Oligomerization nitriles

The cobalt catalyzed cocyclization of alkynes with heterofunctional substrates is not limited to nitriles. cpCo-core complexes are capable of co-oligomerizing alkynes with a number of C,C, C,N or C,0 double bonds in a Diels-Alder-type reaction. Chen, in our laboratories, has observed that these cycloadditions are best performed with the help of stabilizers such as ketones or acetic esters that are weakly coordinated to the cobalt and prevent the alkynes from being cyclotrimerized at the metal center... 

Bicyclic derivatives of furazan A-oxide are prepared by nitrile oxide dimerization reaction. Dioxime 272 (R, R = Me) undergoes cyclization to the corresponding 4,4-tetramethylperhydrocycloocta[c]furazan A-oxide 273 (84% yield) by treatment with NaOCl/HaO/CHaCla at 0°C and then refuxing in toluene (equation 117). However, in the cases of sterically less hindered oximes 272 (R = H, Me R = H) only complex mixtures of oligomerization and cyclization products could be obtained ". Interestingly, the reaction of pyridyl oxime -274 with TsCl afforded 1,2,5-oxadiazole 275 as single product (equation 118). On the other hand, the reaction of Z-isomer of oxime 274 leads only to 0-tosylated oxime. ... 

Side reactions that occur with intramolecular cycloaddition, such as linear oligomerization or dimerization of the nitrile oxide, are not very common when shorter chain lengths n < 1) are used due to the entropically favored intramolecular process. A rather unusual result in this regard involves the formation of a fused cyclooctane instead of the less-strained six-membered ring (also fused) in the cycloaddition of the nitrile oxide derived from p-naphthoquinone (Scheme 6.43). This result is consistent with the effect of electron-withdrawal in the enedione part, leading to increased reactivity (247), and also reflects the known sluggishness of cyclohexenes towards nitrile oxides (cf. Section 6.2.1.2). 

In a recent example, Mironov used the rational replacement of starting materials for oligomerization reactions to discover new MCRs in a systematic way [31]. A reaction library of six alkenes/alkynes, two isonitriles, two nitriles and isoquinoline was set up, giving (n1 — n)/2 different reactions products. A minimum peak height of 30% of the total reaction product was used as a criterion for identifying an efficient MCR. In this way, a novel MCR that yields pyrrolo[2,l-a]isoquinolin-1-ones from electron-deficient olefins, isonitriles and isoquinoline was found (Scheme 10.6). 

It is well known that the addition of ammonia or amines to nitriles leads to formation of various amidines 32 this type of oligomerization can be applied to the synthesis of heterocycles. On heating in the presence of sodium amide (or benzenesulfonic acid in some cases), o-cyanoaniline produces a mixture of the quinazoline (18) by dimerization and the tricycloquinazoline (19) by trimerization the trimer is obtained when the dimer is heated with another molecule of the monomer and sodium amide at 300°. The mechanism of this condensation has been reviewed.33... 

In spite of their bulky substituents, the presence of which is necessary to prevent oligomerization reactions, disilenes undergo numerous addition and cycloaddition reactions these have been summarized in several review articles [1]. For example, stable or marginally stable disilenes react with the C=0 and C=S groups of ketones [2-4] and thioketones [5] as well as with the triple bonds of acetylenes [2, 3] and nitriles [6]. Surprisingly, however, no cycloaddition reactions of disilenes with simple alkenes have yet been reported [1]. 

On the other hand, a slow controlled thermal oligomerization of nitrile groups, in air and at moderate temperatures, leads to highly conjugated, crosslinked, oxidized structures which make the fiber non-flammable and infusible. Such fibers are said to be stabilized. They can be further heat-treated in an inert atmosphere to form the carbon fibers (>99% carbon). 

The Chemistry of Stabilization and Carbonization 3.1 Oligomerization of Nitrile Groups... 

One of the most significant steps in the preparation of carbon fibers from acrylic precursor fibers is the oligomerization of the nitrile groups. This reaction has originally been studied in context with the problem of thermal discoloration of PAN (e.g. McCarthney Grassie and McNeill Grassie and Hay It was supposed to lead to a so-called ladder structure ... 

Evidently, the oligomerization of the nitrile groups is the clue to stabilization in PAN and related polymers. What initiates this beneficial reaction ... 

Although AI values may be useful to estimate the progress of the oligomerization of the nitrile groups and the subsequent oxidation and dehydrogenation processes, they cannot be recommended as a criterion for optimum stabilization, unless a careful investigation has been made as to which lA value is actually optimum for a particular precursor. AI values close to unity may easily be obtained by prolonged heat treat-... 

Note that such cis-dienic structures can be present only if intermolecular oligomerization of nitrile groups has taken place.) The Diels-Alder product interrupts conjugation, thus reducing color formation. In the particular case of maleic anhydride, and under the relatively rough conditions of stabilization, this type of reaction should lead to crosslinking, by cross-anhydridization ... 

The y-protons in a,y6-unsaturated nitriles and the a-protons in 6, /-unsaturated nitriles are also acidic, and anionic intermediates may deprotonate such substrate molecules. Thermodynamically these nitriles are more acidic than water (when compared in DMSO) and more acidic than MeCN. This may lead to double-bond isomerizations or oligomerization/polymerization reactions. An example of a base-induced isomerization is the electrolysis of 3-butennitrile in MeCN/HoO (Et4NOTs). The 3-butennitrile is converted into 8a and the LHD of 8a found as the main product [58]. Oligomers formed via substrate deprotonation contain a double bond, since oligomerization is initiated by... 

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