Azobenzenes-Synthesis and Carbohydrate Applications.Pdf

Tetrahedron 65 (2009) 10105–10123

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Tetrahedron

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Tetrahedron report number 892 Azobenzenesdsynthesis and carbohydrate applications

Florian Hamon a, Florence Djedaini-Pilard b, Francis Barbot a, Christophe Len a,c,* a Universite´ de Poitiers, Synthe`se et Reáctivite´ des Substances Naturelles, UMR CNRS 6514, 40 Avenue du Recteur Pineau, F-86022 Poitiers Cedex, France b Universite´ de Picardie Jules Verne, Laboratoire des Glucides, UMR CNRS 6219, 33 rue Saint Leu, F-80039 Amiens, France c Universite´ de Technologie de Compie`gne, Transformations Inte´greés de la Matie`re Renouvelable, EA 4297 UTC/ESCOM, 1 alleédure´seau Jean-Marie Buckmaster, F-60200 Compie`gne, France article info

Article history: Received 7 August 2009 Available online 29 August 2009

Contents

1. Introduction ...... 10105 2. Synthesis of azobenzene derivatives ...... 10107 2.1. Oxidation reactions of aromatic primary amines ...... 10107 2.2. Reduction reactions of aromatic compounds having nitro groups ...... 10109 2.3. Coupling of primary arylamines with nitroso compounds (Mills reaction) ...... 10110 2.4. Diazo-coupling via diazonium salts ...... 10110 2.5. Oxidation of hydrazo derivatives ...... 10113 2.6. Reduction of azoxybenzene derivatives ...... 10113 2.7. Other methods ...... 10115 3. Synthesis of azobenzene carbohydrates ...... 10116 3.1. Carbohydrate derivatives having one azobenzene moiety ...... 10116 3.2. Dimers and polymers linked with one azobenzene moiety ...... 10120 4. Conclusions ...... 10121 Acknowledgements ...... 10121 References and notes ...... 10121 Biographical sketch ...... 10123

1. Introduction magnetic fields, and light. Various photochemical reactions are described in the literature: E/Z isomerization, tautomerization, Advances in molecular and supramolecular organization are electrocyclic reactions, heterolytic cleavage, and homolytic cleav- a major challenge in organic chemistry. The control of molecular and age. The use of light-powered reactions is the most reliable strategy supramolecular assembly may be obtained under ambient condi- to convert photochemical energy into reversible physical motion tions by external inputs such as electricity, pH, redox potential, without waste (Fig. 1). Azobenzene derivatives have been used as photoresponsive functional devices utilized as smart polymers,1 liquid crystals,2 molecular switches,3 and machines.4 Azobenzene derivatives have Abbreviations: Ac, acetyl; BBCP, bis(2,20-bipyridyl)copper(II) permanganate; Boc, tert-butoxycarbonyl; s-Bu, sec-Butyl; t-Bu, tert-butyl; Bz, benzoyl; CAN, cerium received considerable experimental and theoretical attention. ammonium nitrate; CD, cyclodextrin; DCC, dicyclohexylcarbodiimide; DIB, diac- Azobenzene exists as two isomers: the isomer E (or trans) and the etoxyiodobenzene; DMF, dimethylformamide; DTBB, 4,40-di-tert-butylbiphenyl; Et, isomer Z (or cis) (Fig. 2). The E isomer is 50 kJ mol1 more stable ethyl; Fmoc, fluorenylmethyloxycarbonyl; HABA, 4-hydroxyazobenzene-2-carboxylic thermodynamically than the Z form. Consequently, the different acid; HOBt, 1-hydroxybenzotriazole; Me, methyl; i-Pr, iso-Propyl; PTSA, para-tol- uenesulfonic acid; THF, tetrahydrofuran; p-Tol, para-tolyl; Tf, triflate; Ts, tosyl. spatial arrangements lead to different physical and chemical * Corresponding author. Tel.: þ33 (0)5 49 36 63 89; fax: þ33 (0)5 49 45 37 02. properties. The E to Z-photoisomerization of azobenzene induces E-mail address: [email protected] (C. Len). a change of the dipole moment (m E-azobenzene¼0.5 D vs m

N N cis-trans isomerisation N N

N N tautomerisation H H O O

O O

O O electrocyclic reaction O O O O NO2

heterolytic cleavage - N ONO N O 2 +

N N N 2 homolytic cleavage N . N N

Figure 1. Typical photo-sensor molecules and their photochromic reactions.

UV N N visible, Δ N N

(E)-1 (Z)-1

Figure 2. Reversible photoisomerization of azobenzene moieties upon UV–vis irradiation.

Z-azobenzene¼3.1 D), which in turn determines the hydrophobic and hydrophilic nature of the E and Z isomers. The structure of E-azobenzene is not planar; the dihedral angle N,N,C,C is around 17.5, whereas one of the benzene rings of the Z- isomer occupies a plane tilted by 56 from the plane of the other Figure 4. UV–vis absorption spectra of E- and Z-azobenzene in ethanol. ring. Thus, the distances between the two carbon atoms in the 4- Z-isomer is quite different from that of the E-isomer, with a band at 0 and 4 - positions are 9.0 and 5.0 Å for the E- and Z-isomer, re- 260 nm originating from symmetry-allowed p–p* transitions spectively (Fig. 3). (318 nm for the E-isomer).

4 4'

Figure 3. Geometrical structures of E- and Z-azobenzene.

The UV–visible absorption spectra of E-azobenzene is charac- The mechanism of photoisomerization of azobenzene has been terized by three major bands: (i) a band at 228 nm originates from a fundamental subject and two pathways have been described. It is p–p* transitions localized on the phenyl groups; (ii) a band at accepted that the photoirradiation step from E to Z has a rotational 318 nm originates from symmetry-allowed p–p* transitions, which pathway and/or an inversion pathway, whereas the heating step are delocalized through the molecule including the two nitrogen from Z to E has an inversion pathway (Scheme 1).5 atoms; and (iii) a band at 440 nm originates from symmetry-for- The Rau classiﬁcation of azobenzene distinguishes three fami- bidden n–p* transitions occurring at the central nitrogen atoms lies of compounds: (i) the azobenzene derivative 1; (ii) the amino- (Fig. 4). It is notable that the UV–visible absorption spectrum of the azobenzene derivative 2; and (iii) the push–pull azobenzene F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123 10107

7 8 9 10 11 12 Ag2O, AgMnO4, Ni peroxide, Pb(OAc)4, Ce(OH3)O2H, BBCP, 13 14 or KO2, hypervalent iodides such as PhI(OAc)2, and aerial oxi- N N 15 16 17 dants such as O2-t-BuOK, O2-Cu2Cl2-pyridine, O2-Co3O4, or 18 peroxidase-H2O2 have permitted the preparation of azobenzene from aniline derivatives, but these reagents have not been used recently. inversion The use of manganese dioxide (MnO2) for the oxidation of aniline derivatives 4 and 5 in reﬂuxing hexane was realized to furnish the corresponding azobenzene analogues 1 and 6 in 69 and 61% yields, respectively (Scheme 2).19

N N R1 N NH2 N a N N R (E)-1 (Z)-1 1 R1 4 1 rotation R1 =H R1 =H 5 R1 =Cl 6 R1 =Cl

Scheme 2. Reagents and conditions: (a) MnO2, molecular sieves, hexane, reﬂux (for 1: NN 69%, for 6: 61%).

More recently, Gilbert et al. reported the dimerization of 4- chloroaniline (5) via oxidation with MnO2 in reﬂuxing toluene to afford the corresponding azo compound 6 in a lower yield (49 vs Scheme 1. Isomerization pathway from E-toZ-azobenzene 1 and vice versa. 61%).20 Starting from the aniline derivatives 7–9 having halogen derivative 3 (Fig. 5). Depending up on the nature of the substituents atoms in the meta or para position, application of this procedure on the aromatic groups, the equilibria between the (E) and (Z) permitted the formation of the corresponding azobenzene ana- compounds are variable. logues 10–12 in 89, 88, and 79% yields, respectively (Scheme 3).

NH R2 NH2 2 R1 NH2 a R N N N N 1 N R N N N 1 R2 R O2N 2 (E)-1 (E)-2 (E)-3 5 R1 =H;R2 =Cl 6 R1 =H;R2 =Cl 7 10 azobenzene aminoazobenzene push-pullazobenzene R1 =R2 =Cl R1 =R2 =Cl 8 R1 =Cl;R2 =H 11 R1 =Cl;R2 =H Figure 5. Rau classiﬁcation of azobenzenes. 9 R1 =H;R2 =F 12 R1 =H;R2 =F

The purpose of the present review is not to list all of the azo- Scheme 3. Reagents and conditions: (a) MnO2, toluene, 110 C (for 6: 49%, for 10: 89%, for 11: 88%, for 12: 79%). benzene syntheses that have been described in the literature, but to provide a comprehensive overview of the recent investigations The synthesis of azobenzene derivatives having a protected and the applications in carbohydrate chemistry. Azobenzenes can hydroxyl group in the para position of the aromatic ring has been be obtained by: (i) oxidation of aromatic primary amines; (ii) re- described.21 The protection of the aromatic hydroxyl group was duction of aromatic compounds having a nitro group; (iii) cou- realized by treatment of 4-aminophenol (13) with triisopropylsilyl pling of primary arylamines with nitroso compounds (Mills chloride. The silylated derivative 14 in the presence of MnO2 in reaction); (iv) electrophilic reactions of diazonium salts; (v) oxi- refluxing benzene furnished the azobenzene derivative 15 in 34% dation of hydrazo derivatives; (vi) reduction of azoxybenzene overall yield (two steps) (Scheme 4). derivatives; and (vii) other methods. It is important to remember that the data to be presented and discussed here has come from different sources using experimental conditions that are not uni- RO NH2 b NOR form. It is therefore very difficult and often misleading to form RO N general conclusions and establish trends merely on the basis of comparing these data, because each laboratory employs its own 13 R=H 15 R=iPr Si a 3 experimental conditions and standards. For the sake of clarity, this 14 R=iPr3Si review has been arranged to describe the synthesis of azobenzene Scheme 4. Reagents and conditions: (a) NaH, i-Pr3SiCl, THF; (b) MnO2, benzene, reflux derivatives and of azo-linked carbohydrate moieties in the period (34% for two steps). from 1998 to 2008. Woolley et al. described the synthesis of the azobenzene de- 2. Synthesis of azobenzene derivatives rivative 18 with an ethyl(2-hydroxyethyl)amino substituent in the para position starting from the commercial benzeneaminium 2.1. Oxidation reactions of aromatic primary amines hydrogenosulfate 16 (Scheme 5). Treatment of the salt 16 with sodium bicarbonate generated the amine 17 in 62% yield. Then, The synthesis of symmetrical azobenzene derivatives has been oxidation of 17 with MnO2 gave the azobenzene derivative 18 in 22 reported via the oxidation of primary aromatic amines with MnO2, 29% yield. It was notable that the primary hydroxyl group was not 6 KMnO4,H3BO3, and Hg/I2. Other metal oxides such as Ag2CO3, protected during the oxidation step. 10108 F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123

HO HO HO +- NN N NH3 SO4H a N NH2 b N N OH 16 17 18

Scheme 5. Reagents and conditions: (a) NaHCO3, ethyl acetate, H2O (62%); (b) MnO2, benzene, reﬂux (62%).

Recently, the synthesis of azobenzene derivatives with steric azobenzene derivatives by solvent-free permanganate oxidation hindrance has been described. Yamaguchi et al. reported the syn- was developed.26 Treatment of the primary aromatic amines 4, 5, thesis of a cycloalkyne dimer 20 linked by an azo group. Oxidative and 25 with KMnO4,Al2O3, and CuSO4$5H2O afforded the corre- coupling of 19 with MnO2 in benzene generated the azobenzene 20 sponding azobenzene derivatives 1, 6, and 26 in 75, 80, and 70% in 64% yield (Scheme 6).23 yields, respectively (Scheme 9).

R R

NH2 N a

R R

19 R=CO2n-C10H21 20 R=CO2n-C10H21

Scheme 6. Reagents and conditions: (a) MnO2, toluene, 110 C (64%).

The use of potassium permanganate absorbed onto copper(II) R sulfate pentahydrate was studied. This oxidizing reagent offered 1 R3 R2 R NH2 1 several practical advantages, namely better yields, a simpler ex- a N perimental procedure, and utilization of a commercially available N R R oxidant that is safe to handle. Starting from the aniline derivatives 2 3 R1 R2 R3 4, 21, and 22, Noureldin et al. reported the synthesis of azobenzenes 4 1 1, 23, and 24 using the supported permanganate in 77–87% yield R1 =R2 =R3 =H R1 =R2 =R3 =H 5 R =R =H;R =Cl 6 R =R =H;R =Cl (Scheme 7).24 1 3 2 1 3 2 25 R1 =R3 =R2 =Me 26 R1 =R3 =R2 =Me

R Scheme 9. Reagents and conditions: (a) KMnO4,Al2O3, CuSO4$5H2O (for 1: 75%, for 6: NH2 80%, for 26: 70%). a N N R The use of sodium perborate for the oxidation of aromatic R amines has been reported. Abelt et al. reported the oxidation of 4 R=H 1 R=H aniline derivative 27 with sodium perborate tetrahydrate in the 21 R=secBu 23 R=secBu presence of boric acid in acetic acid to give the azobenzene 28 in 22 R=I 24 R=I 64% yield (Scheme 10) and various amounts of the azoxy com- 27 Scheme 7. Reagents and conditions: (a) KMnO4, CuSO4$5H2O, CH2Cl2 (for 1: 78%; for pound. It was notable that the amino group and the carboxy 23: 77%; for 24: 87%). group, respectively, in the para and meta positions of the aromatic ring, were protected. A few years later, Flatt et al. described the synthesis of com- 25 pound 24 in 15% yield using CHCl3 instead of CH2Cl2 (Scheme 8). NHAc

MeOOC NH MeOOC N I 2 a NCOOMe NH2 a N AcHN N AcHN I I 27 28 22 24 Scheme 10. Reagents and conditions: (a) NaBO3$4H2O, B(OH)3, AcOH (64%).

Scheme 8. Reagents and conditions: (a) KMnO4, CuSO4$5H2O, CHCl3 (15%). Recently, Naeimi et al. reported the synthesis of 4,40-di- In order to limit the use of solvents and to have applications in aminoazobenzene (30)28 as described by Stubbings et al. (Scheme combinatorial chemistry and in green chemistry, the synthesis of 11).29 Starting from the p-aminoacetanilide (29), treatment with F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123 10109 sodium perborate tetrahydrate and boric acid in glacial acetic acid example was reported in the literature concerning the oxidation of followed by treatment with HCl in methanol furnished the azo the primary aromatic amines for the preparation of dissymmetrical compound 30 in 52% overall yield (two steps). azobenzenes. Oxidative hetero-coupling of free base amino- porphyrin 36 and aminoacetal 37 using MnO2 as the oxidant was utilized to furnish the dissymmetrical azobenzene 38 in 50% yield NH 2 (Scheme 15).31 NH N 2 a N

AcHN H2N 29 30 N HN Scheme 11. Reagents and conditions: (a) NaBO3$4H2O, B(OH)3, AcOH then HCl, MeOH (52%). NH2 NH N The synthesis of azobenzenes was described by oxidation with N HN a mercury(II) oxide/iodine reagent. Orito et al. reported the syn- a + thesis of azobenzene derivatives 32 by dimerization of aniline de- NH N rivatives 31 in 87% yield (Scheme 12).30 As described above, the OO N protection of the carboxy group of the compounds was established N before the oxidation reaction. 37 COOEt 36 OO NH N 2 a N

EtOOC EtOOC 38 31 32 Scheme 15. Reagents and conditions: (a) MnO2, toluene, 110 C (50%). Scheme 12. Reagents and conditions: (a) HgO, I2,CH2Cl2 (87%). 2.2. Reduction reactions of aromatic compounds having nitro The authors proposed a plausible mechanism involving a nitro- groups gen–nitrogen coupling of an initially formed cation radical (or aminyl radical), which is probably produced by a one-electron The synthesis of azobenzene derivatives has been reported via transfer, followed by a two-electron oxidation of the resultant the reduction of nitro compounds using zinc in a basic medium, 30 hydrazobenzene 33 (Scheme 13). via catalytic transfer hydrogenation using HCO2H, NEt3/Pb, and via

H H H H H -HgI2 NH2 I HgO + I N 2 -H O N N 2 Hg 2 + or O 4

-HgI2 H H N -H O N HgO + I N N 2 N 2 N Hg H OH I2 133

Scheme 13. Mechanism of oxidation of aniline (4) with HgO and I2.

Flatt et al. described the synthesis of azobenzene 35 from 2- reduction using glucose in a basic medium. Hecht et al. reported the nitro-4-iodoaniline (34) by oxidation using mercury(II) oxide and synthesis of azobenzene-core amphiphilic oligo(meta-phenylene iodine (Scheme 14).25 The poor yield (4%) was presumably due to ethynylene)s.32 After bromination of methyl 3-nitrobenzoate (39), the low reactivity of the electron-deﬁcient aniline and the sterically treatment of 40 with zinc under basic conditions yielded the de- hindered azo product. sired azobenzene bis-acid 41 in 63% yield (two steps) (Scheme 16). As described in this section, the oxidative coupling of amines Cyclic azobenzenes were prepared by reduction reaction using was used for the synthesis of symmetrical azobenzene. One Zn. Sukwattanasinitt et al. reported the synthesis of azobenzene derivative-bridged crown ether p-tert-butylcalix[4]arenes.33 Re-

NO2 I ductive coupling of p-tert-butylcalix[4]arene derivative 42 with NO2 NH2 zinc in the presence of NaOH in isopropanol and water furnished a N N the azobenzene 43 in 8% yield (Scheme 17). Starting from the iso- I mer 44, application of this strategy using high-pressure method NO2 I (3 atm nitrogen) gave the azo compound 45 in 59% yield (Scheme 34 35 17). It was notable that the 4,40-derivative was unstable and none of

Scheme 14. Reagents and conditions: (a) HgO, I2,CH2Cl2 (4%). the desired product could be isolated. 10110 F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123

COOH The synthesis of azobenzene derivatives having an unprotected COOMe COOMe carboxy group in the aromatic ring could be effected using glucose as a carbohydrate reducing agent.35 Starting from 4-nitrobenzoic a b N Br Br N acid (47) in an aqueous solution of NaOH, treatment with an

NO2 Br NO2 aqueous solution of glucose gave the corresponding azo compound 39 40 41 COOH 48 in 76% yield (Scheme 19). Recently, different teams have applied the above synthesis for the preparation of 4,40-azodibenzoic acid 36 Scheme 16. Reagents and conditions: (a) DIB, H2SO4 (90%); (b) Zn, NaOH, EtOH (70%). (49) in 44% yield.

2.3. Coupling of primary arylamines with nitroso compounds (Mills reaction) N NO2 O2N N The synthesis of dissymmetrical azobenzene derivatives has O O O O been reported by treatment of aromatic nitroso compounds and primary arylamines via Mills reactions.37–52 Compound 52 was OH O O O OH O OH OH obtained starting from the nitroso compound 50 and the aniline a derivative 51 in acetic acid at 60 C in quantitative yield (Scheme 20).51 It was notable that, using the Mills reaction, different electron-withdrawing and -donating groups could be present in the ortho-, meta-, and para-positions of both the aromatic nitroso compounds and the aromatic amines. 42 43 NHFmoc NHFmoc a NO O N + N 2 2 HOOC NO H2N N HOOC N N 50 51 52

O O O O Scheme 20. Reagents and conditions: (a) AcOH (quant).

OH O O O OH O Nishihara et al. have described the synthesis of ferrocenylazo- OH OH benzenes.53 3-Ferrocenylazobenzene (54) was prepared by the re- b action of 3-ferrocenylaniline (53) and nitrosobenzene (50) in acetic acid in 24% yield (Scheme 21). Application of this strategy permitted different ferrocenylazobenzenes 55–60 to be obtained (Fig. 6). It was notable that the azobenzene 60 was obtained in the Z 44 45 conﬁguration.

Scheme 17. Reagents and conditions: (a) i-PrOH, Zn, H2O (8%); (b) i-PrOH, Zn, H2O, 3 atm N2 (59%). Fe Fe a

The synthesis of azo compounds by catalytic transfer hydroge- NH2 NN nation of nitroarenes has been studied in order to limit the use of ﬂammable hydrogen gas and pressure equipment. Nitrobenzene (46) and lead powder in methanol in the presence of triethylammonium 53 54 34 formate furnished the azobenzene (1)in92%yield(Scheme 18). Scheme 21. Reagents and conditions: (a) nitrosobenzene (50), AcOH, rt (24%). Gowda et al. proposed34 that the initial reduction of the nitro compound furnished a hydroxylamine as a reaction intermediate.

R R NO2 1 2 Fe a N Fe N R3 N NN N 46 1

Scheme 18. Reagents and conditions: (a) HCOOH, NEt3, Pb, MeOH (92%).

R4 60 55 R1 =R2 =R4 =H;R3 =Cl N COONa 56 R1 =Cl;R2 =R3 =R4 =H NaOOC N 57 R1 =R2 =R4 =H;R3 =Me a 58 48 R1 =R2 =R4 =H;R3 =OH 59 R1 =OH;R2 =R3 =R4 =H

HOOC NO2 Figure 6. 3-Ferrocenylazobenzene derivatives 55–60. N COOH b 47 HOOC N 2.4. Diazo-coupling via diazonium salts 49

Scheme 19. Reagents and conditions: (a) NaOH, H2O, glucose (76%); (b) 1. NaOH, H2O, The synthesis of dissymmetrical azobenzene derivatives has glucose; 2. AcOH (44%). been reported by the coupling of diazo compounds and diazonium F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123 10111 salts in an acidic or basic medium.54–62 Due to the size of the with p-nitrophenyldiazonium tetraﬂuoroborate in 84% yield (Scheme attacking species, substitution is mostly para to the activating 24).65 It was notable that the coupling reaction was carried out on the group. When the para position is already occupied, ortho sub- aromatic ring having a free hydroxyl group in the phenol ring. stitution takes place. The pH of the solution was important both for the amine and phenol. In general, the use of an aromatic amine O2N O2N needed a mildly acidic or neutral solution. If the solution was too NO2 acidic, the reaction did not occur because the concentration of free amine became too small. Phenol was not active enough for the a reaction, but, in slightly alkaline solution, the more reactive S S N N N phenoxide ion afforded the target azobenzene. Neither amines nor S S N N N OH OH OH O phenols, however reacted in moderately alkaline solution because the diazonium ion was converted into the corresponding diazo S S hydroxide. Yang et al. reported the formation of the azobenzene 71 S S OH O derivative 63 by the reaction of phenol (61) and diazo compound 62 OH OH in AcONa at 0 C(Scheme 22).59a

a 72 OH + N OH H2NO2SN2Cl H2NO2S N 61 62 63

Scheme 22. Reagents and conditions: (a) AcONa (nd).

Scheme 24. Reagents and conditions: (a) p-NO2C6H4N2BF4, pyridine (84%). Contemporary calixarene chemistry has involved the synthesis and development of biologically active compounds. The structures Kim et al. reported the diazo-coupling reaction of calix[4]arene of calixarenes are convenient for performing diazonium coupling. 73 with p-nitrobenzenediazonium tetraborate in the presence of pyridine and THF to give the bis-azobenzene derivative 74 in 42% Parola et al. described the diazo-coupling reaction of thiacalix[4]- 66 arenes.63 Starting from thiacalixarene 64, the synthesis of the tet- yield (Scheme 25). Application of the above methodology per- rakis(phenylazo) derivative 65 was realized by treatment with the mitted the preparation of calix[4]arene derivatives 75 and 76 with two propyl groups and a monocrown-6 moiety, respectively corresponding diazonium salt in pyridine and THF in 71% yield 67 (Scheme 23). (Fig. 8).

O2N NO2

a N N N N N N N N N S S S NNN OHOH OH HO a 64 S S S S OH HO OH OH OH HO OH O O HO O O

65 O O O OO O OO Scheme 23. Reagents and conditions: (a) C6H4N2BF4, THF, pyridine (71%).

73 74 Using a similar procedure, Parola et al. and Lhotak et al. reported Scheme 25. Reagents and conditions: (a) p-NO C H N BF , THF, pyridine, rt (42%). the synthesis of the tetraazo compounds 66–70 (Fig. 7).63,64 Bolte 2 6 4 2 4 et al. reported the synthesis of an azobenzene derivative of thiacalix[4]arene 72 by coupling the monoalkylated thiacalix[4]arene 71 O2N NO2

R R O2N R NO2 R N N N N

N N N N N HO N N OH O O NNN N N O O OH O O HO O O S S S S OH OH OH HO 75 76

Figure 8. Calix[4]arene derivatives 75 and 76. 66 R=NO2 67 R=Me 68 R=OMe 69 Dumazet-Bonnamour et al. described the synthesis of azoca- R=Br 68 70 R = COOH lix[4]arenes containing bipyridyl subunits. A diazo-coupling reaction of calix[4]arene 77 with the diazonium salt led to the Figure 7. Tetrakis(phenylazo) derivatives 66–70. corresponding azobenzene derivative 78 in 66% yield (Scheme 26). 10112 F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123

HO HO

N N a NNN R N

PPh2 PPh2 OH O O HO OH O O HO 83 84 R=Me N N N N 85 R=Et N N N N Scheme 28. Reagents and conditions: (a) for 84: 4-MeC6H4N2BF4, AcOH, rt (70%); for 85: 4-EtC6H4N2BF4, AcOH, rt (70%).

77 78 Isolation of arenediazonium salt was essential before addition to the anion of the tertiary phosphine in order to prevent undesirable Scheme 26. Reagents and conditions: (a) C6H5N2BF4, THF (66%). reactions such as oxidation at the phosphorus atom or de- In 2000, Lamartine et al. reported the selective synthesis of new composition of the diazonium salt by quaternization. phenylazocalixarenes.69 A coupling reaction between the tetrazo- Three years later, Pritchard et al. described the synthesis of some nium salt 81 and calix[4]arene 79 in pyridine and THF gave the azo azo-containing phosphine chalcogenides.71 Treatment of the dimer 80 in 15% yield (Scheme 27). Using the same procedure, phenoxide anion of the phosphine derivative with benzenediazo- compound 82 was also synthesized (Fig. 9). nium tetraﬂuoroborate did not lead to the azo compound but only the diphenylphosphine oxide was isolated. In order to obtain the OH OHOH HO target azobenzene derivative, the phosphorus atom was protected and the derivative 86 gave the corresponding azobenzene 87 in 60% yield (Scheme 29). N BF N 2 4 OH N MeO N MeO S a N HO PPh2 a S PPh2 OH OH OH HO OMe 86 87 OMe 79 N2BF4 N Scheme 29. Reagents and conditions: (a) 1. NaH, THF; 2. C6H5N2BF4, MeCN (60%). N 81 Having established that p-hydroxyphenyl(diphenyl)phosphine chalcogenides underwent azo-coupling reactions, Pritchard et al. OH OH OHHO then investigated the coupling reaction with a m-hydroxy analogue 80 88 (Scheme 30).71 Using the same procedure, the coupling reaction

Scheme 27. Reagents and conditions: (a) compound 81, THF (15%). of 88 with the diazonium salt furnished a mixture of two azo compounds 89 and 90. The coupling reaction occurred at the expected ortho-positions to the meta-directing Ph2PS group. Using OH OH OH HO the non-selectivity of this coupling reaction, the authors described the synthesis of the bis-azobenzene derivative 91 in 58% yield.

HO HO N N N + N S PPh2 NN S PPh2

a 89 90 HO S HO N N PPh2 N N 88 a S PPh2 NN

OH OH OH HO

91 82

Scheme 30. Reagents and conditions: (a) 1. NaH, THF; 2. C6H5N2BF4, MeCN (58%). Figure 9. Calix[4]arene derivative 82.

Replacing the Ph2PS moiety with a formally ortho/para-directing Pritchard et al. reported the ﬁrst synthesis and structural char- methyl group was also studied by Pritchard et al.71 In contrast to acterization of a tertiary phosphine containing an azo linkage.70 compound 88, treatment of compound 92 with the diazonium salt After treatment of the phosphine 83 with NaOH in ethanol, the gave selective coupling to afford the azobenzene derivative 93 in arenediazonium tetraﬂuoroborate salt in MeCN was added to fur- 56% yield. Starting from the azo compound 93, a second coupling nish the azo compounds 84 and 85 in 70% yield (Scheme 28). gave the bisazobenzene 94 in 40% yield (Scheme 31). F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123 10113

HO HO Much effort has been directed toward the development of sus- tainable chemistry by reduction of the solvent or the use of the N HO resin. Recently, Mihara et al. reported the synthesis of azobenzene a a N NN NN derivatives by dehydrogenation of hydrazo compounds using solvent-free conditions.73a Commercially available KF/alumina and hydrazobenzene (33) furnished the corresponding azobenzene (1) 92 93 94 in 85% yield (Scheme 34).

Scheme 31. Reagents and conditions: (a) 1. NaH, THF; 2. p-MeC6H4N2BF4, MeCN (for 93: 56%; for 94: 40%). HN a N N 2.5. Oxidation of hydrazo derivatives NH

33 1 The preparation of azobenzene derivatives has been reported by a stoichiometric oxidation process of hydrazo compounds using Scheme 34. Reagents and conditions: (a) KF/Al2O3,O2 (85%). 72 73 19 74 H2O2, O2, MnO2, sodium hydroxide, triarylphenol/potassium ferricyanide/sodium hydroxide,75 iron chloride,76 periodate resin,77 The authors proposed a plausible mechanism for the oxidation Pb(MeCO ) ,78 HgO,79 (NH ) S O ,80 N-bromosuccinimide in pyri- 2 4 4 2 2 8 of the hydrazobenzene via radical species generated on the alumina dine,81 tetrabutylammonium cerium(IV) nitrate82 and arylsulfonyl 83 84 (Scheme 35). peroxide reagents, NaNO2 in acetic anhydride, polyethylene 85 86 glycol/nitrogen dioxide, and NaNO2/NaHSO4 on silica support. The oxidation of hydrazine derivatives using H2O2 as oxidant HN Al2O3 HN O2 HN was studied.87 Oxidation of hydrazobenzenes 95–97 with pyr- NH NH N. idinium tribromide gave the corresponding azobenzenes 98–100 in 33 100, 89, and 26% yields, respectively (Scheme 32). Similarly, the use of t-BuOCl as oxidant furnished the azobenzenes 98 and 99 in 61 . and 45% yields. It was notable that hydrazobenzene 97 bearing N N N N a phosphine selenide moiety was not oxidized by t-BuOCl. . 1

Scheme 35. Reagents and conditions: (a) KF/Al2O3,O2 (85%). X X aorb P HN NH P NN Ph Ph Oxidation of hydrazobenzene was realized using a periodate Ph Ph resin by Rademann et al.77 The authors reported the synthesis of azobenzene (1) (88% yield) starting from hydrazobenzene (33)by treatment with the oxidizing periodate resin 105 (Scheme 36). 95 X=O 98 X=O 96 X=S 99 X=S 97 X=Se 100 X=Se N - þ HN a NIO+ 4 Scheme 32. Reagents and conditions: (a) pyH Br3 , CHCl3 (for 98: 100%; for 99: 89%, N for 100: 26%); (b) tBuOCl, toluene (for 98: 61%; for 99: 45%). NH

33 1 105 The oxidation of hydrazines using iron chloride was reported by 76 Scheme 36. Reagents and conditions: (a) resin 105, MeOH (88%). Wang et al. Treatment of hydrazobenzene 33 with FeCl3 as dehydrogenating agent in acetone furnished azobenzene (1) in 90% yield (Scheme 33). Application of this methodology permitted the In parallel work, it was notable that catalytic oxidation of preparation of the azo compounds 6 and 101–104 in 81–99% yields hydrazo compounds was described using oxygen or H2O2 as oxi- 88 89 90 (Fig. 10). dant, NH4VO3, CuCl2, Co(II) complexes, and FeSO4/KClO3/ 91 H2SO4. Recently, Gozin et al. reported the synthesis of azobenzene (1) by treatment of hydrazo compound 33 in the presence of TiCl3 and HBr (Scheme 37).72 Application of the aforementioned strategy HN a N permitted the synthesis of azobenzene derivatives 106–109 in 85– NH N 95% yields. It was notable that this methodology allowed the 33 1 preparation of the bisazobenzene compound 109 (Fig. 11).

Scheme 33. Reagents and conditions: (a) FeCl3$H2O, acetone (90%).

HN a N NH N R1 N R 2 33 1 R2 N R 1 Scheme 37. Reagents and conditions: (a) TiCl3 solution (15% in HCl aqueous solution, 0.2–0.25 mol %), HBr, H2O2 (96%). 6 R1 =H;R2 =Cl 101 R1 =H;R2 =Me 102 R =Cl;R =H 1 2 2.6. Reduction of azoxybenzene derivatives 103 R1 =Me;R2 =H 104 R1 =Br;R2 =H Selective deoxygenation of organic N-oxides is a subject of Figure 10. For 6: 87%, for 101: 79%, for 102: 99%, for 103: 88%, for 104: 92%. considerable interest in heterocyclic chemistry. In particular, 10114 F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123

O2N NOMeS NOMe N NO 2 N N N N

106 107 108

O N N NN

109

Figure 11. Azobenzene derivatives 106–109. azoxybenzene derivatives were reduced with various reducing reaction time (10 h) led to full reduction to the corresponding agents and with sulfuric acid via a Wallach rearrangement. Aly primary amines. reported the synthesis of trans-azobenzene 111 by the reduction of Konwar et al. described an efﬁcient general system for the de- 95 a mixture of the E and Z azoxy compounds 110 with LiAlH4 in 82% oxygenation of azoxybenzene derivatives in aqueous media. yield (Scheme 38).92 Starting from the azoxybenzenes 112, 115, and 116, treatment with AlCl3$6H2O/KI in acetonitrile and water led to the azobenzenes 1, 11, and 103 in 71–75% yields (Scheme 41). - O + - R1 R2 R1 R2 N N O + N a N N R3 N R3 a R3 N R3 N

R2 R1 R2 R1 110 111

112 R1 =R2 =R3 =H 1 R1 =R2 =R3 =H Scheme 38. Reagents and conditions: (a) LiAlH4, THF (82%). 115 R1 =R3 =H;R2 =Cl 11 R1 =R3 =H;R2 =Cl 116 R1 =Me;R2 =R3 =H 103 R1 =Me;R2 =R3 =H Sanz et al. described the chemoselective deoxygenation of Scheme 41. Reagents and conditions: (a) AlCl3$6H2O/KI, MeCN, H2O (for 1: 71%; for 11: N-oxides 112 and 113 under mild conditions with common phos- 75%; for 103: 72%). phines in the presence of dichlorodioxomolybdenum(VI) to furnish the corresponding azo compounds 1 and 114 in 97 and 96% yields, Reduction of azoxybenzene (112) using ruthenium(III) chloride 93 respectively (Scheme 39). in acetonitrile was described by Sandhu et al. to furnish the de- oxygenated compound 1 in 82% yield (Scheme 42).96 - O + N R N R - R N a R N O + N N N a N 112 R=H 1 R=H 113 R=OMe 114 R=OMe 112 1 Scheme 39. Reagents and conditions: (a) MoO2Cl2(dmf)2 5%, PPh3, THF (for 1: 97%; for 114: 96%). Scheme 42. Reagents and conditions: (a) RuCl3, MeCN (82%).

Various metals were used to deoxygenate azoxybenzene de- A mild and efficient deoxygenation of azoxybenzene 117 was rivatives. These methods were efficient, simple, and selective. reported by Singh et al. using CuI and Zn powder in ethanol to give Yus et al. reported the reduction of azoxy compounds using 97 94 the azo compound 118 in 95% yield (Scheme 43). combinations of NiCl2/Li/DTBB as reducing agents. Starting from the azoxybenzenes 112 and 113, the corresponding azo- - benzenes 1 and 114 were obtained in 83 and 79% yields, re- FFO + spectively (Scheme 40). It was notable that the final products N F N F F N a F N depend upon the reaction conditions used: for a short reaction FF time (1 h), the target compounds were isolated, whereas a longer 117 118

- Scheme 43. Reagents and conditions: (a) CuI/Zn, EtOH (95%). O + N R N R RRN a N With an increasing focus on green methodologies, Sandhu et al. reported a safe and environmentally friendly procedure for the 112 R=H 1 R=H deoxygenation of azoxybenzene 112 using Zn(OTf)2 to give the 113 R=OMe 114 R=OMe corresponding azobenzene (1) in 70% yield. In this works, the use of

Scheme 40. Reagents and conditions: (a) NiCl2$2H2O, Li, DTBB, THF (for 1: 83%; for Cu(OTf)2 instead of Zn(OTf)2 furnished compound 1 in 75% yield 98 114: 79%). (Scheme 44). F. Hamon et al. / Tetrahedron 65 (2009) 10105–10123 10115

- the reaction of 2-p-tolylsuﬁnyl-1,1,4,4-tetramethoxy-2,5-cyclo- O + hexadiene (125) and the hydrazine 123 in the presence of CAN in N N aorb acetonitrile in 85% yield (Scheme 48).102 N N

O O2N 112 1 MeO OMe p O2N -Tol S N NO a 2 Scheme 44. Reagents and conditions: (a) Zn(OTf)2, MeCN (70%); (b) Cu(OTf)2, MeCN + MeO N p HN NO2 (75%). -Tol S H2N O MeO OMe It was notable that the Wallach rearrangement using sulfuric 125 123 126 acid induced the direct conversion of azoxybenzene 112 into the corresponding 4-hydroxyazobenzene (119)(Scheme 45).99 Scheme 48. Reagents and conditions: (a) CAN, MeCN (85%).

- Recently, Kaneda et al. reported the treatment of p-nitro- O + N N OH phenylhydrazine (127) with benzoquinone 128 having an 18- a N N crown-6 moiety to furnish the azobenzene 129 in 25% yield (Scheme 49).103 112 119 Cl Scheme 45. Reagents and conditions: (a) H2SO4. O O O O OO 2.7. Other methods HN NO2 + H2N O O Different methods to obtain azobenzenes were reported using 127 O O isocyanates or aryldiazonium salts and metal reagents or aryl hy- 128 drazines and quinones. Recently, only the last two methods were studied. In 1998, Fochi et al. developed the synthesis of azo- a benzene by an electrophilic coupling reaction of arenediazonium 100 Cl salts with Grignard reagents. Thus, treatment of the arenedi- OH O azonium o-benzenedisulfonimide 120 with a solution of phenyl- O O magnesium bromide (121) afforded azobenzene (1) in 71% yield OO (Scheme 46). O O N O N O 2 N S MgBr + - a N N2 N + S O2 120 121 1 NO2

Scheme 46. Reagents and conditions: (a) THF (71%). 129 Scheme 49. Reagents and conditions: (a) EtOH (25%).

In order to ﬁnd new organometallic compounds useful in organic synthesis, Dughera et al. studied the aforementioned strategy In 2006, azocalix[4]arene having azobenzene 131 has been 104 by replacing the Grignard reagent with a triorganoindium synthesized by Chawda et al. A non-diazo-coupling route was compound.101 also used to synthesize azocalix[4]arene derivatives containing two Quinone chemistry is well documented, but only a few studies free phenolic groups. Treatment of dib