Azo compounds , aliphatic

The addition of halogenated aliphatics to carbon-carbon double bonds is the most useful type of carbon-carbon bond forming synthetic method for highly halogenated substrates Numerous synthetic procedures have been developed for these types of reactions, particularly for the addition of perfluoroalkyl iodides to alkenes using thermal or photolytic initiators of free radical reactions such as organic peroxides and azo compounds [/]... 

In contrast to the aliphatic diazo compounds, which are invariably colored, all the diazirines so far prepared are colorless. The UV absorption of diazirines corresponds approximately to that of the aliphatic azo compounds. Diazirine shows in methanol an absorption maximum at 321 mja. The IR spectrum of the diazirines shows a band at ca. 1580 cm". ... 

In this chapter techniques of block copolymer preparation involving thermally labile azo compounds are reviewed. Upon heating, aliphatic azo compounds evolve nitrogen thus forming two carbon centered free radicals. 

When applied to ketones, this is called Norrish Type / cleavage or often just Type I cleavage. In a secondary process, the acyl radical R —CO can then lose CO to give R radicals. Another example of a category 1 process is cleavage of CI2 to give two Cl atoms. Other bonds that are easily cleaved by photolysis are the 0—0 bonds of peroxy compounds and the C—N bonds of aliphatic azo compounds R—N=N—R. The latter is an important source of radicals R , since the other product is the very stable N2. 

Aliphatic azo compounds in which the carbon containing the azo group is attached to a hydrogen are unstable and tautomerize to the isomeric hydrazones (15), which are therefore the products of the reaction. 

As already discussed in Section 7.4, hexamethyidisiiane 857 (which is produced on a technical scale), in the presence of catalytic amounts of tetrabutylammonium fluoride di- or trihydrate in THF, reduces aromatic heterocyclic N-oxides such as pyridine N-oxide 860, quinoline N-oxide 877, or isoquinoline N-oxide 879 to the heterocycles [95] and nitrones to Schiff-bases. Aromatic nitro compounds such as nitrobenzene are reduced analogously to azo compounds such as azobenzene [96]. As mentioned in Section 7.5, secondary aliphatic nitro groups are reduced to oximes. 

Zollinger, H. "Diazo and Azo Chemistry Aliphatic and Aromatic Compounds" Interscience, New York, 1961. 

Azodicarbonyl compounds, which contain a carbonyl function on both sides of the azo bond, in contrast to aliphatic and aromatic azo compounds, possess a highly reactive N=N group. In recent years they have found wide... 

Aliphatic azo compounds usually undergo photoextrusion of nitrogen (cf. chapter 2.3.3). Nevertheless 1,2-diazetidines are formed efficiently when there is a favourable interaction between the pi-systems of the N—N and the C-C bonds (4.85)499). 

The mechanism of the coupling reaction has been very exhaustively studied. Summarising first what has already been mentioned, it must be noted that the reaction is not confined to the aromatic series, for diazo-compounds condense also with enols and with the very closely related aliphatic aci-nitro-compounds. The final products of these reactions are not azo-compounds, but the isomeric hydrazones formed from them by rearrangement. 

Aliphatic azo compounds Lewis acids with or without co-initiators Lewis bases Mixed catalysts (Ziegler-Natta catalysts)... 

The following types of dipolarophiles have been used successfully to synthesize five-membered heterocycles containing three heteroatoms by [3 + 2]-cycloaddition of thiocarbonyl ylides azo compounds, nitroso compounds, sulfur dioxide, and Al-sulfiny-lamines. As was reported by Huisgen and co-workers (91), azodicarboxylates were noted to be superior dipolarophiles in reactions with thiocarbonyl ylides. Differently substituted l,3,4-thiadiazolidine-3,4-dicarboxylates of type 132 have been prepared using aromatic and aliphatic thioketone (5)-methylides (172). Bicyclic products (133) were also obtained using A-phenyl l,2,4-triazoline-3,5-dione (173,174). 

For perhaps a century, the primary interest in azo compounds has been in dye chemistry. Recently, aliphatic azo compounds, which are thermally less stable than their aromatic counterpart, have enjoyed attention as sources of free radicals for polymerization reactions. In this regard, an initiator such as a,a -azobis(isobutyronitrile) is important for two reasons ... 

Since aliphatic azo compounds can, at best, act only as hydrogen abstracters in oxidation reactions, the color formation associated with oxygen addition (it is due to fragments from peroxide-initiated polymerization) is eliminated or reduced even in the presence of atmospheric oxygen. 

Among the oxidative procedures for preparing azo compounds are oxidation of aromatic amines with activated manganese dioxide oxidation of fluorinated aromatic amines with sodium hypochlorite oxidation of aromatic amines with peracids in the presence of cupric ions oxidation of hindered aliphatic amines with iodine pentafluoride oxidation of both aromatic and aliphatic hydrazine derivatives with a variety of reagents such as hydrogen peroxide, halogens or hypochlorites, mercuric oxide, A-bromosuccinimide, nitric acid, and oxides of nitrogen. 

When the two radicals attached to the azo group are derived from different parent molecules, the IUPAC system places the term azo between the complete names of the (substituted) parent molecules (Rule C-911.2). This system resembles an older numbered azo bridge system. The Chemical Abstracts system names the compound as a parent molecule RH substituted by a radical R N=N— (Rule C-912.4). This system is particularly convenient for unsymmetrically substituted aliphatic azo compounds. 

Two older systems of nomenclature name aliphatic azo compounds diazenes or diimines, as in structure VII. 

By use of aliphatic zinc iodides, in toluene solution, mixed aromatic-aliphatic azo compounds have also been prepared [45b]. Reported properties are... 

While aromatic nitro and azoxy compounds have been reduced to azo compounds with lithium aluminum hydride, aliphatic nitro compounds produced only the corresponding aliphatic amines [65]. The usual technique involves dropwise addition of 1 mole of nitro compound in ether to 1.05-1.15 moles of lithium aluminum hydride in ether solution at Dry Ice temperatures followed by warming to room temperature. If the resulting product is only slightly soluble in ether, hydrolysis should be carried out with dilute sulfuric acid. Then the azo compound simply needs to be filtered off, washed with water, and dried. If the product is ether-soluble, the ether layer is separated, evaporated, and the residue is recrystallized [65, 66]. 

As indicated above (Ohme and Schmitz [38a, p. 339]) primary amines converted into the alkylamides of sulfuric acid can be oxidized with sodium hypochlorite to azo compounds. The reaction appears to proceed by way of an intermediate hydrazine, which is ultimately oxidized [38a], The reaction is suitable for the formation of symmetrically substituted azoalkanes. Highly branched primary aliphatic amines have been oxidized with sodium hypochlorite in an aqueous dioxane medium [77],... 

Since A,A -disubstituted hydrazines are readily available from a variety of sources (see Volume I, Chapter 14), their dehydrogenation constitutes a widely applicable route to both aliphatic and aromatic azo compounds. Such oxidative procedures are of particular value in the aliphatic series because so many of the procedures applicable to aromatic compounds, such as the coupling with diazonium salts, have no counterpart. The oxidation reactions permit the formation not only of azoalkanes, but also of a host of azo compounds containing other functional groups, e.g., a-carbonyl azo compounds [83], a-nitrile azo compounds [84], azo derivatives of phosphoric acid [85], phenyl-phosphoric acid derivatives [86],... 

Similar azo compounds were also prepared by the oxidation of aliphatic hydrazo compounds with nitrogen dioxide [84],... 

The oxidation of both aliphatic and aromatic azo compounds to the corresponding azoxy derivative may be carried out with a variety of reagents. While older techniques favored chromic or nitric acid as the oxidizing agent, newer methods make use of various organic peracids or hydrogen peroxide. In the oxidation of aliphatic azo compounds, relatively weak peracids are favored to reduce the possibility of acid-catalyzed isomerization of azo compounds to hydrazones. Under controlled conditions cis azo compounds may be converted into cis azoxy compounds. 

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