Triple bonds in nitriles

The carbon-nitrogen triple bond in nitriles is reducible to an aminomethyl group in acidic media at lead or mercury electrodes 9 whereas in neutral or alkaline medium cleavage of the C-CN bond takes place 146 148). This duality of mechanism has been studied in great detail for 2- and 4-cyanopyridine 146 and it was demonstrated that both pH and electrode potential is of importance in determining the mechanism. 

Values of /(C-N) across the triple bond in nitriles and isonitriles are negative. (106, 109) In both acetonitrile and methylisocyanide the Fermi contact contribution to /(N-C) is predicted to be positive and, for the methylisocyanide, quite large. The orbital and spin-dipolar mechanisms are predicted to make a negative contribution thus it appears that the latter two mechanisms are important for coupling across triple bonds. 

Nucleophiles other than water can also add to the carbon-nitrogen triple bond of nitriles In the following section we will see a synthetic application of such a nude ophilic addition... 

The carbon-nitrogen triple bond of nitriles is much less reactive toward nucleophilic addition than is the carbon-oxygen double bond of aldehydes and ketones Strongly basic nucleophiles such as Gngnard reagents however do react with nitriles in a reaction that IS of synthetic value... 

Nitriles are susceptible to nucleophilic addition. In their hydrolysis, water adds to the carbon-nitrogen triple bond. In a series of proton-transfer steps, an anide is produced ... 

It has been found that low-valent iridium hydride complexes are effective catalysts for activation of both the a-C-H bond and the CN triple bond of nitrile. Actually, the catalytic cross condensation of nitrile 27 and nitrile 28 in the presence of IrH(CO)(PPh3)3 catalyst (29) was performed under neutral conditions to give the... 

An important feature of the present reaction is the chemoselective addition of activated nitriles to the CN triple bonds of nitriles in the presence of carbonyl groups, because of the strong coordination ability of nitriles toward metals. The iridium-catalyzed addition of ethyl cyanoacetate to 4-acetylbenzonitrile (30) gives ethyl (Z)-3-(4-acetylphenyl)-3-amino-2-cyano-2-propenoate (31, 59%) chemoselec-tively, while the same reaction promoted by a conventional base such as AcONH4 and NaOH gives ethyl 2-cyano-3-(4-cyanophenyl)-2-butenoate (32) ( Z= 55 45) [20]. 

The triple bond of nitriles is attacked by aroyl nitrenes to give rise to oxadiazoles as illustrated in Sch. 25 [22,41]. However, additions of acyl nitrenes to olefmic double bonds can be carried out in acetonitrile solution because the cycloaddition reaction to the solvent is much slower. 

Recently, the transition-metal-catalyzed addition of active methylene C-H bonds to electron-deficient olefins having a carbonyl, a nitrile, or a sulfonyl group has been extensively studied by several research groups. In particular, the asymmetric version of this type of catalytic reaction provides a new route to the enantioselective construction of quaternary carbon centers [88]. Another topic of recent interest is the catalytic addition of active methylene C-H bonds to acetylenes, allenes, conjugate ene-ynes, and nitrile C-N triple bonds. In this section, the ruthenium-catalyzed addition of C-H bonds in active methylene compounds to carbonyl groups and C-C multiple bonds is described. 

In contrast to some of the aforementioned cases, the stretching frequency of the C N triple bond in the nitrile group shifts to the blue when this group accepts a proton in a H-bond. This shift amounts to some 20-30 cm when HCN is paired with HF, and the band is intensified by a factor between two and three. Also shifted toward the blue is the bending frequency of the HCN molecule. The red shift in the HF stretch within the donor molecule is considerably smaller than when HF donates a proton to the more basic amines, as... 

The structure and bonding in nitriles is veiy different from the carboxylic acid derivatives, and resembles the carbon-carbon triple bond of alkynes. 

The triple bond in acetylene, like that in nitrogen, is composed of one a and two tt bonds and here also the tt bonds arc formed between electrons in non-hybridized p states. The a electrons, two to each carbon atom, are located in sp hybrid orbitals, formed by the linear combination of one s and one p wave function, as in beryllium, and the molecule is therefore linear. The arrangement of bonds is shown diagrammatically in Figure si. In a similar way the triple bond in the nitrile group G N is composed also of one n and two tt bonds. 

The azide ion adds to the triple bond of nitriles with formation of the tetrazole ring. The reaction is often very smooth but usually requires a long reaction period49 or heating in a bomb tube.50... 

Nitrile, azo, and nitroso groups, and even the oxygen molecule, take part in such reactions, and acetylenic triple bonds in particular confer reactivity as philodiene. As for dienes, so for philodienes the reactivity depends on the constitution. Activating groups particularly favor addition. The most reactive components include <%,/ -unsaturated carbonyl compounds such as acrolein, acrylic acid, maleic acid and its anhydride, acetylenedicarboxylic acid, p-benzo-quinone and cinnamaldehyde, as well as saturated nitriles and <%,/ -unsaturated nitro compounds. Tetracyanoethylene also reacts with dienes.41,42 Conjugation of the double bond to an active group is not absolutely essential for a philodiene, for dienes add under certain conditions also to philodienes with isolated double bonds examples of the latter type are vinyl esters and vinyl-acetic acid. Ketenes do not undergo the Diels-Alder reaction with dienes, but instead yield cyclobutanone derivatives 43,44... 

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