Photochemical reaction kinetics in optically dense media: The influence of thermal reactions

The kinetics of photochemical reactions in optically dense media essentially free from diffusion was considered. The photochromic isomerization A ↔ B was studied as an example. If thermal isomerization is possible, a stationary state is achieved in time determined by rate constants for the thermal reactions. The concentration wave profile is changed during the photochemical reaction propagation. Low values of thermal reaction constants and decrease in sample optical density during photochemical isomerization were found to be essential for maximal wave penetration into the sample. Sharp concentration gradients of A and B can be observed when both the optical density is increased during photochemical isomerization and the quantum yield of the direct photochemical reaction A → B is higher than that of the reverse photochemical reaction B → A.     

Ruthenium(II) pyridylamine complexes with diimine ligands showing reversible photochemical and thermal structural change

Ruthenium(II)-TPA-diimine complexes, [Ru(TPA)(diimine)]2+ (TPA=tris(2-pyridylmethyl)amine; diimine=2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpm), 1,10-phenanthroline (phen)) were synthesized and characterized by spectroscopic and crystallographic methods. Their crystal structures demonstrate severe steric hindrance between the TPA and diimine ligands. They exhibit drastic structural changes on heating and photoirradiation at their MLCT bands, which involve partial dissociation of the tetradentate TPA ligand to exhibit a facially tridentate mode accompanied by structural change and solvent coordination to give [Ru(TPA)(diimine)(solvent)]2+ (solvent=acetonitrile, pyridine). The incoming solvent molecules are required to have pi-acceptor character, since sigma-donating solvent molecules do not coordinate. The thermal process is irreversible dissociation to give the solvent-bound complexes, which takes place by an interchange associative mechanism with large negative activation entropies. The photochemical process is a reversible reaction reaching a photostationary state, probably by a dissociative mechanism involving a five-coordinate intermediate to afford the same product as obtained in the thermal reaction. Quantum yields of the forward reactions to give dissociated products were lower than those of the backward reactions to recover the starting complexes. In the photochemical process, the conversions of the forward and backward reactions depend on the absorption coefficients of the starting materials and those of the products at certain wavelength, as well as the quantum yields of those reactions. The reversibility of the motions can be regulated by heating and by photoirradiation at certain wavelength for the recovery process. In the bpm system, we could achieve about 90 % recovery in thermal/photochemical structural interconversion.     

Rearrangements of the Diels-Alder cycloadducts obtained from acetylenic sulfones and 1,3-diphenylisobenzofuran

1,3-Diphenylisobenzofuran afforded Diels-Alder cycloadducts 4a,b with n-butyl- and phenyl-substituted acetylenic sulfones 3a,b, respectively. The products underwent various types of rearrangements under pyrolytic, acid-catalyzed, and photochemical conditions. In the presence of acid, or upon heating in xylenes, they afforded the ketones 5a,b. In addition, the dehydration product 7a was produced from the pyrolysis of 4a, and the unexpected transposed ketone 6b was generated under acid-catalyzed or pyrolytic conditions from 4b via a postulated epoxide intermediate. The photolysis of 4a afforded ketone 5a as the sole isolated product, whereas 5b afforded oxepin 8b and indenyl phenyl ketone 9b. The formation of the latter two products can be rationalized by a series of pericyclic reactions. These include an intramolecular [2+2] cycloaddition, followed by a 1,3-dipolar cycloreversion, for the transformation of 4b to 8b and a series of electrocyclic and [1,3]sigmatropic reactions to convert 8b into 9b.     

Photochemical acetalization of carbonyl compounds in protic media using an in situ generated photocatalyst

Carbonyl compounds are conveniently converted into their corresponding dimethyl acetals in good yields and short reaction times by means of a photochemical reaction in methanol with a catalytic amount of chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone, CA) as the sensitizer. Using aldehydes gives better results than using ketones, which also tend to form enol ethers as side products. These results are similar to those of simple acid-catalyzed acetalization reactions, suggesting the involvement of a photochemically generated acid. On the basis of steady state and laser flash photolysis data the reaction is proposed to involve the in situ generation of a photocatalyst (2,3,5,6-tetrachloro-1,4-hydroquinone, TCHQ) via reaction of CA with the solvent. The acetalization process is initiated by ionization of TCHQ, followed by loss of a proton to the solvent or the carbonyl, which starts a catalytic reaction. The photocatalyst is regenerated via a disproportionation reaction.     

A density functional study of propylene glycol conversion to propanal and propanone of various acid-catalyzed reaction models: a water-addition effect

The acid-catalyzed models on reaction mechanisms of pinacol rearrangement of propylene glycol conversion to propanal and propanone have been investigated using the density functional method at 298.15 K. Thermodynamic quantities of activation steps of four water-addition models were obtained. The number of added water interacting with the transition states of three concerted pathways has obviously affected the product ratio. The relative energetic profiles of the conversion reactions of all solvation models have been comparatively displayed. Estimation of the percent ratio of product composition computed from activation free energies of each acid-catalyzed reaction model was carried out. The percent ratios of propanal and propanone were decreased as the number of added water increased.     

Alkali ion exchanged Nafion as a confining medium for photochemical reactions

Methanol-swollen Nafion beads were used as microreactors to control the photochemical reaction pathways. Product selectivity in three unimolecular reactions, namely, the photo-Fries rearrangement of naphthyl esters, Norrish Type I reaction of 1-phenyl-3-p-tolyl-propan-2-one and Norrish Type I and Type II reactions of benzoin alkyl ethers were examined. The influence of cations over the photodimerization of acenaphthylene and cross-photodimerization between acenaphthylene and N-benzyl maleimide included within Nafion were also examined. The photochemical behaviors of the above substrates were significantly altered within Nafion compared with their solution photochemistry. Of particular interest, the product distributions were found to depend on the counter cations of Nafion.     

A facile solution phase combinatorial synthesis of tetrasubstituted pyridines using the Bohlmann-Rahtz heteroannulation reaction

The solution phase combinatorial Bohlmann-Rahtz reaction gives highly functionalized pyridine libraries from enamino esters and alkynones in a single synthetic step. Good product ratios and library purities were obtained in reactions catalyzed by zinc(II) bromide, the acid-catalyzed heteroannulation procedure offering considerable improvements over traditional methodology.     

Reactions of radical cations of acetals: Evidence for unimolecular decomposition

The thermal and photochemical reactions of the methylal radical cation /I/ in freon matrices were studied using selective deuteration for elucidating the structure of the resulting species. /I/ has been shown to decay by unimolecular reaction upon heating to 140 K as well as upon photolysis in CFCl₃ matrix and the product of decay has been assumed to be the complex of formaldehyde radical cation with CFCl₃. Such decay reaction has been demonstrated for 1,3-dioxolan radical cation as well.     

Effects of conformation on the stereochemistry of solvent attack on benzylic beta-hydroxycarbocations: mechanisms of epoxide hydrolysis reactions

The rates and products from the acid-catalyzed and the pH-independent reactions of two diastereomeric 6-methoxy-trans-1,2,3,4,4a,10a-hexahydrophenanthrene 9,10-oxides (5b and 7b), along with their cis and trans chlorohydrins, have been determined in dioxane/water solutions. The mechanisms of the acid-catalyzed hydrolysis of 5b and 7b involve rate-limiting formation of benzylic carbocations (6b and 8b), which have sufficient lifetimes to be trapped by azide ion. Each carbocation is stabilized by the 6-methoxy group and held in single conformation by the adjacent trans-fused cyclohexane ring. The stereochemistry of the attack of water on each carbocation is independent of whether the precursor is an epoxide, a cis chlorohydrin, or a trans chlorohydrin, and the major diol hydrolysis product from each compound results from the axial attack of a solvent molecule on the carbocation intermediate. The hydrolysis of the trans chlorohydrin formed from the reaction of 5b with HCl exhibits a common ion rate depression. The major product from the pH-independent reaction of 5b is a trans diol, and the major product from the pH-independent reaction of 7b is an isomeric ketone. The rate of the pH-independent reaction of 7b is >10(4) times faster than that of 5b.     

Functionalized fluorenes via dicationic electrophiles

Dicationic fluorenyl cations are shown to react with nitriles to provide amide-functionalized fluorenes. A similar reaction with alcohols gives ether derivatives. The chemistry is initiated by the reactions of N-heterocyclic ketones in a superacidic solution. This leads to cyclizations involving 2-biphenyl groups and formation of the reactive fluorenyl cations.     

Rearrangement of Aryl Geranyl Ethers

This article describes the thermal rearrangement reactions of aryl geranyl ethers. These reactions depend on the structure of the aryl moiety of the substrate and the reaction conditions used. The naphthyl ethers underwent a [1,3]-alkyl shift, followed by acid-catalyzed intramolecular cyclization. The microwave-assisted rearrangement of isoquinolinyl ether showed a pattern of an abnormal Claisen rearrangement. The multi step rearrangement of the quinolyl ether afforded a spiro compound. These new reactions were used to synthesize novel heterocyclic compounds.     

Using tandem mass spectrometry to predict chemical transformations of 2-pyrimidinyloxy-N-arylbenzyl amine derivatives in solution

Tandem mass spectrometry is used to predict the chemical transformations of 2-pyrimidinyloxy-N-arylbenzyl amine derivatives. Compound 1, N-2-2-4,6- dimethoxypyrimidin-2-yloxy benzylamino phenyl benzamide was selected as a model to present our idea. The CID reactions of protonated 1 include an intramolecular S(N)2 reaction and a cyclodehydration reaction. Under in-source CID conditions, deprotonated 1 undergoes a Smiles rearrangement reaction and then dissociates to the ion at m/z 349. Theoretical computations were invoked to shed light on the reaction mechanisms of 1 by the semiempirical PM3 method. These studies of gas-phase reactions show the reactivity of some potential reaction centers in this molecule, which inspired us to explore the solution phase analogous reactions of 1. Further experiments show that 1 has two analogous reactions in acidic solution: the acid-catalyzed cyclodehydration reaction and the acid-catalyzed Smiles rearrangement reaction. Moreover, 1 undergoes the base-catalyzed Smiles rearrangement under basic conditions. The present study demonstrates that mass spectrometry can play an important role in predicting the chemical solution phase transformations of 2-pyrimidinyloxy-N-arylbenzyl amine derivatives.     

The Theory of Pericyclic Reactions

Pericyclic reactions include thermal and photochemical cycloadditions and cycloreversions, electrocyclic reactions and sigmatropic shifts. Configuration interaction analysis can be used for making predictions about preferred reaction pathways, about reaction rates, about the nature of possible intermediate steps, and above all about the stereochemistry of such reactions.     

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).     

Dual wavelength asymmetric photochemical synthesis with circularly polarized light

Asymmetric photochemical synthesis using circularly polarized (CP) light is theoretically attractive as a means of absolute asymmetric synthesis and postulated as an explanation for homochirality on Earth. Using an asymmetric photochemical synthesis of a dihydrohelicene as an example, we demonstrate the principle that two wavelengths of CP light can be used to control separate reactions. In doing so, a photostationary state (PSS) is set up in such a way that the enantiomeric induction intrinsic to each step can combine additively, significantly increasing the asymmetric induction possible in these reactions. Moreover, we show that the effects of this dual wavelength approach can be accurately determined by kinetic modelling of the PSS. Finally, by coupling a PSS to a thermal reaction to trap the photoproduct, we demonstrate that higher enantioselectivity can be achieved than that obtainable with single wavelength irradiation, without compromising the yield of the final product.     

Photoproduct selectivity in reactions involving singlet and triplet excited states within bile salt micelles

Generally, photochemical reactions tend to give more than one product. For such reactions to be useful one should be able to control them to yield a single product. Of the many approaches used in this context, the use of reaction media with features different from those of isotropic solutions has been very effective. We provide results of our studies on four reactions within bile salt micelles (cholic acid and deoxycholic acid). These four reactions involve homolytic cleavage of a C-C or C-O bond to yield either a singlet or triplet radical pair. The bile salt micelles control the rotational and translational mobilities of the radical pair, resulting in photoproduct selectivity. The dynamic nature of the bile salt micelles results in differential effects on the singlet and triplet radical pairs.     

Photochemical and thermal reactions of iron pentacarbonyl with dimethyldiacetylene (DMDA): spectral characterization of reaction complexes

A series of photochemical and thermal reactions has been carried out with iron pentacarbonyl and the conjugated alkadiyne dimethyldiacetylene (DMDA). One product (DMDA)Fe(CO)₆, was prepared directly from starting materials in tetrahydrofuran under photochemical conditions. Previously analogs of this complex had been prepared only under thermal conditions from complex intermediates. An X-ray analysis has established the structure of this complex; ¹³C NMR and other spectral data reported here are in agreement. Under thermal conditions in xylene, three isomeric forms of a tricarbonylferroleiron tricarbonyl (DMDA)₂Fe₂(CO)₆ complex were prepared. Separation and purification of two of these isomers was achieved; selected spectral data are reported. Infrared and mass spectra are also reported for the complex (DMDA)₂Fe(CO)₄ resulting from the photochemical reaction of starting materials in cyclohexane.     

Matrix isolation investigation of the photochemical reaction of benzene with CrCl2O2 and OVCl3

The matrix isolation technique, combined with infrared spectroscopy, has been used to characterize the products of the photochemical reactions of benzene with CrCl(2)O(2) and OVCl(3). While initial twin jet deposition of the reagents led to no visible changes in the recorded spectra, strong product bands were noted following irradiation with light of lambda > 300 nm. Wavelength dependence studies determined that light of lambda < 590 nm led to reaction and oxygen atom transfer, forming an eta(1)-complex between 2,4-cyclohexadienone and CrCl(2)O. The identification of the complex was further supported by isotopic labeling ((13)C and (2)H) and by density functional calculations at the B3LYP/6-311G++(d,2p) level. Merged-jet experiments in which thermal reactions are examined were also conducted, at temperatures as high as 150 degrees C. No products were observed.     

Contrasting Effect of Additives on Photoinduced Reactions of SmI2

The work described herein compares the effect of additives (HMPA, methanol, ethylene glycol, pinacol, N-methylethanolamine) on thermal and photochemical reactions of samarium diiodide (SmI₂). In thermal reactions, additives that coordinate to SmI₂ induce a significant increase in reaction rate. In photochemical reactions, the presence of an electronegative atom with a highly localized negative charge on the substrate leads to a rate deceleration. In order to benefit from the columbic interaction with the positively charged samarium cation, these substrates react preferentially by an inner sphere reduction mechanism. The addition of ligands prevents this close interaction causing rate retardation. Furthermore, studies demonstrate that excited state quenching of Sm^II by ethylene glycol and other additives indicate that it is unlikely to be the major cause for the observed rate retardation. This effect provides a simple diagnostic tool to distinguish between an inner and an outer sphere reduction mechanism.     

Stereoselective construction of the tricyclic core of neoliacinic acid

The tricyclic core of the plant-derived sesquiterpene natural product neoliacinic acid was synthesized using a novel synthetic strategy. The pivotal synthetic transformations are construction of the key bicyclic ether-bridged intermediate by sequential deployment of metal carbenoid C-H insertion and ylide-forming reactions and installation of the lactone portion of neoliacinic acid by an acid-catalyzed intramolecular ring-opening reaction of an epoxide with a carboxylic acid.