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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy Petr Klan,́*,†,‡ Tomaś̌solomek,̌ †,‡ Christian G

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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy Petr Klan,́*,†,‡ Tomaś̌solomek,̌ †,‡ Christian G Review pubs.acs.org/CR Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy Petr Klan,́*,†,‡ Tomaś̌Solomek,̌ †,‡ Christian G. Bochet,§ Aurelień Blanc,∥ Richard Givens,⊥ Marina Rubina,⊥ Vladimir Popik,# Alexey Kostikov,# and Jakob Wirz∇ † Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic ‡ Research Centre for Toxic Compounds in the Environment, Faculty of Science, Masaryk University, Kamenice 3, 625 00 Brno, Czech Republic § Department of Chemistry, University of Fribourg, Chemin du Museé 9, CH-1700 Fribourg, Switzerland ∥ Institut de Chimie, University of Strasbourg, 4 rue Blaise Pascal, 67000 Strasbourg, France ⊥ Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, 5010 Malott Hall, Lawrence, Kansas 66045, United States # Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States ∇ Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland 7.3. Arylsulfonyl Group 160 7.4. Ketones: 1,5- and 1,6-Hydrogen Abstraction 160 7.5. Carbanion-Mediated Groups 160 7.6. Sisyl and Other Silicon-Based Groups 161 7.7. 2-Hydroxycinnamyl Groups 161 7.8. α-Keto Amides, α,β-Unsaturated Anilides, CONTENTS and Methyl(phenyl)thiocarbamic Acid 162 7.9. Thiochromone S,S-Dioxide 162 1. Introduction 119 7.10. 2-Pyrrolidino-1,4-Benzoquinone Group 162 2. Arylcarbonylmethyl Groups 121 7.11. Triazine and Arylmethyleneimino Groups 162 2.1. Phenacyl and Other Related Arylcarbonyl- 7.12. Xanthene and Pyronin Groups 163 methyl Groups 121 o 7.13. Retro-Cycloaddition Reactions 163 2.2. -Alkylphenacyl Groups 123 8. Sensitized Release 163 2.3. p-Hydroxyphenacyl Groups 125 p 8.1. Sensitized Release: Photoinduced Energy Position and Requirement for a -Hydroxy Transfer 164 Group 129 8.2. Sensitized Release: Photoinduced Electron Nature of the Leaving Group 130 Transfer 166 Substituent Effects on the Chromophore 132 ff K 8.3. Sensitized Release: Light Upconverting E ect of Media pH and pHP p a 132 Nanoparticles 170 2.4. Benzoin Groups 133 9. Two-Photon Excitation-Induced Photorelease 170 3. Nitroaryl Groups 137 o 10. Chromatic Orthogonality 173 3.1. -Nitrobenzyl Groups 137 11. Photoactivatable Fluorescent Dyes 174 Substitution at the Benzylic Position 140 o 12. Conclusion 178 Substituents on the Aromatic Ring of the - Author Information 178 Nitrobenzyl Chromophore 141 Corresponding Author 178 Two-Photon Absorption 143 Notes 178 Isotopic Substitution 143 Biographies 178 3.2. o-Nitro-2-phenethyloxycarbonyl Groups 143 o Acknowledgments 180 3.3. -Nitroanilides 145 Dedication 180 4. Coumarin-4-ylmethyl Groups 147 List of Abbreviations 180 Synthetic Approaches to Coumarin-Caged Com- References 181 pounds 147 Mechanism of Photorelease and Selected Appli- cations 147 5. Arylmethyl Groups 152 1. INTRODUCTION 5.1. Simple Arylmethyl Groups 152 Photoremovable (sometimes called photoreleasable, photo- 5.2. o-Hydroxyarylmethyl Groups 156 cleavable or photoactivatable) protecting groups (PPGs) provide 6. Metal-Containing Groups 157 spatial and temporal control over the release of various chemicals 7. Miscellaneous Groups 158 7.1. Pivaloyl Group 158 Received: April 29, 2012 7.2. Esters of Carboxylic Acids 158 Published: December 21, 2012 © 2012 American Chemical Society 119 dx.doi.org/10.1021/cr300177k | Chem. Rev. 2013, 113, 119−191 Chemical Reviews Review Table 1. Photoremovable Protecting Groups such as bioagents (neurotransmitters and cell-signaling mole- the biological entity.21 Moreover, the photoreaction cules), acids, bases, Ca2+ ions, oxidants, insecticides, pher- should be clean and should occur with a high quantum ffi Φ Φ omones, fragrances, etc. Following early reports on PPGs for use yield or e ciency for release, rel. The quantum yield rel 1 2 3 in organic synthesis by Barltrop, Barton, Woodward, is equal to the amount of released substrate, nrel/mol, Sheehan4 and their co-workers, applications to biology were divided by the amount of photons at the irradiation ff 5 λ sparked o by Engels and Schlaeger and Kaplan and co- wavelength , np/mol = np/einstein, that were absorbed by 6 fi Φ workers, who rst achieved the photorelease of cyclic adenosine the caged compound: rel = nrel/np. An important measure monophosphate (cAMP) and ATP, respectively. The latter for the efficacy of a PPG is the product of the quantum authors introduced the convenient, if somewhat misleading, term yield and the molar decadic absorption coefficient ε of the “ ” Φ ε λ caged to designate a compound protected by a PPG. Two PPG, rel ( irr), which is proportional to the amount of general perspectives7 and many more specialized reviews release at the given excitation wavelength.22 covering applications of PPGs in synthesis,8 biochemistry and 9 10 11 (ii) Sensitive detection of the response under study often neurobiology, biomedicine, volatiles release, polymeriza- depends not only on the product Φ ε(λ ) but also on the tion,12 and fluorescence activation13 have been published during rel irr background level of activity of the caged compound prior the past decade, and a journal issue themed on these topics has recently been published.14 The present review covers recent to irradiation. Hence, the PPGs must be pure, exhibit low developments in the field, focusing on the scope, limitations, and intrinsic activity, and be stable in the media prior to and applications of PPGs, which are used to release organic during photolysis. molecules. Photoactivation of small inorganic species and ions, (iii) The PPGs should be soluble in the targeted media; they such as NO,15 CO,16 Ca2+,9h,17 Zn2+,18 Cd2+,19 or Cu+,20 is not may further be required to pass through biological barriers covered. Simplified basic structures of the photoremovable such as cell membranes and show affinity to specific target protecting groups discussed in this review are listed in Table 1 components, for example, binding sites on cancer cells or (the leaving groups are shown in red). the active site of an enzyme. The criteria for the design of a good PPG will depend on the (iv) The photochemical byproducts accompanying the application. No single system needs to fulfill all of the following released bioactive reagent should ideally be transparent requirements: at the irradiation wavelength to avoid competitive (i) In general, the PPG should have strong absorption at absorption of the excitation wavelengths. Moreover, they wavelengths well above 300 nm, where irradiation is less must be biocompatible, i.e., they should not react with the likely to be absorbed by (and possibly cause damage to) system investigated. 120 dx.doi.org/10.1021/cr300177k | Chem. Rev. 2013, 113, 119−191 Chemical Reviews Review (v) To study the kinetics of rapid responses to a released agent are strong, and internal conversion to the S1 state is very fast (e.g., in samples such as brain tissue or single live cells, the PPG ∼100−260 fs for acetophenone).28 The electronic transitions of must be excited by a short light pulse and the appearance aromatic ketones are sensitive to solvent polarity and to rate constant kapp of the desired free substrate must exceed substitution on the phenyl ring. Hydrogen bonding of protic the rate constant of the response under investigation. solvents to the carbonyl oxygen stabilizes the oxygen non- Commonly, there are several reaction steps involving bonding orbital, giving rise to a hypsochromic shift of the n,π* ground- and excited-state intermediates that precede the absorption band. Both electron-donating groups and polar actual release of the free substrate. Therefore, detailed solvents tend to stabilize the π,π* states. The strong bath- knowledge of the reaction mechanism is needed; in ochromic shift induced by para-amino substituents is attributed 29 λ particular, the rate-determining step in the reaction path to a CT interaction (for example, max for p-amino- τ ∼ π π* 24 and its lifetime rd or kapp must be known, unless the benzaldehyde is 325 nm ( , ) in cyclohexane). Aromatic appearance of the free substrate (k ) can be monitored ketones are highly phosphorescent and only weakly fluorescent30 app − directly by time-resolved techniques. due to their fast (>1010 s 1), very efficient intersystem crossing to Nitrobenzyl, nitrophenethyl compounds, and their dimethoxy the triplet state, for which two energetically close lying states π* π π* derivatives (nitroveratryl) (section 3) are by far the most (n, and , ) seem to play a crucial role, possibly due to a S1/ 31 − commonly used PPGs. The decay of their primary quinonoid T2/T1 intersection. The singlet triplet energy gap is much intermediates on the microsecond time scale does not generally larger for π,π* than for n,π* states. The lowest π,π* and n,π* correspond to the rate-determining step of the overall reaction, triplet states are nearly degenerate, and substitution on the and the release of the free substrate may be orders of magnitude phenyl ring as well as polar solvents may lead to triplet-state slower. Moreover, photolysis of these compounds forms inversion.27,32 Some of the important photophysical properties potentially toxic and strongly absorbing byproducts such as o- of acetophenone, the parent aryl ketone, are summarized in nitrosobenzaldehyde. Quite a number of alternative PPGs have Table 2. Examples of the absorption spectra of other been developed that do not suffer from these disadvantages. representative PPG aryl ketones are provided in Figure 1. The appearance rate constant kapp of the desired product is The carbonyl group of aromatic ketones is usually the center of equal to the inverse of the rate-determining intermediate’s the photochemical reactivity. Scheme 2 shows the most τ τ lifetime rd, kapp =1/ rd, which often depends on the solvent as important photoreactions that lead to the liberation of a leaving well as on the concentrations of acids and bases including those group (X) and are discussed in the following paragraphs.
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