2.3. [4 + 4] Cycloadditions 1058 2.4. Photocycloadditions of Aromatic Compounds 1058 2.4.1. Benzene Derivatives 1058 2.4.2. Condensed Aromatic Compounds 1060 3. Photochemical Rearrangements 1061 4. Cyclizations 1064 4.1. Pericyclizations 1064 4.2. Norrish−Yang Reaction 1066 5. Photochemical Extrusion of Small Molecules 1067 6. Photochemical Electron Transfer 1068 6.1. Photochemical Electron-Transfer Reactions with a Catalytic Sensitizer 1068

Photochemical Reactions as Key Steps in Organic Synthesis

Since their first detection and bulk production, the fullerenes have
gained a primary role on the scientific scene, reaching their climax when
the 1996 Nobel Prize for Chemistry was awarded to Kroto, Curl and Smalley
for their seminal discovery. The unique physical and chemical properties
of these new forms of carbon led many scientists to predict several
technological applications. This created a heavy disappointment when it
was clear that fullerene-based materials would not soon be ready for the
market. However, the fullerenes have so far delighted several dozens of
researchers who found that C
 60
 and its relatives undergo a
variety of chemical reactions. In most cases, the new derivatives retain
the main properties of the original fullerene, and it is now not unlikely
that some functionalized fullerenes may find useful applications in the
field of materials science and technology. In this Article we summarize
the basic principles of the organic chemistry of fullerenes, together with
a description of the physicochemical properties that have made these
carbon cages popular in materials science, and review the most recent
achievements in the functionalization of fullerenes aimed at the
production of new molecular materials.

[60]Fullerene chemistry for materials science applications

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4, whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.5b00723

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2+2] Photocycloaddition Reactions

Photochemical reactions contribute in a significant way to the existing repertoire of carbon-carbon bond-forming reactions by allowing access to exceptional molecular structures that cannot be obtained by conventional means. In this Review, the most important photochemical transformations that have been employed in natural product synthesis are presented. Selected total syntheses are discussed as examples, with particular attention given to the photochemical key step and its stereoselectivity. The structural relationship between the photochemically generated molecule and the natural product is shown, and, where necessary, the consecutive reactions in the synthesis are illustrated and classified.

Photochemical reactions as key steps in natural product synthesis

More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.

Strategies and tactics for the metal-directed synthesis of rotaxanes, knots, catenanes, and higher order links.

New insights into an old mechanism: [2 + 2] photocycloaddition of enones to alkenes

A new class of [2]catenanes containing zinc(II)−porphyrin (ZnP) and/or [60]fullerene (C60) as appended groups has been prepared. A complete description of the convergent synthetic approach based on Cu(I) template methodology and “click” 1,3-dipolar cycloaddition chemistry is described. This new electron donor−acceptor catenane family has been subjected to extensive spectroscopic, computational, electrochemical and photophysical studies. 1H NMR spectroscopy and computational analysis have revealed that the ZnP−C60−[2]catenane adopts an extended conformation with the chromophores as far as possible from each other. A detailed photophysical investigation has revealed that upon irradiation the ZnP singlet excited state initially transfers energy to the (phenanthroline)2−Cu(I) complex core, producing a metal-to-ligand charge transfer (MLCT) excited state, which in turn transfers an electron to the C60 group, generating the ZnP−[Cu(phen)2]2+−C60•− charge-separated state. A further charge shift from the [Cu(phen...

/pdf/2-catenanes-decorated-with-porphyrin-and-60-fullerene-groups-3glzonh8f7.pdf

[2]Catenanes Decorated with Porphyrin and [60]Fullerene Groups: Design, Convergent Synthesis, and Photoinduced Processes

Aromatic triazoles have been frequently used as π-conjugated linkers in intramolecular electron transfer processes. To gain a deeper understanding of the electron-mediating function of triazoles, we have synthesized a family of new triazole-based electron donor–acceptor conjugates. We have connected zinc(II)porphyrins and fullerenes through a central triazole moiety—(ZnP–Tri–C60)—each with a single change in their connection through the linker. An extensive photophysical and computational investigation reveals that the electron transfer dynamics—charge separation and charge recombination—in the different ZnP–Tri–C60 conjugates reflect a significant influence of the connectivity at the triazole linker. Except for the m4m-ZnP–Tri–C6017, the conjugates exhibit through-bond photoinduced electron transfer with varying rate constants. Since the through-bond distance is nearly the same for all the synthesized ZnP–Tri–C60 conjugates, the variation in charge separation and charge recombination dynamics is mainly a...

Triazole Bridges as Versatile Linkers in Electron Donor–Acceptor Conjugates

We now report functionalization of C[sub 60] by [2 + 2] photocycloaddition of cyclic enones, allowing the simple preparation of a large new class of C[sub 60] derivatives. This is another application of a classic photochemical reaction which is widely used synthetically. The mechanistic features also are of considerable interest. We have shown (a) that the new technique of electrospray ionization mass spectrometry (ESI-MS) is a uniquely suitable tool for studying the reactions of fullerenes, (b) that a variety of functionally substituted derivatives of C[sub 60] can be prepared by [2 + 2] photocycloaddition to cyclic enones, and (c) that these adducts arise by the stepwise addition of enone triplets to the fullerenes. 29 refs., 1 fig.

David I. Schuster

Papers

New insights into an old mechanism: [2 + 2] photocycloaddition of enones to alkenes

[2]Catenanes Decorated with Porphyrin and [60]Fullerene Groups: Design, Convergent Synthesis, and Photoinduced Processes

Triazole Bridges as Versatile Linkers in Electron Donor–Acceptor Conjugates

A new reaction of fullerenes: [2+2]-photocycloaddition of enones

Enone photochemistry. Dynamic properties of triplet excited states of cyclic conjugated enones as revealed by transient absorption spectroscopy