Photochemical transformation of a dithioacetal S-oxide into the corresponding aldehyde

Irradiation of an aldehyde dithioacetal S-oxide gives the corresponding aldehyde. The mechanism of this photochemical transformation is discussed and its application to organic synthesis is also described.     

N‐Heterocyclic Carbene Catalyzed Photoenolization/Diels–Alder Reaction of Acid Fluorides

The combination of light activation and N‐heterocyclic carbene (NHC) organocatalysis has enabled the use of acid fluorides as substrates in a UVA‐light‐mediated photochemical transformation previously observed only with aromatic aldehydes and ketones. Stoichiometric studies and TD‐DFT calculations support a mechanism involving the photoactivation of an ortho‐toluoyl azolium intermediate, which exhibits “ketone‐like” photochemical reactivity under UVA irradiation. Using this photo‐NHC catalysis approach, a novel photoenolization/Diels–Alder (PEDA) process was developed that leads to diverse isochroman‐1‐one derivatives.     

Synthesis and crystalline state photochromism of 3,3′-diaryl biindenylidenedione derivatives

A new series of 3,3′-diaryl biindenylidenedione derivatives were synthesized through Grignard reaction. Some of their stereoisomers were obtained by photochemical transformation upon heating and a plausible reaction mechanism was proposed. Most of these compounds exhibited photochromism in crystalline states as well as generation of stable organic radicals. The absolute configurations of the stereoisomers were determined by single crystal X-ray crystallography. The results showed that the position of substituent could dramatically affect molecular structure and photochemical properties of the biindenylidenedione derivatives.     

Photoinduced Proton‐Transfer Reactions for Mild O‐H Functionalization of Unreactive Alcohols

Hexafluoroisopropanol is typically considered as an unreactive solvent and not as a reagent in organic synthesis. Herein, we report on a mild and efficient photochemical reaction of aryl diazoacetates with hexafluoroisopropanol that enables, under stoichiometric reaction conditions, the synthesis of fluorinated ethers in excellent yield. Mechanistic studies indicate there is a preorganization of hexafluoroisopropanol and the diazoalkane acts as an unreactive hydrogen‐bonding complex. Only after photoexcitation does this complex undergo a protonation‐substitution reaction to the reaction product. Investigations on the applicability of this photochemical transformation show that a broad variety of acidic alcohols can be subjected to this transformation and thus demonstrate the feasibility of this concept for O‐H functionalization reactions (54 examples, up to 98 % yield).     

Polymerization of methyl methacrylate in a tetrahydrofuran–maleic anhydride system. Part II

A donor–acceptor complex consisting of tetrahydrofuran and maleic anhydride initiates photochemical and thermal polymerization of methyl methacrylate. The mechanism of the transformation of this complex was investigated by studying changes in its electrical conductivity, its chemiluminescence, and various influences on its initiating capability (water, air, DPPH, substitution of styrene for methyl methacrylate and of 1,4-dioxane for tetrahydrofuran). It has been shown that initiation by radicals cannot be clearly excluded and that ionic radicals form in the system and can initiate the anionic growth of the chain.     

Plasmonic Activation of a Fluorescent Carbazole–Oxazine Switch

The covalent attachment of a carbazole fluorophore to an oxazine photochrome permits the reversible activation of fluorescence under optical control. Ultraviolet irradiation with a pulsed laser opens the oxazine ring to shift bathochromically the absorption of the carbazole component. Concomitant visible illumination excites selectively the carbazole fluorophore of the photochemical product to produce fluorescence. The photogenerated and fluorescent species reverts spontaneously on a submicrosecond timescale to the initial nonemissive state of the carbazole–oxazine dyad. The photochemical and photophysical properties engineered into this particular molecular switch allow the convenient monitoring of plasmonic effects on photochemical reactions with fluorescence measurements. In close proximity to silver nanoparticles, visible illumination with a continuous‐wave laser also results in fluorescence activation. The metallic nanostructures enable the two‐photon excitation of the oxazine component to induce the photochromic transformation and then facilitate the one‐photon excitation of the photochemical product to generate fluorescence. Thus, these operating principles offer the opportunity to avoid altogether the need of pulsed ultraviolet irradiation to trigger the photochromic transformation and, instead, allow fluorescence activation with a single visible source operating at low illumination power.     

Photocatalyzed Transition-Metal-Free Oxidative Cross-Coupling Reactions of Tetraorganoborates**

Readily accessible tetraorganoborate salts undergo selective coupling reactions under blue light irradiation in the presence of catalytic amounts of transition-metal-free acridinium photocatalysts to furnish unsymmetrical biaryls, heterobiaryls and arylated olefins. This represents an interesting conceptual approach to forge C−C bonds between aryl, heteroaryl and alkenyl groups under smooth photochemical conditions. Computational studies were conducted to investigate the mechanism of the transformation.     

Photochemical conversion and storage of solar energy

The possibilities for the photochemical storage of solar energy are examined from the standpoint of maximum efficiency and mechanism. Loss factors are considered for a general endergonic photochemical reaction and it is concluded that a realistic maximum solar energy storage efficiency for any photochemical system is 15–16%. The natural process of photochemical solar energy storage, namely, photosynthesis, is analyzed and it is found that the maximum solar energy storage efficiency of photosynthesis is 9.5 ± 0.8%. Kinetic and thermodynamic limitations on a photochemical energy storage process are identified and it is shown that the desirable production of hydrogen and oxygen from water probably cannot be sensitized with visible light if only one photochemical step is employed. However, by analogy with the mechanism of photosynthesis, two photochemical reactions operating in series permit a full utilization of the photochemically active part of the solar spectrum. A possible scheme is described and analyzed as to its possibilities and potential difficulties. Finally, some practical considerations are presented not only for the photochemical production of hydrogen but also for solid state photovoltaic devices.     

Efficient photochemical merocyanine-to-spiropyran ring closure mechanism through an extended conical intersection seam. A model CASSCF/CASPT2 study

A mechanism of the thermal and photochemical bleaching of merocyanine to spiropyran is proposed on the basis of CASSCF/CASPT2 calculations on the 6-(2-propenyliden)cyclohexadienone model system. Our results suggest that this photochemical transformation takes place in two steps. First, the initially pumped 1(pi-pi) S2 undergoes radiationless decay to 1(n-pi) S1 via an extended S2/S1 conical intersection seam that runs approximately parallel to the trans-to-cis isomerization coordinate, a few kilocalories per mole higher in energy. Thus, S2 --> S1 internal conversion is possible at all values of the S2 trans-to-cis reaction coordinate. Second, on the S1 potential energy surface, there is a barrierless ring closure reaction path from the S1 cis minimum that leads to a peaked S1/S0 conical intersection where the deactivation to the ground state takes place. The inertia of the moving nuclei then drives the system toward the ground-state minimum of the 2H-chromene product. Thus, the extended seam topology of the S2/S1 conical intersection and the coordinate of the branching space of the S1/S0 conical intersection are essential to explain the efficiency and high speed of this reaction.     

Photochemical Degradation of the Plant Growth Regulator 2‐(1‐Naphthyl) acetamide in Aqueous Solution Upon UV Irradiation

The photochemical degradation of 2‐(1‐naphthyl) acetamide (NAD) in aqueous solution using simulated sunlight excitation as well as UV light within the 254–300 nm range was investigated to obtain an insight into the transformation mechanism that could occur under environmental conditions. Several photoproducts were identified using HPLC/MS/MS techniques. The degradation quantum yield was found to be independent of the excitation wavelength, but showed a dependence of oxygen concentration. This increased by a factor of approximately 3 from aerated to oxygen‐free solutions. There is a clear involvement of both triplet and singlet excited states in NAD photoreactivity. The participation of singlet oxygen as a significant route in NAD degradation was ruled out by comparison with the behavior using Rose Bengal as a photosensitizer. A mechanistic pathway implying hydroxylation process through NAD radical cation species as well as an oxidation reaction by molecular oxygen is proposed. The photochemical behavior of NAD appears to mainly involve the aromatic moieties without any participation of the amide side chain. Toxicity tests clearly show that the generated primary photoproducts are responsible for a significant increase in the toxicity. However, upon prolonged irradiation this toxicity tends to decrease.     

Monitoring Photoinduced Interparticle Chemical Communication In Situ

Monitoring interparticle chemical communication plays a critical role in the nanomaterial synthesis as this communication controls the final structure and stability of global nanoparticles (NPs). Yet most ensemble analytical techniques, which could only reveal average macroscopic information, are unable to elucidate NP-to-NP interactions. Herein, we employ stochastic collision electrochemistry to track the morphology transformation of Ag NPs in photochemical process at the single NP level. By further statistical analysis of time-resolved current transients, we quantitatively determine the dynamic chemical potential difference and interparticle communication between populations of large and small Ag NPs. The high sensitivity of stochastic collision electrochemistry enables the in situ investigation of chemical communication-dependent transformation kinetics of NPs in photochemical process, shedding light on designing nanomaterials.     

Beta-lactam-forming photochemical reactions of N-trimethylsilylmethyl- and N-tributylstannylmethyl-substituted alpha-ketoamides

Two mechanisms have been proposed for the beta-lactam-forming photochemical reactions of alpha-ketoamides. One, suggested by Aoyama, involves excited-state H-atom abstraction while the other, put forth by Whitten, follows a sequential SET-proton-transfer route. The photochemical properties of N-trimethylsilylmethyl- and N-tributylstannylmethyl-substituted alpha-ketoamides were explored in order to gain information about the mechanism of this process and to develop a regioselective method for beta-lactam formation. The results of this effort show that (1) photoreactions of N-trimethylsilylmethyl-substituted alpha-ketoamides proceed by competitive H-atom abstraction and sequential SET-desilylation pathways and (2) a sequential SET-destannylation pathway is preferentially followed in photochemical reactions of the tributylstannylmethyl-substituted alpha-ketoamides.     

维生素C稀溶液光化分解导致的振荡行为

光化学振荡是近十多年才发现和研究的一种新的时空振荡行为目前的文献屈指可数,机制的解释各说不一。维生素C的化学振荡1982年以来仅有个别的报导。我们发现在一定浓度,温度条件下,局部光照维生素C稀溶液,若不搅拌,经过一段前置期可以获得规整性和重现性相当好的衰减式周期性振荡波。本文还研究了它的特征,形成条件及其可能的偶合机制,对维生素C稀溶液光化学反应动力学和机理也进行了探讨。     

Proton or Carbene Transfer? On the Dark and Light Reaction of Diazoalkanes with Alcohols

The formal alkylation reaction of OH groups with diazoalkanes under catalyst-free reaction conditions finds broad application in organic synthesis. However, even today, this reaction is mainly limited to the use of diazomethane as reaction partner. In this combined experimental and theoretical study, we aim at a fundamental understanding of the reaction of diazoalkanes with alcohols to make this transformation amenable to a generalized approach towards formal alkylation reactions of alcohols with diazoalkanes. Experimental and theoretical studies suggest a direct proton transfer only in exceptional cases. In a more general setting, such O−H functionalization proceed both under dark and photochemical conditions via a key hydrogen-bonded singlet carbene intermediate that undergoes a protonation–addition mechanism. We conclude with applications of this approach in O−H functionalization reactions of alcohols, including simple fluorinated, halogenated and aliphatic alcohols and showcase functional-group tolerance of this method in the reaction of biologically active and pharmaceutically relevant alcohols.     

Photochemical electron transfer mediated addition of naphthylamine derivatives to electron-deficient alkenes

Using photochemical electron transfer, N,N-dimethylnaphthylamine derivatives are added to α,β-unsaturated carboxylates. The addition takes place exclusively in the α-position of electron-deficient alkenes and mainly in the 4-position of N,N-dimethylnaphthalen-1-amine. A minor regioisomer results from the addition in the 5-position of this naphthylamine. A physicochemical study reveals that the fluorescence quenching of N,N-dimethylnaphthalen-1-amine is diffusion-controlled and that the back electron transfer is highly efficient. Therefore no transformation is observed at lower concentrations. To overcome this limitation and to induce an efficient transformation, minor amounts of water or another proton donor as well as an excess of the naphthylamine derivative are necessary. A mechanism involving a contact radical ion pair is discussed. Isotopic labeling experiments reveal that no hydrogen is directly transferred between the substrates. The hydrogen transfer to the furanone moiety observed in the overall reaction therefore results from an exchange with the reaction medium. An electrophilic oxoallyl radical generated from the furanone reacts with the naphthylamine used in excess. Concerning some mechanistic details, the reaction is compared with radical and electrophilic aromatic substitutions. The transformation was carried out with a variety of electron-deficient alkenes. Sterically hindered furanone derivatives are less reactive under standard conditions. In a first experiment, such a compound was transformed using heterogeneous electron transfer photocatalysis with TiO(2).     

Molecule‐Induced Radical Formation (MIRF) Reactions—A Reappraisal

Radical chain reactions are commonly initiated through the thermal or photochemical activation of purpose‐built initiators, through photochemical activation of substrates, or through well‐designed redox processes. Where radicals come from in the absence of these initiation strategies is much less obvious and are often assumed to derive from unknown impurities. In this situation, molecule‐induced radical formation (MIRF) reactions should be considered as well‐defined alternative initiation modes. In the most general definition of MIRF reactions, two closed‐shell molecules react to give a radical pair or biradical. The exact nature of this transformation depends on the σ‐ or π‐bonds involved in the MIRF process, and this Minireview specifically focuses on reactions that transform two σ‐bonds into two radicals and a closed‐shell product molecule.     

Solvent and substituent effects on the photochemistry of norbornadiene-diarylacetylene Pauson-Khand adducts

The photochemistry of Pauson-Khand cycloadducts of norbornadiene with a series of bis-aryl alkynes has been studied. Two types of photochemical transformation take place: photorearrangement to tricyclic ketones or photochemical 6π electrocyclization. High selectivity levels have been attained for each pathway, controlled by the polarity of the solvent, irradiation wavelength, and presence (or absence) of oxygen.     

Template-controlled topochemical photodimerization based on "organometallic macrocycles" through single-crystal to single-crystal transformation

Organometallic macrocycles and undergo [2+2] photochemical cycloaddition to form and in quantitative yield, accompanied by a single-crystal to single-crystal transformation.     

正戊烷与SO2气相光化学反应自由基机理的ESR验证

烷烃与SO2的气相光化学作用为自由基反应[1].Penzhorn等[2]对C4以下的气相烷烃与SO2光化学反应产物的复杂性和多样性进行了推测,此后对该光化学反应机理的研究均以反应产物(特别是凝聚态产物)为基础进行的[3].为验证烷烃与SO2光化学反应体系中确实存在自由基,Makarov等[4]向正戊烷与SOz光化学反应体系中引入NO,通过对反应起始阶段的产物的光谱分析和反应动力学研究,论证了该反应的自由基过程.ESR技术是检测自由基的有效方法,Stokes等[5]利用自旋捕集-ESR技术成功地测得了气相羟基自由基的存在.     

Trapping and spectroscopic identification of the photointermediates of bacteriorhodopsin at low temperatures

Light-driven transmembrane proton pumping by bacteriorhodopsin occurs in the photochemical cycle, which includes a number of spectroscopically identifiable intermediates. The development of methods to crystallize bacteriorhodopsin have allowed it to be studied with high-resolution X-ray diffraction, opening the possibility to advance substantially our knowledge of the structure and mechanism of this light-driven proton pump. A key step is to obtain the structures of the intermediate states formed during the photocycle of bacteriorhodopsin. One difficulty in these studies is how to trap selectively the intermediates at low temperatures and determine quantitatively their amounts in a photosteady state. In this paper we review the procedures for trapping the K, L, M and N intermediates of the bacteriorhodopsin photocycle and describe the difference absorption spectra accompanying the transformation of the all-trans-bacteriorhodopsin into each intermediate. This provides the means for quantitative analysis of the light-induced mixtures of different intermediates produced by illumination of the pigment at low temperatures.