical properties of Ru(bpy)(3)(2+), which enables either one-electron reduction or one-electron oxidation of interesting organic substrates under appropriate conditions.

A Dual-Catalysis Approach to Enantioselective [2 + 2] Photocycloadditions Using Visible Light

A Dual Approach to 2 + 2 Asymmetric catalysis generally accelerates the pathway to one specific product geometry that can be manipulated by reducing the temperature to slow down competing reactions. It is more difficult to be selective in photochemical reactions, but in the [2 + 2] coupling of olefins to make four-membered rings, Du et al. (p. 392; see the Perspective by Neier) used a ruthenium catalyst that absorbs visible light to activate the substrates below the frequency threshold where they absorb intrinsically. Then a second—a chiral Lewis acid—catalyst directs the product stereochemistry. A major advantage of the dual reactions is that each catalyst can be tuned independently. A two-catalyst combination offers long-sought selectivity in intermolecular coupling of olefins to form four-membered rings. [Also see Perspective by Neier] In contrast to the wealth of catalytic systems that are available to control the stereochemistry of thermally promoted cycloadditions, few similarly effective methods exist for the stereocontrol of photochemical cycloadditions. A major unsolved challenge in the design of enantioselective catalytic photocycloaddition reactions has been the difficulty of controlling racemic background reactions that occur by direct photoexcitation of substrates while unbound to catalyst. Here, we describe a strategy for eliminating the racemic background reaction in asymmetric [2 + 2] photocycloadditions of α,β-unsaturated ketones to the corresponding cyclobutanes by using a dual-catalyst system consisting of a visible light–absorbing transition-metal photocatalyst and a stereocontrolling Lewis acid cocatalyst. The independence of these two catalysts enables broader scope, greater stereochemical flexibility, and better efficiency than previously reported methods for enantioselective photochemical cycloadditions.

[3+2] cycloadditions of aryl cyclopropyl ketones by visible light photocatalysis.

We report a new method for the formal [3+2] reaction of aryl cyclopropyl ketones with olefins to generate highly substituted cyclopentane ring systems. The key initiation step in this process is the one-electron reduction of the ketone to the corresponding radical anion, which is accomplished using a photocatalytic system comprising Ru(bpy)(3)(2+), La(OTf)(3), and TMEDA.

Radical Cation Diels–Alder Cycloadditions by Visible Light Photocatalysis

Ruthenium(II) polypyridyl complexes promote the efficient radical cation Diels-Alder cycloaddition of electron-rich dienophiles upon irradiation with visible light. These reactions enable facile [4 + 2] cycloadditions that would be electronically mismatched under thermal conditions. Key to the success of this methodology is the availability of ligand-modified ruthenium complexes that enable rational tuning of the electrochemical properties of the catalyst without significantly perturbing the overall photophysical properties of the system.

Visible Light Photocatalysis of [2+2] Styrene Cycloadditions by Energy Transfer†

Hip to be square: Styrenes participate in [2+2] cycloadditions upon irradiation with visible light in the presence of an iridium(III) polypyridyl complex. In contrast to previous reports of visible light photoredox catalysis, the mechanism of this process involves photosensitization by energy transfer and not electron transfer.

Enantioselective Conjugate Additions of α-Amino Radicals via Cooperative Photoredox and Lewis Acid Catalysis

We report the highly enantioselective addition of photogenerated α-amino radicals to Michael acceptors. This method features a dual-catalyst protocol that combines transition metal photoredox catalysis with chiral Lewis acid catalysis. The combination of these two powerful modes of catalysis provides an effective, general strategy to generate and control the reactivity of photogenerated reactive intermediates.

Photocatalytic reductive cyclizations of enones : Divergent reactivity of photogenerated radical and radical anion intermediates

Photocatalytic reactions of enones using metal polypyridyl complexes proceed by very different reaction manifolds in the presence of either Lewis or Brønsted acid additives. Previous work from our lab demonstrated that photocatalytic [2+2] cycloadditions of enones required the presence of a Lewis acidic co-catalyst, presumably to activate the enone and stabilize the key radical anion intermediate. On the other hand, Brønsted acid activators alter this reactivity and instead promote reductive cyclization reactions of a variety of aryl and aliphatic enones via a neutral radical intermediate. These two distinct reactive intermediates give rise to transformations differing in the connectivity, stereochemistry, and oxidation state of their products. In addition, this reductive coupling method introduces a novel approach to the tin-free generation of β-ketoradicals that react with high diastereoselectivity and with the high functional group compatibility typical of radical cyclization reactions.