C-CN bonds

Although a C—CN bond is normally strong, one or two cyano groups in TCNE can be replaced easily, about as easily as the one in an acyl cyanide. The replacing group can be hydroxyl, alkoxyl, amino, or a nucleophilic aryl group. Thus hydrolysis of TCNE under neutral or mildly acidic conditions leads to tricyanoethenol [27062-39-17, a strong acid isolated only in the form of salts (18). 

Our groups developed a catalytic C-CN bond cleavage of organonitriles catalyzed by the Fe complex (Scheme 49) [163, 164]. In this reaction, an organonitrile R-CN and EtsSiH are converted into EtsSiCN as a result of the C-CN bond cleavage and the Si-CN bond formation, and the R-H product. This is the first example of the catalytic C-CN bond cleavage of acetonitrile. 

Structural parameters, such as the fairly short C—NO, C—NO2 and C—CN bond lengths, together with the planarity, indicate the presence of delocalized jr-bonds over the whole anion. MO and NBO calculations displayed the existence of an w 7r-electron, m-center bond unit in all methanides (Scheme 20, with n = 2 + 4x + 2y+2z and m = 1 + 3x + 2y - -2z X = number of NO2 groups, y = number of NO groups and z = number of CN groups in the anion plane). 

PNIPAM containing pendant triphenylmethane leuconitrile groups also showed photostimulated phase transition from the phase separated state to the homogeneous state [13]. The triphenylmethane leuconitrile group is known to change the polarity more pronouncedly than the azobenzene chromophore by the ionic photodissociation of C-CN bond [14]. 

The quantum yields for a number of triarylmethane leuconitriles have been reported to be close to unity (44,45,47-49). Therefore, internal conversion of the electronic excited state energy into the vibrational mode of the C—CN bond must take place with almost 100% efficiency. 

Interestingly, the crystal structures of 8-methoxy-l-naphtonitrile and 8-nitro-1-naphtonitrile have been determined by X-ray analysis by Procter, Britton, and Dunitz (24). The structure of the methoxy derivative corresponds to 61 where the exocyclic C —0 bond is bent inward (toward the nitrile group), the exocyclic C-CN bond is bent outward (away from the methoxy group). The C-C = N bond angle is 174° instead of 180°. A similar observation has been made with 8-nitro-l-naphtonitrile. Crystals of this compound contain two symmetry independent molecules which differ in structure. Both show a bent C CN bond and a short 0—C = N distance Icf. 62), but the orientation of the nitro group is different with the result that in one molecule the 0i—Cii distance is 2.69 A whereas in the other, it is 2.79 A. This analysis is in complete agreement with the theoretical calculations and the experimental results presented above. Thus, it can be concluded that the nucleophilic addition on triple-bond (and the reverse process) is strongly influenced by stereoelectronic effects which favor the anti mode of addition. 

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. 

An interesting square molecule with mixed bridges was obtained as the result of oxidative C-CN bond cleavage in benzylnitrile (172). The dinuclear cation [Cu°2(PD 0)(OOH)] + contains the active hydroperoxo group that attacks... 

The active nickel catalyst contains one bidentate phos-phinite ligand and the overall mechanism of the reaction is believed to be similar to butadiene hydrocyanation except that the final reductive elimination step is irreversible under the conditions of the reaction. jr-Allyl intermediates (7) are believed to play an important role in the exclusive formation of the branched nitrile product observed. Formation of the C-CN bond in the final reductive ehmination from the r-allyl intermediate occurs at C(2) and not C(4), because the aromaticity of the naphthalene ring is preserved only when the bond forms with C(2). A a-alkyl complex see a-Bond) with the Ni bound to C(l), which could give the linear (anti-Markovnikov) nitrile product, does not contribute because of the much greater stability of intermediate (7), accounting for the high regioselectivity observed. 

The electrochemical cleavage of nitriles can also be performed in anhydrous amine media [146], making possible C-CN bond cleavage in aliphatic nitriles, such as cyclohep-tanecarbonitrile and heptanecarbonitrile. This reaction was interpreted as due to reductive cleavage by solvated electrons. This reaction has also been accomplished with electrogenerated solvated electrons [147]. 

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]. 

The a-C—C bonds adjacent to carbonyls, alcohols, and nitriles are often the targets for activation [92]. This section focuses on recent developments of transition metal-catalyzed C—CN bond cleavage. 

Such structural studies can also be useful to explain the reactivity of quinolizidine systems. For example, molecular mechanics calculations showed that the more stable conformation of quinolizidine-9-carbonitrile and its indolizidine analogue was the antiperiplanar structure (18), which explained the isolation of (20) on treatment with lithium aluminum through cleavage of the C—CN bond and subsequent reduction of the iminium ion (19) (Scheme 1). On the other hand, the equivalent pyrrolizine derivative, which has a cis orientation of the lone pair and the cyano group, is reduced to amine <87JHC47>. 

Disconnection of a C—CN bond is particularly attractive when the CN unit is attached to a primary or a secondary carbon, which reflects the relative ease of formation of this bond by the Sn2 reaction and the variety of functional group transformations available. The disconnection for nitriles can be simplified as ... 

Upon photolysis of CVCN in the presence of 150-fold excess of ethanethiol, MGCN is produced in 20% yield (only 1% of CVH was isolated). This may be the result of the radical substitution reaction, which implies that the C-CN bond homolysis takes place with (j) > 0.02. Because the intervention of the homolytic bond cleavage has not been seen elsewhere in our study, we assume that it is a singlet state reaction and that the recombination of the singlet radical pair within the solvent cage is rapid (Scheme 1.13) [66, 30]. 

Among possible alternatives to DPMR we discarded the cx.cx-elimination of benzidine 12 from biradical-zwitterions 2 and 3 and formation of carbene 23 based on the absence of trapped carbene products in experiments with cyclohexene and ethanethiol (Xgxc = 300 nm). However, norcaradiene 6 produces carbene 23 when subjected to 350-nm irradiation. Another plausible alternative to DPMR involves initial C-CN bond homolysis, which is energetically favorable in both excited states, followed by a photochemical reaction of Crystal Violet radical. We discarded this mechanism because none of the expected products of the photochemical reactions of Crystal Violet radical has been detected during the course of our work. Nevertheless, isolation of MGCN in the trapping experiment with ethanethiol points to the possible involvement of the C-CN bond homolysis in S. Finally, although norcaradiene, cycloheptatriene, and norbomadiene are valence tautomers and are easily interconverted into each other [188], products of the latter two kinds have been neither detected nor isolated in our study [66, 30]. 

Controlled potential electrolysis at a Hg pool cathode in the absence of proton donors at potentials corresponding to the first electron transfer consumed le/molecule, yielding the radical anion with no C—CN bond breaking. When proton donors were added under otherwise similar conditions, 1,2-dicyanobenzene gave benzonitrile and cyanide ion (2e/... 

---