Photochemical Isomerization Reactions Research Articles

A comprehensive spectroscopic, solvatochromic and photochemical analysis of 5-hydroxyquinoline and 8-hydroxyquinoline mono-azo dyes

A series of novel substituted-azo dyes 8-(aryldiazenyl)quinolin-5-ol (5a-i) were synthesized by the coupling reaction of 5-hydroxyquinoline with diazotized aniline derivatives in the presence of NaNO2 in HCl/H2O mixture. The study of the spectroscopic and solvatochromic properties were performed by FT-IR, 1H and 13C-NMR and UV-Visible spectroscopies. The tautomerism of these dyes was studied using the deuteration technique and solvatochromic measurements. Photochromic properties of these 5-hydroxyquinoline azo dyes were also examined via E/Z and Z/E photochemical isomerization reactions and compared with the existing 8-hydroxyquinoline analogous. The novel substituted-azo dyes exhibited higher Z/E thermal isomerization rates and have larger absorbance wavelength range than their 8-hydroxyquinoline analogous, making them potential molecular switches.

Dewar Heterocycles as Versatile Monomers for Ring-Opening Metathesis Polymerization

We report the utility of readily available heterocycles as precursors to unique ring-opening metathesis polymerization (ROMP) monomers. Photochemical valence isomerization reactions of pyridones, dihydropyridines, and pyrones dearomatize the parent heterocycles to their highly strained Dewar isomers, which readily engage in controlled ROMP reactions using Grubbs catalysts. This strategy is used to access polymer backbones that contain strained β-lactam and azetidine cores, which can be further derivatized using post-polymerization chemistries. We demonstrate this through the synthesis of water-soluble β-amino acid polymers that have potential applications as biomedical materials, along with the synthesis of highly-soluble poly(acetylene) derivatives, which have potential applications as organic conductive materials derived from bio-feedstock chemicals.

Mechanistic Investigations on the Photoisomerization Reactions of Five-Membered Ring Heterocyclic Molecules Containing Sulfur and Selenium Atoms.

The restricted active space self-consistent field method in the 26-electron/27-orbital active space and the 6-311(d) basis set has been used to investigate the mechanisms of the photochemical isomerization reactions concerning the model systems of 1,2,3-thiadiazole and 1,2,3-selenadiazole. The computational works suggest that the preferred reaction paths for both 1,2,3-thiadiazole and 1,2,3-selenadiazole are as follows: reactant → Franck–Condon region → conical intersection → intermediate → transition states → photoproducts. As a result, the structures of the conical intersections, which play a decisive role in these photoisomerization reactions, are obtained. In particular, the present theoretical evidences demonstrate that the potential energy surfaces for the formation of 1,3-diradicals are quite flat. This may explain why their experimental detections are so difficult.

Photochemical isomerization reactions of acrylonitrile. A mechanistic study.

The mechanisms for the photochemical isomerization reactions are determined theoretically using the acrylonitrile model molecule. The CASSCF (twelve-electron/eleven-orbital active space) and MP2-CAS methods are respectively used with the 6-311G(d,p) and 6-311++G(3df,3pd) basis sets. The structure of the conical intersection that plays a prominent role in the photoisomerization of acrylonitrile is obtained. The intermediates and the transition structures of the ground states are also calculated, to allow a qualitative explanation of the reaction pathways. These model studies suggest that the preferred reaction route is: acrylonitrile → Franck–Condon region → conical intersection → isoacrylonitrile → transition state → intermediate complex → transition state → cyanoacetylene. The theoretical evidence suggests that conical intersections found in this paper can give a better understanding of the photochemical reactions of acrylonitrile and support the experimental observations.

Mechanistic Investigations of the Photochemical Isomerizations of [(CO)5MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes.

The mechanisms for the photochemical isomerization reactions are determined theoretically using group 6 Fischer carbene complexes (CO)5M=C(Me)(OMe) (M = Cr, Mo, and W) and the complete-active-space self-consistent field (CASSCF) (10-orbital/8-electron active space) and second-order Møller–Plesset perturbation (MP2-CAS) methods with the Def2-SVPD basis set. The structures and energies of the singlet/singlet conical intersections and the triplet/singlet intersystem crossings, which play a decisive role in these photoisomerizations, are determined. The former is applied to the chromium and molybdenum systems because their photoproducts are essentially from the singlet excited states. The latter is applied to the tungsten complex because its photoproducts are formed from a low-lying triplet excited state. Two reaction pathways are examined in this work: photocarbonylation (path I) and CO-photoextrusion (path II). The model studies strongly indicate that in the photochemistry of Cr and Mo Fischer carbene systems, the formation of metallaketene intermediates may occur at higher excitation wavenumbers, whereas the five-coordinated complexes that are attached by a solvent molecule are obtained at lower excitation wavenumbers. However, in the W analogue, because the activation barriers for path I are greater than that for path II and path I has more reaction steps than path II, the quantum yields for the metallaketene intermediate should be smaller than those for the five-coordinated species, which is also attached by a solvent molecule. These theoretical studies also suggest that the conical intersection and the spin crossover mechanisms that are identified in this work explain the process well and support the experimental observations.

Photocatalytic E → Z Isomerization of Alkenes

The current renaissance of organic photochemistry and photocatalysis has emboldened non-specialists to explore this rich landscape in search of solutions to long-standing challenges in synthesis. Many iconic stoichiometric photochemical transformations that were often platforms for mechanistic explorations have found limited application in contemporary synthesis, likely due to a perceived difficulty in reaction execution and the dearth of scope. Fortunately, many of these valuable processes are now being translated into robust methods, often with the aid of catalysis. The selective E → Z photochemical isomerization of olefins exemplifies this growing trend. Whilst examples of photochemical isomerization reactions are ubiquitous in physical organic textbooks, their intricate mechanistic delineations often contrast starkly with highly restricted substrate scopes: In the case of carbogenic scaffolds, electron-rich styrenes and stilbenes continue to dominate despite their limited synthetic potential. In this Account , our recent development of a photocatalytic isomerization of activated olefins is discussed. Inspired by the antipodal Z → E process with retinal that constitutes the basis of the mammalian visual system, and a qualitative report describing the reverse E → Z directionality enabled by crystalline (–)-riboflavin localized in the eye, critical noncovalent interactions have been distilled allowing this latter biological process to be reengineered. The result is an expansive, operationally trivial isomerization of substrates containing the cinnamyl motif using an inexpensive, readily available photocatalyst. Reliant on a selective excitation mechanism, this process complements the pioneering work of Hammond, Arai, and others by expanding the scope of olefin isomerization. In the case of carboxylic acids, it was possible to generate the coumarin scaffold directly by exploiting the two discrete activation modes of riboflavin sequentially, namely energy transfer (E T ) followed by single electron transfer (SET). This first example of a “ one catalyst, two activation modes ” strategy constitutes a photocatalytic variant of covalent cascade catalysis and provides a new entry to an important class of pharmaceutical building blocks. Reaction development is placed in an historical context, and key findings from other laboratories that facilitated the study are highlighted. 1 Introduction 2 A Brief Introduction to Photochemistry 3 Photochemical Isomerization of Olefins 4 The Bioinspired Isomerization of Activated Olefins 5 Emulating Coumarin Biosynthesis with (–)-Riboflavin 6 Conclusions

ChemInform Abstract: Photochemical Reactions of Fac‐Rhenium(I) Tricarbonyl Complexes and Their Application for Synthesis

AbstractReview: 79 refs.

A model study on the photochemical isomerization of cyclic silenes.

The mechanism for the photochemical isomerization reaction is investigated theoretically using a model system of a five-membered-ring silene and the CAS(8,8)/6-311G(d) and MP2-CAS-(8,8)/6-311++G(3df,3pd)//CAS(8,8)/6-311G(d) methods. These model investigations indicate that the preferred reaction route for a five-membered-ring silene, which leads to the photorearrangement product, is as follows: reactant → Franck-Condon region → conical intersection→ photoproduct. In other words, the direct mechanism is a one-step process that has no barrier. These theoretical results agree with the available experimental observations.

Mechanistic Study of the Photochemical Isomerization Reactions of Silabenzene

The mechanisms for photochemical isomerization reactions were examined theoretically using a model system of a parent silabenzene with CAS(6,6)/6-311G(d) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d) methods. Five reaction pathways leading to five types of photoisomers have been investigated. The theoretical computations indicate that conical intersections play a prominent role in the photoisomerization of silabenzenes. The model investigations reveal that the preferred reaction route for silabenzene should result in the corresponding silabenzvalene, rather than the Dewar silabenzene isomer. Moreover, the theoretical computations suggest that all of the photochemical mechanisms of silabenzene should proceed as follows: reactant → Franck–Condon region → conical intersection → photoproduct. In other words, the photochemical mechanism for silabenzene should be a barrierless, single-step process. The computational results agree well with available experimental observations.

A theoretical study of the photochemical isomerization reactions of (+)-2-carene-4α-methanol from the triplet state

The mechanisms for the photochemical isomerization reactions are investigated theoretically using a model system of (+)-2-carene-4α-methanol (2-carene) 1, using the CASSCF (eight-electron/six-orbital active space) and MP2-CAS methods and the 6-311(d) and 6-311++G(3df, 3pd) basis sets, respectively. The structures of the intersystem crossings, which play a crucial role in these photo-isomerization reactions, are identified. The intermediates and transition structures of the ground state are also calculated to provide a qualitative explanation of the reaction pathways. The model investigations suggest that the preferred reaction route for 1 is as follows: singlet reactant → Franck–Condon region → triplet reactant → triplet transition state → intersystem crossing → intermediate → singlet transition state → photoproduct. In particular, the intersystem-crossing mechanism in this work gives a better explanation and supports the available experimental observations. Two types of reaction pathways that lead to final photoproducts are identified: (path I) ring expansion to form a cycloheptene ring and (path II) ring closure to form a methylcyclohexene structure. Both exhibit a biradical character. These theoretical results agree well with the available experimental observations.

Model study of the photochemical rearrangement pathways of 1,2,4-oxadiazole.

The mechanisms of photochemical isomerization reactions are investigated theoretically by using a model system of 1,2,4- oxadiazole with the CAS(14,9)/6-311G(d) and MP2-CAS-(14,9)/ 6-311++G(3df,3pd)//CAS(14,9)/6-311G(d) methods. Three reaction pathways are examined, including 1) the direct mechanism, 2) the ring contraction-ring expansion mechanism, and 3) the internal cyclization-isomerization mechanism, which lead to two types of photoisomers. The theoretical findings suggest that conical intersections play a crucial role in the photorearrangement of 1,2,4-oxadiazoles. These model investigations also indicate that the preferred reaction route for 1,2,4-oxadiazole, which leads to phototransposition products, is as follows: reactant → Franck-Condon region → conical intersection → photoproduct. In other words, the direct mechanism is a one-step process that has no barrier. These theoretical results agree with the available experimental observations.

A model study on the photochemical isomerization of isothiazoles and thiazoles.

The mechanisms for photochemical isomerization reactions are studied theoretically using three model systems: 3-methylisothiazole, 4-methylisothiazole and 5-methylisothiazole. The CASSCF (ten-electron/seven-orbital active space) and MP2-CAS methods are used with the 6-311G(d) and 6-311++G(3df,3pd) basis sets, respectively. Three mechanisms, the internal cyclization-isomerization route (path A), the ring contraction-ring expansion route (path B) and the direct route (path C), are used to determine the actual photochemical reaction mechanism for these three model molecules. The structures of the conical intersections, which play a crucial role in these photo-transpositions, are determined. The intermediates and transition structures of the ground states are also calculated, to give a qualitative explanation of the reaction pathways. These model investigations suggest that the preferred reaction route is: reactant → Franck-Condon region → conical intersection → photoproduct. Particularly, the direct mechanism (path C) described in this work gives a better explanation than other pathways proposed before and is supported by the experimental observations. The results obtained allow a number of predictions to be made.

Mechanistic Investigations on the Photoisomerization Reactions of 1,2‐Dihydro‐1,2‐Azaborine

The mechanisms of the photochemical isomerization reactions were investigated theoretically by using a model system of 1,2-dihydro-1,2-azaborine with the CAS(6,6)/6-311G(d,p) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d,p) methods. Three reaction pathways, which lead to three kinds of photoisomers, have been examined. The structures of the conical intersections, which play a decisive role in such photorearrangements, were obtained. The thermal (or dark) reactions of the reactant species have also been examined by using the same level of theory to assist in providing a qualitative explanation of the reaction pathways. The model investigations suggest that the preferred reaction route for 1,2-dihydro-1,2-azaborine, which leads to the Dewar 1,2-dihydro-1,2-azaborine photoproduct, is as follows: reactant→Franck-Condon region→conical intersection→photoproduct. The results obtained allow a number of predictions to be made.

A Computational Study of the Mechanisms of the Photoisomerization Reactions of Monocyclic and Bicyclic Olefins

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of cyclohexene (1), cycloheptene (2), norbornene (3), and two bicyclic olefins (4 and 5) using the CASSCF (six-electron/six-orbital active space) and MP2-CAS methods with the 6-311(d,p) basis set. The structures of the conical intersections, which play a decisive role in such photoisomerizations, were obtained. The intermediates and transition structures of the ground state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Two photoreaction pathways were examined in the present work. The first can produce a photoproduct with an extra ring. The other can yield a photoproduct with a smaller ring with an external double bond. Both pathways involve cyclic carbene intermediates. Also, our model investigations suggest that both reaction pathways follow a similar photochemical pattern as follows: reactant → Franck-Condon region → conical intersection → cyclic carbene intermediate → transition state → photoproduct. Moreover, these two reaction pathways can compete with each other since the energetics of their conical intersection points are quite similar. Our present theoretical results agree with the available experimental observations.

Computational Study of the Mechanisms of the Photoisomerization Reactions of Bicycloalkene

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system, bicyclo[4,1,0]hept-3-ene (1), with the CASSCF (six-electron/six-orbital active space) and MP2-CAS methods and the 6-311(d,p) basis set. The structures of the conical intersections, which play a decisive role in such phototranspositions, were obtained. The intermediates and transition structures of the ground state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route for bicyclo[4,1,0]hept-3-ene is as follows: reactant → Franck-Condon region → conical intersection → intermediate → transition state → photoproduct. Two reaction paths, which lead to final photoproducts, have been identified: (path I) ring expansion and (path II) ring closure. The former is more favorable than the latter. Also, our theoretical findings strongly indicate that there is a substantial interaction between the cyclopropane moiety and the isolated carbon-carbon double bond in the excited state of 1.

Efficient Syntheses of 5-X-B10H13 Halodecaboranes via the Photochemical (X = I) and/or Base-Catalyzed (X = Cl, Br, I) Isomerization Reactions of 6-X-B10H13

High yield syntheses of the 5-X-B(10)H(13) (5X) halodecaboranes have been achieved through the photochemical (X = I) or base-catalyzed (X = Cl, Br, I) isomerization reactions of their 6-X-B(10)H(13) (6X) isomers. 5I was obtained in 80% isolated yield upon the UV photolysis of 6I. Treatment of 6X (X = Cl, Br, I) with catalytic amounts of triethylamine at 60 degrees C led to the formation of 78:22 (Cl), 82:18 (Br), and 86:14 (I) ratio 5X/6X equilibrium mixtures. The 5X isomers were then separated from these mixtures by selective crystallization (Br and I) or column chromatography (Cl), with the supernatant mixtures in each case then subjected to another round of isomerization/separation to harvest a second crop of 5X. The combined isolated yields of pure products after two cycles were 71% 5-Cl-B(10)H(13), 83% 5-Br-B(10)H(13), and 68% 5-I-B(10)H(13). The previously proposed structures of 5-Br-B(10)H(13) and 5-I-B(10)H(13) were crystallographically confirmed. Deprotonation of 6X and 5X with 1,8-bis(dimethylamino)naphthalene (PS) resulted in the formation of [PSH(+)][6X(-)] and [PSH(+)][5X(-)]. Density functional theory-gauge-independent atomic orbital (DFT/GIAO) calculations and crystallographic determinations of [PSH(+)][6Cl(-)] and [PSH(+)][6Cl(-)] confirmed bridge-deprotonation at a site adjacent to the halogen-substituted borons. NMR studies of the 6-Br-B(10)H(13) isomerization induced by stoichiometric amounts of PS showed that following initial deprotonation to form 6-Br-B(10)H(12)(-), isomerization occurred at 60 degrees C to form an equilibrium mixture of 6-Br-B(10)H(12)(-) and 5-Br-B(10)H(12)(-). DFT calculations also showed that the observed 5-X-B(10)H(13)/6-X-B(10)H(13) equilibrium ratios in the triethylamine-catalyzed reactions were consistent with the energetic differences of the 5-X-B(10)H(12)(-) and 6-X-B(10)H(12)(-) anions. These results strongly support a mechanistic pathway for the base-catalyzed 6X to 5X conversions involving the formation and subsequent isomerizations of the 6X(-) anions. While triethylamine did not catalyze the isomerization reactions of either 6-(C(6)H(13))-B(10)H(13) or 6,9-(C(6)H(13))(2)-B(10)H(12), it catalyzed the isomerization of 6-X-9-(C(6)H(13))-B(10)H(12) to 5-X-9-(C(6)H(13))-B(10)H(12) resulting from halo, but not alkyl rearrangement. Comparisons of the chemical shift values found in the temperature-dependent (11)B NMR spectra of 6Cl(-) and 6F(-) with DFT/GIAO chemical shift calculations indicate the fluxional behavior observed for these anions results from a process involving hydrogen migration around the open face that leads to the averaging of some boron resonances at higher temperatures.

Theoretical Investigations of the Photochemical Isomerizations of Indoxazene and Isoxazole

The mechanisms of the photochemical isomerization reactions were investigated using two model systems of indoxazene (1) and isoazole-3-carboxylate (5) with the CASSCF and MP2-CAS methods using the 6-311G(d,p) basis set. The active space of the former consists of 14 electrons in 11 orbitals, while that of the latter consists of 10 electrons in seven orbitals. Two reaction pathways were examined in the present work. They are referred to as the internal cyclization-isomerization path (paths I and III) and the direct path (paths II and IV). Our model investigations suggest that the preferred reaction route for these species is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the direct (conical intersection) mechanism found in this work gives a better explanation than the previous proposed mechanism and also supports the available experimental observations. Additionally, a simple p-pi orbital topology model is proposed that can be used as a diagnostic tool to predict the location of the conical intersections as well as the geometries of the phototransposition products of various heterocycles.

A computational study of the photosomerization reactions of bicyclic cyclopropene

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of bicyclic cyclopropene 1 using the CASSCF (six-electron/six-orbital active space) and MP2–CAS methods and the 6-311(d,p) basis set. Our model investigations suggest that the preferred reaction route for 1 is as follows: reactant → Franck–Condon region → conical intersection → intermediate → transition state → photoproduct. In particular, the conical intersection mechanism found in this work gives a good explanation and supports the experimental observations. In addition, we suggest a cyclic vinylcarbene model, which can be used as a guide tool to predict the location at which conical intersections are likely to occur, as well as the conformations of the photoisomerization products.

A computational study of photochemical isomerization reactions of thiophenes

The mechanisms of the photochemical isomerization reactions were investigated theoretically using three model systems; 2-methylthiophene, 2-cyanothiophene, and 2-phenylthiophene. The CASSCF (10-electron/eight-orbital active space) and MP2-CAS methods were employed with the 6-311(d) basis set. Three mechanisms, i.e., the internal cyclization-isomerization route (path A), the zwitterion-tricyclic route (path B), and the direct route (path C), have been used to explore the real photochemical reaction mechanism of these three model molecules. The structures of the conical intersections, which play a key role in such phototranspositions, were obtained. The intermediates and transition structures of the ground states were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route is as follows: reactant --> Franck-Condon region --> conical intersection --> photoproduct. In particular, the conical intersection mechanism described in this work gives a better explanation than either the previously proposed internal cyclization-isomerization (path A) or the zwitterion-tricyclic pathway (path B) mechanisms, and is supported by the experimental observations. The results obtained allow a number of predictions to be made.

Quantum Mechanical Study of the Photoisomerizations of Bicyclo[4,1,0]hept-2-ene (2-Norcarene)

The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of bicyclo[4,1,0]hept-2-ene (2-norcarene) 1 with the CASSCF (eight-electron/eight-orbital active space) and MP2-CAS methods and the 6-311(d) basis set. The structures of the conical intersections and intersystem crossings, which play a crucial role in such photoisomerization reactions, were obtained. The intermediates and transition structures of the ground-state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred singlet photoreaction route for 1 is as follows: singlet reactant --> Franck-Condon region --> conical intersection --> intermediate --> transition state --> photoproduct. On the other hand, our theoretical findings indicate that the preferred triplet photoreaction route for 1 is as follows: singlet reactant --> Franck-Condon region --> triplet minimum --> triplet transition state --> intersystem crossing --> intermediate --> singlet transition state --> photoproduct. In particular, the intersystem crossing mechanism found in this work gives a better explanation and supports the available experimental observations. Two kinds of reaction pathways, which can lead to final photoproducts, have been identified: (paths I or III) ring-expansion to form a cycloheptene ring and (paths II or IV) ring-closure to form a methylcyclohexene structure. Both exhibit biradical character. Also, our theoretical investigations strongly indicate that substantial interaction occurs between the cyclopropane moiety and the isolated carbon-carbon double bond in the excited state of (1).