Carbon-Nitrogen Triple Bonds Nitriles

Nitriles are not reduced by LAH on SiOj [KH2], alkaline borohydrides in alcohol media or in ethers at room temperature [BK5, PSl], (CF3COO)2BH [MMl], cya-noborohydrides [GNl, LI], or diisopropoxytitanium tetrahydroborate [RC2]. Nevertheless, the 2-cyano- or 4-cyanopyridines are reduced to amines by NaBH4 in EtOH under reflux [HH3, KK2]. 

The use of NaEt2AlH2 in the presence of a Lewis acid for converting aliphatic nitriles to aldehydes has also been described [YK2]. Sodium triaminoaluminohy-drides or diethylpiperidinoaluminohydride reacts with aromatic nitriles in THF at r.t., allowing their transformation into the corresponding aldehydes [CJl, YA2]. 

The reduction of nitriles to amines can be carried out by LAH or SAH in an ether medium, LAH-N-methylpyrrolidine complex, AIH3 in Et20 or AlH3-Et3N [BK5, CBS, CB7, E2, FSl, H3, L2, M3, MCI, MP2, PSl]. Li(MeO)3AlH or Red-Al at 80°C reduces aromatic nitriles, while aliphatic nitriles remain untouched [M3]. This transformation can also be accomplished with BH3 THF or aminoboranes under 

A C—CN bond cleavage has been observed in the reduction of 5-cyano-5-isopropylsulfonylnorbom-2-ene with LAH in THF, likely via a SET process in the propagation chain [MS 10]. 

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Though not as common as double bonds, both carbon-carbon triple bonds (alkynes) and carbon-nitrogen triple bonds (nitriles) are important. Both occur in the 2,600-2,100 cm region of the spectrum. They are usually very sharp. The carbon-nitrogen triple bond tends to give a more intense peak than the alkyne peak. 

Chapter 22 continues the study of carbonyl compounds with a detailed look at nucleophilic acyl substitution, a key reaction of carboxylic acids and their derivatives. Substitution at sp hybridized carbon atoms was introduced in Chapter 20 with reactions involving carbon and hydrogen nucleophiles. In Chapter 22, we learn that nucleophilic acyl substitution is a general reaction that occurs with a variety of heteroatomic nucleophiles. This reaction allows the conversion of one carboxylic acid derivative into another. Every reaction in Chapter 22 that begins with a carbonyl compound involves nucleophilic substitution. Chapter 22 also discusses the properties and chemical reactions of nitriles, compounds that contain a carbon-nitrogen triple bond. Nitriles are in the same carbon oxidation state as carboxylic acids, and they undergo reactions that form related products. 

Carbon-nitrogen triple bonds (nitriles) undergo addition reactions, as do carbonyl compounds. 

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... 

Reductions of Nitriles. In the reduction of nitriles, hydrogen is added progressively across the carbon—nitrogen triple bond, forming first the imine and then the amine. 

In the above examples the polymerisation takes place by the opening of a carbon-carbon double bond. It is also possible to open carbonyl carbon-oxygen double bonds and nitrile carbon-nitrogen triple bonds. An example of the former is the polymerisation of formaldehyde to give polyformaldehyde (also known as polyoxymethylene and polyacetal) (Figure 2.3). 

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 ... 

Here is a list of all dangerous reactions that are related to nitrile functional group behaviour. By active polymerisation is meant the polymerisation that affects the carbon-nitrogen triple bond. Polymerisations that are related to an ethylene double bond will be dealt with on p.336. So far as stability is concerned, it is difficult to say whether certain spontaneous reactions of certain nitriles are... 

Mixing trichlorosilane, acetonitrile and diphenylsulphoxide, carried out at 10°C, detonated. This accident was put down to the exothermic addition reaction of the silicon-hydrogen bond on the carbon-nitrogen triple bond of nitrile. Other interpretations are possible for instance, the effect of traces of hydrogen chloride formed by the hydrolysis of chlorosilane on acetonitrile. 

The carbon-nitrogen triple bond of aryl thiocyanates acts as a dipolarophile in 1,3-dipolar cycloadditions. Reactions with nitrile oxides yield 5-arylthio-1,2,4-oxadiazoles 227 (X = O Y = S). Aryl selenocyanates behave similarly forming 5-arylseleno-l,2,4-oxadiazoles 227 (X = 0 Y = Se). Reactions of 5-aryl-... 

Nitriles absorb with variable strength in the infrared in the region 2000 cm-1 to 2300 cm-1, due to stretching vibrations of the carbon-nitrogen triple bond. 

The nitrile group in cyanopyridine can participate in intramolecular reactions. The presence in chalcone 118 of an ortho-hydroxyl substituent can lead to its addition to a carbon-nitrogen triple bond, with the formation of the 2/7-pyran-2-iminic moiety (compound 119) [126, 127] (Scheme 3.36). 

These reactions are mentioned here because of the apparent similarity between the carbon-carbon and the carbon-nitrogen triple bonds, but because the reactive excited state is that of the arene, they do not fall in the category of alkynes and maleimides treated in this section. A somewhat more detailed discussion on photoaddition of nitriles to the benzene ring can be found in Section VI. 

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. 

RC=N Cl i3ch2ch2c=n Nitrile (The functional group is the carbon-nitrogen triple bond.)... 

The highlighted bond joins a nitrile carbon atom to a methyl carbon atom. The nitrile carbon atom, being joined to a nitrogen atom via a carbon-nitrogen triple bond, can only be joined to one additional atom. Therefore, this atom is sp hybridized. However, the methyl carbon is joined to the nitrile carbon and three hydrogen atoms. Therefore, because the methyl carbon is bound to four separate atoms, this carbon is sp3 hybridized. Based on the atomic hybridizations, the nitrile carbon is connected to its bound atoms in a linear geometry, and the methyl carbon is connected to its bound atoms in a tetrahedral geometry. 

Reactions of organomagnesium and organolithium compounds with nitriles normally proceed via initial addition to the carbon-nitrogen triple bond or via initial a-deprotonation ... 

Reactions with other carbon triple bonded functional groups. The substitution of nitrile for alkynes does not lead to pyridines or quinolines in the benzannulation reaction.Instead noncyclic products are obtained that are the result of insertion of the carbon-nitrogen triple bond into the metal-carbene bond. On the other hand, in a very recent report it was found that X -phosphaalkynes will undergo the benzannulation reaction to produce phosphaarene chromium tricafbonyl complexes. 

Such compounds are highly reactive, even more so than the carbon-nitrogen triple bond characteristic of nitriles. 

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