Reactions with Azobenzene

A fourth example of a radical-type reaction of Grignard reagents is the reaction with azobcn-zene [971. It was found that a/obcnzcnc in the reaction with mcthylmagnesium bromide produces hydra/obenzene and ethane (98). To be able to reduce a/obenzene the reagent has to act as an election donor in an induced homolysis  

In a mechanistic study [99 the use of a variety of Grignard reagents showed that liydrazobenzene was normally the main product (806 or above) accompanied by 10-156 1,2-addiiion produci. 

The rates of reaction for the different Grignard reagents were largely linearly correlated to the anodic oxidation potentials of the reagents. 5-Hcxenylmagnesium bromide produced I -addition product of which 509 was cycli/cd. 

The conclusion from all observations is that the Grignard reagents transfer magnesium to azo-henzene to produce a radical pair in a stepwise mechanism. The transfer, however, takes place in a transition siale which is very similar to the concerted six-centre Whitmore Mosher complex and has rather strict steric requirements. The reaction is radical-concerted. This, lor example, explains that in the reaction of x-hulylmagncsinm bromide with azobenzene, l-hinene is formed in a 4 1 ratio relative lo 2-huiene. while this ratio in reactions with other electron acceptors is below unity. Since the radical pail is formed with a conformation which is ideal for transfer of /I-hydrogen this is the main product, but the radicals 

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The THT and SMe2 adducts have structures of the type (18-B-V). Their chemistry has been extensively studied and it is summarized in Fig. 18-B-7. The diverse, and in some cases unique, reactivity of these compounds includes substitution with preservation of the geometry or with conversion to (MX4)2(/t-X)2 species, oxidative-addition,53 cluster formation, splitting of C—N bonds,54 and above all coupling of the molecules with triply bonded carbon atom.55 They catalytically trimerize and polymerize terminal acetylenes, and dimerize nitriles and isonitriles with incorporation of the new ligand into the complex. Another remarkable reaction of M2C16L3 is the metathesis of M=M and N=N bonds into two M=N bonds upon reaction with azobenzene. 

Reactions which have rale-determining FT have transition slates with charge development and should be accelerated in the more polar solvent THF. Since the various coordination equilibria existing in the reaction mixtures are seriously shifted, reactivity changes have to be carefully analysed. Although the FT reaction with azobenzene is accelerated in Till, the F.T reaction between t-butylmagnesium chloride and di-r-hulylperoxide is extremely slow in THF 1441... 

The formation of M—C bonds in this way is also observed in reactions with azobenzene, N,vV-dimethylbenzylamine and similar compounds,32 e.g.,... 

With KCg in benzene it gave [ U(T) -C5Me5)2 2([Jt-ii i1 -C6H6)] [106], which itself proved to be also quite interesting in the area of multi-electron reduction chemistry, as shown by its reaction with azobenzene in which it functions as an eight-electron reductant [104]. Thus, it is clear that the complex (ii -CsMes) shows quite an interesting two- to four-electron reduction chemistry in a similar way to that of the com-... 

The only other reaction with an aromatic substance is the C-H insertion into ferrocene [85], giving 41,which illustrates the highly electrophilic character of the phosphinidene complex. Other aromatic C-H insertions have been observed, but these likely occur by means of intermediate P,0- and P,N-ylids,such as the reaction of (0C)5W=PR withbenzophenone and azobenzene that give 42 and 43,respectively [56a, 86]. 

However, cycloauration is, sometimes, difficult to achieve with direct activation of a C—H bond and transmetallation reaction of organomercury or organotin compounds with the appropriate gold compounds is, hence, frequently used. This procedure has been used with azobenzene, 36 A, A -dimethylbenzilamine,1937,1938 4,4 -dimethyl-2-phenyl-l, 3-oxaazoline,1938 1 - (dii zene,... 

The highly oxophilic nature of the cobalt powder was readily demonstrated by its reaction with nitrobenzene at room temperature. Reductive coupling was quickly effected by 2 to give azo- and azoxy derivatives. Nitrobenzene reacted with 2 to give azobenzene in yields up to 37%. In some cases small amounts of azoxybenzene were also formed. With 1,4-diiodonitrobenzene, 2 reacted to give low yields of 4,4-diiodoazoxy-benzene and 4,4-diiodoazobenzene. 

Vogtle and co-workers first reported a photoswitchable dendrimer [33] with six peripheral azobenzene groups, which took advantage of the efficient and fully reversible photoisomerization reaction of azobenzene-type compounds (Scheme 7). In a follow-up study [34], polypropylene imine) dendrimers bearing azobenzene moieties (p-Im-Gn, n = 1-4) on the periphery were synthesized. These dendrimers displayed similar photoisomerization properties as the azobenzene monomers. Irradiation of the all-E azobenzene dendrimers at 313 nm led to the Z-form dendrimers, while irradiation at 254 nm or heating could convert the Z-form dendrimers back to the E-form dendrimers. The observation that the... 

By Dismutation.—Hydrazobenzene (1-2 g.) is melted in a test tube over a small flame. The orange-red liquid thus produced is carefully heated until the aniline which has been formed begins to boil. On cooling, a semi-solid mixture of red azobenzene and aniline is obtained. The aniline can be shaken out with water and identified by means of the bleaching powder reaction. The azobenzene may be recrystallised from alcohol as described above. If it is desired to isolate the aniline also, when larger amounts of hydrazobenzene are used, the base is separated from the azobenzene by means of dilute acetic acid. From the solution of its acetate the aniline is then liberated with concentrated alkali hydroxide solution, extracted with ether, and purified in the manner already described. 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

The theory for the reaction of an adsorbed redox couple (2.146) has been exemplified by experiments with methylene blue [92], and azobenzene [79], Both redox couples, methylene blue/leucomethylene, and azobenzene/hydrazobenzene adsorb strongly on the mercury electrode surface. The reduction of methlylene blue involves a very fast two-step redox reaction with a standard rate constants of 3000 s and 6000 s for the first and second step, respectively. Thus, for / < 50 Hz, the kinetic parameter for the first electron transfer is log(m) > 1.8, implying that the reaction appears reversible. Therefore, regardless of the adsorptive accumulation, the net response of methylene blue is a small peak, the peak current of which depends linearly on /J. Increasing the frequency above 50 Hz, the electrochemical... 

All ECi adsorption coupled mechanisms have been verified by experiments with azobenzene/hydrazobenzene redox couple at a hanging mercury drop electrode [86,128,130]. As mentioned in Sect. 2.5.3, azobenzene undergoes a two-electron and two-proton chemically reversible reduction to hydrazobenzene (reaction 2.202). In an acidic medium, hydrazobenzene rearranges to electrochemically inactive benzidine, through a chemically irreversible follow-up chemical reaction (reaction 2.203). The rate of benzidine rearrangement is controlled by the proton concentration in the electrolyte solution. Both azobenzene and hydrazobenzene, and probably benzidine, adsorb strongly on the mercury electrode surface. 

Figure 13-35 illustrates the mechanism for the formation of p-hydroxyazoben-zene beginning with the reaction of benzenediazonium chloride with an aromatic system activated by an -OH. The mechanism for the reaction with an amine is similar. Figure 13-36 illustrates the reaction of a diazonium salt with an amine. The product of the reaction in Figure 13-36 is p-(dimethylamino) azobenzene. 

This method has been employed for the synthesis of aluminum hydrazides in only few, but quite interesting cases. The reaction of azobenzene with an arylaluminum dihydride [aryl = tri(tert-butyl)phenyl] was reported to afford a... 

The hydrogenation of nitrobenzene, nitrosobenzene and azobenzene has been studied singly and competitively. A kinetic isotope effect was observed with nitrobenzene but not with nitrosobenzene. Nitrosobenzene inhibits nitrobenzene hydrogenation in a competitive reaction, whereas azobenzene and nitrobenzene co-react but at lower rates. Taken together a more detailed mechanistic understanding has been obtained. 

The palladium-catalyzed amination reaction with a monofunctional primary amine 167 effects i / -diarylation to afford the tertiary amine polymers 168. Hence, polycondensation of 167 with bifunctional arylene dihalides leads to 168 (Equation (81)). Polycondensation of a primary amine bearing an azobenzene moiety with dibromides in the presence of Pd2(dba)3 (2.5mol%)- Bu3P (P/Pd = 3 l) and NaO Bu at 100°G for 24h in toluene gives 168 in 81-93% yields, as reported by Kanbara. 

Since it was observed that absorption ceased after 3.3 atoms of hydrogen were taken up per mole of nitrobenzene, Equation 14 is shown as producing 4 moles of cyanocobaltate(II) per mole of substrate via reaction of the latter with CoH. Since further absorption of hydrogen occurred only upon introduction of alkali, it is implied that an intermediate complex, X, is formed which is not subject to further reaction with CoH but may be decomposed by alkali. The stoichiometry of this equation requires formulation of complex X as [Co(CN)5(C6H5NH)]—3. However, since absorption ceased after two atoms of hydrogen had been absorbed per atom of cobalt present, it is implied that a binuclear complex is formed, perhaps involving phenylhydroxylamine, azobenzene, or some other reduction intermediate. 

An elegant extension of this approach is the combination of the nitroalkanes to ketone reaction with prior EGB-catalysed Michael condensation involving nitronate anions. Azobenzene works well as the catalyst probase and the condensation and... 

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