Key words
azobenzene - diazocine - molecular switches - cross-coupling reactions - C–H activation
- metal-catalyzed
1
Introduction
Azobenzene and its derivatives are among the most investigated molecular switches.[1] They can interconvert photochemically and thermally between their metastable (E)- and (Z)-isomers (Figure [1]).[2]
Figure 1 Reversible isomerization of azobenzene[2]
Several physicochemical characteristics are affected by this photoisomerization, e.g.
geometry and end-to end distance,[3] electronic properties,[4] and polarity.[2] Whereas (E)-azobenzene is planar[3a] and without a dipole moment,[4] (Z)-azobenzene shows a non-planar geometry[3b] and a dipole moment of 3.0 D.[4] Consequently, azobenzene derivatives have gained great interest, for example for
applications in data storage materials,[5] dynamic molecular devices,[1] or in photonics.[6]
(from left to right) Melanie Walther studied chemistry and business administration at the University of Kiel. After research
stays at Cardiff University as well as at Stockholm University, she joined the Staubitz
group for her master’s thesis, dealing with photoswitchable polysiloxanes. During
her following doctoral study at the University of Bremen she wants to expand the synthetic
scope of molecular switches and their application into materials.
Waldemar Kipke studied biochemistry at Leibniz University in Hannover and obtained his bachelor’s
degree in 2016. He wrote his master’s thesis about new ethylene-bridged molecular
switches and obtained his master’s degree in October 2018. During his Ph.D., he continues
to work on molecular switches and new heterocycles containing B, Zr, and Sn.
Sven Schultzke studied chemistry at the University of Kiel and joined the Staubitz group for his
bachelor’s thesis about organogold(I) cross-coupling reactions. His upcoming research
has been based on photoswitchable molecules, starting with a research exchange to
the University of British Columbia in Vancouver for his master thesis and now his
doctoral study, where he designs smart materials for ‘soft grippers’.
Souvik Ghosh completed his M.Sc. from SVNIT, India. During his studies, he was a DAAD scholar
at KIT, Germany and a OIST research scholar at OIST, Japan. In 2018, he joined the
Staubitz group as Ph.D. student focusing on the synthesis of novel switchable molecules
and their application in materials and polymer sciences.
Anne Staubitz was an assistant professor at the University of Kiel, before moving to the University
of Bremen in 2015, where she has a full professorship for organic functional materials.
Her main interests are light and force sensitive materials. The primary research focus
is on their syntheses and properties, as well as applications. The second large research
area in the group is comprised of compounds and materials that contain unusual combinations
of main group elements and heavier elements.
Thus, manifold synthetic procedures have been developed for the preparation of azobenzene
derivatives,[7] most with the formation of the diazenyl group as the key step. Methods to obtain
symmetric azobenzenes range from reductive coupling of nitrobenzenes[8] or oxidative coupling of anilines.[9] The Mills reaction[10] or azo-coupling reactions[11] can be used to prepare asymmetric azobenzene derivatives. However, functionalized
azobenzenes obtained in this way usually require prefunctionalized starting materials[7] which limits the synthetic modification possibilities. An additional problem is
the susceptibility of the diazenyl group towards oxidizing[11b]
[12] and reducing[13] agents. Therefore, late-stage modification through cross-coupling reactions provides
a valuable alternative to access a wider variety of azobenzene derivatives. This short
review aims to give a broad, but not exhaustive, overview of the synthetic possibilities
offered by cross-coupling reactions on azobenzenes and diazocines. In this short review,
we distinguish between azobenzenes as formally electrophilic and formally nucleophilic
components because of the different requirements and the corresponding difficulties
in the synthesis of the azobenzene precursors, especially for nucleophilic derivatives.
Azobenzenes as Formally Electrophilic Components
2
Azobenzenes as Formally Electrophilic Components
(Pseudo)halogenated azobenzenes are used as an electrophilic component in cross-coupling
reactions with a large variety of organometallic (nucleophilic) coupling partners.
These (pseudo)halogenated species are usually obtained by employing prefunctionalized
building blocks.[7] There are very few reported examples of the direct halogenation of azobenzene derivatives.[14] The relatively low reactivity of azobenzenes towards electrophilic halogenation
reactions results from the electronic properties of the diazenyl group, which can
form adducts with halogen halides leading to low yields.[14a]
[b] The use of elemental halogens often results in inseparable mixtures of mono-, di-,
tri-, and tetrahalogenated products.[14a,c,d] Due to the lone-electron pairs on the nitrogen atoms, the diazenyl group can coordinate
to metal catalysts facilitating substitution in the ortho-position.[14e]
ortho-Halogenation is thus possible via metal-catalyzed C–H activation.[14c]
[e]
[f] However, different coordination patterns of the metal catalyst on the azobenzene
moiety have been detected.[14d] Thus, selective halogenation remains challenging.[14e]
2.1
Palladium Catalysis
Palladium catalysts are the most frequently used catalysts in cross-coupling reactions.
Therefore, the high number of palladium-catalyzed cross-coupling reactions of azobenzene
derivatives that serve as a formally electrophilic component is no surprise.
2.1.1
Suzuki–Miyaura Cross-Coupling Reactions
The Suzuki–Miyaura cross-coupling reaction of (pseudo)halogenated azobenzenes with
boronic acids or esters is one of the most frequently used cross-coupling reactions
for the modification of azobenzenes. Due to its convenience, reliability, and high
yields, it is often used as the final synthetic step to combine large building blocks.[15] Of the many available examples, in this review we place a certain focus on polymers
or molecules that self-assemble: such larger molecules are often not easy to prepare
and this is where the benefits of the Suzuki–Miyaura cross-coupling are most relevant.
Consequently, the Suzuki–Miyaura cross-coupling reaction gives access to many azobenzene
derivatives with new applications in self-assembled materials[15a] or many liquid crystals,[15`]
[c]
[d]
,
[16] compounds that show tunable fluorescence,[17] photoswitchable porphyrin systems,[15`]
[f]
[g] dendrimers,[15h] polymers,[15`]
[j]
[k]
[l]
[m]
[n]
[o] metal-organic frameworks (MOFs),[15p] as well as molecular machines such as rotaxanes.[15q]
[r]
The first successful Suzuki–Miyaura cross-coupling reaction of an azobenzene derivative
was described in a polymerization reaction (Scheme [1]);[15i] different conjugated polymers were synthesized with molecular weights up to Mn = 9700 (in yields of 80–99%).
Scheme 1 Suzuki–Miyaura cross-coupling reaction for polymerization of an azobenzene derivative[15i]
Besides a benzene ring,[15i] more complex motifs such as a fluorene ring[15j]
[l]
[n]
[o] or a carbazole ring[15l,m,o] were successfully integrated in the conjugated main chain.
For the synthesis of rotaxanes, azobenzene derivatives were connected with two α-cyclodextrin
units resulting in a [3]-rotaxane,[18] and later on a [1]-rotaxane.[15q]
[r] The azobenzene motif was either trapped in or bonded directly to the α-cyclodextrin
units and subsequently capped by benzo[de]isoquinoline derivatives via cross-coupling.[15q,r,18]
Further possibilities are demonstrated by the implementation of two consecutive Suzuki
cross-coupling reactions. Starting from a 4,4′-diiodoazobenzene, initial coupling
with 4-bromophenylboronic acid gave a 4,4′-bis(4-bromophenyl)azobenzene that underwent
a second cross-coupling reaction with a 4-substituted phenylboronic acid to give an
azoterphenyl derivative (37–54% over 2 steps).[19]
Since 2016, several cross-coupling reactions have been performed using asymmetric
azobenzene derivatives; in this way, molecules capable of precise self-assembly with
additional non-covalent interactions were prepared in yields ranging from 26% to 94%
(Scheme [2], A–C).[15c]
[16]
[17] Suzuki–Miyaura cross-coupling was also applied to functionalize nickel porphyrin
systems with azobenzene moieties in excellent overall yields (Scheme [2], D).[15f] Pd(PPh3)4 served as Pd(0) catalyst with K2CO3 as base and the reaction was carried out in a toluene/EtOH/water mixture at 90 °C
leading to good and sometimes excellent yields. The coupling was even successful with
an azopyridine and with adjusted conditions for an azoimidazole unit. For the latter,
the free amine of the imidazole was N-methylated to prevent a possible side reaction
with PdCl2(dppf) as the Pd(II) catalyst.[15f]
Scheme 2 Pd-catalyzed Suzuki–Miyaura cross-coupling reactions for enlarging a planar system
(A–C)[15c]
[16]
[17] and for the functionalization of porphyrin systems (D)[15f]
2.1.2
Sonogashira Cross-Coupling Reactions
The palladium-catalyzed cross-coupling reaction of a terminal alkyne with halogenated
azobenzenes represents another widely used functionalization possibility. In this
way, an azobenzene unit can be connected with relatively long, rigid, and π-conjugated
linkers in good to excellent yields.[20] The incorporated linkers then serve a specific function in the molecule: For example,
the functionalization of ethynyl-1,6-methano[10]annulenes with azobenzene demonstrated
the synthesis of electron-donor/acceptor systems that are suitable substrates for
nonlinear optics or liquid crystals (Scheme [3], A).[20a] In different azobenzene systems for the synthesis of photochromic self-assembled
monolayers, the rigid linker ensures sufficient control over the distance from the
headgroup to the surface and features a cooperative switching behavior of the azobenzene
units.[20`]
[c]
[d]
Scheme 3 Pd-catalyzed Sonogashira cross-coupling reactions of halogenated azobenzenes[20a]
[f]
[g]
,
[l]
[m]
[o]
Hydrophobic fluorescent azobenzenes were transformed into water-soluble fluorescent
2-borylazobenzenes by incorporating ionic functional groups via Sonogashira coupling.[20e] Employing 4,4′-diiodoazobenzene as the starting material enabled a double cross-coupling;
in this way, an azobenzene moiety containing two paramagnetic nitroxide spin labels
was synthesized in which the ethynyl groups supported the formation of spin exchange
coupling (Scheme [3], B left).[20f] Low-molecular organogelators were obtained by double cross-coupling of 3,3′-diiodoazobenzene
with acetylene derivatives (Scheme [3], B right).[20g] The two urethane moieties were required for strong hydrogen bonding, whereas the
two cholesterol units led to relatively weak van der Waals interactions.[20g] Moreover, an azobenzene bisporphyrin system[20d] as well as different para-alkynylazobenzene ligands and their corresponding organometallic cobalt complexes
were obtained.[20h] As azotolanes usually show liquid crystallinity as well as highly birefringent features,
this method was utilized for the synthesis of several azotolane monomers[20i]
[j] or polymers with azobenzene in the side chain[20k] or main chain[20l–n] (Scheme [3], C), respectively. Additionally, photoresponsive and fluorescent co-polymers (Scheme
[3], D),[20o] polyamide-phenyleneethynylenes[20p] or a semiconducting colloidal porous organic polymer[20q] were obtained. The scope of electrophilic azobenzene cross-coupling partners was
successfully broadened to bistriflates for the synthesis of rigid dendrimers in an
acceptable yield.[20s]
[t]
2-Iodoazobenzene reacted with (trimethylsilyl)acetylene under Sonogashira conditions,
but even after optimization of the reaction conditions the yield of 2-[(trimethylsilyl)ethynyl]azobenzene
remained 50%.[21] Additionally, the product decomposed during workup because of the lability of the
protecting group and the instability of the deprotected diazene. Coupling with more
robust (triisopropylsilyl)- and (triethylsilyl)acetylene solved both problems and
the product 2-[(trialkylsilyl)ethynyl]azobenzenes were obtained in 97% and 87% yield,
respectively.[21]
2.1.3
Buchwald–Hartwig Cross-Coupling Reactions
This type of cross-coupling reaction is used to form C–N bonds. In 2020, the coupling
of 3,5-dibromoazobenzene with N-Boc-N-(4-methoxyphenyl)hydrazide to give 3,5-bis[N′-Boc-N′-(4-methoxyphenyl)hydrazino]azobenzene in 58% yield was reported (Scheme [4]).[22] The obtained product was then oxidized to yield a C
2-symmetric 3,5-bis(4-methoxyphenylazo)azobenzene. Unsymmetric tris(arylazo)benzenes
were accessible by sequential coupling.[22]
Scheme 4 Pd-catalyzed Buchwald–Hartwig cross-coupling reaction of 3,5-dibromoazobenzene[22]
2.1.4
Heck Reactions
The Heck reaction can be employed in order to preserve double bonds within the starting
material for later functionalization. As with the Suzuki–Miyaura cross-coupling, we
mainly discuss reports of larger functional polymers and assemblies. One interesting
example is the functionalization of cage silsesquioxanes with azobenzene units via
the Heck reaction (Scheme [5]).[23] The synthesis of new azobenzene-doped hybrid porous polymers was thus possible.[24]
Scheme 5 Pd-catalyzed Heck reaction of 4-bromoazobenzene[23]
Poly(phenylenevinylene)-based conjugated polymers with azobenzene derivatives incorporated
directly in the π-conjugative building units were prepared in quantitative yield and
with a high molecular weight (Mn >10000) by coupling polymerization of divinylbenzenes with 4,4′-dihaloazobenzenes.[25] The Heck reaction of nipecotic acid (piperidine-3-carboxylic acid) derivatives with
azobenzene triflates and iodides yielded vinyl ethers in good yields. However, the
coupling was not possible for ortho-substituted azobenzenes. In this case, the Heck reaction needed to be performed with
1-iodo-2-nitrobenzene with the formation of the azobenzene by an azo coupling in a
later step.[26]
2.1.5
Stille Reactions
In a Stille cross-coupling reaction, an organotin compound is reacted with a halide.
Organostannanes are easy accessible and stable in air and moisture so that a broad
range of functional groups can be used under mild conditions.[20l]
[25]
[27] In this way, 4,4′-dibromoazobenzene was coupled with tributylvinyltin to yield 4,4′-divinylazobenzene
in 70% yield.[25] It was also possible to introduce heteroaromatic compounds into a polymer backbone
via a Stille cross-coupling: The monomer 4,4′-diiodoazobenzene was reacted with four
different bis(trimethylstannyl)-substituted heteroaromatic compounds to give poly(phenylene)based-polymers
that were soluble in common organic solvents in moderate to excellent yields (Scheme
[6]).[27] Due to the extended main-chain conjugation, the thiophene-, furan-, and N-methylpyrrole-containing poly(phenylenes) showed strongly red-shifted absorptions
in the visible region. Only the pyridine-containing poly(phenylene) had a low degree
of main-chain conjugation, but contrary to other examples, it showed in solution reversible
photoisomerization of azobenzene units with an accompanied change of the electrochemical
properties. The (Z)-enhanced polymer was less susceptible to oxidation.[27]
Scheme 6 Pd-catalyzed Stille cross-coupling reactions of diiodoazobenzenes[27]
2.2
Nickel Catalysis
Although palladium complexes are the most common catalysts in cross-coupling reactions,
attempts have been made to replace palladium by less expensive metals such as nickel.
For example, a nickel-catalyzed Heck reaction of aryl triflates with vinyl ethers
proceeded under mild reaction conditions, using a catalytic system consisting of bis(cyclooctadiene)nickel(0),
1,1′-bis(diphenylphosphino)ferrocene (DPPF), and tertiary amine Cy2NMe, followed by hydrolysis to give the corresponding acetyl-substituted products
with good functional group tolerance. It was also possible to incorporate a photoswitchable
unit by the olefination of an azobenzene triflate followed by hydrolysis to give the
corresponding acetyl derivative (Scheme [7]).[28]
Scheme 7 Ni-catalyzed Heck reaction of an azobenzene triflate derivative[28]
2.3
Copper Catalysis
Copper catalysts are another alternative to palladium catalysts in cross-coupling
reactions to obtain substrates otherwise not accessible.
2.3.1
Cadiot–Chodkiewicz Reactions
The copper-catalyzed Cadiot–Chodkiewicz reaction enables the formation of conjugated
dienes. A synthetic route towards large-scale highly ordered porous structures from
organometallic precursors via spontaneous self-assembly was established by using a
two-step Cadiot–Chodkiewicz cross-coupling. Several neutral platinum–acetylide complexes
with azobenzene groups in the center and long alkyl chains on both ends of the molecule
were obtained in excellent to quantitative yields (Scheme [8]).[29] In a similar fashion, poly(platinaynes) were synthesized with both meta- or para-substituted azobenzene spacers to compare their optoelectronic properties. In these
complexes, the acetylide-functionalized azobenzene ligands could still undergo photoisomerization
reversibly, although the switching process appeared to be more facile for para-substituted systems and with lower photoisomerization in solution in comparison to
smaller systems.[30]
Scheme 8 Cu-catalyzed two-step Cadiot–Chodkiewicz cross-coupling reaction of alkynyl-functionalized
azobenzenes[29]
2.3.2
Ullmann Reactions
The Ullmann reaction is a powerful tool for C–N bond formation. The Ullmann coupling
of 4,4′-dibromoazobenzene with 3,6-bis(9H-carbazol-9-yl)-9H-carbazole gave a bis(tercarbazole)azobenzene derivative in 46% yield that was used
as a precursor for the fabrication of photoresponsive microporous films (Scheme [9]).[31]
Scheme 9 Cu-catalyzed Ullmann coupling of dibrominated azobenzene[31]
2.4
Cobalt Catalysis
Another alternative to palladium catalysis is the use of cobalt as an inexpensive
metal. For example, the C(sp2)–P cross-coupling of vinyl, styryl, and aryl halides with diphenyl phosphine oxide
and dialkyl phosphinate using a unique Co/Cu catalytic system gave the corresponding
phosphoryl-substituted products. This protocol showed robust functional group tolerance
that enabled the coupling of 4,4′-diiodoazobenzene with diisopropyl phosphite to give
4,4′-bis(diisopropoxyphosphoryl)azobenzene in 76% yield (Scheme [10]).[32]
Scheme 10 Co/Cu-catalyzed C(sp2)–P cross-coupling reaction of 4,4′-diiodoazobenzene[32]
Azobenzenes as Formally Nucleophilic Components
3
Azobenzenes as Formally Nucleophilic Components
In cross-coupling reactions, the formal nucleophile is an (organo)metallic species.
Organometallic, nucleophilic azobenzene derivatives can be obtained either by halogen–metal
exchange of the (pseudo)halogenated azobenzene or by applying an appropriate cross-coupling
reaction with a dimetallic reagent (Scheme [11], A) (see later for C–H activation).[7b]
[33] However, in the case of azobenzenes, halogen–metal exchange can lead to the reduction
of the azo group as a dominating side reaction (Scheme [11], B).[34] From the perspective of the formally nucleophilic azobenzene, the main limitation
is access to the azobenzene starting material. There has been very little research
performed in this area in terms of systematic investigations and thus, it is difficult
to distill common principles or indeed select the most seminal papers.
Scheme 11 (A) Stille–Kelly cross-coupling reaction of 4-iodo-4′-methylazobenzene with hexamethyldistannane;[33] (B) halogen–metal exchange of a halogenated azobenzene with the possible reduction
of the diazenyl group[35]
3.1
Palladium Catalysis
Palladium catalysts are also most commonly used in cross-coupling reactions involving
azobenzene derivatives as the formally nucleophilic component. In terms of the obtained
product structure, the same criteria apply for the selection of the specific cross-coupling
reaction as are utilized for electrophilic azobenzene derivatives. However, a key
consideration is the availability of the metalated azobenzene.
3.1.1
Suzuki–Miyaura Cross-Coupling Reactions
The first use of an azobenzene derivative as a nucleophile in a Suzuki–Miyaura cross-coupling
reaction was reported in 2007;[36] the coupling of a boronic acid pinacol ester functionalized azobenzene with diverse
iodoarenes gave arylate azobenzenes in 41–72% yields.[36] While the cross-coupling reactions themselves are relatively unremarkable, the importance
is in the synthesis of the starting material by cross-coupling of a (pseudo)halogenated
azobenzene with the boronic ester.[36]
[37] A second approach is the condensation of a nitrosobenzene and aniline boronic acid
ester; the boronic acid ester is unaffected by the condensation reaction.[36] Due to the efficiency of this method, a number of synthetic targets[38] were assessed. Moreover, an azobenzene-4-boronic acid pinacol ester derivative was
used as the nucleophile and 4-bromo-2,2,2′,2′-tetrafluoroazobenzene derivatives as
the electrophile, which enabled the use of azobenzene as both cross-coupling components.
The resulting product undergoes orthogonal switching, where the azobenzene units are
switched separately to give 4 different isomers by green, blue, or ultraviolet light
or electrocatalytic isomerization (Scheme [12]).[37]
Scheme 12 Pd-catalyzed Suzuki–Miyaura cross-coupling reaction of an azobenzene-4-boronic acid
pinacol ester and 4-bromo-2,2,2′,2′-tetrafluoroazobenzene derivatives[37]
3.1.2
Sonogashira Cross-Coupling Reactions
In 2014, the preparation of an azobenzene liquid crystal was reported by the Sonogashira
cross-coupling reaction of an ethynyl-substituted azobenzene with 1-bromooctane.[39] This protocol was utilized in 2017 for the coupling of an azobenzene derivative
with aryl bromides (Scheme [13], A).[15d] The Sonogashira cross-coupling reaction has also been used for the synthesis of
artificial helical oligomers[40] or polymers[41] in which the photoisomerization of the azobenzene moieties triggers a geometric
change. Novel azobenzene-containing hydroxyphenylglycine-derived poly(m-phenyleneethynylene)s were synthesized by polymerization through the Sonogashira
couplings (thus formally a polycondensation) of 3,5-diethynylazobenzenes with various
diiodinated amides (Scheme [13], B).[41a]
Scheme 13 Pd-catalyzed Sonogashira cross-coupling reactions of alkynyl-functionalized azobenzenes[15d]
[41a]
Furthermore, the Sonogashira reaction was used to prepare a hairy-rod like π-conjugated
polymer with a fluorene unit in the backbone.[42] The late-stage functionalization of poly(aryl ethers) with azobenzene moieties was
feasible, in which polymer bromo side groups react with 4-(dimethylamino)-3′-ethynylazobenzene.[43]
3.1.3
Buchwald–Hartwig Cross-Coupling Reactions
The Buchwald–Hartwig cross-coupling reaction can be used to form C–N bonds. The Buchwald–Hartwig
amination of various polystyrene and poly(iminoarylene) derivatives was reported to
give the corresponding products with aminoazobenzene groups in the side chain (Scheme
[14]).[44] The absence of characteristic stretching vibrations of the starting materials in
the IR spectrum indicated a full loading of the obtained polymer.[44]
Scheme 14 Pd-catalyzed Buchwald–Hartwig amination of 4-(phenylamino)azobenzene[44]
This methodology was applied to the synthesis of amorphous materials such as branched
triarylamine derivatives,[45] a spiro-linked bifluorene[46] as well as a perfluorocyclobutane (PFCB) aryl ether polymer[47] or a poly(arylimino) derivative.[48] It was even possible to prepare ferrocenophanes with azobenzene derivatives in the
ligand and to use them as a redox-active and chromophore site showing potential as
electron- or acid-responsive organic materials.[49]
3.1.4
Heck Reactions
The Heck reaction of dihaloazobenzenes with divinylarenes as well as the reverse case,
the coupling of 4,4′-divinylazobenzene with dihaloarenes, to produce photoresponsive
poly(phenylenevinylene)s was investigated. However, the obtained polymers were largely
insoluble in common organic solvents, hence this route was discarded.[25]
3.1.5
Stille Reactions
An efficient microwave-assisted method to prepare stannylated azobenzenes was developed
to circumvent the possible reduction of the diazenyl group during halogen–metal exchange.
These organostannyl-substituted azobenzenes subsequently served as nucleophiles in
high-yielding Stille cross-coupling reactions (Scheme [15]).[33]
Scheme 15 Pd-catalyzed Stille cross-coupling reaction using stannylated azobenzene as nucleophile[33]
3.2
Copper Catalysis
A copper-catalyzed Ullmann cross-coupling reaction was the method of choice for the
synthesis of bis[4-(phenyldiazenyl)phenyl]amine and tris[4-(phenyldiazenyl)phenyl]amine
by varying the stoichiometric quantities of the electrophilic component (Scheme [16]).[50]
Scheme 16 Cu-catalyzed Ullmann cross-coupling reaction of 4-iodoazobenzene with amino-azobenzene[50]
The Ullmann cross-coupling reaction is also useful for generating phenol ethers through
C–O bond formation. In this way, a series of azobenzene-functionalized poly(ether
sulfone)s were prepared, using a catalyst system of CuI and 2,2,6,6-tetramethylheptane-3,5-dione
(TMHD), that had high glass transition temperatures (T
g >199 °C) (Scheme [17]).[51] Irradiation and writing/erasing experiments indicated a large photoinduced birefringence
and good stability of the photoinduced orientation of the polymers. This makes them
interesting for applications in reversible optical storage.[51]
Scheme 17 Cu-catalyzed Ullmann cross-coupling reaction of poly(ether sulfone)s with bromine
side groups and 4-[(4-methoxyphenyl)diazenyl]phenol with different functionalization
degrees (x = 20, 45, 100)[51]
This synthetic procedure was expanded to the synthesis of an azobenzene-containing
poly(aryl ether) with carboxyl side groups capable of coordination to rare earth complexes.[43]
3.3
C–H Activation Reactions
C–H Activation reactions catalyzed by different transition metals have played an important
role especially in the functionalization of azobenzene derivatives in the ortho-position.
3.3.1
Palladium-Catalyzed C–H Activation Reactions
The ortho-directing property of the azo group has been exploited in palladium-catalyzed C–H
activation reactions.[14e]
[52] In many such reactions, the azobenzene is transformed by reaction with the diazenyl
group. For example, azobenzenes were used for the synthesis of indazole backbones
through palladium-catalyzed C–H functionalization and subsequent intramolecular cyclization.[52a] In a similar approach, 3H-indazol-3-ones were prepared from azobenzene derivatives using formic acid as carbon
monoxide source (Scheme [18], A).[52g]
ortho-C–H Amination of azoarenes with trimethylsilyl azide yielded 2-aryl-2H-benzotriazoles (Scheme [18], B).[52b] In this reaction, electron-donating substituents (alkyl, alkoxy) give higher product
yields (58–87%) than electron-withdrawing groups, such as CF3 (8%).
Scheme 18 Pd-catalyzed C–H activation reactions of azobenzene derivatives yielding heteroaromatic
compounds[52b]
[g]
Late-stage functionalization of azobenzenes in the ortho-position was reported by the Trauner group (Scheme [19], A).[14e] These tetra-ortho-chlorinated azobenzenes are of special interest because of their redshifted isomerization
to cis at λ ≈ 560 nm. Azoarenes were functionalized with phenylhydrazine using a Pd(II)
catalyst with atmospheric oxygen as the oxidant (Scheme [19], B).[52c] Wu, Wang, and co-workers reported the acylation of azobenzene derivatives with benzylic
ethers (Scheme [19], C).[52d]
Scheme 19 Pd-catalyzed C–H activation reactions of azobenzene derivatives[14e]
[52c]
[d]
3.3.2
Rhodium-Catalyzed C–H Activation Reactions
Rhodium has been shown to be a potent ortho C–H activator of azobenzene derivatives. There are several reported examples of the
formation of C–N bonds,[53] 2-aryl-2H-benzotriazoles,[54] and indazoles and indoles[55] similar in yields to the palladium-catalyzed reactions. A very useful reaction is
the ortho-heteroarylation of azobenzenes by rhodium-catalyzed cross-dehydrogenative coupling
(Scheme [20]).[56] Such conjugated biaryls might be of special interest for luminous materials.
Scheme 20 Rh-catalyzed C–H activation reactions of azobenzene derivatives[56]
3.3.3
Ruthenium-Catalyzed C–H Activation Reactions
Ruthenium-catalyzed C–H activation reactions of azobenzene derivatives have been reported.[57] Of particular interest is the meta/ortho-selective C–H alkylation of azoarenes. Using a carboxylic acid promoted Ru(II)-catalyzed
CAr–H alkylation reaction of 4,4′-substituted azobenzenes with secondary and tertiary
alkyl bromides gave selectively the meta-product (Scheme [21]),[58] while under the same reaction conditions, primary alkyl groups gave the ortho-product. To our knowledge this is the only reported reaction so far which allows
C–H activation in the meta position. Coupling with alkyl chlorides was unsuccessful. Furthermore, bulky groups,
such as tert-butyl (Y = t-Bu), on the azobenzene, prevented the reaction.[58]
Scheme 21 Ru-catalyzed C–H activation reactions of azobenzenes[58]
3.3.4
Cobalt-Catalyzed C–H Activation Reactions
Cobalt-catalyzed C–H activation reactions are largely unknown on azobenzene derivatives
and only a few examples exist.[59] A synthetic procedure for the azo-directed selective 1,4-addition of maleimides
by Co(III)-catalyzed C–H activation was reported (Scheme [22]).[59c] Worth noting is the use of low amounts of additives, as well as the fact that it
does not require the use of a copper source.[59c]
Scheme 22 Co-catalyzed C–H activation reactions of azobenzene derivatives[59c]
3.3.5
Iridium-Catalyzed C–H Activation Reactions
The Ir(III)-catalyzed [4+2] cyclization of azobenzenes with diazotized Meldrum’s acid
via a two-step reaction with an initial C–H alkylation, followed by intramolecular
annulation gave 3-oxo-2,3-dihydrocinnoline-4-carboxylic acids or esters depending
on the solvent used (Scheme [23]).[60]
Scheme 23 Ir-catalyzed C–H activation reaction of azobenzene[60]
3.3.6
Copper-Catalyzed C–H Activation Reactions
Azobenzenes can be functionalized by a Cu(II)-catalyzed aerobic oxidative amidation
with amides yielding the corresponding 2-(acylamino)azobenzenes in moderate to excellent
yields (Scheme [24]).[61]
3.3.7
Iron-Catalyzed C–H Activation Reactions
To date only one example of an iron-catalyzed C–H activation reaction has been reported.
An iron hydride complex bearing a 2,5-bis(di-tert-butylphosphinomethyl)pyrrolide ligand reacted with azobenzene. However, further functionalization
was unsuccessful since the pentacoordinated aryl–iron complex was inert toward various
reagents (Scheme [25]).[62]
Scheme 24 Cu-catalyzed C–H activation reactions of azobenzene derivatives[61]
Scheme 25 Fe-catalyzed C–H activation of azobenzene[62]
Azobenzenes as Ligands in Catalysts
4
Azobenzenes as Ligands in Catalysts
In addition to their use as a reactant in cross-coupling reactions, azobenzene derivatives
can be also employed as ligands for catalysts in cross-coupling reactions.[63] Here, the N-donor capability of the diazenyl group (due to the lone electron pair
on the nitrogen atoms) is used to form transition metal complexes. The incorporation
of azo chromophores has enabled the synthesis of complexes with interesting physicochemical
properties such as photoluminescence. Complexes with multidentate azoaromatic ligands
are significantly stabilized because of the enhanced π-acceptor behavior compared
to monocyclopalladated azobenzenes.[63e] Although the synthesis of azo-containing phosphine Pd(II) and Pt(II) complexes was
reported in 1999, and the first results of their catalytic use were demonstrated in
Heck reactions,[64] it took a further decade before this possibility was explored in more detail.[63]
In 2010, the synthesis of a polystyrene-anchored Pd(II) azo complex (Figure [2], A) and its application in the Suzuki–Miyaura as well as Sonogashira cross-coupling
reactions was reported; various aryl halides were reacted with phenylboronic acids
or terminal alkynes in excellent yields (68–100% yield, 27 examples) under phosphine-free
and aerobic reaction conditions in aqueous medium.[63b] A similar catalytic system showed comparable recyclability, but, in addition, it
could even be employed in Heck reactions (89–96% yield, 5 examples).[63d]
A single core palladacyclic azobenzene catalyst with CNCN chelation was successfully synthesized (Figure [2], B) and used successfully in Suzuki–Miyaura and Heck reactions. However, it was
only moderately active (27–70% yield, 4 examples) and required high temperatures that
led to decomposition of the catalyst.[63c]
A symmetric bisazobenzene derivative was used as chelating ligand to obtain unsymmetric
CNN pincer palladacycles (Figure [2], C) that showed high turnover numbers (TONs) even under the harsh conditions of
the Heck reaction (60–93% yield, 9 examples, TONs up to 93000).[63e]
Phosphine-free Pd(II) complexes with 2,2′-bis(alkylamino)azobenzene ligands were obtained
in good yields by reaction of the ligands with sodium tetrachloropalladate. In this
work, the benzyl derivative (Figure [2], D) showed high catalytic activity in Suzuki–Miyaura and Heck reactions under mild
conditions in the presence of air and moisture (65–93% yield, 22 examples).[63f]
It should be noted that the photoswitchability of the azobenzenes in Figure [2] was not exploited.
Figure 2 Evolution of palladium azo catalysts used in different cross-coupling reactions[63b]
[c]
[e]
[f]
5
Diazocines
(Z)-11,12-Dihydrodibenzo[c,g][1,2]diazocines (diazocines) are ethylene-bridged azobenzenes that can be switched
from their thermodynamic stable (Z)- to the metastable (E)-isomer by using blue light at λ ≈ 370–400 nm and back from the (E)- to the (Z)-isomer by green light at λ ≈ 480–550 nm (Figure [3]).[65]
Figure 3 Structure and isomerization of diazocine
The switching properties of unsubstituted diazocines are different to unsubstituted
azobenzenes. Diazocines show better resolution of absorption bands between the two
isomeric states and switching is possible with light in the visible range.[65] However, the substituents have a great impact on the switching properties of both
azobenzenes and diazocines. For example, tetra-ortho-chlorinated azobenzenes can be switched to (Z) at λ ≈ 560 nm, which exceeds the redshift of regular diazocines. Amino substituents
on diazocines have also been shown to reduce the separation of the absorption bands
yielding low amounts of (E)- isomers (25–30%).[66] The synthesis of diazocines is more demanding compared to azobenzenes, which is
why only few applications have been reported to date.[65]
[67]
5.1
Synthesis
The key step in any diazocine synthesis is the cyclization to form the diazene moiety.
This has been performed by reduction of 2,2′-dinitrobibenzyls,[65]
[67]
[68] or the oxidation of 2,2′-ethylenedianilines (Figure [4]).[69]
Figure 4 Retrosynthetic approach towards diazocines[65]
,
[67]
[68]
[69]
[70]
A novel route involving a cross-coupling reaction has been introduced by connecting
the C–N bond instead of the N–N bond (Scheme [26]).[70] The diazocine ring in this route is formed via consecutive cross-coupling reactions
between a 2,2′-dihalobibenzyl and di-tert-butyl hydrazodicarboxylate.[70]
Scheme 26 Synthesis of diazocine through cross-coupling reaction[70]
5.2
Cross-Coupling Reactions
To date there are only a few reports of palladium-catalyzed cross-coupling reactions
of diazocines. Therefore, a comparison with cross-coupling reactions of azobenzenes
is, at present, of little informative value. Due to the different electronic and geometric
structures of azobenzenes and diazocines, a different reactivity can be expected (as
the lone pairs in diazocine are not aligned with the π-systems of the aromatic rings).
At first glance the yields seem to be lower for cross-coupling reactions on diazocines,
but this might be misleading since it is unknown if the reaction conditions were optimized.
The Heck reaction of an 8-substituted 3-bromodiazocine with a glutamate derivative
yielded diazocine ligands capable of light-controlling neural receptors (Scheme [27]).[67c]
Scheme 27 Pd-catalyzed Heck coupling of diazocine derivatives[67c]
A Buchwald–Hartwig coupling on 3-bromodiazocines with tert-butyl carbamates was successfully performed (Scheme [28], A).[69b] Furthermore, the coupling of benzophenone imine with bromodiazocines was reported
(Scheme [28], B).[71] The synthesis of a diazocine with turn-on fluorescence was achieved by the coupling
of 3,8-dibromodiazocine with diphenylamine (Scheme [28], C).[72]
Scheme 28 Buchwald–Hartwig cross-coupling reactions of diazocine derivatives[69b]
[71]
[72]
Turn-on fluorescence diazocines were prepared by a Stille cross-coupling reaction
(Scheme [29], A).[72] Furthermore, it was possible to obtain a pyroglutamate diazocine derivative via
the Stille cross-coupling reaction (Scheme [29], B).[67e]
Scheme 29 Pd-catalyzed Stille cross-coupling reactions of diazocine derivatives[67e]
[72]
6
Conclusion
Cross-coupling reactions have proved to be a powerful tool for the late-stage modification
of both electrophilic and nucleophilic azobenzene derivatives, with palladium catalysis
being most prevalent. The Suzuki–Miyaura and Sonogashira cross-coupling reactions
are the most widely used. First examples of cross-coupling reactions catalyzed by
other transition metals than palladium, such as nickel or cobalt, have been published
thus broadening the scope of cross-coupling reactions towards new bond formations
that are not possible with palladium catalysts. At present, the number of examples
of the use of azobenzenes as formally electrophilic reactants is much greater than
that for their use as formally nucleophilic reactants. Most likely, this does not
reflect intrinsic problems with nucleophilic azobenzenes, but rather that their accessibility
is limited at present and requires more research. The reported yields do not significantly
differ for cross-coupling reactions with azobenzene as formally the electrophilic
or as nucleophilic component. However, due to the difficulties in the synthesis of
nucleophilic azobenzene derivatives, cross-coupling reactions involving formally electrophilic
azobenzene derivatives are favored. Normally, cross-coupling reactions with formally
nucleophilic azobenzene derivatives are only used if the as nucleophilic coupling
partner in the reaction of electrophilic azobenzene derivative cannot be synthesized.
Most reported examples of cross-coupling reactions involving azobenzene derivatives
employ the para-isomer; there are few examples of the use of the meta- or even the ortho-isomer. In fact, some groups specifically pointed out that cross-coupling reactions
on ortho-azobenzene derivatives were unsuccessful. So cross-coupling reactions in the ortho-position of azobenzenes are almost, but not completely, unknown. The different reaction
behavior of ortho-azobenzene derivatives in comparison to their corresponding meta- and para-isomers can be attributed to the nature of the diazenyl group; due to the adjacent
lone electron pair on the nitrogen atoms, the diazenyl group can interact with substituents
in the ortho-position. For example, ortho-halogenated precursors that are easily accessible can be directly lithiated because
the ortho-lithiated species is significant stabilized by N→Li coordination. Although the diazenyl
group has this directing and stabilizing effect also for transition metal insertions,
only very few examples of metalated ortho-azobenzenes exist. C–H Activation is, therefore, a valuable alternative especially
for the modification of ortho-azobenzenes. In almost all examples of C–H activation on azobenzenes, the ortho-position was functionalized. Here, the use of palladium complexes as catalysts was
not as dominant as for the cross-coupling reactions. Another promising field is the
use of complexed azobenzene derivatives acting as ligands or promotors for catalysts.
New synthetic procedures for the preparation of diazocines means that they are accessible
in good yields. Therefore, further functionalization possibilities through cross-coupling
reactions can now be explored.
