4.2.8. Reductive Coupling 3109 5. Reaction of Aromatic Compounds 3110 5.1. Electrophilic Substitutions 3110 5.2. Radical Substitution 3111 5.3. Oxidative Coupling 3111 5.4. Photochemical Reactions 3111 6. Reaction of Carbonyl Compounds 3111 6.1. Nucleophilic Additions 3111 6.1.1. Allylation 3111 6.1.2. Propargylation 3120 6.1.3. Benzylation 3121 6.1.4. Arylation/Vinylation 3121 6.1.5. Alkynylation 3121 6.1.6. Alkylation 3121 6.1.7. Reformatsky-Type Reaction 3122 6.1.8. Direct Aldol Reaction 3122 6.1.9. Mukaiyama Aldol Reaction 3124 6.1.10. Hydrogen Cyanide Addition 3125 6.2. Pinacol Coupling 3126 6.3. Wittig Reactions 3126 7. Reaction of R,â-Unsaturated Carbonyl Compounds 3127

Organic Reactions in Aqueous Media with a Focus on Carbon−Carbon Bond Formations: A Decade Update

The review covers the knowledge on photoremovable protecting
groups and includes all relevant chromophores studied in the
time period of 2000–2012; the most relevant earlier works are
also discussed.

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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

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

Biologically active compounds which are light-responsive offer experimental possibilities which are otherwise very difficult to achieve. Since light can be manipulated very precisely, for example, with lasers and microscopes rapid jumps in concentration of the active form of molecules are possible with exact control of the area, time, and dosage. The development of such strategies started in the 1970s. This review summarizes new developments of the last five years and deals with "small molecules", proteins, and nucleic acids which can either be irreversibly activated with light (these compounds are referred to as "caged compounds") or reversibly switched between an active and an inactive state.

Biologically active molecules with a "light switch".

Spatial and temporal control over chemical and biological processes plays a key role in life, where the whole is often much more than the sum of its parts. Quite trivially, the molecules of a cell do not form a living system if they are only arranged in a random fashion. If we want to understand these relationships and especially the problems arising from malfunction, tools are necessary that allow us to design sophisticated experiments that address these questions. Highly valuable in this respect are external triggers that enable us to precisely determine where, when, and to what extent a process is started or stopped. Light is an ideal external trigger: It is highly selective and if applied correctly also harmless. It can be generated and manipulated with well-established techniques, and many ways exist to apply light to living systems--from cells to higher organisms. This Review will focus on developments over the last six years and includes discussions on the underlying technologies as well as their applications.

Light-Controlled Tools

The diastereofacial selectivity in reactions of a series of alkyl-substituted cyclohexyl radicals has been investigated. In additions of cyclohexyl radicals to alkenes, it has been found that only substituents bound at the olefinic center being attacked by the radical influence the equatorial-axial selectivity. Substituents bound to the radical center or axial substituents p to the radical center lead to increased axial attack. Equatorial /3-substituents or axial ?-substituents increase the amount of equatorial attack. The same trends are observed for halogen and hydrogen abstraction reactions; the amount of axial reaction product is usually somewhat higher than in the addition reactions. The stereoselectivities can be explained with steric and torsional effects very similar to those suggested for nucleophilic addition reactions to cyclohexanones. A MM2 force field has been parameterized to gain further insight into the stereochemistry of the reaction.

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Diastereofacial Selectivity in Reactions of Substituted Cyclohexyl Radicals. An Experimental and Theoretical Study

Mo-bound methyl group. (ii) The coupling constant between the methyl protons and the 31P nuclei increases from 1.8 Hz at-20 O C (at lower temperatures the corresponding signal is unresolved) to 3.0 Hz at 20 \"C. For comparison, in the structurally characterized met hyl-tungsten c o m p l e x e ~ ~ ~ + ~ W (CH3)(L-L) (CO),-(PMe3)2 (L-L = acac, S2CNR2, S2COR) this coupling ranges from 3.5 to 8 Hz. (iii) The solution IR spectrum of 4 (20 \"C) is more complex than those of 1-3 and shows, in addition to bands due to the carbonyl functionalities in the agostic and q2-acyl isomers, two absorptions at ca. 191 2 and 1836 cm-I which can be tentatively assigned to the terminal carbonyl ligands in the methyl dicarbonyl species Mo(CHJ (S2CO-i-Pr) (C02) (PMe3)2. In conclusion, our results indicate that in the system under investigation there are small energy differences among the isomeric structures A, B, and C , so that both types of acyl coordination, B and C, are kinetically and thermodynamically accessible from their isomeric alkyl-xrbonyl structure A. Since most of the acyl complexes of molybdenum known to date have q2 structures while the only known agostic acyls are those discussed in this paper, it seems that the C-He-Mo interaction becomes structurally and thermodynamically competitive only in the presence of strongly electron releasing ligands such as the dithiocarbamates and xanthates. Note however that the analogous tungsten complexes have alkyl-carbonyl structure^.^ Therefore it is clear that very subtle electronic effects must be responsible for the observation in the present system of the three isomeric structures A, B, and C. Carbon-centered radicals are nucleophilic or electrophilic species, depending upon the substituent at the radical center. Electron-donating substituents like alkyl or alkoxy groups increase the nucleophilicity' of radicals whereas electron-withdrawing substituents like ester or nitrile groups augment their electrophilic9 behavior. Calculations for a variety of cases have shown that nucleophilic radicals approach the olefinic carbon atoms at angles between 1 0 4 O and 108\" at the UHF/3-21G level.3 Figure 1 shows the geometry for the addition of the methyl radical to ethylene at the UHF/6-31G* leveLJb Figure 1. UHF/6-3lG* transition state for the addition of the methyl radical to ethylene.

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Transition states of electrophilic radical additions to alkenes

Sequentially Photocleavable Protecting Groups in Solid-Phase Synthesis

Phototriggered bod cleavage has found wide application in chemistry as well as in biology. Nevertheless, there are only a few methods available for site-specific photochemical induction of DNA strand scission despite numerous potential applications. In this study we report the development of new photocleavable nucleotides based on the photochemistry of o-nitrobenzyl esters. The light-sensitive moieties were generated through introduction of o-nitrophenyl groups at the 5'C position of the nucleoside sugar backbone. The newly synthesized, modified nucleosides were incorporated in oligonucleotides and are able to build stable DNA duplexes. In such a way modified oligonucleotides ca cleaved site-specifically upon irradiation with > 360 nm light with high efficiency. Furthermore, we show that these modifications can be bypassed in DNA synthesis promoted by Thermus aquaticus DNA polymerase.

Bernd Giese

Papers

Diastereofacial Selectivity in Reactions of Substituted Cyclohexyl Radicals. An Experimental and Theoretical Study

Transition states of electrophilic radical additions to alkenes

Sequentially Photocleavable Protecting Groups in Solid-Phase Synthesis

New light-sensitive nucleosides for caged DNA strand breaks

The Curtin-Hammett principle: Stereoselective radical additions to alkenes