Disproportionation with nitriles

Irradiation of a degassed benzene-d6 solution of 72 at 90°C slowly affords a mixture of 2,3-dimethyl-2-butene and tetramethylsuccinodinitrile in almost quantitative yield together with molecular nitrogen and traces of isobutyronitrile (81TH1 92TH2 96T1965). The two nitriles are probably formed by dimerization and disproportionation of the a-cyanoisopropyl... 

Not only alkenes and arenes but also other types of electron-rich compound can be oxidized by oxygen. Most organometallic reagents react with air, whereby either alkanes are formed by dimerization of the metal-bound alkyl groups (cuprates often react this way [80]) or peroxides or alcohols are formed [81, 82]. The alcohols result from disproportionation or reduction of the peroxides. Similarly, enolates, metalated nitriles, phenolates, enamines, and related compounds with nucleophilic carbon can react with oxygen by intermediate formation of carbon-centered radicals to yield dimers (Section 5.4.6 [83, 84]), peroxides, or alcohols. The oxidation of many organic compounds by air will, therefore, often proceed faster in the presence of bases (Scheme 3.21). 

Benzonitrile oxide (C in Figure 15.44) is an isolable 1,3-dipole. It can be generated from benzaldoxime and anNaOH/Cl2 solution. Under these reaction conditions the oxime/nitroso anion (A B) is initially formed and chlorine disproportionates into Cl—O and chloride. An SN reaction of the negatively charged C atom of the anion A B at the Cl atom of Cl— O or of Cl—O—H affords the oc-chlorinated nitroso compound E, which tautomerizes to the hydroxamic acid chloride D. From that species, the nitrile oxide C is generated via a base mediated 1,3-elimination. Isoxazoles are formed in the reactions of C with alkynes (Figure 15.44), while isoxazolines would be formed in its reactions with alkenes. 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

There is only sparse literature on [4 + 2] cycloadditions of nitriles with 1,3-dienes. In general, simple alkyl and aryl nitriles will only react at high temperatures and under these conditions the product dihy-drc yridines usually disproportionate.However, certain types of electron-deficient nitriles appear to be reactive dienophiles under milder conditions. For example, aiylsulfonyl cyanides cycloadd to 1,3-dienes to afiord adducts as shown in Scheme 1This methodology has not been widely explored and little is known about the regiochemistry of the process. 

Redox processes involving 178 have also been studied.Anodic oxidation of thianthrene has been eifected in a wide variety of solvents. Use of trifluoracetic acid gives stable solutions of 178 and, if perchloric acid is included, the solid perchlorate salt may be isolated on evaporation of the solvent after electrolysis. Dichloromethane at low temperatures has been used and, at the opposite extreme, fused aluminum chloride-sodium chloride mixtures. " Propylene carbonate permits the ready formation of 178, whereas the inclusion of water in solvent mixtures gives an electrochemical means of sulfoxidizing thianthrene. Reversible oxidation of 178 to thianthrenium dication may be brought about in customary solvents such as nitriles, nitro compounds, and dichloromethane if the solvent is treated with neutral alumina immediately before voltammetry addition of trifluoracetic anhydride to trifluoracetic acid equally ensures a water-free medium. The availability of anhydrous solvent systems which permit the reversible oxidation and reduction of 178 has enabled the determination of the equilibrium constants for the disproportionation of the radical and for its equilibria with other aromatic materials. ... 

Activation with sulfonic acid chlorides is more general, rendering amides with the possible participation of symmetric anhydrides after disproportionation. This method has also found use in the synthesis of modem 3-lactam antibiotics. However, in peptide chemistry this activation method leads to unwanted side reactions, like formation of nitriles in the cases of glutamine and asparagine, and racemization. In a convenient one-pot procedure, the carboxylic acids are activated by sulfonyl chlorides under solid-liquid phase transfer conditions using solid potassium carbonate as base and a lipophilic ammonium salt as catalyst. ... 

Hydrolysis with powdered potassium hydroxide or potassium fluoride on alumina in r-butyl alcohol converts nitriles to amides without further hydrolysis to carboxlic acids. Under similar conditions, addition of alkyl halides gives iV-alkylcarboxyamides. Less drastic acidic or basic hydrolysis conditions involve disproportionation of alkaline hydrogen peroxide with concomitant hydration of the nitrile (equation 21). 

Few reagents are available for the preparation of organometallic and coordination complexes of the early transition metals. The anhydrous metal halides often lead to disproportionation reactions, and the nitrile derivatives,1 MX (NCR)>, are not suitable because of the reactivity of the nitrile with many other reagents. The procedures described below provide simple, high-yield routes to the tetrahydrofuran (THF) complexes of selected, early transition metal halides.2 They have been useful for the synthesis of a wide variety of organometallic complexes.3... 

As with the aryl compounds the reaction scheme is simplified for the sake of clarity. The reactions observed are various and include reductive coupling (ketones, nitriles), insertion (diphenylacetylene, isocyanate, isonitrile, CO2 and CO), and disproportionation (diphenylacetylene, CS2). The course of the reactions and the complexity of the product mixtures strongly depend on the nature of the alkyl groups and the substituents on the substrate molecule. As examples we will discuss reactions with ketones, acetylenes and CO. The reaction with ketones strongly resembles the coupling of nitriles R C=N on Cp2TiR (R = aryl). The coupling proceeds more slowly when R, R and R" increase in size. 

The 1,3-dipolar character of triazaallenium salts, from now on referred to as l,3-diaza-2-azoniaallene salts, is evidenced by the many [3+2] cycloaddition reactions these types of compounds can participate in. The l,3-diaza-2-azoniaallene salts are generated in situ and trapped with suitable dipolarophiles. For example, l,3-diaza-2-azoniaallene salts undergo stereospecific [3+2] cycloaddition reactions with alkynes and olefins. However, they fail to react with isocyanates, isothiocyanates and azo compounds. Also, [3+2] cycloadditions to carbodiimides and cyanamides are observed. In contrast, nitriles fail to react. 1,3-Diaza-2-azoniaallene salts are obtained in the oxidation of 1,3-disubstituted triazenes with t-butyl hypochlorite. The resultant Al-chlorotriazenes react with antimony pentachloride to form the salts as reactive intermediates. Above —25°C, l,3-diaza-2-azoniaaUene salts disproportionate into diazonium salts and azo compounds. 

An unusual reaction in which a nitrile, RCN, is reduced to an amine, RCH2NH2, has been reported. The reaction of a-[Re2Cl4(dppbe)2] [dppbe = l,2-6w(diphenylphosphino)benzene] with RCN in the presence of HCl yields the amido complex [ReCl3(NCH2R)(dppbe)]. The reaction involves the disproportionation of the dirhodium(II) complex into a rhodium(I) and rhodium(III) species, followed by the reduction of the nitrile by rhodium(I). ... 

---