Solvent and Temperature Effects on Diastereodifferentiating Paternó−Büchi Reaction of Chiral Alkyl Cyanobenzoates with Diphenylethene upon Direct versus Charge-Transfer Excitation

Unveiling the impact of the light-source and steric factors on [2+2] heterocycloaddition reactions

Information gained from in-depth mechanistic investigations can be used to control the selectivity of reactions, leading to the expansion of the generality of synthetic processes and the discovery of new reactivity. Here, we investigate the mechanism of light-driven [2+2] heterocycloadditions (Paternò-Büchi reactions) between indoles and ketones to develop insight into these processes. Using ground-state UV-Vis absorption and transient absorption spectroscopy (TAS), together with DFT calculations, we found that the reactions can proceed via an exciplex or electron-donor-acceptor (EDA) complex, which are key intermediates in determining the stereoselectivity of the reactions. We used this discovery to control the diastereoselectivity of the reactions, gaining access to previously inaccessible diastereoisomeric variants. When moving from 370 to 456 nm irradiation, the EDA complex is increasingly favoured, and the diastereomeric ratio (d.r.) of the product moves from >99:<1 to 47:53. In contrast, switching from methyl to ⁱpropyl substitution favours the exciplex intermediate, reversing the d.r. from 89:11 to 16:84. Our study shows how light and steric parameters can be rationally used to control the diastereoselectivity of photoreactions, creating mechanistic pathways to previously inaccessible stereochemical variants.

Mechanisms and Synthetic Strategies in Visible Light-Driven [2+2]-Heterocycloadditions

The synthesis of four membered heterocycles usually requires multi-step procedures and prefunctionalized reactants. A straightforward alternative is the photochemical [2+2]-heterocycloaddition between an alkene and a carbonyl derivative, conventionally based on the photoexcitation of this latter. However, this approach is limited by the absorption profile of the carbonyl, requiring in most of the cases the use of high-energy UV-light, that often results in undesired side reactions and/or the degradation of the reaction components. The development of new and milder visible light-driven [2+2]-heterocycloadditions is, therefore, highly desirable. In this Review, we highlight the most relevant achievements in the development of [2+2]-heterocycloadditions promoted by visible light, with a particular emphasis on the involved reaction mechanisms. The open challenges will also be discussed, suggesting new possible evolutions, and stimulating new methodological developments in the field.

Investigating 3,3-diaryloxetanes as potential bioisosteres through matched molecular pair analysis

Oxetanes have received increasing interest in medicinal chemistry as attractive polar and low molecular weight motifs. The application of oxetanes as replacements for methylene, methyl, gem-dimethyl and carbonyl groups has been demonstrated to often improve chemical properties of target molecules for drug discovery purposes. The investigation of the properties of 3,3-diaryloxetanes, particularly of interest as a benzophenone replacement, remains largely unexplored. With recent synthetic advances in accessing this motif we studied the effects of 3,3-diaryloxetanes on the physicochemical properties of 'drug-like' molecules. Here, we describe our efforts in the design and synthesis of a range of drug-like compounds for matched molecular pair analysis to investigate the viability of the 3,3-diaryloxetane motif as a replacement group in drug discovery. We conclude that the properties of the diaryloxetanes and ketones are similar, and generally superior to related alkyl linkers, and that diaryloxetanes provide a potentially useful new design element.

Chemoselective Photoreaction of Enamides - Divergent Reactivity Towards [3+2]-photocycloaddition vs Paternò-Büchi Reaction

Photoreaction of enamides tethered to a phenyl ketone leads to either [3+2]-photocycloaddition or Paternò-Büchi reaction. This divergence in chemical reactivity originating from the same excited state was dependent on the reaction temperature. At low temperatures, Paternò-Büchi reaction was preferred, while at higher temperatures there was preference towards formation of [3+2]-photoproduct.

The Paternò–Büchi reaction – a comprehensive review

This review represents the most complete description of the scientific results obtained on a photochemical reaction described 110 years ago by an Italian scientist.

A leaning amine-ketone dyad with a nonconjugated linker: solvatofluorochromism and dual fluorescence associated with intramolecular charge transfer

Dyad 4, comprising a triphenylamine (TPA) electron donor and 1,4-pentadien-3-one (pentadienone) electron acceptor tethered by a nonconjugated linker, displays solvatofluorochromism (SFC) and dual fluorescence associated with intramolecular charge transfer (ICT) in the excited state. While the fluorescence arises from a locally excited state of 4 (LE-4*) in saturated hydrocarbon solvents, the fluorescence from the ICT state of 4 (ICT-4*) occurs in aprotic solvents. ICT-4* has a much greater dipole moment than its corresponding ground state. The results of theoretical calculations suggest that the conversion of LE-4* to ICT-4* involves a unique structural change like a leaning of the pentadienone moiety. Two factors are responsible for the significant SFC displayed by 4, the first being the high electron-donating and -accepting abilities of the respective locally excited TPA and pentadienone moieties in LE-4* and the other being a rigid ethano bridge that links the two moieties in ICT-4*. The former property facilitates photoinduced electron-transfer (PET) and the latter prevents full single electron transfer (SET) by prohibiting direct π-conjugation and the spatial approach of the two dyad components. Consequently, these electronic and geometrical features lead to SFC arising from a large dipole moment change caused by ICT and partial intramolecular SET.

Structurally Divergent Lithium Catalyzed Friedel-Crafts Reactions on Oxetan-3-ols: Synthesis of 3,3-Diaryloxetanes and 2,3-Dihydrobenzofurans

The first examples of 3,3-diaryloxetanes are prepared in a lithium-catalyzed and substrate dependent divergent Friedel-Crafts reaction. para-Selective Friedel-Crafts reactions of phenols using oxetan-3-ols afford 3,3-diaryloxetanes by displacement of the hydroxy group. These constitute new isosteres for benzophenones and diarylmethanes. Conversely, ortho-selective Friedel-Crafts reactions of phenols afford 3-aryl-3-hydroxymethyl-dihydrobenzofurans by tandem alkylation-ring-opening reactions; the outcome of the reaction diverging to structurally distinct products dependent on the substrate regioselectivity. Further reactivity of the oxetane products is demonstrated, suitable for incorporation into drug discovery efforts.

Toroidal Interaction and Propeller Chirality of Hexaarylbenzenes. Dynamic Domino Inversion Revealed by Combined Experimental and Theoretical Circular Dichroism Studies

Hexaarylbenzenes (HABs) have greatly attracted much attention due to their unique propeller-shaped structure and potential application in materials science, such as liquid crystals, molecular capsules/rotors, redox materials, nonlinear optical materials, as well as molecular wires. Less attention has however been paid to their propeller chirality. By introducing small point-chiral group(s) at the periphery of HABs, propeller chirality was effectively induced, provoking strong Cotton effects in the circular dichroism (CD) spectrum. Temperature and solvent polarity manipulate the dynamics of propeller inversion in solution. As such, whizzing toroids become more substantial in polar solvents and at an elevated temperature, where radial aromatic rings (propeller blades) prefer orthogonal alignment against the central benzene ring (C6 core), maximizing toroidal interactions.

Contrasting Behaviour of Exciplex Ensembles in the Diastereodifferentiating Paternò–Büchi Reaction of Chiral Cyanobenzoate with Naphthyl- and Phenylethenes on Direct or Charge-Transfer Excitation

The diastereodifferentiating Paternò–Büchi reaction of chiral cyanobenzoate with 1-(1-naphthyl)-1-phenylethene was compared with those with 1,1-diphenylethene on direct and charge-transfer excitations. By desymmetrization of the donor, four diastereomeric oxetane products were formed on photolysis in excellent combined yields. Increase in donor strength induced a stronger charge-transfer interaction both in ground and excited states. Thus, the difference in diastereoselectivities with two different excitation modes (i.e. direct versus charge-transfer) became less significant with a naphthyl derivative as donor. A subtle change of donor–acceptor interaction was shown to have profound effect on the nature of the excited-state complexes and thus the product (stereo)selectivities. Despite a small temperature dependence, an Eyring-type study on the diastereoselectivities confirmed that the excited charge-transfer complex is an excited species distinct from the conventional exciplex.

Exciplex ensemble modulated by excitation mode in intramolecular charge-transfer dyad: effects of temperature, solvent polarity, and wavelength on photochemistry and photophysics of tethered naphthalene-dicyanoethene system

Solvent, temperature, and excitation wavelength significantly affected the photochemical outcomes of a naphthalene-dicyanoethene system tethered by different number (n) of methylene groups (1-3). The effect of irradiation wavelength was almost negligible for 2a but pronounced for 3a. The temperature dependence and theoretical calculations indicated the diversity of exciplex conformations, an ensemble of which can be effectively altered by changing excitation wavelength to eventually switch the regioselectivity of photoreactions.

Oxetane synthesis through the Paternò-Büchi reaction

The Paternò-Büchi reaction is a photochemical reaction between a carbonyl compound and an alkene to give the corresponding oxetane. In this review the mechanism of the reaction is discussed. On this basis the described use in the reaction with electron rich alkenes (enolethers, enol esters, enol silyl ethers, enanines, heterocyclic compounds has been reported. The stereochemical behavior of the reaction is particularly stressed. We pointed out the reported applications of this reaction to the synthesis of naturally occuring compounds.

Organic aspects. Oxygen-containing functions

In this chapter, most of the reported work deals with the photochemistry of carbonyl compounds; however, the photoreactions of other functions, such as the photo-Claisen rearrangement or the photocleavage of cyclic ethers, are also included. In the present volume, time coverage is 2010–2011, and only original research articles are quoted. In general, reviews or purely theoretical calculations are not systematically included. As usually, the material is organized according to established types of reactions (e.g., Norrish I/II, hydrogen abstraction, Paternò-Büchi, photoelimination, photo-Fries/photo-Claisen, etc.). After presenting the basic photochemical aspects, more specific findings are reported. They include synthetic applications, stereoselectivity, and biological or technological implications. Next, the attention is focused on photochemical reactions in anisotropic media, including (micro)heterogeneous or supramolecular systems, solid matrixes or fully organized crystals. Finally, mechanistic studies based on direct experimental evidence are highlighted, especially when transient absorption spectroscopy or related ultrafast detection are employed.

Hydroxy-group effect on the regioselectivity in a photochemical oxetane formation reaction (the Paternò-Büchi Reaction) of geraniol derivatives

The Paternò-Büchi (PB) reaction of geraniol derivatives 1, which contain allylic alcohol functionality and unfunctionalized double bonds, with benzophenone was investigated to see the effect of the hydroxyl group on the regioselectivity of the oxetane formation, i.e., 2/3. At low concentration of geraniol (1a), oxetanes 2a and 3a were formed in a ratio of 2a/3a = ca. 50/50. The oxetane 2a is derived from the PB reaction at the allylic alcohol moiety, whereas the PB reaction at the unfunctionalized double bond produces the oxetane 3a. The PB reaction of the hydroxy-protected methyl ether 1b and acetate 1c gave selectively oxetanes 3b,c derived from the reaction at the more nucleophilic double bond, 2/3∼ 15/85. The hydroxyl-group effect was found to be small, but apparently increased the formation of 2a in the PB reaction with geraniol (1a).