Radical reactions of nitro compounds

A few more free radical reactions of nitro compounds should be mentioned. Some of these reactions are photo-induced and are described in the chapter on photochemistry of nitro compounds. 

Scheme 4. CAN-mediated radical reaction of nitro compound Id...

Fluoro-penianitrobenrcnc 144 Free radical reactions of nitro compounds 112-I 5(I/ 212). mechanism of 114 Free radicals. 

Because reductive cleavage of aliphatic nitro compounds with Bu3SnH proceeds via alkyl radicals, nitro compounds are also used as precursors to alkyl radicals. Reactions using nitro compounds may have some advantages over other ones, since aliphatic nitro compounds are available from various sources. For example, the sequence of the Michael additions of nitro compounds provides an excellent method for the construction of quaternary carbon compounds (Eq. 7.79).126 Newkome has used this strategy for the construction of dendritic polymers (Eq. 7.80).127... 

The fragmentation of the radical cations of nitro compounds may be initiated by a surprisingly high number of different mechanisms. The following reaction mechanisms appear as particularly important and will be discussed comprehensively ... 

Today, many stable radicals are known, as shown in Figures 1.11 and 1.12. However, most of them are nitroxyl radicals like NO or N02. Standard generation methods of nitroxyl radicals are as follows. One is the oxidation of amines or hydroxyamines by Pb02, or by less toxic oxidants such as oxone, Cu(OAc)2, mCPBA (eqs. 1.13 and 1.14). Another one is the reaction of nitro compounds with Grignard reagents (eq. 1.15) [9-14]. 

Reaction of nitro compounds such as nitro-tert-butane with Bu3SnH or (Me3Si)3SiH produces a nitroxyl radical (27). This is just an addition product of Bu3Sn or (Me3Si)3Si onto the nitro group [15, 16]. This radical is also a tt radical (eq. 1.16). 

Finally attention must be drawn to the fact that the orienting effect of the nitro group in nucleophile and radical reactions usually differs from that in electrophilic reactions, and instead of meta orientation, ortho or para orientation takes place. The corresponding observations are referred to in chapters dealing with nucleophile and radical substitutions of nitro compounds (pp. 204, 207 and 212 respectively). 

Another example of the homolytic reaction of nitro compounds with free radicals is their inhibiting effect on addition polymerization of vinyl compounds, especially by the higher nitrated compounds which have the most pronounced inhibiting properties. 

Mechanism of Richter reaction Nucleophilic substitution in gas phase Reactions of radical ions Radical anions of nitro compounds Free radical reactions... 

E.s.r. observations of the reactions of nitro compounds with reducing agents have given evidence about a number of the individual steps in the overall processes. The first step in the reaction with a one-electron reducing agent is the formation of the corresponding radical-anion. Subsequent steps depend on the structiu e of the nitro compound. 

N KORNBLUM, Radical Anion Reactions of Nitro Compounds, The Chemistry of Amino, Nitroso and Nitro Compounds and their Derivatives, (Ed. S. Patai) Wiley, New York, 1982. 

Aromatic nitro-compounds have also seen use as inhibitors in polymerization and as additives in radical reactions. The reactions of these compounds with radicals are very complex and may involve nitroso-compounds and nitroxide intermediates.20" 206 In this case, up to four moles of radicals may be consumed per mole of nitro-compound. The overall mechanism in the case of nitrobenzene has been written as shown in Scheme 5.18. The alkoxyamiuc 40 can be isolated in... 

Recently, nitration of organolithiums and Grignards with N204 has been developed for the preparation of certain kinds of nitro compounds (Eqs. 2.14 and 2.15).31 The success of this process depends on the reaction conditions (low temperature) and the structure of substrates. For example, 3-nitrothiophene can be obtained in 70% overall yield from 3-bromothiophene this is far superior to the older method. 3-Nitroveratrole cannot be prepared usefully by classical electrophilic nitration of veratrole, but it can now be prepared by direct o>7/ o-lithiation followed by low-temperature N204 nitration. The mechanism is believed to proceed by dinitrogen tetroxide oxidation of the anion to a radical, followed by the radical s combination. 

The ability of a nitro group in the substrate to bring about electron-transfer free radical chain nucleophilic substitution (SrnI) at a saturated carbon atom is well documented.39 Such electron transfer reactions are one of the characteristic features of nitro compounds. Komblum and Russell have established the SrnI reaction independently the details of the early history have been well reviewed by them.39 The reaction of p-nitrobenzyl chloride with a salt of nitroalkane is in sharp contrast to the general behavior of the alkylation of the carbanions derived from nitroalkanes here, carbon alkylation is predominant. The carbon alkylation process proceeds via a chain reaction involving anion radicals and free radicals, as shown in Eq. 5.24 and Scheme... 

Photoreduction of aromatic and aliphatic nitro compounds gives hydroxylamines or amines, which is well reviewed.125 The radical reaction of primary nitro compounds with tin hydride does not give the denitrated product (see Chapter 7), but give the corresponding oximes (Eq. 

Recently, it was reported that a nitromethyl group was reduced to the corresponding oxime by reaction with Bu3SnH in the presence of radical initiator (Eq. 7.61).68 Interestingly, primary nitro groups are selectively reduced to oximes in the reaction of the compounds containing both primary and secondary nitro groups (Eq. 7.62).69 The product of Eq. 7.60 may not be correct, but may be the oxime. 

Early kinetic experiments on the thermal decomposition of nitro compounds established that for the simplest derivative, nitromethane, the process was first order, but that the reaction was chemically complex owing to further reactions between the products and nitromethane. Cottrell et re-examined the nitromethane pyrolysis and reported values of = 53.2 kcal.mole" and log A = 13 for the Arrhenius parameters of the homogeneous decomposition a radical mechanism was proposed, initiated by C-N cleavage... 

Further examination of these reactions has shown that in some cases the spectrum of the nitro compound is further split by protons from the donor species. For example, Griffiths et al. (1966) found a tripletsplitting of 0-5 G when nitrobenzene was reduced by CH2OH. It is unlikely that this is due to ion-pairing between the products of electron transfer, PhNO " and CH2OH+ it is more probable that the radical adds to an oxygen atom of the nitro group (cf. the addition of the benzyl radical to aromatic nitro compounds Jackson and Waters, 1960) ... 

In reaction of aliphatic nitro compounds with alkyl radicals 6 7 generated from ethers or alcohols aminyloxides 69b could be detected79. Moreover dialkylaminyl-oxides 74b are formed, 67 being trapped from the corresponding nitroso compound. Reduction of nitro compound to nitroso compound probably occurs by electron transfer from alkyl radical 67 to nitro compound, subsequent dissociation of the resulting complex 68b giving nitro anion radical which finally disproportionates. 

The reaction of phenols with nitrous acid gives the ortho- and para-nitroso products, which are formed through a neutral dienone intermediate, the proton loss from the latter being the rate-limiting step" " . It has been shown that the nitrous acid can act as a catalyst for the formation of the nitro derivatives. Thus the conventional preparation of nitro compounds by the oxidation of nitroso compounds may be replaced by methods using an electron-transfer pathway in certain cases. In the latter method, the phenoxide reacts with nitrosonium ion to give the phenoxy radical and nitric oxide radical. The nitric oxide radical is in equilibrium with the nitronium radical by reaction with nitronium ion. The reaction of the phenoxy radical with the nitroninm radical resnlts in the formation of the ortho- and para-mixo prodncts" . Leis and coworkers carried ont kinetic stndies on the reaction of phenolate ions with alkyl nitrites and fonnd that the initially formed product is the 0-nitrite ester, which evolves by a complex mechanism to give the ortho-and the para-nitro products". ... 

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