A concept of an electron transfer stopped-flow method

A new concept of an electron transfer stopped-flow (ETSF) method is proposed. In the ETSF method, unstable cation radicals are formed via the electron transfer reactions with stable cation radicals whose oxidation potential is positive to that of unstable ones. Thus, by combining the electron transfer reactions with the stopped-flow operation, absorption spectroscopic measurements for short-lived cation radical become possible with a reduced dead time from the point of the generation of cation radicals to the point of the optical measurement, in particular, with a help of a rapid-scan spectrophotometer. By mixing an acetonitrile (AN) solution of tris(p-bromophenyl)amine cation radical (TBPA^⋅+) with an AN solution of methyldiphenylamine (MDPA) or diphenylamine (HDPA), the kinetic analyses could be carried out successfully for the dimerization reactions of MDPA^⋅+ and HDPA^⋅+ as well as the observations of their absorption spectra. It was found that fast reactions whose second order rate constants are around 10⁷ M⁻¹s⁻¹ can be analyzed using the ETSF method.     

Photosensitized (electron transfer) carbon-carbon bond cleavage of radical cations : The diphenylmethyl system

The photosensitized (electron transfer) reaction of methyl 2,2-diphenylethyl ether (1), 1,1,2,2-tetraphenylethane (5), 2-methyl-1,1,2-triphenylpropane (6), and 2-methoxy-2-diphenylmethylnorbornane (11 endo and exo) with 1,4-dicyanobenzene (4) in acetonitrile-methanol leads to products indicating cleavage of an intermediate radical cation to give the diphenylmethyl radical and a carbocation. The diphenylmethyl radical is then reduced by the radical anion of the photosensitizer and protonated to yield diphenylmethane. The carbocation fragment reacts with methanol to yield ether and/or acetals. The effect of temperature on the efficiency of cleavage of 5 and 6 has been analyzed. The increase in efficiency observed at higher temperatures reflects an activation energy for the cleavage of the radical cations. In cases where no cleavage is observed, the activation energy for cleavage may be so high that back electron transfer from the radical anion of the pbotosensitizer is the dominant reaction. The C—C bond dissociation energies of the radical cations of 5 and 6 were estimated by analysis of the thermochemical cycle using the bond dissociation energies and the oxidation potentials of the neutral molecules and the oxidation potential of the diphenylmethyl and cumyl radicals. The direction of cleavage of the radical cation is explained in terms of the relative oxidation potentials of the two possible radicals.     

Die photolytische α-spaltung von benzoinalkyläthern in sauerstoffhaltigem methanol

The primary step in the photolytical α-cleavage of benzoin alkyl ether in oxygen saturated methanol at room temperature is the formation of a benzoyl (1) and an α-alkoxybenzyl radical (2), which react via their peroxi radicals 3 und 4 to the final products yielding perbenzoic acid (5), alkyl benzoate (7), benzaldehyde (11), benzaldehyde dimethylacetal (10) and benzil to 100%. Product analysis of the final products of the photolysis of specifically deuterated benzoin methyl ethers shows that methyl benzoate, benzaldehyde and benzaldehyde dimethylacetal are formed via the α-methoxybenzyl radical (2) while the perbenzoic acid results from the benzoyl radical (1). Both the peroxi radicals 3 and 4 have an independent reaction pathway to the final products. Hydrogen abstraction of 3 and 4 from the solvent methanol give rise to hydroxy methyl radicals which yields formic acid, hydrogen peroxide, performic acid and formaldehyde via their hydroxy methyl peroxi radicals. The reaction pathway of the primary radicals is discussed.     

Zur konformation sterisch gehinderter allyl-radikale; hyperkonjugation in verdrillten π-radikalen—VII

For the sterically hindered allyl radicals 2 and 6a, which are produced as model compounds for 1,1,3,3-tetramethylallyl radical, twist angles of about 10° are determined on the basis of their ESR coupling constants. Even the more hindered radicals 8a,b are twisted to only about 16°. These results are in contrast to the interpretation of experimental results regarding 1,1,3,3-tetramethylallyl radical and the corresponding cation.     

Organische schwefelverbindungen—VI : Thiaxanthonsynthese durch photoumlagerung von diaryl-α-ketosulfiden (thiolestern), eine neue lichtinduzierte aromatische substitutionsreaktion

4′-Tolyl-2-methylsulfono-thio-benzoate (1) undergoes a new type of photorearrangement to give 2-methyl-thiaxanthone (4). The isomeric 3-methylsulfono-thio-benzoate (2) suffers cleavage of the S-benzoyl bond, giving rise to the corresponding thiyl and benzoyl radicals. The resulting radicals form bis-4′-tolyl disulfide (5) by dimerization and 3-methylsulfono-benzaldehyde (6) by hydrogen abstraction respectively. No thia-photo-Fries products are formed by the photolysis of 1 and 2. In contrast to 1 4′-tolyl-2-methylsulfono-benzoate (3) displays the normal photo-Fries reaction and affords the benzophenone derivative 8. A mechanism for the photorearrangement of thiol ester 1 into thiaxanthone derivative 4 is discussed.     

N,N-Dimethylaniline and 1-(trifluoromethyl)benzene-functionalized tetrakis(ethynyl)pyrenes: synthesis,photophysical, electrochemical and computational studies

We have synthesized a series of tetrakis(ethynyl)pyrenes functionalized with N,N-dimethyaniline and 1-(trifluoromethyl)benzene as a peripheral electron-donor and electron-acceptor moiety, respectively. In solvatochromic studies, compounds with one peripheral donor and three peripheral acceptors (2), with two donors and two acceptors (3 and 4), with three donors and one acceptor (5) show enhanced charge transfer compared with tetra-donor (6) and tetra-acceptor (1) compounds. The redox peak reversibility depends on the number of peripheral donors and acceptors appended to tetrakis(ethynyl)pyrenes as well as on their substitution pattern as revealed by cyclic voltammetric studies. The photophysical and electrochemical properties of compounds 1–5 have been compared with compound (6) reported recently by J.-W. Oh et al. [Angew. Chem., Int. Ed. 2009, 48, 2522–2524]. The density functional theory (DFT) based calculations such as spin density distribution (SDD) of cation/anion radicals, electrostatic potential (ESP) density distribution, non-adiabatic reduction potentials (NRP) for cation radicals, and vertical detachment energy (VDE) for anion radicals supported the experimental observations. The differences in oxidation peak reversibility for different substitution pattern have been rationalized by calculated static first hyperpolarizability (β). Our observations would be helpful in designing new ECL-active materials, where ECL (electrogenerated chemiluminescence) efficiency can be improved through improving radical stability.     

The chemistry of small ring compounds—XX : Cidnp effects in the oxidation of 2,2-dimethylcyclo- propanone methyl hemiacetal

Mild oxidation of 2,2-dimethylcyclopropanone methyl hemiacetal (1) with oxygen, t-butyl- hydroperoxide or di t-butylperoxalate leads to hydrogen abstraction from the OH group of 1. The cyclopropyl ring probably breaks simultaneously to give ring-opened tertiary radical 3a and primary radical 3b. In CDCl₃, CIDNP effects are observed in the saturated disproportionation product of 3a and also in products resulting from disproportionation and combination of 3a with solvent derived CCl3₃ and CDCl₂ radicals. The large difference in g-values between individual radical components in each of these latter pairs explains the observed emissions.In benzene, solvent does not interfere and CIDNP effects mainly arise from radical pairs, consisting of identical components, e.g. two radicals 3a or at least of components with practically equal g-values. Thus disproportionation leads to absorption-emission (multiplets effects) for the unsaturated esters 5 and 6 and the isovalerate 8.     

Photo-Reimer-Tiemann reaction of phenols,anilines and indolines

The photo-reaction of phenol (1) with chloroform in the presence of diethylamine gave salicylaldehyde (2) and 4-hydroxybenzaldehyde (3) in fair yield. Dihydroxybenzenes, sensitive to alkali, under similar conditions without the base gave corresponding aldehydes and a cyclohexa-2,5-dienone (15). The photo-reaction was also applied to diethylaniline (18), indoline (21) and N-methylindoline (24). In carbon tetrachloride instead of chloroform in an alcohol solution the same type of reaction took place to give esters. The mechanism involving the coupling of a phenoxy radical or a radical cation with dichloromethyl radical and not involving dichlorocarbene is proposed.     

Radical cations of cyclopentadiene dimers—facets of an intriguing energy surface

The radical cations of various cyclopentadiene dimers can be generated by photoinduced electron transfer to strong electron acceptors. Nuclear spin polarization effects observed during these reactions provide insight into the radical cation structures. Three of the systems studied, endo-[4 + 2]-(1), anti-[2+2]-(2), and exo-(4+2]-dicyclopentadiene (5) give rise to unusual singly linked, delocalized radical cations, whereas the anti-[4 + 4]-dimer (3) and 1,3-bishomocubane (4) give rise to more conventional radical cations. The reactions of spiroheptadiene (9) and di(spiroheptadiene) (10) provide evidence for two different dimer radical cations, a doubly linked (D) and a singly linked (S) species. This finding supports a stepwise mechanism for the radical cation Diels-Alder reaction of 9.     

Azulenoisoindigo: A building block for π-functional materials with reversible redox behavior and proton responsiveness

Azulene, one of representative nonbenzenoid aromatic hydrocarbons, exhibits unique molecular structure and distinctive physical and chemical properties. Herein, azulenoisoindigo（Az II）, an azulene-based isoindigo analogue, is designed and synthesized, which has a twisted molecular backbone and R/Sisomers in single crystals. Interestingly, Az II shows the characteristics of both isoindigo and azulene,such as reversible redox behavior and reversible proton responsiveness. UV-vis-NIR,¹ H...     

The photochemical reaction of 1,4-naphthalenedicarbonitrile with aromatic pinacols and pinacol ethers

Irradiation of 1,4-naphthalenedicarbonitrile (NDN) in deoxygenated acetonitrile in the presence of aromatic pinacols (1) leads to reduction of the former to the dihydro derivative 4 and the tetrahydro derivative 5 as well as to oxidative cleavage of the latter to yield ketones or aldehydes (3). Reaction with pinacol ethers (2) leads to product types 3, 4 and 5 as well as to 1(1-methoxybenzyl)-1,2-dihydro NDN derivatives (two diastereoisomers, 6 and 7). Only one of these adducts is formed in the reaction of NDN with benzyl methyl ether (8). The reaction involves electron transfer to singlet excited NDN and cleavage of the radical cations 1XXX and 2XXX to yield α-hydroxy and α-methoxy radicals, respectively. These react with NDN by proton transfer in the first case, and by carbon-carbon bond formation in the latter. The stereoselectivity observed in the adduct formation with 8 is explained by deprotonation of the radical cation and reaction of the corresponding radical with NDNXXX in the geminate solvent cage. The mechanism of these reactions is discussed in the light of a recent flash-photolysis study.     

Time resolved absorption and resonance Raman spectroscopic studies of the oxidation of benzidine in aqueous solution

We report a time resolved resonance Raman study of transient radicals produced in the pulse radiolytic oxidation of benzidine in aqueous solution. The intense and structured transient absorption in the 400–470 nm region, observed at microsecond times in the acidic medium, is attributed to the benzidine radical cation. The Raman spectrum, observed by excitation in resonance with this absorption, exhibits eight prominent bands which are assigned to planar phenyl vibrations. The ring breathing mode (v₁) at 844 cm^-1 is most highly resonance enhanced, indicating an overall expansion of the ring CC bonds in the excited state. The interring CC bond, with partial double bond character, is characterized by an intense (v₁₃) Raman band at 1335 cm^-1. The frequency of the in-phase v_7a CN stretching vibration is 1540 cm^-1. These frequencies and the presence of weak bands attributable to non-planar phenyl vibrations indicate the radical to be slightly non-planar. The pK_a for the proton loss from the radical cation is 10.87, four units higher than for the aniline radical cation. At high pH the observed transient has a broad and structureless absorption at ∽ 380 nm. It is identified from its resonance Raman features as the 4(4′aminophenyl)anilino radical formed by proton loss from the radical cation. The interring CC bond is characterized by a Raman band at 1292 cm^-1, indicating it to be a single bond. The structure of this neutral radical is highly nonplanar, with little conjugation between the two ring systems so that electronic excitation is primarily confined to the anilino moiety. The acidic and basic forms of the radical react rapidly in second order processes to produce products which absorb strongly at, respectively, 360 and 410 nm.     

DFT studies of unique stereoelectronic effects of substituents on divergent reaction pathways of methylenecyclobutanone radical cations

The results of DFT investigation suggest that C2–C3 bond cleavage of the 2,2-dianisyl-3,3-dimethyl-4-methylenecyclobutanone radical cation (2b⁺) is preferred from both a thermodynamic and a kinetic perspective while C1–C2 bond cleavage is both thermodynamically and kinetically favored in the parent methylenecyclobutanone radical cation (MCB⁺) and the 2,2-diphenyl- and 2,2-dianisyl-4-isopropylidenecyclobutanone radical cations (1a-b⁺). The DFT calculations also suggest that a bonding character exists in C2–C3 bond of the 2,2-diphenyl-3,3-dimethyl-4-methylenecyclobutanone radical cation (2a⁺) but not in that of 2b⁺. Consequently, the unique reactivity of 2b⁺ can be accounted for by the existence of the steric hindrance between methyl and anisyl substituents, which favors C2–C3 bond cleavage, and the absence of C2–C3 bonding character. Those results support that the previous experimental results of photoinduced electron-transfer reactions of 1 and 2. The combined factors comprise stereoelectronic substituent effects that lead to a drastic change in the reaction pathways followed by 2b⁺ relative to that of other methylenecyclobutanone-type radical cations.     

The Divergent Dimerization Behavior of N‐Substituted Dicyanomethyl Radicals: Dynamically Stabilized versus Stable Radicals

Herein, we demonstrate that the dimerization behavior of amine‐substituted dicyanomethyl radicals can be switched from σ‐ to π‐dimerization simply by varying the electron‐donating substituents. For dicyanomethyl radicals with a 4,4′‐ditolylamine ( DT^. ) or a phenothiazine ( PT^. ) substituent, the monomeric radical form and the corresponding dimer connected by a reversible C−C bond (σ‐dimer) are in equilibrium in solution. On the other hand, the radical with the julolidine skeleton ( JD^. ) does not undergo σ‐dimerization and was isolated as a stable radical in spite of the absence of bulky protecting groups. X‐ray single‐crystal analysis revealed that JD^. forms the π‐dimer in the crystalline state, and variable‐temperature spectroscopy showed that JD^. is in equilibrium with the π‐dimer in toluene solution. DFT calculations point to the importance of electrostatic interactions as a driving force for the π‐dimerization of JD^. because of its polarized structure.     

5-Isothiazolidinonyl and 5-isoxazolidinonyl radicals : Approaches to the biogenetic-type synthesis of β-lactams

A possible mechanism for penicillin biosynthesis involves the rearrangement of a 5-isothiazolidinonyl radical (6) to a β-lactam. However, generation of the model radicals (13 and 20) did not lead to β-lactams. The analogous reaction of a 5-isoxazolidinonyl radical is the basis for a potential synthesis of clavulanic acid, but again the rearrangement was not observed in the model radical (24).     

Electroreduction of 9-fluoro-10-cyanoanthracene

The electroreduction of 9-fluoro-10-cyanoanthracene (1) was voltammetrically studied. In contrast to expectations, no halide elimination was observed at the radical anion level but after a radical ion coupling the dimer 9,9^′-bianthryl-10,10^′-dicarbonitrile (2) was formed in quantitative yield. The mechanism was studied using digital simulation. Thermodynamic and kinetic parameters were determined. The low activation energy of the dimerization step indicates a diffusion-controlled radical ion coupling, which produces a σ-dimer but not a π-dimer as often suggested in the literature.     

Ring contraction mechanisms in substituted cyclohexylium cations

The effect of an electron donating or withdrawing group on the ring contraction mechanism of a cyclohexylium (cyclohexane-derived) cation has been studied using density functional theory. The barrier to rearrangement of the parent cyclohexane cation (1) was previously calculated to be 7.7 kcal/mol using PBE/6-311++G(2d, 2p). We show in this work that addition of an electron withdrawing group (CF₃) lowers the average barrier to ring contraction, while an electron donating group (CH₃) increases the average barrier, relative to the parent, unsubstituted, cyclohexane cation. Calculated barrier heights for going from a 6-membered to 5-membered ring range from 4.3 to 23.3 kcal/mol for methyl-cyclohexylium (2), but only from 0.6 to 14.0 kcal/mol for trifluoromethyl-cyclohexylium (3). The lower barriers for 3 can be explained by (a) the starting structures involved, and (b) the use of dative bonding as a catalyst in stabilizing intermediates and transition states. For 1 and 3 the reaction involves starting from secondary cations and going downhill in energy through secondary intermediates to a tertiary product. However, 2 does not benefit from such favorable energetics since it more likely starts from a tertiary cation and has to first rearrange to secondary intermediates to derive the tertiary methyl-cyclopentane-type product.     

A study of radicals arising in the low-temperature radiolysis of thiophene and some of its derivatives

EPR was used to investigate the radicals formed by radiolysis of furan, thiophene, and some of their substitution products having the general formula C₄H₃RS, where R = Cl, Br, COOH, C(CH₃)₃, and of di-tert-butylthiophene. By analyzing the EPR spectra it is shown that radicals formed by radiolysis of furan, thiophene, their alkyl substitution products, and 2-thiophene carboxylic acid arise in primary radiation-chemical events with splitting of the C-H bond in both ring and side chain. Characteristic of the radiolysis of these compounds is formation of secondary radicals by addition of hydrogen atoms at the double bond of the heteroaromatic ring. The values of the hyperfine splittings in the spectra of the secondary radicals are, for protons of methylene group 32 e [positions 2(5)] or 40 e [positions 3(4)], and 13 e for protons of the thiophene ring. In radiolysis of 2-chlorothiophene and 3-bromothiophene the main products are radicals with the unpaired electron localized on the sulfur atom.     

Studies on the interaction of peroxy radicals with transition metals complexes: I. Effect of 2-cyano-2-propyl and 2-cyano-2-propyl peroxy radicals on cobaloximes

Reactions of CH₃Co(DH)₂py (1) and [Co(DH)₂py]₂ (2) with (CH₃)₂(CN)C (r) and (CH₃)₂(CN)COO (rO₂) radicals were investigated. At 60°C, reaction or r with (1) results in non-homogeneous ligand decomposition, whereas for 2, complex (CH₃)₂CNCCo(DH)₂py (6) and a precipitate are formed. Ligand decomposition also took place at 60°C when the reaction of rO₂ radicals with 1 and 2 was investigated. However, the same reaction with rO₂ radicals at −10°C, yielded two complexes, CH₃OOCo(DH)₂py (3) and Co(DH)₂py (4) with 1, and complex 6 for the reaction of rO₂ with 2.     

Highly selective conversion of N-aroyl-α-dehydronaphthylalaninamides into 3,4-dihydrobenzoquinolinone derivatives via photoinduced intermolecular electron transfer

The irradiation of substituted (Z)-N-aroyl-α-dehydronaphthylalaninamides [(Z)-1] in methanol containing triethylamine (TEA) with Pyrex-filtered light was found to give 3,4-dihydrobenzoquinolinone derivatives (2) in high yields along with minor amounts of 4,5-dihydrooxazole derivatives (3). Analysis of the substituent effects on product composition revealed that both the photoreactivity of 1 and the selectivity of 2 are decreased with increasing electron-withdrawing ability of the substituent introduced at the para-position on the N-benzoyl benzene ring. From the analysis of the dependence of the quantum yield for the formation of 2 on the TEA concentration, it was found that back electron transfer occurs efficiently within an (E)-1 anion radical-TEA cation radical pair intermediate.