Singlet Oxygen: Discovery, Chemistry, C60 -Sensitization.

This review article refers to the discovery of excited molecular oxygen, in particular on its lower singlet excited state (1 Δg , 1 O2 ). After a short report on singlet oxygen generation, the review is focused on the chemistry of this reactive species. Specifically, the three major reactions of 1 O2 with unsaturated organic substrates, namely the [4 + 2] and [2 +2] cycloadditions as well as the ene reaction, are reviewed. The proposed mechanisms of these reactions, through the years, based on experimental and computational work, have been presented. Selected examples of singlet oxygen-synthetic applications are also mentioned. The [60]fullerene and fullereno-materials photosensitized oxidations in homogeneous, as well as in heterogeneous conditions, are also comprehensively discussed. Finally, the self-sensitized photooxidation of open cage fullerenes as well as fullerenes bearing oxidizable groups is reported.

Decatungstate-Photocatalyzed Addition of Lactones to [60]Fullerene.

A new, one step functionalization of C60 with lactones has been developed. This photochemical approach utilizes a variety of lactonyl radicals deriving from lactones through a hydrogen atom abstraction process mediated by tetrabutylammonium decatungstate [(n-Bu4N)4W10O32]. This reaction provides access to a previously unknown class of materials, namely 1-lactonyl-2-hydro[60]fullerenes. A mechanism for this new reaction has been proposed based mainly on the structure of reaction products and deuterium-incorporated experiments.

Isomerization of Epoxides into Allylic Alcohols Catalyzed by Supported Au Nanoparticles on TiO2: A Mechanistic Insight.

Based on primary and ß-secondary isotope effects, it is established that the isomerization of epoxides into allylic alcohols catalyzed by supported Au nanoparticles proceeds via an unsymmetrical concerted transition state in which there is partial positive charge developing on the C-O carbon atom progressing C-H abstraction. A carbocationic intermediate can be formed only in cases of sufficient stabilization of the positive charge by appropriate substituents.

Ion formation pathways of crown ether-fullerene conjugates in the gas phase.

The ion formation of crown ether-[60]fullerene conjugates of the type (crown - H)-C60-H with crown = 12cr4, 15cr5 and 18cr6 has been studied with matrix-assisted laser desorption/ionisation (MALDI) and electrospray ionisation mass spectrometry (ESI MS). In total five different ways of ion formation are presented, including metalation (MALDI, ESI), protonation and oxidation (both in MALDI) in the positive-ion mode and deprotonation (MALDI, ESI) and reduction (MALDI) in the negative-ion mode. In line with thermochemistry, the deprotonation and electron transfer processes involve the C60 moiety as the charge-carrying entity, while protonation and metalation occur at the crown ether. Particular emphasis has been placed on the study of metal cation attachment in MALDI varying the crown ether size in the conjugate and using different alkali metal chlorides in the target preparation. Dissociation reactions of the metalated conjugates are influenced by the interaction strength of the metal cation to the crown ether fullerene conjugate. The data confirm an increase in bond strength with smaller metal cations, supporting the notion of charge density-driven interactions.

Photocycloaddition of biscyclopropyl alkenes to C60: an unprecedented approach toward cis-1 tricyclic-fused fullerenes.

A novel, simple, and entirely regioselective tandem cycloaddition of biscyclopropyl-substituted alkenes to [60]fullerene has been revealed. This reaction affords cis-1 tricyclic-fused organofullerenes bearing the hitherto elusive 5-4-5 fused tricyclic ring system.

Hydroxyalkylation of [60]fullerene: free radical addition of alcohols to C60.

A new general reaction for the preparation of isomerically pure hydroxyalkylated C(60) monoadducts, via the free radical photochemical addition of alcohols to [60]fullerene, is described.

Unraveling the mechanism of the singlet oxygen ene reaction: recent computational and experimental approaches.

The mechanism of the singlet oxygen ene reaction has been a subject of renewed interest within the last few years. The main question being whether this reaction proceeds through a concerted mechanism or if it involves discrete intermediates. In general, the majority of experimental and computational studies support a traditional stepwise mechanism involving a perepoxide-like intermediate. In this minireview we highlight the most prominent and recent theoretical, as well as experimental results relating to the challenging mechanism of the singlet oxygen ene oxyfunctionalization.

Radical reactivity of aza[60]fullerene: preparation of monoadducts and limitations.

Six aza[60]fullerene monoadducts were synthesized by the thermal reaction between the azafullerene radical C(59)N* and 9-alkyl-substituted fluorenes, 9,10-dihydroanthracene, or xanthene. Unlike fluorenes, dihydroanthracene, and xanthene, the structurally related substituted diphenylmethanes, ethylbenzene, cumene, 1,2-diphenylethane, 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene, 10,11-dihydro-5H-dibenzo[a,d]cycloheptene, 9-methylanthracene, and 9-benzylanthracene do not lead to the isolation of azafullerene monoadducts. Moreover, 1,2-dichlorobenzene, the most commonly utilized solvent for azafullerene reactions, reacts slowly with the azafullerenyl radical C(59)N* affording the corresponding aza[60]fullerene monoadduct.

Open-cage fullerenes: towards the construction of nanosized molecular containers.

Open-cage fullerene derivatives have excited organic chemists' creativity over the past decade. These adducts, generated via consecutive cleavage of sigma- and pi-carbon-carbon bonds on the fullerene cage, allow small atoms or molecules to pass through their opening and be placed inside the cavity. Restoration of the ruptured fullerene back to the pristine fullerene cage affords the corresponding endohedral complexes. This "molecular surgery" approach has been proposed as an alternative to the synthesis of endohedral fullerenes via the conventional physical methods of production, which restrict the availability of endohedral fullerenes to milligram quantities after laborious isolation procedures. In this critical review, we survey all published techniques for the creation of an orifice, as well as for the expansion of an existing one, on the fullerene framework. Successful encapsulation experiments employing cage-opened fullerene derivatives are also comprehensively discussed (160 references).

Photosensitized oxidations of substituted pyrroles: unanticipated radical-derived oxygenated products.

Photooxidation of pyrrole adducts 7-10 has been investigated in order to establish a general reaction pattern and mechanism for the formation of the resulting oxygenated products. The reactions were performed in several solvents utilizing both type I and type II sensitizers. In most cases, photooxidations gave complex mixture of products. Among these products, 5,5- or 6,5-bicyclic lactams (11, 15, and 19), maleimide 12 unsaturated gamma-lactams (16 and 20), 5-hydroxylactams (13, 17, and 21), and 5-methoxylactams (14, 18, and 22) were isolated and characterized. Photooxidation of 2,5-dimethyl-substituted pyrrole 10 in aprotic solvents unexpectedly afforded aldehyde 23 as the major product. Moreover, photooxidation of pyrrole adduct 10 in protic solvents exclusively gave the unprecedented solvent-trapped products 24-27. The formation of products 11-22 was rationalized by the intermediacy of a common endoperoxide intermediate, which could be formed by both type I and type II mechanisms. Compounds 23-27 were most probably formed via an electron-transfer mechanism.

Decatungstate as an efficient photocatalyst in organic chemistry.

In view of its unique photocatalytic properties, decatungstate (W(10)O(32)(4-)) is rapidly emerging as a promising tool in organic chemistry. This tutorial review surveys recent developments in the chemistry of decatungstate, including mostly synthetic, and to a lesser extent mechanistic aspects. We have chosen to present several representative examples that illustrate the diverse uses of decatungstate in organic synthesis. Thus, the decatungstate-mediated radical functionalization of several classes of organic compounds such as alkanes, alkenes, alcohols, aldehydes and sulfides, under both aerobic and anaerobic conditions, represents reactions of fundamental and practical interest in academia and industry. Several new discoveries concerning the heterogenization of decatungstate for the development of sustainable methods with broad applications in catalysis, such as the photooxidation or photodegradation of various organic substrates, are also presented.

One-step synthesis of fullerene hydride C(60)H2 via hydrolysis of acylated fullerenes.

The hitherto unexplored class of acylated fullerene compounds has been shown to be excellent C(60)H2 precursors. Upon a simple treatment with basic Al2O3, they are hydrolyzed quantitatively into C(60)H2. This key feature led to the development of a new, straightforward protocol for the selective synthesis of the simplest [60]fullerene hydride, C(60)H2. This protocol may offer an advantageous alternative to previously known methods for the synthesis of C(60)H2 allowing for a rapid access to C(60)H2 in good yield and high purity without tedious separating processes.

Concerning the reactivity of PTAD with isomeric dienes: the mechanism of the Diels-Alder cycloaddition.

Cyclopropyl substituted dienes are employed as mechanistic probes in the triazolinedione Diels-Alder (DA) reaction. In aprotic and protic solvents, apart from the DA adducts that bear an intact cyclopropyl group, complicated and rearranged products are also obtained. These results provide solid evidence for the involvement of an open intermediate with a lifetime greater than 2 x 10(-12) s.

Acyl radical reactions in fullerene chemistry: direct acylation of [60]fullerene through an efficient decatungstate-photomediated approach.

A versatile and highly efficient photochemical methodology for the direct acylation of C(60) has been developed. This approach utilizes a wide variety of acyl radicals derived from aldehydes through a hydrogen atom abstraction process mediated by tetrabutylammonium decatungstate [(n-Bu(4)N)(4)W(10)O(32)]. The single addition reaction of these acyl radicals to [60]fullerene proceeded selectively to afford a novel class of previously unexplored fullerene-based materials. Product analysis of this reaction showed that decarbonylation and acylation pathways compete when a tertiary or phenylacetyl aldehyde is the starting material. However, a decrease of the reaction temperature was found to be effective in overcoming the decarbonylation encountered in certain acyl radical additions to C(60); the carbonyl radical addition precedes decarbonylation even in the cases where the decarbonylation rate constant exceeds 10(6) s(-1) (i.e., phenylacetaldehyde). The regiochemistry of the t-butyl radical addition was also found to be thermally controlled. The present methodology is directly applicable even in the cases of the cyclopropyl-substituted aldehydes, where rapid rearrangement of the cyclopropyl acyl radical intermediate can potentially occur. A mechanistic approach for this new reactivity of C(60) has been also provided, based mainly on intra- and intermolecular deuterium isotope effect studies.

Solvent-dependent changes in the triazolinedione-alkene ene reaction mechanism.

The influence of the solvent on the triazolinedione-alkene ene reaction mechanism has been investigated. Both inter- and intramolecular kinetic isotope effects with tetramethylethylenes and 2,2,2-(trideuterio)methyl-7-methyl-2,6-octadiene-[D3]-1,1,1 provide, for the first time, strong evidence for changes in the mechanism of the reaction on going from non-protic to polar protic solvents. In non-protic polar or apolar solvents, an aziridinium imide that equilibrates to an insignificant extent with an open intermediate (a dipolar or a polarized biradical) is formed irreversibly in the first, rate-determining step of the reaction, which is followed by fast hydrogen abstraction. On the contrary, in polar protic solvents, hydrogen abstraction is rate limiting, allowing the main dipolar intermediate to equilibrate with its open intermediate(s) as well as with the starting reagents.

Stereochemistry of the singlet oxygenation of simple alkenes: a stereospecific transformation.

The stereochemistry of the allylic oxidation (ene reaction) mediated by singlet oxygen ((1)O2), using the optically active alkene (S,S)-cis-1,4-diphenyl-2-butene-1,4-d2 , in MeOH and aprotic solvents was investigated. Our findings indicate that the title reaction is a highly stereospecific suprafacial process, independent of solvent polarity. The observation of an isotope effect, which matches the stereogenic ratio exactly, rules out biradical or open dipolar intermediates.

The cyclopropyl group as a hypersensitive probe in the singlet oxygen ene reaction mechanism.

Cyclopropyl-substituted olefins are employed as mechanistic probes in the singlet oxygen-alkene ene reaction. In MeOH and aprotic solvents [CHCl(3), (CH(3))(2)CO, CH(3)CN], only the allylic hydroperoxides bearing an intact cyclopropyl group are detected. The reaction mechanism is independent of solvent polarity. Our findings, to a certain experimental limit, exclude a biradical or dipolar intermediate.

Decatungstate-mediated radical reactions of C60 with substituted toluenes and anisoles: a new photochemical functionalization strategy for fullerenes.

A convenient, highly efficient, decatungstate-mediated chemical methodology to functionalize fullerenes is demonstrated. A variety of radicals have been generated by the photochemical interaction of tetrabutylammonium decatungstate [(n-Bu4N)4W10O32] and para-substituted toluenes, anisoles, and thioanisole and effectively trapped by the [60]fullerene affording the corresponding 1,2-dihydro[60]fullerene monoadducts in moderate to good yields.