Addition and cycloaddition reactions via photoinduced electron transfer. In: Mattay J, editor. Electron Transfer I. Berlin: Springer Berlin Heidelberg; 1994. pp. 301-46. [DOI: 10.1007/3-540-57565-0_79]

Enantioselective Cyanofunctionalization of Aromatic Alkenes via Radical Anions

Alkene radical ions constitute an integral and unique class of reactive intermediates for the synthesis of valuable compounds because they have both unpaired spins and charge. However, relatively few synthetic applications of alkene radical anions have emerged due to a dearth of generally applicable and mild radical anion generation approaches. Precise control over the chemo- and stereoselectivity in alkene radical anion-mediated processes represents another long-standing challenge due to their high reactivity. To overcome these issues, here, we develop a new redox-neutral strategy that seamlessly merges photoredox and copper catalysis to enable the controlled generation of alkene radical anions and their orthogonal enantioselective cyanofunctionalization via distonic-like species. This new strategy enables highly regio-, chemo-, and enantioselective hydrocyanation, deuterocyanation, and cyanocarboxylation of alkenes without stoichiometric reductants or oxidants under visible light irradiation. This protocol provides a new blueprint for the exploration of the transformation potential of alkene radical anions.

Recent Visible Light and Metal Free Strategies in [2+2] and [4+2] Photocycloadditions

When aiming to synthesize molecules with elevated molecular complexity starting from relatively simple starting materials, photochemical transformations represent an open avenue to circumvent analogous multistep procedures. Specifically, light-mediated cycloadditions remain as powerful tools to generate new bonds begotten from non-very intuitive disconnections, that alternative thermal protocols would not offer. In response to the current trend in both industrial and academic research pointing towards green and sustainable processes, several strategies that meet these requirements are currently available in the literature. This Minireview summarizes [2+2] and [4+2] photocycloadditions that do not require the use of metal photocatalysts by means of alternative strategies. It is segmented according to the cycloaddition type in order to give the reader a friendly approach and we primarily focus on the most recent developments in the field carried out using visible light, a general overview of the mechanism in each case is offered as well.

Visible-light mediated oxidative ring expansion of anellated cyclopropanes to fused endoperoxides with antimalarial activity

Hetero- and carbocyclic anellated cyclopropanes were converted in one step by a visible light induced photooxidation to their corresponding polycyclic endoperoxides, which show promising antimalarial activity.

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4 , whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

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.

Photoinduced tandem three-component coupling of propanedinitrile, 2,5-dimethylhexa-2,4-diene, and cyanoarenes

A tandem three-component coupling photoreaction proceeds upon photoirradiation of MeCN/H2O solutions containing propanedinitrile (1, malononitrile), 2,5-dimethylhexa-2,4-diene (2), and polycyanoarenes in the presence of phenanthrene and carbonate, leading to selective alpha-monoalkylation of 1. The reaction proceeds via photo-NOCAS (Nucleophile-Olefin Combination, Aromatic Substitution) type mechanism: nucleophilic attack of the anion of 1 to photogenerated 2(*+) is followed by ipso-substitution on the radical anion of the polycyanoarene. It advances under mild, safe, and environmentally friendly conditions such as proceeding at ambient temperature without metals and halogens, and in the presence of weak base. The reaction also represents a novel and metal-free cross-coupling reaction that leads to ipso-substitution on polycyanoarene via aryl-cyano bond cleavage. In addition, the reaction is a rare example of introducing carbon nucleophile in the photoinduced electron transfer reaction, except that of cyanide ion.

Transition metal complexes coordinated by an NAD(P)H model compound and their enhanced hydride-donating abilities in the presence of a base

The ruthenium(II) and rhenium(I) complexes containing an NAD(P)H model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), as ligand, [Ru(tpy)(bpy)(BNAH)]2+ (1 a) and [Re(bpy)(CO)3(BNAH)]+ (1 b), were quantitatively produced by the reaction of the corresponding metal hydrido complexes with BNA(+) (1-benzylnicotinamidium cation). In the presence of base with pK(a) = 8.9, 1 a and 1 b have much greater reducing power than "free" BNAH. The oxidation potentials of 1 a in the absence and the presence of triethylamine were 0.55 V and -0.04 V, respectively, versus Ag/AgNO(3), whereas that of "free" BNAH was 0.30 V. Spectroscopic results clearly showed that the base extracts a proton from the carbamoyl group on 1 a and 1 b to give the deprotonated BNAH coordinating to the transition-metal complexes [Ru(tpy)(bpy)(BNAH-H+)]+ (3 a) and [Re(bpy)(CO)3(BNAH-H+)] (3 b); this deprotonation underlies the enhancement in reducing ability. The deprotonated forms 3 a and 3 b can efficiently reduce other NAD(P) models to give the corresponding 1,4-dihydro form, resulting in the deprotonated BNA+ being coordinated to the metal complexes [Ru(tpy)(bpy)(BNA(+)-H+)]2+ (2 a) and [Re(bpy)(CO)3(BNA+-H+)]+ (2 b); "free" BNAH and the protonated adducts 1 a and 1 b cannot act in this way. X-ray crystallography was performed on the PF6- salt of 2 a, and showed that the deprotonated nitrogen atom on the carbamoyl group coordinates to the ruthenium(II) metal center with a bond length of 2.086(3) Angstroms. Infrared spectral data suggested that the deprotonated carbamoyl group on the reduced forms 3 a and 3 b is converted to the imido group, and that the oxygen atom coordinates to the metal center.

Photoinduced Electron Transfer Reactions of alpha-Cyclopropyl- and alpha-Epoxy Ketones. Tandem Fragmentation-Cyclization to Bi-, Tri-, and Spirocyclic Ketones

Reductive photoinduced electron transfer (PET) reactions have been performed with various bicyclic alpha-cyclopropyl-substituted ketones and tertiary amines. The reaction resulted in a regioselective cleavage of one cyclopropyl bond under formation of an exocyclic radical with an endocyclic enolate unit. In the case of bicyclic ketones with an unsaturated side chain, various bicyclic, spirocyclic, and tricyclic products are accessible via radical cyclization, depending on the position of the alkenyl substituent. In addition to triethylamine, N-silylated amines have also been used as electron donors, leading to a variety of compounds, among them are silylated fragmentation products, indicating that a proton is transferred from not only the amine radical cation but also the cationic silyl group. The intramolecular Paternó-Büchi reaction has also been studied for cyclopropane derivatives of the jasmone type leading to tetracyclic oxetanes. Finally, alpha-epoxy-substituted ketones have been investigated under PET conditions, yielding ring-opened products.