微反应器中的不对称光化学反应

化学反应的手性诱导一直备受化学家的关注,虽然不对称热化学合成和手性技术已经取得了巨大的进展,但不对称光化学反应的研究远远没有取得相应的成功.激发态寿命短、活化能低是导致其对映选择性低的主要原因.最新的研究表明,采用含手性空间或经手性修饰的微环境可以使光化学反应的立体选择性大大提高.本文针对这一热点问题,综述在微反应器中进行不对称光化学反应的研究进展.     

Chirally modified zeolites as reaction media: photochemistry of an achiral tropolone ether

[structure: see text] Zeolites modified with chiral inductors serve as media for performing chiral induction during photochemical reactions of organic molecules. The photochemical behavior of achiral tropolone ethyl phenyl ether illustrates this unique feature of a zeolite.     

Chiral Inductions in Excited State Reactions: Photodimerization of Alkyl 2‐Naphthoates as a Model

Enantioselectivity in organic transformations continues to be a topic major interest in organic photochemistry. In the last decade, synergistic combination of photocatalysis and organocatalysis has emerged as a powerful strategy to gain enantioselectivity in photochemical reactions, and remarkable achievements have been obtained. In this strategy, the asymmetric induction is provided in ground state. In contrast, in the conventional enantioselective photochemistry, the chiral induction is controlled in electronic excited state, and to achieve high stereoselectivity is still a formidable challenge. Because the reactions of excited states often yield strained products with unique structures in single step that are difficult to form by thermal reactions, the development of new strategies attempted to achieve enantioselectivity in excited state reactions is still highly desired. Since the short excited state lifetime and low activation energy for reaction in excited state leave little room for manipulating the chiral induction, in order to gain enantioselectivity the substrate molecule has to already reside in a chiral environment during the excitation step. Chiral auxiliaries and chiral supramolecular hosts can provide such environments. In this presentation, we summarize the studies employing chiral auxiliary and chiral microreactor approaches to achieve high asymmetric inductions in excited state reactions performed in our laboratory. We chose the photodimerization of alkyl 2‐naphthoates as a reaction model to give deeper insights into the basic factors controlling chiral induction in excited state.     

Enantioselective photoreduction of arylalkyl ketones via restricting the reaction to chirally modified zeolite cages

Obtaining a high enantiomeric excess during a photoreaction within a zeolite is hampered by the statistical distribution of reactant and chiral inductor molecules within the cages of a zeolite. By restricting the photoreactions to only those cages that contain both the reactant and a chiral inductor, one should be able to avoid reactions that yield racemic products. This approach is illustrated with the photoreduction of an arylalkyl ketone by a chiral inductor with an amino group.     

Value of zeolites in asymmetric induction during photocyclization of pyridones, cyclohexadienones and naphthalenones

Two strategies, namely chiral inductor and chiral auxiliary approaches, have been examined within zeolites with the aim of achieving asymmetric induction during the photocyclization of cyclohexadienone, naphthalenone and pyridone derivatives. Within zeolites, enantioselectivity as high as 55% and diastereoselectivity as high as 88% have been obtained. The observed stereoselectivities are significant given the fact that these reactions gave very little stereoselectivities in isotropic solution media. The results obtained on the photocyclization of dienones, naphthalenones and N-alkyl pyridones within zeolites compliment our earlier investigations on the photocyclization of tropolone derivatives, the geometric isomerization of 1,2-diphenylcyclopropanes and 2,3-diphenyl-1-benzoyl cyclopropanes, and the Norrish type II reaction of alpha-oxoamides, phenyl adamantyl ketones, phenyl norbornyl ketones and phenyl cyclohexyl ketones. With the help of these examples, we have established the importance of zeolite and its charge compensating cations in effecting asymmetric induction in photochemical reactions.     

PHOTOCHEMISTRY OF ARYLMETHYL ESTERS ADSORBED ON ZSM-5 ZEOLITE──SIZE/SHAPE SELECTIVITY

本文研究了对甲基苯乙酸苄基酯(BCO₂A)及苯乙酸1-萘基酯(ACO₂N)在ZSM-5沸石中的光化学反应。研究发现,光解BCO₂A在ZSM-5沸石中主要生成BA,而光解ACO₂N只生成AA及NN.两种酯的产物分布和在乙腈溶液中的光解完全不同。这些结果是由ZSM-5沸石孔腔对底物分子的尺寸及形状具有选择性而引起的。     

Role of cations and confinement in asymmetric photochemistry: enantio- and diastereo-selective photocyclization of tropolone derivatives within zeolites

Asymmetric induction in photochemical reactions has been explored using the photochemistry of tropolones as a model. Three approaches have been examined: chiral inductor, chiral auxiliary and [chiral inductor + chiral auxiliary]. All three methods gave excellent asymmetric induction in zeolite and very little or zero induction in solution. Results presented on tropolones clearly illustrate the remarkable influence that a confined space studded with cations can have on asymmetric induction. Tropolone derivatives, upon irradiation undergo 4pi-electron electrocyclization to yield a bicyclic product and a rearranged product. Enantiomeric excess up to 68% has been achieved in the cyclized product. In systems where a chiral inductor has been covalently linked, diastereomeric excess as high as 88% has been achieved within a zeolite while the same system in solution gave 10%.     

Enantioselective Catalysis of Photochemical Reactions

The nature of the excited state renders the development of chiral catalysts for enantioselective photochemical reactions a considerable challenge. The absorption of a 400 nm photon corresponds to an energy uptake of approximately 300 kJ mol^?1. Given the large distance to the ground state, innovative concepts are required to open reaction pathways that selectively lead to a single enantiomer of the desired product. This Review outlines the two major concepts of homogenously catalyzed enantioselective processes. The first part deals with chiral photocatalysts, which intervene in the photochemical key step and induce an asymmetric induction in this step. In the second part, reactions are presented in which the photochemical excitation is mediated by an achiral photocatalyst and the transfer of chirality is ensured by a second chiral catalyst (dual catalysis).     

Supramolecular photochemistry concepts highlighted with select examples

‘Supramolecular photochemistry’ (SP) deals with a study of the properties of molecules in their excited states where the medium plays a significant role. While ‘molecular photochemistry’ (MP) deals with studies in isotropic solution, the SP deals with reactant molecules that interact weakly with their surroundings. The surroundings in general are highly organized assemblies such as crystals, liquid crystals, micelles, and host–guest structures. The behavior of exited molecules in SP unlike in isotropic solution is controlled not only by their inherent electronic and steric properties but also by the immediate surroundings. The weak interactions that control the chemistry include van der Walls, hydrophobic, CH⋯π, π⋯π and several types of hydrogen bonds. In this review the uniqueness of SP compared to MP is highlighted with examples chosen from reactions in crystals, micelles and host–guest assemblies. In spite of distinctly different structures (crystals, micelles, etc.) the influence of the medium could be understood on the basis of a model developed by G.M.J. Schmidt for photoreactions in crystals. The principles of reaction cavity model are briefly outlined in this review. There are a few important features that are specific to SP. For example, highly reactive molecules and intermediates could be stabilized in a confined environment; they enable phosphorescence to be observed at room temperature and favor chiral induction in photochemical reactions. Using such examples the uniqueness of SP is highlighted. The future of SP depends on developing efficient and unique catalytic photoreactions using easily available reaction ‘containers’. In addition, their value in artificial photosynthesis should be established for SP to occupy a center stage in the future.     

有机化学新领域──固态光化学的研究

介绍了有机化学新领域──固相光化学研究的重要意义和特点。通过色氨酸类、核酸碱基衍生物同稠环化合物异种分子间固态光化学的研究，揭示出这种反应的高选择性和专一性，对反应历程也进行了初步研究，为合成化学提供了新方法。利用晶格控制物质，使不易进行光化学反应的物质可顺利地定向进行光化学反应，亦可直接用于合成Ｄ或Ｌ手性化合物，省去拆分。对Ｃ－Ｔ络合物（电荷转移）和分子化合物的固态光化学也进行了研究。     

Radiation-induced reactions via the lowest excited states in cinnamic acid crystals

Radiation-induced reactions of cinnamic acid derivatives have been examined and compared with photoreactions in the crystalline state; all the reaction products were exactly the same as those of the photoreactions, indicating that the reactions proceed only via the lowest excited state to give [2 + 2] cycloadducts, E/Z isomerization products, or starting molecules.     

Enhanced enantio- and diastereoselectivity via confinement and cation binding: yang photocyclization of 2-benzoyladamantane derivatives within zeolites

Irradiation of 2-benzoyladamantane derivatives in zeolites yields the endo-cyclobutanols as the only photoproduct via a gamma-hydrogen abstraction process. The cyclobutanols readily undergo retro-aldol reaction to give delta-ketoesters. The enantiomeric excess (ee) in the endo-cyclobutanols is measured by monitoring the ee in the ketoesters. Whereas in solution the ee in the product ketoester is zero, within achiral NaY zeolite, in the presence of a chiral inductor such as pseudoephedrine, ee's up to 28% have been obtained. The influence of zeolite on several chiral esters of 2-benzoyladamantane-2-carboxylic acids has also been examined. Whereas in solution the diastereomeric excess is <15%, in zeolite the delta-ketoesters are obtained in 79% de (best examples). Ab initio computations suggest that enhancement of chiral induction within zeolites is likely to be due to cation complexation with the reactant ketone. Alkali ion-organic interaction, a powerful tool, is waiting to be fully exploited in photochemical and thermal reactions. In this context zeolites could be a useful medium as one could view them as a reservoir of "naked" alkali ions that are only partially coordinated to the zeolite walls.     

Photo‐Organocatalytic Enantioselective Radical Cascade Reactions of Unactivated Olefins

Radical cascade processes are invaluable for their ability to rapidly construct complex chiral molecules from simple substrates. However, implementing catalytic asymmetric variants is difficult. Reported herein is a visible‐light‐mediated organocatalytic strategy that exploits the excited‐state reactivity of chiral iminium ions to trigger radical cascade reactions with high enantioselectivity. By combining two sequential radical‐based bond‐forming events, the method converts unactivated olefins and α,β‐unsaturated aldehydes into chiral adducts in a single step. The implementation of an asymmetric three‐component radical cascade further demonstrates the complexity‐generating power of this photochemical strategy.     

Mechanistic Organic Photochemistry

Organic photochemical reactions can be understood as transformations of the electronically excited states of the reactant molecules. By considering Lewis structure or molecular orbital representations of these excited states it is possible to outline the several possible reactions available in the case of a given reactant. A number of different types of photochemical transformations are now reasonably well understood. In these cases one finds the same common controlling feature, namely the tendency for an excited state species to follow mechanistic pathways of minimum energy and the requirement for continuous electron redistribution in following these pathways. These preferred transformations can often be selected by inspection of relative bond orders for different types of bonding, by comparison of the potential energy surfaces available to the excited state molecules, and by use of correlation diagrams. The reactions derive from both singlet and triplet states, and one of the more reliable methods now available for identifying excited states reacting is termed the “fingerprint method”. Examples of the author's mechanistic approach are given both for ketone and for hydrocarbon photochemistry.     

受限介质中光化学反应的研究

提高化学反应的选择性始终是化学家关注的热点. 本工作以光Fries重排反应、Norrish/Yang光环化反应、9-取代蒽衍生物的光二聚反应以及不对称光化学反应为例, 结合近年来在受限介质中光化学反应研究的最新进展, 总结了利用受限介质控制化学反应选择性的一般规律.     

Nafion as an efficient reaction medium for diastereoselective photochemical reactions

The ability of alkali ion exchanged Nafion interior to enhance the influence of chiral auxiliaries in photochemical reactions was examined with three independent asymmetric photoreactions, namely, photoisomerization of trans,trans-2,3-diphenylcyclopropane-1-carboxylic acid derivatives, electro-cyclization of 2-oxo-1,2-dihydropyridine-1-acetic acid derivatives and oxa di-π methane rearrangement of 1,2-dihydro-2,2′-dimethylnapthalenone derivatives. Chiral auxiliaries that lead to 1:1 mixture of diastereomers in organic solvents yielded products in 5-21% diastereomeric excess when the photoreactions were carried out within the nanoclusters of alkali ion exchanged Nafion. The investigated systems also serve as probes to understand the mode of distribution of guest molecules within Nafion framework upon inclusion.     

Asymmetric induction during photocyclization of chiral and achiral alpha-oxoamides within achiral zeolites

The photochemistry of 31 alpha-oxoamides capable of undergoing gamma-hydrogen transfer has been examined within zeolites. These molecules, upon excitation, yield two products--a beta-lactam and oxazolidinone--in solution, both resulting from gamma-hydrogen transfer. While in benzene the major product is oxazolidinone, within an MY zeolite, the main product is a beta-lactam. In this investigation, we have focused our attention on asymmetric induction in the formation of the beta-lactam product. Two approaches--using a chiral inductor and chiral auxiliary--have been employed. While in solution, in the presence of chiral inductors, achiral alpha-oxoamides yield beta-lactams with zero enantioselectivity; within zeolites, an ee of up to 44% has been achieved. Alpha-oxoamides appended with a chiral auxiliary gave beta-lactams with less than 5% diastereoselectivity in solution while within zeolites, the same alpha-oxoamides gave the products with de's of up to 83%. Such a remarkable influence of zeolites is attributed to an alkali ion interaction with the reactant alpha-oxoamides and to the confined environment of the zeolite interior. At this stage, we have not been able to provide a model with predictive power and further work is needed to understand this valuable asymmetric induction strategy.     

Use of chirally modified zeolites and crystals in photochemical asymmetric synthesis

Three different approaches to asymmetric induction in the cis-to-trans photoisomerization of a number of 1-benzoyl-2,3-diphenylcyclopropane derivatives are reported: the use of chiral inductors and covalent chiral auxiliaries in MY zeolites and the use of ionic chiral auxiliaries in crystals. High levels of asymmetric induction were achieved using the latter two methods-up to 71% through the use of covalent chiral auxiliaries in zeolites and a remarkable 99% via the solid state ionic chiral auxiliary approach. In the zeolite method, the diastereomeric excess was found to depend strongly on the nature of the zeolite cation, M(+), and in the ionic chiral auxiliary approach, evidence is presented that it is the fixed orientation of the benzoyl group with respect to the cyclopropane ring that controls enantioselectivity in the crystalline state-a finding that is directly relevant to theoretical work on this topic.     

The primary photodynamics of aqueous nitrate: formation of peroxynitrite

We have examined the photochemical reactions occurring after irradiation at 200 nm of the aqueous nitrate ion, NO3(-)(aq). Using femtosecond transient absorption spectroscopy over the range 194-388 nm, we have characterized the formation and subsequent relaxation of the primary photoproducts of nitrate photolysis. The dominant photoproduct is the cis-isomer of peroxynitrite, which accounts for 48% of the excited state molecules initially produced. A slightly smaller fraction, 44%, of the excited molecules return to the electronic ground state of NO3(-) and relax to the vibrational ground state in 2 ps. The remaining 8% of the molecules initially excited react via the *NO + *O2(-) or the NO- + O2 dissociation channels. Formation of NO2(-) and *NO2 is not observed, suggesting that the previous observations of these species in steady-state photolysis are caused by reactions occurring on a longer time scale.     

微反应器控制的化学反应的选择性

研究利用分子筛、Nafion薄膜、低密度聚乙烯薄膜和囊泡作为微反应器控制有机光化学反应的方向,提高反应的选择性和可能性.在NaY沸石或低密度聚乙烯薄膜中,带有长烷基链或醚链的二芳基化合物光二聚只生成分子内的加成产物,而不生成分子间的加成产物,从而在底物浓度很大的情况下,高选择性地合成了大环化合物.通过控制底物和敏化剂分子在ZSM-5沸石、Nafion薄膜和囊泡中的分布高选择性地控制烯烃光敏氧化反应的方向,单一地生成单重态氧的氧化产物或超氧负离子的氧化产物.利用Nafion薄膜作为介质进行光诱导电子转移,得到超长寿命的电荷分离态.