Synthetic feasibility of oxygen-driven photoisomerizations of alkenes and polyenes

Role of curvature in acridone for 1 O2 oxidation of a natural product homoallylic alcohol: A novel iso-hydroperoxide intermediate

A density functional theoretical (DFT) study is presented, implicating a 1 O2 oxidation process to reach a dihydrobenzofuran from the reaction of the natural homoallylic alcohol, glycocitrine. Our results predict an interconversion between glycocitrine and an iso-hydroperoxide intermediate [R(H)O+ -O- ] that provides a key path in the chemistry which then follows. Formations of allylic hydroperoxides are unlikely from a 1 O2 'ene' reaction. Instead, the dihydrobenzofuran arises by 1 O2 oxidation facilitated by a 16° curvature of the glycocitrine ring imposed by a pyramidal N-methyl group. This curvature facilitates the formation of the iso-hydroperoxide, which is analogous to the iso species CH2 I+ -I- and CHI2 + -I- formed by UV photolysis of CH2 I2 and CHI3 . The iso-hydroperoxide is also structurally reminiscent of carbonyl oxides (R2 C=O+ -O- ) formed in the reaction of carbenes and oxygen. Our DFT results point to intermolecular process, in which the iso-hydroperoxide's fate relates to O-transfer and H2 O dehydration reactions for new insight into the biosynthesis of dihydrobenzofuran natural products.

Bluer Phototruncation: Retro-Diels-Alder of Heptamethine Cyanine to Trimethine Cyanine through an Allene Hydroperoxide Intermediate

The photoconversion of heptamethine to pentamethine cyanines and of pentamethine to trimethine cyanines was recently reported. Here, we report mechanistic studies and initial experimental evidence for a previously unexplored 4-carbon truncation reaction that converts the simplest heptamethine cyanine to the corresponding trimethine cyanine. We propose a DFT-supported model describing a singlet oxygen (1O2)-mediated formation of an allene hydroperoxide intermediate and subsequent 4-carbon loss through a retro-Diels-Alder process. Fluorescence and mass spectrometry measurements provide evidence of this direct conversion process. This 4-carbon truncation reaction adds to a growing body of cyanine reactivity and may provide an optical tool leading to a substantial blue-shift (Δλem) of ∼200 nm.

Probing the Transition State-to-Intermediate Continuum: Mechanistic Distinction between a Dry versus Wet Perepoxide in the Singlet Oxygen "Ene" Reaction at the Air-Water Interface

A mechanistic study is reported for the reactions of singlet oxygen (¹O₂) with alkene surfactants of tunable properties. Singlet oxygen was generated either top-down (photochemically) by delivery as a gas to an air-water interface or bottom-up (chemically) by transport to the air-water interface as a solvated species. In both cases, reactions were carried out in the presence of 7-carbon (7C), 9-carbon (9C), or 11-carbon (11C) prenylsurfactants [(CH₃)₂C═CH(CH₂)_nSO₃^- Na⁺ (n = 4, 6, 8)]. Higher "ene" hydroperoxide regioselectivities (secondary ROOH 2 to tertiary ROOH 3) were reached in delivering ¹O₂ top-down through air as compared to bottom-up via aqueous solution. In the photochemical reaction, ratios of 2:3 increased from 2.5:1 for 7C, to 2.8:1 for 9C, and to 3.2:1 for 11C. In contrast, in the bubbling system that generated ¹O₂ chemically, the selectivity was all but lost, ranging only from 1.3:1 to 1:1. The phase-dependent regioselectivities appear to be correlated with the "ene" reaction with photochemically generated, drier ¹O₂ at the air-water interface vs those with wetter ¹O₂ from the bubbling reactor. Density functional theory-calculated reaction potential energy surfaces (PESs) were used to help rationalize the reaction phase dependence. The reactions in the gas phase are mediated by perepoxide transition states with 32-41 kJ/mol binding energy for C═C(π)···¹O₂. The perepoxide species, however, evolve to well-defined stationary structures in the aqueous phase, with covalent C-O bonds and 85-88 kJ/mol binding energy. The combined experimental and computational evidence points to a unique mechanism for ¹O₂ "ene" tunability in a perepoxide continuum from a transition state to an intermediate.

Diels-Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application

Upon irradiation at λ = 350 nm, cyclohept-2-enone undergoes an isomerization to the strained (E)-isomer. The process was studied by XMS-CASPT2 calculations and found to proceed by two competitive reaction channels on either the singlet or the triplet hypersurface. (E)-Cyclohept-2-enone is a reactive dienophile in thermal [4 + 2] cycloaddition reactions with various dienes. Ten different dienes were probed, most of which─except for 1,3-cyclohexadiene─underwent a clean Diels-Alder reaction and gave the respective trans-fused six-membered rings in good yields (68-98%). The reactions with furan were studied in detail, both experimentally and by DLPNO-CCSD(T) calculations. Two diastereoisomers were formed in a ratio of 63/35 with the exo-product prevailing, and the configuration of both diastereoisomers was corroborated by single crystal X-ray crystallography. The outcome of the photoinduced Diels-Alder reaction matched both qualitatively and quantitatively the calculated reaction pathway. Apart from cyclohept-2-enone, five additional cyclic hept-2-enones and cyclooct-2-enone were employed in their (E)-form as dienophiles in the Diels-Alder reaction with 1,3-cyclopentadiene (80-98% yield). The method was eventually applied to a concise total synthesis of racemic trans-α-himachalene (four steps, 14% overall yield).

Azobenzene Photoswitching with Near-Infrared Light Mediated by Molecular Oxygen

Efficient photoisomerization between the cis and the trans states of azobenzenes using low-energy light is desirable for a range of applications in, e.g., photobiology yet challenging to accomplish directly with modified azobenzenes. Herein, we utilize molecular iodine as a photocatalyst to induce indirect cis-to-trans isomerization of 4,4'-dimethoxyazobenzene with 770 nm near-infrared light, showing robustness during more than 1000 cycles in ambient conditions. Intriguingly, the catalysis is mediated by molecular oxygen, and we demonstrate that other singlet-oxygen-generating photosensitizers besides iodine, i.e., palladium phthalocyanine, catalyze the isomerization as well. Thus, we envision that the approach can be further improved by employing other catalysts with suitable photoelectrochemical properties. Further studies are needed to explore the applicability of the approach with other azobenzene derivatives.

Defining the Basis of Cyanine Phototruncation Enables a New Approach to Single-Molecule Localization Microscopy

The light-promoted conversion of extensively used cyanine dyes to blue-shifted emissive products has been observed in various contexts. However, both the underlying mechanism and the species involved in this photoconversion reaction have remained elusive. Here we report that irradiation of heptamethine cyanines provides pentamethine cyanines, which, in turn, are photoconverted to trimethine cyanines. We detail an examination of the mechanism and substrate scope of this remarkable two-carbon phototruncation reaction. Supported by computational analysis, we propose that this reaction involves a singlet oxygen-initiated multistep sequence involving a key hydroperoxycyclobutanol intermediate. Building on this mechanistic framework, we identify conditions to improve the yield of photoconversion by over an order of magnitude. We then demonstrate that cyanine phototruncation can be applied to super-resolution single-molecule localization microscopy, leading to improved spatial resolution with shorter imaging times. We anticipate these insights will help transform a common, but previously mechanistically ill-defined, chemical transformation into a valuable optical tool.

Photosensitization Reactions of Biomolecules: Definition, Targets and Mechanisms

Photosensitization reactions have been demonstrated to be largely responsible for the deleterious biological effects of UV and visible radiation, as well as for the curative actions of photomedicine. A large number of endogenous and exogenous photosensitizers, biological targets and mechanisms have been reported in the past few decades. Evolving from the original definitions of the type I and type II photosensitized oxidations, we now provide physicochemical frameworks, classifications and key examples of these mechanisms in order to organize, interpret and understand the vast information available in the literature and the new reports, which are in vigorous growth. This review surveys in an extended manner all identified photosensitization mechanisms of the major biomolecule groups such as nucleic acids, proteins, lipids bridging the gap with the subsequent biological processes. Also described are the effects of photosensitization in cells in which UVA and UVB irradiation triggers enzyme activation with the subsequent delayed generation of superoxide anion radical and nitric oxide. Definitions of photosensitized reactions are identified in biomolecules with key insights into cells and tissues.