Electroorganic chemistry. XIX. Anodic oxidation of nonconjugated dienes

Generation of Flavor-Active Compounds by Electrochemical Oxidation of (R)-Limonene

Terpenes may be converted by electrochemical oxidation to various oxidized products with appealing aroma properties. In this study, (R)-limonene was anodically oxidized in the presence of ethanol, and the resulting mixture exhibited a pleasing fruity, herbal, citrus-like, and resinous odor. The aroma-active compounds were purified by means of preparative high-performance liquid chromatography, and their structures were elucidated by means of gas chromatography (GC)-mass spectrometry and nuclear magnetic resonance spectroscopy. In addition, the odor of the isolated compounds was determined by means of GC-olfactometry. Seventeen compounds were isolated, and for only four of them, analytical data had been reported previously in the literature. Furthermore, only for two of the compounds, an odor description had been available in the literature.

The Use of Some Non-conventional Methods in Chemistry of Bicyclohomofarnesenic Methyl Esters

The present paper reports the results of microwave irradiation assisted method for the preparation of bicyclohomofarnesenic methyl esters versus classical Stoll and Hinder method. Moreover, the chemical transformations of bicyclohomofarnesenic methyl esters via anodic electrooxidation and dye-sensitized photooxidation were performed. A new method for the preparation of methyl 7-oxo-13,14,15,16-tetranorlabd-6,8(8)-dien-12-oate and the mechanism of electrochemical products formation are presented. The structure of all synthesized compounds was fully confirmed by spectral methods.

Energy Storage in Strained Organic Molecules: (Spectro)Electrochemical Characterization of Norbornadiene and Quadricyclane

We have investigated the electrochemically triggered cycloreversion of quadricyclane (QC) to norbornadiene (NBD), a system that holds the potential to combine both energy storage and conversion in a single molecule. Unambiguous voltammetric traces are obtained for pure NBD and pure QC, the latter a strained polycyclic isomer of the former. The difference in redox potentials is smaller than the energy difference between the neutral molecules. This is owing to a significant energy difference between the corresponding radical cations, as demonstrated by density functional theory (DFT) calculations. The vibrational modes of each pure compound are characterized experimentally in the fingerprint region and identified by DFT methods. Thermal and electrochemical transformations of NBD and QC are monitored in situ by IR spectroelectrochemical methods. The kinetics of the cycloreversion of QC to NBD, which is catalyzed by oxidizing equivalents, can be controlled by an applied electrode potential, which implies the ability to adjust in real time the release of thermal power stored in QC.

Triplet acetylenes as synthetic equivalents of 1,2-bicarbenes: phantom n,pi state controls reactivity in triplet photocycloaddition

Diaryl acetylenes, in which one of the aryl groups is either a pyridine or a pyrazine, undergo efficient triplet state photocycloaddition to 1,4-cyclohexadiene with formation of 1,5-diaryl substituted tetracyclo[3.3.0.0(2,8).0(4,6)]octanes (homoquadricyclanes). In the case of pyrazinyl acetylenes, the primary homoquadricyclane products undergo a secondary photochemical rearangement leading to diaryl substituted tricyclo[3.2.1.0(4,6)]oct-2-enes. Mechanistic and photophysical studies suggest that photocycloaddition proceeds through an electrophilic triplet excited state whereas the subsequent rearrangement to the tricyclooctenes proceeds through a singlet excited state. Chemical and quantum yields for the cycloaddition, in general, correlate with the electron acceptor character of aryl substituents but are attenuated by photophysical factors, such as the competition between the conversion of acetylene singlet excited state into the reactive triplet excited states (intersystem crossing: ISC) and/or to the radical-anion (photoelectron transfer from the diene to the excited acetylene: PET). Dramatically enhanced ISC between pi-pi S(1) state and "phantom" n,pi triplet excited state is likely to be important in directing reactivity to the triplet pathway. The role of PET can be minimized by the judicious choice of reaction conditions (solvent, concentration, etc.). From a practical perspective, such reactions are interesting because "capping" of the triple bond with the polycyclic framework orients the terminal aryl (4-pyridyl, 4-tetrafluoropyridyl, phenyl, etc.) groups in an almost perfect 60 degrees angle and renders such molecules promising supramolecular building blocks, especially in the design of metal coordination polymers.

The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 11: Involving (R)-(+)-α-terpineol and (R)-(+)-limonene, substituting on 1,4-dicyanobenzene

Irradiation of an acetonitrile–methanol (3:1) solution of 1,4-dicyanobenzene (1), biphenyl (5), and (R)-(+)-limonene (21) leads to formation of the 1:1:1 (methanol:21:1) photo-NOCAS adducts: 4-[(1R,2S,4R)-4-isopropenyl-2-methoxy-1-methylcyclohexyl]benzonitrile (23, 30%), 4-[(1S,2R,4R)-4-isopropenyl-2-methoxy-1-methylcyclohexyl]benzonitrile (24, 2%), 4-[(1R,2R,5R)-5-isopropenyl-2-methoxy-2-methylcyclohexyl]benzonitrile (25, 3%), and 4-[(1S,2S,5R)-5-isopropenyl-2-methoxy-2-methylcyclohexyl]benzonitrile (26, 1%). When an acetonitrile solution (no added methanol) of 1,4-dicyanobenzene (1), biphenyl (5), and (R)-(+)-α-terpineol (22) was irradiated under these conditions, the products were the cyclized 1:1 (22:1) photo-NOCAS adducts: (1R,2S,5R)-2-(4-cyanophenyl)-2,6,6-trimethyl-7-oxabicyclo[3.2.1]octane (27,21%) and (1S,4R,6R)-6-(4-cyanophenyl)-1,3,3-trimethyl-2-oxabicyclo[2.2.2]octane (28, 2%). Structural assignments were based primarily upon detailed analysis of 1H and 13C nmr spectra and, for four of the products (24, 26, 27, and 28), structures were firmly established by X-ray crystallography. The mechanism for the formation of these products is discussed, with emphasis on the intramolecular reactions of the intermediate alkene radical cations. Molecular mechanics (MM3) calculations gave information regarding the structure and energy of the conformers of 21 and 22 that was useful for predicting/explaining the observed reactivity on the basis of approach vector analysis; the transition state for cyclization incorporates the nucleophile and the alkene radical cation carbon atoms at the vertices of an obtuse triangle orthogonal to the plane of the π-system. Key words: photoinduced electron transfer, radical cations, cyclization, terpenes.