Tin-functionalized azobenzenes as nucleophiles in Stille cross-coupling reactions

Synthesis of a Series of 12-Membered Azobenzene Macrocycles and Tuning of the Half-Life of the Thermal Z-E Isomerization

Azobenzene macrocycles (AzMs) represent a class of azobenzene that are typically photoswitchable with good switching yields of E and Z isomers at certain photostationary states. Here, the synthesis and versatile functionalization of 12-membered AzMs is presented to obtain various meta- and para-aryl-substituted AzMs in high yields of 71-98%. At different positions in the periphery, these substituents significantly impact on the thermal half-lives of the less-stable Z isomers. Para-substitution leads to faster thermal relaxation than meta-substitution, and electron-donating groups lead to a faster relaxation than electron-withdrawing groups.

Pd(II)-catalyzed coupling of C-H bonds of carboxamides with iodoazobenzenes toward modified azobenzenes

In this paper, we report a synthetic protocol for the construction of biaryl motif-based or π-extended azobenzene and alkylated azobenzene derivatives via the Pd(II)-catalyzed bidentate directing group (DG)-aided C-H activation and functionalization strategy. In the past, the synthesis of biaryl motif-based azobenzenes was accomplished through the traditional cross-coupling reaction involving organometallic reagents and aryl halides or equivalent coupling partners. We have shown the direct coupling of C-H bonds of aromatic/aliphatic carboxamides (possessing a DG) with iodoazobenzenes as the coupling partners through the Pd(II)-catalyzed bidentate DG-aided, site-selective C-H functionalization method. Azobenzene-containing compounds are a versatile class of photo-responsive molecules that have found applications across branches of chemical, biological and materials sciences and are prevalent in medicinally relevant molecules. Accordingly, the synthesis of new and functionalized azobenzene-based scaffolds has been an attractive topic of research. Although the classical methods are efficient, they need pre-functionalized starting materials. This protocol involving the Pd(II)-catalyzed, directing group-aided site-selective C-H arylation of aromatic and aliphatic carboxamides using iodoazobenzene as the coupling partner affording azobenzene-based carboxamides is an additional route and also a contribution towards enriching the library of modified azobenzenes. We have also shown the photoswitching properties of representative compounds synthesized via the Pd(II)-catalyzed directing group-aided site-selective C-H functionalization method.

Modification of Azobenzenes by Cross-Coupling Reactions

Azobenzenes are among the most extensively used molecular switches for many different applications. The need to tailor them to the required task often requires further functionalization. Cross-coupling reactions are ideally suited for late-stage modifications. This review provides an overview of recent developments in the modification of azobenzene and its derivatives by cross-coupling reactions.

1 Introduction

2 Azobenzenes as Formally Electrophilic Components

2.1 Palladium Catalysis

2.2 Nickel Catalysis

2.3 Copper Catalysis

2.4 Cobalt Catalysis

3 Azobenzenes as Formally Nucleophilic Components

3.1 Palladium Catalysis

3.2 Copper Catalysis

3.3 C–H Activation Reactions

4 Azobenzenes as Ligands in Catalysts

5 Diazocines

5.1 Synthesis

5.2 Cross-Coupling Reactions

6 Conclusion

Photoswitchable Azo- and Diazocine-Functionalized Derivatives of the VEGFR-2 Inhibitor Axitinib

In this study, we aimed at the application of the concept of photopharmacology to the approved vascular endothelial growth factor receptor (VEGFR)-2 kinase inhibitor axitinib. In a previous study, we found out that the photoisomerization of axitinib’s stilbene-like double bond is unidirectional in aqueous solution due to a competing irreversible [2+2]-cycloaddition. Therefore, we next set out to azologize axitinib by means of incorporating azobenzenes as well as diazocine moieties as photoresponsive elements. Conceptually, diazocines (bridged azobenzenes) show favorable photoswitching properties compared to standard azobenzenes because the thermodynamically stable Z-isomer usually is bioinactive, and back isomerization from the bioactive E-isomer occurs thermally. Here, we report on the development of different sulfur–diazocines and carbon–diazocines attached to the axitinib pharmacophore that allow switching the VEGFR-2 activity reversibly. For the best sulfur–diazocine, we could verify in a VEGFR-2 kinase assay that the Z-isomer is biologically inactive (IC₅₀ >> 10,000 nM), while significant VEGFR-2 inhibition can be observed after irradiation with blue light (405 nm), resulting in an IC₅₀ value of 214 nM. In summary, we could successfully develop reversibly photoswitchable kinase inhibitors that exhibit more than 40-fold differences in biological activities upon irradiation. Moreover, we demonstrate the potential advantage of diazocine photoswitches over standard azobenzenes.

Synthesis and crystal structure of (E)-1,2-bis-[2-(methyl-sulfan-yl)phen-yl]diazene

The title compound, C14H14N2S2, was obtained by transmetallation of 2,2'-bis-(tri-methyl-stann-yl)azo-benzene with methyl lithium, and subsequent quenching with dimethyl di-sulfide. The asymmetric unit comprises two half-mol-ecules, the other halves being completed by inversion symmetry at the midpoint of the azo group. The two mol-ecules show only slight differences with respect to N=N, S-N and aromatic C=C bonds or angles. Hirshfeld surface analysis reveals that except for one weak H⋯S inter-action, inter-molecular inter-actions are dominated by van der Waals forces only.

Optical manipulation of sphingolipid biosynthesis using photoswitchable ceramides

Ceramides are central intermediates of sphingolipid metabolism that also function as potent messengers in stress signaling and apoptosis. Progress in understanding how ceramides execute their biological roles is hampered by a lack of methods to manipulate their cellular levels and metabolic fate with appropriate spatiotemporal precision. Here, we report on clickable, azobenzene-containing ceramides, caCers, as photoswitchable metabolic substrates to exert optical control over sphingolipid production in cells. Combining atomic force microscopy on model bilayers with metabolic tracing studies in cells, we demonstrate that light-induced alterations in the lateral packing of caCers lead to marked differences in their metabolic conversion by sphingomyelin synthase and glucosylceramide synthase. These changes in metabolic rates are instant and reversible over several cycles of photoswitching. Our findings disclose new opportunities to probe the causal roles of ceramides and their metabolic derivatives in a wide array of sphingolipid-dependent cellular processes with the spatiotemporal precision of light.

Synthesis, Structure, Thermal Behavior and cis/trans Isomerization of 2,2'-(EMe₃)₂ (E = C, Si, Ge, Sn) Substituted Azobenzenes

The synthesis of a series of 2,2′-bis(trimethyltetrel) azobenzenes is reported, evaluating the different synthetic approaches that different group 14 element substituents individually require. The synthetic access to the carbon substituted congener is very different from the heavier tetrels, in that the key step is the formation of the N=N bond in azobenzene, rather than the azobenzene-C bond. Sn could be introduced with a cross-coupling route, whereas the Si and Ge congeners were prepared by a stannylation-lithiation-electrophilic quenching sequence. Iodo-lithium exchange was also a possible route to obtain the dilithiated species, which can be attributed to the chelating effect of the nitrogen atoms. However, the organo-lead species could not be obtained via these routes. The resulting structures were fully characterized (NMR, FTIR, HRMS and XRD). Furthermore, their thermal properties (TGA and DSC) and their photoswitching behavior in solution (UV-VIS & NMR experiments) were investigated and compared for the different tetrels (C, Si, Ge, Sn).

Crystal structures of 3,3'-bis-(hy-droxy-dimethylsilan-yl)azo-benzene and 4,4'-bis-(hy-droxy-dimethyl-silane)azo-benzene

The title compounds {systematic names (E)-[diazene-1,2-diylbis(3,1-phenyl-ene)]bis-(di-methyl-silanol) and (E)-[diazene-1,2-diylbis(4,1-phenyl-ene)]bis-(di-methyl-silanol)}, both of the sum formula C16H22N2O2Si2, were obtained by transmetallation of the respective bis-stannylated azo-benzenes with di-chloro-dimethyl-silane and esterification followed by hydrolysis. The asymmetric unit of 3,3'-diazenediylbis[dimeth-yl(phen-yl)silanol] (with the silanol functional group in a meta position) consists of two mol-ecules, of which one occupies a general position, whereas the second is located on a centre of inversion. In 4,4'-diazenediylbis[dimeth-yl(phen-yl)silanol] (with the silanol functional group in a para position) likewise two mol-ecules are present in the asymmetric unit, but in this case both occupy general positions. Differences between all mol-ecules can be found in the torsions about the N=N bond, as well as in the dihedral angles between the benzene rings. In both structures, inter-molecular O-H⋯O hydrogen bonding is observed, leading to the formation of layers parallel to (01-1) for (I) and to chains parallel to the a axis for (II).

Azobenzenes and catalysis

Azobenzene is the most extensively used class of chromophore in a large variety of applications.

Microwave-Assisted Stille Cross-Coupling Reaction Catalysed by in Situ Formed Palladium Nanoparticles

The Stille coupling of organotin compounds with aryl iodides and aryl bromides catalysed by in situ formed nanoparticles from commercially available palladium dichloride in PEG-400 in the presence of DMAP under microwave irradiation has been developed. For tetraphenylstannane, the reaction was carried out in an atom-efficient way, as 4 equiv. of aryl halides coupled effectively with 1 equiv. of tetraphenylstannane to furnish 4 equiv. of the corresponding functionalised biaryls in high yields. Under the same conditions, PhSnBu3 also reacted with aryl halides to produce biphenyls.

High-yield lithiation of azobenzenes by tin-lithium exchange

The lithiation of halogenated azobenzenes by halogen-lithium exchange commonly leads to substantial degradation of the azo group to give hydrazine derivatives besides the desired aryl lithium species. Yields of quenching reactions with electrophiles are therefore low. This work shows that a transmetalation reaction of easily accessible stannylated azobenzenes with methyllithium leads to a near-quantitative lithiation of azobenzenes in para, meta, and ortho positions. To investigate the scope of the reaction, various lithiated azobenzenes were quenched with a variety of electrophiles. Furthermore, mechanistic (119) Sn NMR spectroscopic studies on the formation of lithiated azobenzenes are presented. A tin ate complex of the azobenzene was detected and characterized at low temperature.