Photochemical Cycloaddition Reactions Research Articles

Template-Controlled Supramolecular Photodimerization of 2-Substituted Anthracenes Mediated by Metal–NHC Complexes

The photochemical cycloaddition reaction has been extensively investigated for many years due to its efficient accessibility for intriguing photochemical products. Nevertheless, such a photoreaction is still an ongoing challenge to control regio- and stereoselectivity during the reaction process. Herein, a series of mononuclear M(I)–NHC (M = metal, NHC = N-heterocyclic carbene) complexes [M(1)2](PF6) (M = Ag, Au; 1 = 1a–c) were designed and synthesized from 2-substituted anthracene derivatives H-1(PF6) (1 = 1a–c) and characterized by nuclear magnetic resonance (NMR) spectroscopy, electrospray ionization-mass spectrometry (ESI-MS), and single-crystal X-ray diffraction analyses. Upon irradiation at 365 nm, the [4 + 4] photodimerization of complexes [M(1)2](PF6) (M = Ag, Au; 1 = 1a–c) successfully proceeded with 100% conversion and gave the syn-head-to-head (HH) photodimers syn-HH-[M(2)](PF6) (M = Ag, Au) as major products. Based on M(I)–NHC templates, syn-HH-[Au(2)](PF6) (2 = 2a–c) was generated in 87–92% relative yields and syn-HH-[Ag(2)](PF6) (2 = 2a–c) in 65–70% relative yields as determined by 1H NMR spectra. This phenomenon revealed the efficient supramolecular control of the M(I)–NHC templates on the regio- and stereoselectivity of the [4 + 4] photodimerization in solution, which can be applied to a series of photoreactions in a controllable manner.

Synthesis of I(III)/S(VI) reagents and their reactivity in photochemical cycloaddition reactions with unsaturated bonds

The development of novel methodologies for the introduction of the sulfoxonium group under mild conditions is appealing but remains underexplored. Herein we report the synthesis of a class of hypervalent iodine reagents with a transferrable sulfoxonium group. These compounds enable mixed iodonium-sulfoxonium ylide reactivity. These well-defined reagents are examined in visible-light-promoted cyclization reactions with a wide range of unsaturated bonds including alkenes, alkynes, nitriles, and allenes. Two distinct cyclization pathways are identified, which are controlled by the substituent of the unsaturated bond. The cycloaddition protocol features simple operation, mild reaction conditions, and excellent functional group tolerance, affording a broad range of sulfoxonium-containing cyclic structures in moderate to excellent yields. Furthermore, the sufoxonium group in the product can be transformed into diverse functional groups and structural motifs via single electron transfer and transition-metal catalysis.

Synthesis of 1,2,5‐Oxadiazinane Derivatives by Photochemical Cycloaddition of Nitrones with Diaminomethanes

Abstract1,2,5‐Oxadiazinanes have a unique heterocyclic skeleton and are expected to be applicable to the synthesis of drugs. However, there have been few reported methodologies to synthesize these structures. We report a facile synthesis of 1,2,5‐oxadiazinanes that is accomplished by the unprecedented photochemical cycloaddition reaction of nitrones with diaminomethanes. The photoreaction is a formal [3+3] cycloaddition, where a nitrone and a diaminomethane act as the 1,3‐dipole and the 1,3‐synthon, respectively. The reaction is significantly accelerated with benzophenone as a photosensitizer. A high degree of diastereoselective addition (d.r. >99 : 1) is observed in the reaction of (Z)‐N‐benzyl‐1‐phenylmethanimine oxide with di(pyrrolidin‐1‐yl)methane.

Modeling and synthesis of novel oxime derivatives as potential cholinesterase inhibitors

Promising test results on biological activity of our previously described benzobicyclo[3.2.1]octadienes, synthetically obtained by intramolecular photochemical cycloaddition reaction, prompted us to pursue with the further functionalization of this basic skeleton toward oxime derivatives. The free energies of formation of complexes between planned new oximes showing promising ADME properties and the active sites of cholinesterases were calculated using docking and density functional theory, showing that thermodynamical stability of some of the examined complexes is the same as the stability of the complex formed with well known and efficient cholinesterase inhibitor huperzine A. On the basis of calculated stabilities of complexes, the synthesis of several representative new compounds was succesfully performed. In some cases, the furan ring opened on two different ways so different opened oximes and oxime ethers were also formed. All new prepared oximes and oxime ethers present good material for further experimental investigation as inhibitors/reactivators of cholinesterases.

A new class of flavonoids bearing macrocyclic polyethers by stereoselective photochemical cycloaddition reaction

A new class of flavonoids bearing cyclic polyethers involving a phenyl ring was conveniently provided by the intramolecular photochemical dimerization of 2-chromonecarboxylic esters. Irradiation of 2-chromonecarboxylate with a polyether tethered at both ends promoted intramolecular [2 + 2] cyclobutane formation leading to 14- to 27-membered cyclic polyethers. The efficiency depended on the substituted position of the phenyl ring, with ortho- and meta-substituted derivatives giving cycloadducts in good chemical yields and quantum efficiencies, whereas the para-derivatives were inert toward photolysis. X-ray crystallographic analysis revealed that the stereochemistry of the macrocyclic cycloadducts exhibited C2-symmetry.

Spin-crossover modulation via single-crystal to single-crystal photochemical [2 + 2] reaction in Hofmann-type frameworks.

This study reports the first modulation of spin-crossover (SCO) behavior via a photochemical [2 + 2] cycloaddition reaction. Here we construct two no-solvent Fe(ii)-Ag(i) bimetallic Hofmann-type frameworks, [Fe(4-spy)2{Ag(CN)2}2] (1) and [Fe(2,4-bpe)2{Ag(CN)2}2] (2) (4-spy = 4-styrylpyridine, 2,4-bpe = trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene). For 1, the dimerization of 4-spy results in a single-crystal to single-crystal (SCSC) transformation from 2D interdigitated layers to a 3D interpenetrated structure. Additionally, a 3D → 3D structural transformation accompanied with Ag(i)-N bond breaking is achieved via the photochemical cycloaddition reaction of 2,4-bpe in 2. More importantly, both the spin transition temperatures and the SCO character are effectively modulated; thus, this approach provides a new strategy for constructing photo-responsive SCO magnetic materials.

Reversible changes of axial chirality of naphthamide by photochemical and thermal reactions

Abstract 2-Propoxy-1-naphthamide possessing a chiral 2-methylpyrrolidine group exists as a mixture of four diastereomers owing to the rotation of the Ar (C O) and C( O) N bonds. The diastereomers existed in the ratio of 32:32:14:22 in CD3OD. When the MeOH solution of the mixture of diastereomers was irradiated with Pyrex filtered UV light (>290 nm) from a high-pressure mercury lamp, the ratio changed to 27:4:43:26. The diastereomeric ratio was reversibly controlled by irradiation and heat. The change was accurately reflected in the intensity of the CD spectra. Furthermore, the reversible axial chirality was transferred to the optical activity of asymmetric reaction products via the photochemical cycloaddition reaction with 9-cyanoanthracene.

Investigation of the photochemical changes of chlorogenic acids induced by ultraviolet light in model systems and in agricultural practice with Stevia rebaudiana cultivation as an example.

Mono- and diacyl chlorogenic acids undergo photochemical trans-cis isomerization under ultraviolet (UV) irradiation. The photochemical equilibrium composition was established for eight selected derivatives. In contrast to all other dicaffeoylquinic acid derivatives, cynarin (1,3-dicaffeoylquinic acid) undergoes a [2 + 2] photochemical cycloaddition reaction, constituting a first example of Schmidt's law in a natural product family. The relevance of photochemical isomerization in agricultural practice was investigated using 120 samples of Stevia rebaudiana leave samples grown under defined cultivation conditions. Ratios of cis to trans chlorogenic acids were determined in leaf samples and correlated with climatic and harvesting conditions. The data indicate a clear correlation between the formation of cis-caffeoyl derivatives and sunshine hours prior to harvesting and illustrate the relevance of UV exposure to plant material affecting its phytochemical composition.

Oxygen-initiated stereoselective thermal isomerisation of a cyclobutane derivative in the solid state.

Solid-state [2+2] photochemical cycloaddition reactions have been extensively studied after the classical work of Schmidt in the 1960s. Of these, trans-1,2-bis(4'-pyridyl)ethylene (bpe) is one of the well-studied alkenes to synthesize tetrakis(4-pyridyl)cyclobutane (tpcb). However, almost all the solid-state [2+2] cycloaddition reactions of bpe yielded, almost exclusively, one of the four possible isomers, namely, the rctt-tpcb (r=regio c=cis and t=trans). Here we describe a stereoselective synthesis of the tetrahedrally disposed rtct-tpcb by the solid-state thermal isomerization of the rctt-isomer in atmospheric air. We propose that this isomerization occurs through a topochemical unimolecular mechanism by a radical chain pathway, initiated by molecular oxygen. This is supported by the nature of products formed in air and nitrogen, detection of a radical in ESR spectral studies, ESI-MS crossover experiments, VT PXRD studies along with QM, MD and docking calculations. The formation of a unique isomer by thermal isomerization may be a general phenomenon to quantitatively synthesize other useful stereoisomers from the existing isomers of cyclobutane derivatives.

Photoreactive gold(i) macrocycles with diphosphine and trans,trans-muconate ligands

A pair of trans,trans-muconate ligands have been successfully aligned in two novel Au(I) macrocycles by design from phosphino metal precursors that undergo photochemical cycloaddition reactions quantitatively, resulting in the formation of cyclooctadiene derivatives.

ChemInform Abstract: Radical Clocks as Probes of 1,4-Biradical Intermediates in the Photochemical Cycloaddition Reactions of 2-Cyclopentenone with Alkenes.

The products of the photochemical reaction of 2-cyclopentenone with 1,6-heptadiene and with vinylcyclopropane have been examined. With 1,6-heptadiene the products were cyclobutanes, which arise from 2 + 2 photocycloaddition between the cyclopentenone carbon–carbon double bond and one of the two terminal double bonds of the heptadiene. The 1,4-biradical that is an intermediate in this reaction contains a derivative of a 1-hexenyl radical; no products derived from cyclization of this intermediate to a cyclopentylmethyl radical were observed. With vinylcyclopropane some of the products isolated also arose from 2 + 2 photocycloaddition; in addition, comparable quantities of products resulting from rearrangement of the intermediate 1,4-biradical were obtained. In this case the initially produced intermediate 1,4-biradical contains a cyclopropylmethyl radical that can rearrange to a homoallylic radical; the products isolated were derived from intramolecular disproportionation in the new 1,7-biradical containing...

ChemInform Abstract: Synthesis of 6,13-Bis(thymidinyl)-5,12-dioxocyclams and the Molecular Structure of the (R,S)-Isomer.

Recently, Hegedus et al. have found an unusual andefficient route for the synthesis of the related 1,4,8,11-tet-raaza 5,12-dions, which involves the acid-catalyzed cleav-age/dimerization of azapenams produced by the photo-chemical cycloaddition reaction between N-protected imida-zolines and chromium (alkoxy)carbene complexes.

Exo-selective formation of bicyclic oxetanes in the photocycloaddition reaction of carbonyl compounds with vinylene carbonate: the important role of intermediary triplet diradicals in the stereoselectivity

The photochemical (2+2) cycloaddition reaction, the Paterno-Buchi (PB) reaction, of carbonyl compounds 1a-d with vinylene carbonate (2) was used to synthesize oxetanodioxolanones 3. The reaction involves the exo-selective (exo/endo > 83/17) formation of the bicyclic oxetanes 3b-d. Computational studies were performed to reveal the equilibrium structures of the intermediary triplet diradicals and their conformational distribution, which play an important role in determining the stereoselectivity of such PB reactions.

Synthetic Chemistry of Quadricyclane

This articale is the first attempt to review chemical transformation of one most interesting synthon among strained, saturated hydrocarbons - tetracyclo[3.2.0.02,704,6]heptane (quadricyclane, Q), from the point of view of synthetic organic chemistry. Thermal [2+2+2] cycloaddition reactions of Q with olefins, acetylenes, carbonyl, nitrogen, sulfur containing compounds, carbenes and silenes along with cycloaddition reactions involving transition metals are reviewed in a first part. Second part of the review covers photochemical transformations, ionic reactions (electrophilic and nucleophilic), oxidation and reduction of Q. Keywords: Quadricyclane, thermal cycloaddition reactions, photochemical cycloaddition reactions, electrophilic reactions, lithiation of C-H bond, oxidation, reduction

Quick Execution of [2 + 2] Type Photochemical Cycloaddition Reaction by Continuous Flow System Using a Glass‐Made Microreactor.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Quick Execution of [2+2] Type Photochemical Cycloaddition Reaction by Continuous Flow System Using a Glass-made Microreactor

Abstract A photochemical [2+2] cycloaddition reaction was executed in a microflow system using glass-made microchannels having 1000-μm width and 500-μm depth. The reaction of cyclohexenones with vinyl acetates in a microflow system under irradiation (300 W, Hg lamp) gave [2+2] cycloaddition products in good yield with a residence time of 2 h, which is a remarkably shortened reaction time compared with a batch system using the same light source.

Crystal structure of methyl endo-8-cyano-exo-8-methyl-3-oxo-2-oxabicyclo[2.2.2]oct-5-ene-6-carboxylate.

A photochemical cycloaddition reaction of methyl 2-pyrone-5-carboxylate with methacrylonitrile mainly gave a [4 + 2] cycloadduct, assigned as methyl endo-8-cyano-exo-8-methyl-3-oxo-2-oxabicyclo[2.2.2]oct-5-ene-6-carboxylate, whose structure was determined by an X-ray diffraction study. The formation of the cycloadduct was reasonably explained by considering a biradical intermediate having a less steric hindrance between the methoxycarbonyl group and the cyano group.

Photochemistry of Cyclic Enones. Part 131. Intermolecular Addition of Styrene and 1-Phenylprop-1-Enes with 3-Phenylcyclopentenone and 3-Phenylcyclohexenone

Arylalkenes undergo photochemical cycloaddition reactions with cyclic enones to give products with specific regio-chemistry and controlled stereochemistry. The regio- and stereo-chemistries can be rationalised in terms of the stability of intermediate radicals.

Geometry optimisation on a hypersphere. Application to finding reaction paths from a conical intersection

A new method for performing constrained optimisation of critical points on the surface of an hypersphere using Newton-Raphson or rational function optimisation (RFO) steps is described. Through the parametrisation of a unitary matrix, it is possible to incorporate the constraint of the fixed distance from the centre of the hypersphere without using Lagrange multipliers. The method is applied to the problem of locating the initial relaxation directions from the vertex of a conical intersection and to the problem of following an intrinsic reaction coordinate along a ridge. An application to the [2π s + 2π s ] ethylene-ethylene photochemical cycloaddition reaction path, which occurs via a rhomboid conical intersection, shows that this reaction occurs in a single step only the reaction channel towards cyclobutane exists and any path to tetramethylene is avoided.

Determination of the Relative Rates of Formation, Fates, and Structures of Triplet 1,4-Biradicals Generated in the Photochemical Cycloaddition Reactions of 2-Cyclopentenones with 2-Methylpropene

Determination of the Relative Rates of Formation, Fates, and Structures of Triplet 1,4-Biradicals Generated in the Photochemical Cycloaddition Reactions of 2-Cyclopentenones with 2-Methylpropene