Cyano-substituent

As a class of compounds, nitriles have broad commercial utility that includes their use as solvents, feedstocks, pharmaceuticals, catalysts, and pesticides. The versatile reactivity of organonitnles arises both from the reactivity of the C=N bond, and from the abiHty of the cyano substituent to activate adjacent bonds, especially C—H bonds. Nitriles can be used to prepare amines, amides, amidines, carboxyHc acids and esters, aldehydes, ketones, large-ring cycHc ketones, imines, heterocycles, orthoesters, and other compounds. Some of the more common transformations involve hydrolysis or alcoholysis to produce amides, acids and esters, and hydrogenation to produce amines, which are intermediates for the production of polyurethanes and polyamides. An extensive review on hydrogenation of nitriles has been recendy pubHshed (10). 

Vinylpyridine (23) came into prominence around 1950 as a component of latex. Butadiene and styrene monomers were used with (23) to make a terpolymer that bonded fabric cords to the mbber matrix of automobile tires (25). More recendy, the abiUty of (23) to act as a Michael acceptor has been exploited in a synthesis of 4-dimethylaminopyridine (DMAP) (24) (26). The sequence consists of a Michael addition of (23) to 4-cyanopyridine (15), replacement of the 4-cyano substituent by dimethylamine (taking advantage of the activation of the cyano group by quatemization of the pyridine ring), and base-cataly2ed dequatemization (retro Michael addition). 4-r)imethyl aminopyri dine is one of the most effective acylation catalysts known (27). 

Azo compounds having functional groups that stabilize the radical products are especially reactive. The stabilizing effect of the cyano substituent is responsible for the easy decomposition of azobis(isobutyronitrile) (AIBN), which is frequently used as a radical initiator. 

The relatively slower rearrangement of alkynyl and cyano substituents can be attributed to the reduced stability of the intermediate derived by cyclization of the triply bonded substituents ... 

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

Stork s elegant use of a protected cyanohydrin function in the synthesis of PGF2a (2) is also noteworthy. The electron-withdrawing cyano substituent in intermediate 21 (Scheme 7) confers nucleophilic potential to C-9 and permits the construction of the saturated cyclopentane nucleus of PGF2a (2) through intramolecular alkylation. In addition, the C-9 cyanohydrin function contained within 40 is stable under the acidic conditions used to accomplish the conversion of 39 to 40 (see Scheme 7), and it thus provides suitable protection for an otherwise labile /J-hydroxy ketone. 

The displacement of halogeno by cyano substituents has not been used frequently in the quinoxaline series of late. Such a reaction is often easy in the 2-and 3-positions but somewhat more difficult in the 5-8-positions an alternative indirect route via a trimethylammonioquinoxaline salt is sometimes preferable (see Section 6.3.2.4). The following examples illustrate the direct route. 

Sodium borohydride is sometimes used in conjunction with CeCl3 (Luche s reagent).70 The active reductants under these conditions are thought to be alkoxyborohydrides. Sodium cyanoborohydride is a useful derivative of sodium borohydride.71 The electron-attracting cyano substituent reduces reactivity and only iminium groups are rapidly reduced by this reagent. 

When phenylacetonitrile is converted to its anion in the presence of excess LDA and then allowed to react with 2-bromo-4-methyl-l-methoxybenzene, the product contains both a benzyl and cyano substituent. Propose a mechanism for this reaction. 

In a similar manner, the diffusion of hexane into dichloromethane solutions containing mixtures of the alkylammonium salts of bromide and the olefinic acceptors o-CA and TCNE result in the formation of brown-red crystals [23]. X-ray analysis reveals the (1 1) complex of bromide with o-CA, in which the anion is located over the center of the C - C bond of the acceptor moiety (Fig. 15b) and Br - C contacts are shortened by as much as 0.6 A relative to the sum of van der Waals radii (Table 3). In bromide complexes with TCNE, the location of the anion relative to the acceptor is variable. In fact, a 2 1 complex [(Br )2,TCNE] is isolated in which both anions reside over the olefinic bond when the tetraethylammonium salt of bromide is used. In comparison, if the tetrapropyl- or tetrabutylammonium salts of the same anion are employed, the (1 1) complexes [Br ,TCNE] are formed in which the bromide donors are shifted toward the cyano substituents (Fig. 15a). In both cases however, the short intermolecular separations that are characteris-... 

Among the more important of these reactions of general synthetic significance is one in which ethene carries a cyano-substituent (acrylonitrile, 91). Attack of Y or Y on the unsubstituted carbon, followed... 

The (benzimidazol-2-yl)pyridine 312, upon treatment with ethyl chloroformate in pyridine, undergoes cyclization to the benzimidazopyridopyrimidine 313. When the pyridine ring carries a cyano substituent, a further cyclization occurs, to give the pentacycle 314 (Scheme 77) <2000JCCS961>. 

Interestingly, for the vinylogous triafulvene 96 the reduction potential is drastically lowered to -0.53 V, whilst the oxidation potential stays in the same range as the above examples (+1.58 V). This effect might well reflect the increase of resonance stabilization for the radical anion 477 contributed by the cyano substituents. 

Consistent with this hypothesis, Smalley and coworkers have found that singlet 2-cyanophenylnitrene (12a) undergoes ring expansion to afford the product formed via cyclization toward the cyano substituent, as well as the product formed via cyclization away from the cyano group.94 Similar results have been found in the ring expansion of singlet o-acetylphenylnitrene.95... 

Because no unpaired spin appears at the meta carbons of lb, a cyano substituent at one of these carbons should have only a small effect on the barrier to cyclization at either of the two, non-equivalent ortho carbons. However, if a para cyano substituent tends to localize spin at the carbon to which it is attached, the concomitant decrease in unpaired spin density at the ortho carbons might raise the barrier to cyclization. 

X. Wu, Y. Liu, and D. Zhu, Synthesis and characterization of a new conjugated polymer containing cyano substituents for light-emitting diodes, J. Mater. Chem., 11 1327-1331, 2001. 

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