Elimination nitrile-forming

A plausible mechanism for the [2+2+2] cycloaddition reactions between diynes and heterocumnlenes (or nitriles) is shown in Scheme 5.16. Initially [2+2] oxidative addition of one alkyne and the heterocnmnlene (or nitrile) forms the five-mem-bered intermediate 54 compound 55 is formed after the insertion of the second alkyne and finally the seven-membered compound 55 undergoes reductive elimination to afford the prodnct 56 and regenerate the Ni(0) catalyst. 

Nitrile-forming anti eliminations from the (Z)-oximes (31) and (32) have also been found to proceed by the E2 mechanism the symmetrical transition state is little... 

Elimination reactions of ( )- and (Z)-benzaldehyde Opivaloyloximes (19a) and (19b) with DBU in MeCN have been found to occur by a nitrile-forming E2 mechanism which is ca 2000-fold faster for the latter isomer in each case.15 The corresponding Hammett substituent constants, activation parameters, and primary deuterium isotope effects, suggest that the anti elimination from (19b) (for which p = 2.4 0.1, H/ D = 2.7 0.3, A/H = 12.5 0.2 kcal mol-1, and A= —31.0 0.6eu) proceeds to (20) via a more symmetrical transition state with a smaller degree of proton transfer, less charge development at the jS-carbon and greater extent of triple bond formation than for syn elimination from (19a) (for which p = 1.4 0.1, kn/kn = 7.8 0.3, AH = 8.8 0.1 kcal mol 1 and A= -23.6 0.4 eu). 

The deuterium KIE in the nitrile-forming elimination of 467 (equation 275), promoted by Et3N-MeCN, t-BuOK-t-BuOH-and t-BuOK-DMSO, has been found533 to be dependent on the base-solvent variation from t-Bu-OH to DMSO. 

Palomo75 report that various aromatic aldehydes can be converted to nitriles in 94-97% yield by refluxing the aromatic aldehyde, hydroxylamine hydrochloride, and magnesium sulfate in toluene or xylene, with p-toluencsulfonic acid as catalyst for 1.5 to 3 hr. The microwave-assisted process may prove better for aliphatic aldehydes and may be made even more attractive if the above process conditions could be refined to reduce or eliminate NMP—for instance, if both aldehyde and nitrile form a homogeneous liquid at the reaction temperature. 

The selective synthesis of the 2-allyltetrazoles 55 by the three-component coupling reaction of the cyano compounds 54, allyl methyl carbonate 5b, and trimethylsilyl azide 42 was accomplished in the presence of Pd2(dba)3.CHCl3 and P(2-furyl)3 (Scheme 19) [55,56]. Most probably, the formation of (r)3-allyl)( ]5-tetrazoyl)-palladium complex 56 took place through [3 + 2] dipolar cycloaddition of 7r-allylpalladium azide 44 with the nitrile 54. The complex 56 thus formed would undergo reductive elimination to form the products 55. 

In the dehydration of aldehydes to form nitriles, Ley and colleagues used a polymer-supported hydrazine (Scheme 6.3). The immobilized hydrazine was reacted with aldehydes to give a hydrazone. Subsequent oxidation with mCPBA (meta-chloroperoxybenzoic acid) rendered the corresponding N-oxide, which spontaneously eliminated to form the desired nitrile [10]. Polyvinylpyridine was used as a scavenger for excess mCPBA. 

Rearrangement-cyclization. 6-Oximino nitriles form 2-acetamidopyridines. n treatment with AC2O-ACCI under reflux. The reaction proceeds from rearrangement of iV-acetoxyenamines to C-acetoxy imines, which undergo elimination and csclization. 

The preparation of 6,y-unsaturated ketones requires particularly mild conditions if conjugation of the double bond is to be avoided. Nevertheless two useful approaches to these molecules have been reported. The first uses a Peterson elimination to form the double bond in a nitrile oxide derived 6-hydroxy ketone (Scheme... 

Oximes can react with triflic anhydride to afford amidines or enamines after a nucleophUic capture of the iminocarhocation formed (111). The activated oxime can also eliminate to form the corresponding nitrile in the presence of imidazole or he converted into henzisoxazoles through an intramolecular cyclization process. ... 

Nitrile-forming eliminations from 2,4,6-trinitrobenzaldehyde 0-oxime derivatives promoted by amines in MeCN have been studied kinetically. The reactions are second-order and exhibit substantial Hammett p and Brpnsted ft values. The second-order rate constant for elimination from ( )-2,4,6-trinitrobenzaldehyde 0-pivaloyloxime promoted by /-Pr2NH in MeCN falls on a single line in the Hammett plot for different j0-aryl substituents, which have been shown to react by the E2 mechanism. The change of the /3-aryl group from phenyl to thienyl to furyl shifted the reaction mechanism from E2 to ElcE) f ... 

Reactions of mtemal olefins are more complex than reactions of terminal olefins (Scheme 16.3). As mentioned previously, terminal nitriles are often formed from reactions of internal olefins. The formation of terminal nitriles results from insertion of the internal olefin to form a branched alkylmetal intermediate (A in Scheme 16.3) that undergoes isomerization to the terminal alkyl intermediate B prior to reductive elimination of the final linear nitrile faster than it undergoes reductive elimination to form the branched nitrile. Internal olefins react more slowly than terminal olefins, and this relative rate can be traced to the slower insertion of internal olefins into metal hydrides. Lewis acids, such as ZnClj and AlClj, promote these reactions of isolated alkenes. 

The isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile is thought to occur by elimination and re-addition of HCN. A number of labeling experiments have been conducted to reveal the order and reversibility of the steps of hydrocyanation an experiment that addresses the mechanism of isomerization of the branched nitrile is depicted in Equation 16.8. As shown on the left of this equation, allylic transposition of the nitrile group without elimination would lead to 5-deuterio-3-pentenenitrile as the only isotopomer of 3-PN-iij. However, elimination to form an H-Ni-CN complex and free 1-deuteriobutadi-ene would lead to a mixture of two labeled 3-PN-iij isotopomers after re-insertion of the labeled butadiene and reductive elimination of the free labeled 3-PN. A mixture of the two isotopomers was formed, and this result indicates that isomerization occurs by elimination and re-addition of HCN. 

A kinetic study of nitrile-forming elimination from ( )-2,4-dinitrobenzaldehyde 0-aryloximes has been carried out in acetonitrile, with catalysis by tertiary amines. The Brpnsted Rvalue for this dehydration ranges from0.83to 1.0, with l/ igl = 0.41-0.46. The results are consistent with a highly iCb-like TS. 

Nitrile-forming eliminations from (E)-2,4-dinitrobenzaldehyde 0-aryloximes promoted by tertiary amines in acetonittile have been studied kinetically. The results have been interpreted in terms of a highly ElcB-like TS with extensive C -H bond cleavage... 

Related to the P-carbon elimination of late transition metal alcoholates, P Carbon elimination from rhodium iminyl complexes has also been reported (Scheme 1.44) [61]. Aryl nitriles were eliminated to form arylrhodium species. 

Tertiary amines capable of eliminating a secondary amine to form a conjugated system can react with hydrogen cyanide to form y-keto nitriles by amine replacement. Thus (I) yields p-benzoylpropionitrile (IV) ... 

The allyl cyanoacetate 731 can be converted into an a, /3-unsaturated nitrile by the decarboxylation-elimination reaction[460], but allyl malonates cannot be converted into unsaturated esters, the protonation and allylation products being formed instead. 

Esters are most commonly prepared by the reaction of a carboxyHc acid and an alcohol with the elimination of water. Esters are also formed by a number of other reactions utilizing acid anhydrides, acid chlorides, amides, nitriles, unsaturated hydrocarbons, ethers, aldehydes, ketones, alcohols, and esters (via ester interchange). Detailed reviews of esterification are given in References 1—9. 

The use of the triphenylphosphine-carbon tetrachloride adduct for dehydration reactions appears to be a very simple way of synthesizing nitriles from amides, carbodi-imides from ureas, and isocyanides from monosubstituted formamides. All of these reactions involve the simultaneous addition of triphenylphosphine, carbon tetrachloride, and tri-ethylamine to the compound to be dehydrated. The elimination of the elements of water is stepwise. An adduct, e.g. (46), is first formed, chloroform being eliminated, which decomposes to produce hydrogen chloride and the dehydrated product. 

Aldoximes are readily dehydrated with N, N -carbonyldiimidazole (CDI). An intermediate azolide is formed in the process under elimination of one mole of imidazole, which fragments into a nitrile through elimination of CO2 and a second mole of imid-azole.[1],[2]... 

It is believed that SCR by hydrocarbons is an important way for elimination of nitrogen oxide emissions from diesel and lean-burn engines. Gerlach etal. [115] studied by infrared in batch condition the mechanism of the reaction between nitrogen dioxide and propene over acidic mordenites. The aim of their work was to elucidate the relevance of adsorbed N-containing species for the F>cNOx reaction to propose a mechanism. Infrared experiments showed that nitrosonium ions (NO+) are formed upon reaction between NO, NOz and the Brpnsted acid sites of H—MOR and that this species is highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, the propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can further be hydrolysed to yield... 

Alkyl and silyl nitronates are, in principle, /V-alkoxy and /V-silyloxynitrones, and they can react with alkenes in 1,3-dipolar cycloadditions to form /V-alkoxy- or /V-silyloxyisoxaz.olidine (see Scheme 8.25). The alkoxy and silyloxy groups can be eliminated from the adduct on heating or by acid treatment to form 2-isoxazolines. It should be noticed that isoxazolines are also obtained by the reaction of nitrile oxides with alkenes thus, nitronates can be considered as synthetic equivalents of nitrile oxides. Since the pioneering work by Torssell et al. on the development of silyl nitronates, this type of reaction has become a useful synthetic tool. Recent development for generation of cyclic nitronates by hetero Diels-Alder reactions of nitroalkenes is discussed in Section 8.3. 

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