OH-Induced shift from carbon to oxygen acidity in the side-chain deprotonation of 2-, 3- and 4-methoxybenzyl alcohol radical cations in aqueous solution: results from pulse radiolysis and DFT calculations

DFT calculations have been carried out for 2-, 3- and 4-methoxybenzyl alcohol radical cations (1⁺, 3⁺ and 4⁺, respectively) and the α-methyl derivatives 2⁺ and 5⁺ using the UB3LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of 1⁺-5⁺ under acidic and basic conditions. In acidic solution, the decay of 1⁺-5⁺ proceeds by cleavage of the C-H bond, while in the presence of ⁻OH all the radical cations undergo deprotonation from the α-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The ⁻OH induced α-O-H deprotonation is consistent with a charge controlled reaction, whereas the C-H deprotonation, observed when the base is H₂O, appears to be affected by frontier orbital interactions.     

Twisted molecular geometry and localized electronic structure of the triplet excited gem-diphenyltrimethylenemethane biradical: substituent effects on thermoluminescence and related theoretical calculations

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals (³2a) were explored by using thermoluminescence (TL) spectroscopy and density functional theory (DFT) including time-dependent (TD) DFT. Linear free energy (Hammett) analyses of TL energies of a variety of para-substituted aryl derivatives of ³2* gave reasonable correlations with the substituent constant, σ. The slope of Hammett plots of the data are nearly identical to one obtained from a similar analysis of the photoluminescence (PL) energies of the structurally-related 1,1-diarylethyl radicals (3*). The results suggest that TL of ³2* and PL of 3* derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of ³2 and 3. Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2⁺ and 1⁻, which generates ³2*, revealed that substituents not only alter the TL energies but also the TL intensities of ³2*. The observations made in this effort demonstrate that ³2* as well as ³2 and 2⁺ have greatly twisted molecular geometries and highly localized electronic structures.     

The first observation in an organic medium and DFT calculation of the TMM radical anion generated via a single electron reduction of a methylenecyclopropane

γ-Irradiation of 2,2-diphenyl-1-methylenecyclopropane (3) in a degassed 2-methyltetrahydrofuran glassy matrix at 77 K gave an intense UV/vis absorption band with λ_ab at 496 nm. This result and calculations based on density functional theory for its radical anion 3⁻ and the corresponding trimethylenemethane radical anion (2⁻) strongly suggest that single electron reduction of 3 followed by ready ring opening affords 2⁻, whose molecular geometry is largely twisted (θ = 45.5°), and the negative charge and spin are localized mainly in the diphenyl methyl and allyl moieties, respectively.     

Syntheses, crystal structure and properties of two novel coordination polymers with the flexible tetrazole-1-acetic acid (Htza)

Two new coordination polymers, [Ag(tza)]_∞ (1) (Htza=tetrazole-1-acetic acid) and [Cu(tza)₂]_∞ (2) have been prepared at room temperature and characterized by X-ray crystallography, IR, UV-vis, fluorescence spectra and magnetism analysis. Compound 1 exhibits extended helical chains through bridging ligand tza. The AgAg interactions between the adjacent chains form a 3-D framework featuring the extended tza-connected Ag chains that obviously affect the photoluminescent property. Compound 2 features undulated layered structure with hourglass-shaped [Cu₄(tza)₄] as subunits with the weak ferromagnetic interactions between Cu(II) ions, which are further stabilized by inter-lamellar CHO hydrogen bonds in the resulting 3-D supramolecular framework.     

Hydroxycinnamic acid clustered by a calixarene platform: radical scavenging and antioxidant activity

Novel hydroxycinnamic acid-calix[4]arene hybrids 4 and 5 were synthesized. Their radical scavenging and antioxidant activities were determined by using DPPH radical and AIBN-induced linoleic acid peroxidation test, respectively. Preliminary studies showed that compounds 4 and 5 possess enhanced activity with respect to the corresponding hydroxycinnamic acid and phenetidine derivative. Kinetic solvent effects were taken in account to understand the different antioxidative behaviour of the synthesized compounds.     

Spectroscopic and DFT evidence for a nonclassical radical cation derived from 7-benzhydrylidenenorbornene

Nanosecond time-resolved UV/vis absorption spectroscopy on laser flash photolysis was conducted for photoinduced electron-transfer reactions of 7-benzhydrylidenenorbornene (1) and 7-benzhydrylidenenorbornane (5). The differences in the observed absorption bands and the structures of 1⁺ and 5⁺ were evaluated successfully using calculations based on (time dependent) density functional theory, confirming the nonclassical nature of 1⁺.     

C-S bond cleavage in the sensitized photooxygenation of tert-alkyl phenyl sulfides. The role of superoxide anion

The N-methylquinolinium tetrafluoroborate (NMQ⁺)-photosensitized oxidation of tert-alkyl phenyl sulfides 1a-c (1a, tert-alkyl=tert-butyl; 1b, tert-alkyl=2-phenyl-2-propyl; 1c, tert-alkyl=1,1-diphenylethyl) and benzyl phenyl sulfide (2) were investigated in CH₃CN by nanosecond laser flash photolysis (LFP) and steady-state irradiation either under nitrogen or in the presence of O₂. By laser irradiation, the formation of sulfide radical cations 1a⁺-c⁺ in the monomeric form (λ_max=520 nm) and of 2⁺ in both the monomeric (λ_max=520 nm) and dimeric form (λ_max=780 nm) were observed within the laser pulse. In both cases, the radical cations decayed by second-order kinetics without any apparent formation of transients attributable to C-S bond rupture. In line with these results, very small amounts of photoproducts were obtained under nitrogen thus suggesting that the sulfide radical cations mainly undergo a back electron transfer process with the reduced N-methylquinolinium (NMQ). A different situation was found in the presence of O₂ since steady-state photolysis produced substantial amounts of C-S bond cleavage products (alcohols, alkenes, and ketones from 1a-c and benzaldehyde from 2), in contrast with LFP experiments. Formation of products was, however, significantly reduced in the presence of benzoquinone, a trap for O₂⁻ generated by NMQ and O₂. For the tert-alkyl phenyl sulfides, 1a-c, these results have been interpreted by suggesting that C-S bond cleavage products in the presence of oxygen mostly derive from the decomposition of a thiadioxirane 6 formed by the reaction of the sulfide radical cation with O₂⁻. In this cleavage a sulfinate and a carbocation formed. The former is oxidized to sulfonate, whereas the carbocation can react with adventitious water to form the alcohol (and the alkene therefrom) and with O₂⁻ to produce the ketone. For 2 (a sulfide with α-CH bonds) probably a different mechanism holds, benzaldehyde coming from the α-phenylthio carbon radical formed from deprotonation by O₂⁻ of 2ⁿ⁺.     

Electron delocalization in vinyl ruthenium substituted cyclophanes: Assessment of the through-space and the through-bond pathways

Pseudo-para[2.2]paracyclophane- and [2.1]orthocyclophane-bridged diruthenium complexes 2 and 3 with two interlinked electroactive styryl ruthenium moieties have been prepared and investigated. Both complexes undergo two reversible consecutive one-electron oxidation processes which are separated by 270 or 105 mV. Stepwise electrolysis of the neutral complexes to first the mixed-valent radical cations and then the dioxidized dications under IR monitoring reveal incremental shifts of the charge-sensitive Ru(CO) bands and allow for an assignment of their radical cations as moderately or very weakly coupled mixed-valent systems of class II according to Robin and Day. Ground-state delocalization in the mixed-valent forms of these complexes as based on the CO band shifts is considerably larger for the “closed” paracyclophane as for the “half-open” orthocyclophane. Experimental findings are backed by the calculated IR band patterns and spin density distributions for radical cations of slightly simplified model complexes 2^Me+ and 3^Me+ with the PⁱPr₃ ligands replaced by PMe₃. Radical cations 2⁺ and 3⁺ feature a characteristic NIR band that is neither present in their neutral or fully oxidized forms nor in the radical cation of the monoruthenium [2.2]paracyclophane complex 1 with just one vinyl ruthenium moiety. These bands are thus assigned as intervalence charge-transfer (IVCT) transitions. Our results indicate that, for the radical cations, electronic coupling “through-space” via the stacked styrene decks is significantly more efficient than the “through-bond” pathway.     

Intramolecular nucleophilic capture of radical cations by tethered hydroxy functions

A range of systems bearing hydroxy functions tethered to the molecular framework gives rise to a family of interesting radical cations, 5⁺-11⁺, upon electron transfer to photo-excited cyanoaromatics. Geraniol (5), nerol (6), citronellol (7), chrysanthemol (8), homochrysanthemol (9), trans-1-o-hydroxyphenyl-2-phenylcyclopropane (10), and endo-5-hydroxymethylnorbornene (11), generate a series of mono-, bi-, or tricyclic ethers via a series of four- to seven-membered transition states. Two of the radical cations, 5⁺ and 6⁺, undergo tandem cyclizations where 1,5- and/or 1,6-C-C cyclizations precede nucleophilic capture.     

The role of oxygen acidity on the side-chain fragmentation of ring methoxylated benzocycloalkenol radical cations

The reactivity of 2,2-dimethyl-5-methoxyindan-1-ol (1) and 2,2-dimethyl-6-methoxytetral-1-ol (2) radical cations has been studied both in acidic and basic solution. At pH≤4 both 1⁺ and 2⁺ undergo C_αH deprotonation as the exclusive reaction with k=4.6×10⁴ and 3.2×10⁴ s⁻¹, respectively. In basic solution 1⁺ and 2⁺ behave as oxygen acids undergoing ⁻OH-induced αOH deprotonation in a diffusion controlled process (k_−OH≈10¹⁰ M⁻¹ s⁻¹). An intermediate alkoxyl radical is formed which undergoes a 1,2-hydrogen atom shift in competition with CC β-scission (with 1⁺) or as the exclusive pathway (with 2⁺). A behavior which is interpreted in terms of the greater ease of ring-opening of a five membered ring as compared to a six-membered one.     

Pyrazino-tetracyanonaphthoquinodimethanes: sterically deformed electron acceptors affording zwitterionic radicals

The X-ray analyses of the title electron acceptors (1) revealed their butterfly-shaped deformed geometry, which is not affected by the pyridyl group attached at 2-position of the pyrazino-TCNNQ skeleton. Small differences between the first and second reduction potentials (ca. 0.1 V) in pyrazino-TCNNQs show that their anion radicals (1⁻) are prone to disproportionate into the neutral (1) and dianionic (1²⁻) species. The thermodynamically unstable anion radical species based on the pyrazino-TCNNQ skeleton could be isolated as inner salts upon electrochemical reduction of the derivatives having an N-methylpyridinium moiety at 2-position (2⁺). The zwitterionic open-shell species (2) constitute a novel class of radicals that exhibit semiconducting behavior as a single component thanks to the high electrochemical amphotericity.     

Chemical capture of an unprecedented oxatetramethyleneethane radical cation with a through-space electronic coupling

A photoinduced electron-transfer reaction of 2,2-dianisyl-4-isopropylidene-3,3-dimethylcyclobutanone (5) in acetonitrile containing molecular oxygen or water gave 4,4′-dimethoxybenzophenone (7) and 2,2-dianisyl-4-isopropylidene-5,5-dimethylhydrofuran-3-one (8), demonstrating the chemical capture of an unprecedented oxatetramethyleneethane-type radical cation intermediate (6⁺). A density functional theory calculation suggests through-space electronic coupling between the tetramethylallyl and joined dianisylmethyl carbonyl subunits in 6⁺.     

Proton reduction catalysis by manganese vinylidene and allenylidene complexes

The present paper reports the unprecedented observation of a catalytic electrochemical proton reduction based on metallocumulene complexes. Manganese phenylvinylidene (η⁵-C₅H₅)(CO)(PPh₃)MnCC(H)Ph (1) and diphenylallenylidene (η⁵-C₅H₅)(CO)₂MnCCCPh₂ (3) are shown to catalyze the reduction of protons from HBF₄ into dihydrogen in CH₂Cl₂ or CH₃CN media at −1.60 and −0.84 V (in CH₃CN) vs. Fc, respectively. The working potential for 3 (−0.84 V vs. Fc in CH₃CN) is the lowest reported to date for protonic acids reduction in non-aqueous media. The similar catalytic cycles disclosed here include the protonation of 1, 3 into the carbyne cations [(η⁵-C₅H₅)(CO)(PPh₃)MnC-CH₂Ph]BF₄ ([2]BF₄), [(η⁵-C₅H₅)(CO)₂MnC-CHCPh₂]BF₄ ([4]BF₄) followed by their reduction to the corresponding 19-electron radicals 2, 4, respectively. Both carbyne radicals undergo a rapid homolytic cleavage of the C_β-H bond generating an H-radical producing molecular hydrogen with concomitant recovery of the neutral metallocumulenes thereby completing a catalytic cycle.     

Matrix isolation and DFT calculations of the TMM radical cation generated via the single electron oxidation of a methylenecyclopropane

A γ-irradiation of 2,2-diphenyl-1-methylenecyclopropane (3) in a degassed n-butyl chloride glassy matrix at 77 K produced an intense UV/vis absorption band with λ_ab at 432 nm. This result and calculations based on density functional theory for its radical cation 3⁺ and the corresponding trimethylenemethane radical cation (2⁺) strongly suggest that a single electron oxidation of 3 followed by ready ring opening affords 2⁺, whose molecular geometry is largely twisted (θ = 44.0°), and the positive charge and spin are localized mainly in the diphenyl methyl and allyl moieties, respectively.     

Diverse reactions of nitroxide-radical adducts of silylene, germylene, and stannylene

The results of the thermolysis of 1:2 adducts of stable group-14 element divalent compounds [R₂M:, R₂=1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl; 1b, M=Ge; 1c, M=Sn] to TEMPO radical are discussed in detail. Whereas the thermal reactions of the 1:2 adducts [R₂M(OR^′)₂, R^′=2,2,6,6-tetramethylpiperidin-N-yl; 3b, M=Ge; 3c, M=Sn] are understood to proceed by the initial homolysis of an M-O bond to give the corresponding aminoxy-substituted group-14 element radicals [R₂(R^′O)M; 2b, M=Ge; 2c, M=Sn] and TEMPO, the subsequent reactions of 2b and 2c were remarkably different to each other; 2b favors the N-O bond fission (path b) to give the corresponding germanone, while 2c prefers the M-O bond fission (path a) to give stannylene (1c). In combining with our previous results for aminoxysilyl radical (2a) [R₂(R^′O)Si], the origin of the remarkable differences in the reactivity among group-14 element radicals 2a-2c is discussed on the basis of the theoretical calculations for model reactions.Improved syntheses of the precursor dichlorogermane and dichlorostannane of germylene (1b) and stannylene (1c), respectively, are described in Section 3.     

Paramagnetic intermediates in the photoinduced reaction between dodecamethylcyclohexasilane and 9,10-phenanthraquinone: Time-resolved CIDNP study

The reaction between dodecamethylcyclohexasilane (Me₂Si)₆1 and 9,10-phenanthraquinone 2 has been studied by means of CIDNP method. In the polar solvent, the photodecomposition of 1 is shown to proceed via triplet radical ion pair formed by phenanthraquinone radical anion and cyclohexasilane radical cation. Its transformation leads to the cyclic reaction product - 10-membered cyclic dioxahexasilecine 8 - formally resulting from the addition of linear 1,6-silicon-centered biradical Si(Me)₂-Si₄(Me₂)₄-(Me)₂Si to CO bonds of quinone. Product 8 is unstable, after several hours it converts to dioxasilole 4 via sequential repeated elimination of dimethylsilylenes 3.     

Redox-photosensitized amination of alkenes and alkadienes with ammonia and alkylamines

Using 1,2,4-triphenylbenzene as a photosensitizer, the photoamination of alkenes and alkadienes (1), which had no absorption at >300 nm proceeded efficiently in the presence of p-dicyanobenzene to give addition products by incorporating both amino and p-cyanophenyl groups. The reaction efficiency was discussed in terms of the relationships between 1 and the photosensitizer in their oxidation potentials and the distribution of positive charge on the reaction site of the cation radical of 1 (1⁺).