Imines from nitriles

Besides the weak bonds listed in the previous table, there are other multiple bonds that endow the molecules in which they are situated with a positive enthalpy of formation. Such compounds are termed endothermic compounds. The danger they represent does not necessarily come from the fact that they are unstable, but is related to the exothermicity of their decomposition reaction. The most convincing examples are the acetylenic compounds, and in particular, acetylene. It is also the case for ethylene, aromatic compounds, imines and nitriles. 

The addition of Grignard reagents to aldehydes, ketones, and esters is the basis for the synthesis of a wide variety of alcohols, and several examples are given in Scheme 7.3. Primary alcohols can be made from formaldehyde (Entry 1) or, with addition of two carbons, from ethylene oxide (Entry 2). Secondary alcohols are obtained from aldehydes (Entries 3 to 6) or formate esters (Entry 7). Tertiary alcohols can be made from esters (Entries 8 and 9) or ketones (Entry 10). Lactones give diols (Entry 11). Aldehydes can be prepared from trialkyl orthoformate esters (Entries 12 and 13). Ketones can be made from nitriles (Entries 14 and 15), pyridine-2-thiol esters (Entry 16), N-methoxy-A-methyl carboxamides (Entries 17 and 18), or anhydrides (Entry 19). Carboxylic acids are available by reaction with C02 (Entries 20 to 22). Amines can be prepared from imines (Entry 23). Two-step procedures that involve formation and dehydration of alcohols provide routes to certain alkenes (Entries 24 and 25). 

The formation of several pyrazolylpyrazolino[60]fullerene adducts (208a-c) from nitrile imines have been described (Scheme 9.64) [111]. The nitrile imines are generated in situ from the corresponding hydrazone 205 and NBS in the presence of Et3N... 

The tetrachloroferrate or tetralluoroborate salts of alkylated alkyl- or aryl-nitriles (nitrilium ions) are reduced to imines with triethylsilane. Subsequent hydrolysis of the intermediate imines leads to aldehydes in good yields, thus providing an excellent overall route to aldehydes from nitriles (Eq. 338).28,562... 

Poly(ethylene glycol) supported liquid-phase syntheses by both the reaction of (polyethylene glycol (PEG))-supported imines with nitrile oxides, generated in situ from aldoximes, (300) and 1,3-dipolar cycloadditions of nitrile oxide, generated in situ on soluble polymers with a variety of imines (301, 302) have been described. The solid-phase synthesis of 1,2,4-oxadiazolines via cycloaddition of nitrile oxide generated in situ on solid support with imines has also been elaborated (303). These syntheses of 1,2,4-oxadiazolines provide a library of 1,2,4-oxadiazolines in good yields and purity. 

Laser flash photolysis (LFP) studies have been carried out on the generation of N,C-diaryl nitrile imines from sydnones and from tetrazoles in solution at 77 K. They were found to have hfetimes of milliseconds and were quenched by dimethyl acetylenedicarboxylate (DMAD) (A q=5-9 x 10 M s ) and by carboxyhc acids (feq= lO -lO s ) (13). The strong dependence of v ax on the nature of the aromatic substituents in N,C-diaryl nitrile imines was interpreted by a hnear free energy relationship as due to intramolecular charge transfer (14). 

The oxidation of hydrazones 143 provides, in principle, a very convenient route to nitrile imines from easily accessible starting materials. However, the earliest reagent used, lead tetraacetate, was of limited effectiveness as yields were low and the reaction often gave high yields of diacyUiydrazides as byproducts. Work has been done on the apphcation of several other oxidants to this process to produce a more effective general route. The one that has proved most popular is chloramine T (A(-chloro-A(-sodio-p-toluenesulfonamide, CAT) which is used under mild conditions and has been shown to work well for both cycloaddition (79) (e.g., in the preparation of 146 from 145) and electrocychzation (80) reactions. 

Nitrile imines generated by the benzaldehyde phenylhydrazone-chloramine-T route have been added to 1-azetines (219) (X = 0, S) (95,124). However, the adducts 220 (X=0, S) proved to be less stable than those derived from nitrile ylides (186) (94) and in many cases fragmented spontaneously to give the 5-butenyl-1,2,4-triazoles (221) as the isolated products. It was found, however, that... 

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. 

An important, but not widely recognized effect in electron transfer chemistry is the phenomenon that the thermochemical stability order of several tautomeric systems can be inverted upon one-electron oxidation. Hitherto, most of the data stem from gas-phase measurements (photoelectron and mass spectrometry data) or from calculations at several levels of theory (Table 2). While the explicit numbers still differ for each method they all agree on the inversion of the stability order for keto/enol, alkine/allene, imine/enamine, nitrile/isonitrile and aldimine/aminocarbene pairs. One-electron reduction, on the other hand, does not necessarily lead to a thermochemical stability inversion, as demonstrated in the case of acetaldehyde 7cthenol [41]. 

From nitriles, isonitriles, ketene imines, carbodiimides and cyanamides 546... 

Acid amide-triphenylphosphine dihalide adducts (4) have found wide application in organic synthesis. - Synthetic equivalents are adducts (5) from acid amides and triphenylphosphine/CCU, which are prepared in situ from the educts. - With these reagents the following transformations have been performed dehydration of amides or aldoximes to nitriles, preparation of isonitriles from secondary form-amides, preparation of imidoyl halides from amides or acylhydrazines and preparation of ketene imines from amides. Using polymer-supported triphenylphosphine the work-up procedure is much easier to achieve. Triphenylphosphine can be replaced by tris(dialkylamino)phosphines. - Instead of CCI4 hexa-chloroethane, hexabromoethane or l,l,2,2-tetrabromo-l,2-dichloroethane can be used " the adducts thus formed are assumed to be more effective than those from the triphenylphosphine/CCU system. 

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