Radical Reactions, Catalytic Hydrogenation, Reductions

No new material relevant to this topic has appeared since CHEC-11(1996), but a few cases of interest have not been reviewed previously. These will be included here after a brief synopsis of the material treated in CHEC(1984) and CHEC-IK1996). 

Attack of the 1,2,3-triazine system by an organic radical is used in the introduction of a carboxamide function at C-5 by the treatment of 4,6-disubstituted l,2,3-triazin-2-ium 2-dicyanomethylides 34 (R = H R, R = Me, Ph, Et) with formamide in the presence of ammonium persulfate at 80 °C. Under these conditions, the 1,2,3-triazine is restored by loss of dicyanomethyl radical. The corresponding 2-methyl-l,2,3-triazin-2-ium iodides gave the demethylated 

5-aminocarbonyl-l,2,3-triazines in good yields 1991H(32)855, 1991H(32)2015 . Aside from this case, mostly catalytic hydrogenations, dissolving metal reductions, and a few electroreductions have been carried out. 

4-dihydro-l,2,3-benzotriazin-4-imines upon treatment with SnCl2 in ethanol under reflux 1964JCS3663 , 

4-imino-2-alkyl-3,4-dihydro-l,2,3-benzotriazin-2-ium-3-ides with hydrazine/Raney-nickel (N2H4/Ra-Ni) at 60-65 °C in ethanol 1970JG2289 , 

---

Reactions of 1,2,4-thiadiazoles with radicals and carbenes are virtually unknown. Catalytic hydrogenations and dissolving metal reductions usually cleave the N-S bond in a reversal of the oxidative cyclization procedures used in synthesis of 1,2,4-thiadiazoles (see Section 5.08.9.4). 

The addition of hydrogen to carbon-carbon multiple bonds (reduction) may be achieved using gaseous hydrogen in the presence of a finely divided noble metal catalyst. This is termed catalytic hydrogenation. It is not a radical reaction as we have seen... 

It is convenient to discuss oxidative attack on ring carbon in the same chapter with reduction of heteronines as many reported syntheses involved various oxidative/reductive sequences and reagent combinations. Examples of oxidative transformations involve radical as well as electrophilic oxidizing agents, while reductive syntheses include both chemical reduction and reactions on surfaces via catalytic hydrogenation. 

Aliphatic hydroxylamines are not easily reduced electrolytically. The polarographic wave of such compounds is not well defined and is visible only in a narrow pH region, around pH 7. In acid solution the wave is masked by hydrogen evolution, and the hydroxylamines are catalytically active in that reaction. It has been suggested [73] that a branching in the reaction takes place after the uptake of the first electron, which leads either to a further reduction of the radical or to hydrogen evolution ... 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

Free radical attack at ring carbon atoms. 6.3 Electrochemical reactions and reactions with free electrons. 6.4 Catalytic hydrogenation and reduction by dissolving metals Reactions with Cyclic Transition States. 7.1 Diels-Alder reactions and 1,3-dipolar additions. 7.2 Photochemical cycloadditions... 

It was found that the action of OH radicals on nitrobenzene or benzoic acid produced all the three isomeric phenolic compounds. OH radicals produced, for example, by reaction (1) can be used to generate other free radicals or atoms from inorganic or organic compounds. These reactions have been studied more recently by Kolthoff and Medalia (13), Merz and Waters (46), Stein and Weiss (45), and others. Reaction (1) also proved useful in the study of the so-called active oxalic acid (47). The free radical mechanism of hydrogen peroxide has been discussed also in connection with the mechanism of the action of the enzymes catalase and peroxydase, the prosthetic groups of which are iron porphyrin complexes which presumably also undergo oxido-reduction processes in the course of their catalytic activity (48). 

---