Design and synthesis of a photoswitchable guanidine catalyst

• Philipp Viehmann and
• Stefan Hecht

Beilstein J. Org. Chem. 2012, 8, 1825–1830, doi:10.3762/bjoc.8.209

• photochromic properties demonstrated the reversible switchability of its photosensitive azobenzene moiety. Its activity in the ring-opening polymerization (ROP) of rac-lactide was investigated as well. The obtained results are discussed, and an additional guanidine was synthesized and utilized in the ROP of

• hydrogen bonds to an acceptor (A) and activated by light-triggered dissociation of such intramolecular hydrogen bonds (Scheme 2). Herein, we report the synthesis of the first photoswitchable guanidine, as well as its photochromic behavior. Upon irradiation, the incorporated azobenzene is supposed to

• was converted by known procedures into its corresponding nitroso derivative 7 [19], followed by a Mills coupling with 4-aminoacetophenone (8) to give azobenzene 9 in 93% yield over two steps. Nitroazobenzene 9 was transformed into its amino derivative 10 by catalytic hydrogenation. Note that under the

Metal–ligand multiple bonds as frustrated Lewis pairs for C–H functionalization

• Matthew T. Whited

Beilstein J. Org. Chem. 2012, 8, 1554–1563, doi:10.3762/bjoc.8.177

• of isocyanates to ruthenium(II) silylenes [58] (Scheme 8). These complexes do not react with nonpolar substrates (although a possible cycloaddition with azobenzene was reported), and the overall cycloaddition was found to proceed through initial nucleophilic attack at an electrophilic silylene

Mannose-decorated cyclodextrin vesicles: The interplay of multivalency and surface density in lectin–carbohydrate recognition

• Ulrike Kauscher and
• Bart Jan Ravoo

Beilstein J. Org. Chem. 2012, 8, 1543–1551, doi:10.3762/bjoc.8.175

• (intervesicular binding) and hence cause vesicle aggregation. We have previously observed this effect for homobifunctional guest molecules equipped with two azobenzene, methylbenzoyl, or tert-butylbenzyl groups [32][33][34]. It should be noted that in the case of guest 3 the aggregation due to cross-linking is

Cation affinity numbers of Lewis bases

• Christoph Lindner,
• Raman Tandon,
• Boris Maryasin,
• Evgeny Larionov and
• Hendrik Zipse

Beilstein J. Org. Chem. 2012, 8, 1406–1442, doi:10.3762/bjoc.8.163

• -position are significantly more effective than aryl substituents in stabilizing the pyridinium ions formed through acetyl cation addition. Photo-switchable 3,4-diaminopyridines including a diazo moiety are potentially useful as special-purpose catalysts. The azobenzene substituent itself is electron

Control over molecular motion using the cis–trans photoisomerization of the azo group

• Estíbaliz Merino and
• María Ribagorda

Beilstein J. Org. Chem. 2012, 8, 1071–1090, doi:10.3762/bjoc.8.119

• . This transformation induces a molecular movement and a significant geometric change, therefore the azobenzene unit is an excellent candidate to build dynamic molecular devices. We describe selected examples of systems containing an azobenzene moiety and their motions and geometrical changes caused by

• external stimuli. Keywords: azobenzenes; molecular switches; nanomachines; photoisomerization; Review This review is based on an article published in 2009 in Anales de Química (Real Sociedad Española de Química) [1]. Azobenzene was described for the first time in 1834 [2] and one century later, in 1937

• , G. S. Hartley published a study of the influence of light on the configuration of N=N double bonds [3]. The exposure of a solution of azobenzene in acetone to light allowed the discovery of the cis isomer. This finding was the starting point of the development of one of the best organic molecular

Recent advances towards azobenzene-based light-driven real-time information-transmitting materials

• Jaume García-Amorós and
• Dolores Velasco

Beilstein J. Org. Chem. 2012, 8, 1003–1017, doi:10.3762/bjoc.8.113

• fast relaxation times for azobenzene-based photochromic molecular switches is still a challenge. This review focuses on the most recent achievements on azobenzene-based light-driven real-time information-transmitting systems. Besides, the main relationships between the structural features of the azo

• applications are certainly azobenzenes. Azobenzene, a photochromic T-type system, exhibits a reversible isomerisation process between its trans and cis isomers of different stability. In this process, the photoreaction simply causes the rearrangement of the electronic and nuclear structure of the molecule

• radicals may occur and dediazotation can be observed [19][20]. Azobenzene-based photochromic systems are under kinetic control; that is, after a photochemical conversion, whose rate depends mainly on the intensity of the excitation beam, the spontaneous thermal back reaction occurs. While the photo-induced

Diarylethene-modified nucleotides for switching optical properties in DNA

• Sebastian Barrois and
• Hans-Achim Wagenknecht

Beilstein J. Org. Chem. 2012, 8, 905–914, doi:10.3762/bjoc.8.103

• photoswitchable components in nucleic acids and are suitable for the control of a variety of different biological functions. The photoinduced cis–trans isomerization of azobenzene nucleosides can reversibly switch between the formation and dissociation of DNA duplexes [14][15][16][17], photoregulate DNA

Building photoswitchable 3,4'-AMPB peptides: Probing chemical ligation methods with reducible azobenzene thioesters

• Gehad Zeyat and
• Karola Rück-Braun

Beilstein J. Org. Chem. 2012, 8, 890–896, doi:10.3762/bjoc.8.101

• two regioisomeric azobenzene building blocks 3,4'-AMPB thioester 1b and 4,4'-AMPB thioester 2b were employed in the ligation reactions. While 4,4'-AMPB requires the 4,5,6-trimethoxy-2-mercaptobenzyl auxiliary to minimize reduction of the diazene unit, 3,4'-AMPB can be used in combination with the

• azoproteins for ambitious biophysical studies, and also for intracellular applications. In this respect, an increased and predictable stability of azobenzene building blocks under reducing conditions seems to be a prerequisite in light of synthetic challenges, but also when considering the reducing

• intracellular environment. We recently reported on the synthesis, properties and applications of a series of novel azobenzene ω-amino acids, with a preference for meta-substitution patterns. Our purpose was amongst other things (i) to increase flexibility, and (ii) to suppress resonance effects in order to

Intramolecular bridges formed by photoswitchable click amino acids

• Christian Hoppmann,
• Ronald Kühne and
• Michael Beyermann

Beilstein J. Org. Chem. 2012, 8, 884–889, doi:10.3762/bjoc.8.100

• presence of glutathione as an additional thiol the click reaction of the PSCaa occurs intramolecularly with the cysteine rather than with the glutathione, indicating that the click reaction may be used even under reducing conditions occurring in living cells. Keywords: azobenzene; helical conformation

• ; isomerization; molecular switches; photoswitchable click amino acid; thiol–ene click; Introduction Photoswitchable bridges that are site-specifically incorporated into proteins allow the conformation and activity of proteins to be modulated by light. In contrast to common bivalent thiol reactive azobenzene

• switches [1][2][3][4][5], the PSCaa described here (Scheme 1) is an α-amino acid containing, besides an azobenzene unit, a vinyl function that can react specifically with a cysteine within a putative conformational domain of a peptide or a protein by light-induced thiol–ene click reaction [6][7][8][9]. The

meta-Oligoazobiphenyls – synthesis via site-selective Mills reaction and photochemical properties

• Raphael Reuter and
• Hermann A. Wegner

Beilstein J. Org. Chem. 2012, 8, 877–883, doi:10.3762/bjoc.8.99

• revealed that the overall degree of switching was independent of the connected azo-units. However, one of the azobonds in the bis-azobiphenyl is isomerized preferentially despite the high structural similarity. Keywords: azobenzene; cross-coupling reaction; foldamer; molecular switches; photochromism

• ; Introduction The possibility to control structures on the molecular level in a reversible manner by using an external stimulus has fascinated scientists for a long time. One of the earliest reports on a molecular entity that can be influenced in this way concerns the azobenzene scaffold. Since this discovery

• by Hartley in 1937 [1] many more compounds showing such a photochromism have been reported [2]. However, the azobenzene moiety still remains one of the most popular “work horses” in this respect [3][4][5][6]. The reason can probably be found in its relatively easy synthetic accessibility combined

Molecular switches and cages

• Dirk Trauner

Beilstein J. Org. Chem. 2012, 8, 870–871, doi:10.3762/bjoc.8.97

• (Rück-Braun, Hoppmann). Two reviews discuss cis-azobenzenes with rapid thermal isomerization kinetics (Velasco), and highlight the azobenzene moiety as one of the smallest light-driven molecular motors conceivable (Merino). Issues of bistability are also addressed in an account on shape-persistent

Azobenzene dye-coupled quadruply hydrogen-bonding modules as colorimetric indicators for supramolecular interactions

• Yagang Zhang and
• Steven C. Zimmerman

Beilstein J. Org. Chem. 2012, 8, 486–495, doi:10.3762/bjoc.8.55

• Yagang Zhang Steven C. Zimmerman Department of Chemistry, 600 South Mathews Avenue, University of Illinois, Urbana, IL 61801, USA 10.3762/bjoc.8.55 Abstract The facile coupling of azobenzene dyes to the quadruply hydrogen-bonding modules 2,7-diamido-1,8-naphthyridine (DAN) and 7-deazaguanine urea

• (DeUG) is described. The coupling of azobenzene dye 2 to mono-amido DAN units 4, 7, and 9 was effected by classic 4-(dimethylamino)pyridine (DMAP)-catalyzed peptide synthesis with N-(3-dimethylaminopropyl)-N’-ethyl carbodiimide hydrochloride (EDC) as activating agent, affording the respective amide

• products 5, 8, and 10 in 60–71% yield. The amide linkage was formed through either the aliphatic or aromatic ester group of 2, allowing both the flexibility and absorption maximum to be tuned. Azobenzene dye 1 was coupled to the DeUG unit 11 by Steglich esterification to afford the product amide 12 in 35

Liquid-crystalline nanoparticles: Hybrid design and mesophase structures

• Gareth L. Nealon,
• Romain Greget,
• Cristina Dominguez,
• Zsuzsanna T. Nagy,
• Daniel Guillon,
• Jean-Louis Gallani and
• Bertrand Donnio

Beilstein J. Org. Chem. 2012, 8, 349–370, doi:10.3762/bjoc.8.39

• to emphasise the wider applicability of this approach to nanohybrid science. Fundamental studies into the photochemical activity of lipophilic azobenzene- and stilbene thiols (10, Figure 12) on Au NPs surfaces were reported [66], which indicated a chromophore-to-NP distance dependence of the

• photoisomerisation and photodimerisation efficiency in these systems. Interesting examples of the potential application of this type of approach were shown by the grafting of similar azobenzene ligands 11 (Figure 12) to the surfaces of γ-Fe2O3 [67] and FePt [68] NPs, which exhibited a photoswitchable change in

Continuous proline catalysis via leaching of solid proline

• Suzanne M. Opalka,
• Ashley R. Longstreet and
• D. Tyler McQuade

Beilstein J. Org. Chem. 2011, 7, 1671–1679, doi:10.3762/bjoc.7.197

• , and finally azobenzene by product disproportionation [53][54][55]. Byproduct suppression is both solvent and temperature dependent. Hayashi reported that when the reaction is performed at room temperature in acetonitrile with 30 mol % proline, the reaction is complete in 10 min, but achieves only 29

Synthetic incorporation of Nile Blue into DNA using 2′-deoxyriboside substitutes: Representative comparison of (R)- and (S)-aminopropanediol as an acyclic linker

• Daniel Lachmann,
• Sina Berndl,
• Otto S. Wolfbeis and
• Hans-Achim Wagenknecht

Beilstein J. Org. Chem. 2010, 6, No. 13, doi:10.3762/bjoc.6.13

• ′-deoxyribose substitutes by influencing the stacking orientation of an attached azobenzene dye. The configuration of the linker decides if the dye protrudes towards the major or minor groove, which subsequently leads to an enhanced or diminished stability of the whole DNA duplex [42]. Herein, we want to