Triple bond Subject

The reaction of disubstituted diacetylenes with hydrazine hydrate was reported by Darbinyan et al. (70AKZ640). In the first stage the addition of hydrazine to the terminal carbon atom of the diacetylene system is analogous to that of primary amines to diacetylene (69ZC108 69ZC110). With monosubstituted diacetylenes (R = H), hydrazine adds to the terminal triple bond. This leads to the formation of vinylacetylenic hydrazine 22 which cyclizes to dihydropyrazole 23 subjected to further isomerization to the pyrazole 25. It is possible that hydrazine 22 undergoes hydration to the ketone 24 which can easily be cyclized to the pyrazole 25... 

One of the most important publications on this subject, considered frorn the chemical side, is that of Gertrude Woker. This investigator drew attention to the importance of multiple bonding. The double bond is often accompanied by a pleasant, but the triple or acetylenic linkage generally produces a disagreeable smell a multiplicity of double bond can produce an effect equivalent to a triple bond. 

The most comprehensive modern works on the subject are the relevant volumes of Patai s series The Chemistry of Functional Groups, namely the two volumes on diazonium and diazo groups (Patai, 1978), the two volumes on hydrazo, azo, and azoxy groups (Patai, 1975) and the two Supplement C volumes on triple-bonded groups (Patai and Rappoport, 1983). Supplement C contains chapters on arene- and alkene-diazonium ions and on dediazoniation reactions. 

Over the last decade, the chemistry of the carbon-carbon triple bond has experienced a vigorous resurgence [1]. Whereas construction of alkyne-con-taining systems had previously been a laborious process, the advent of new synthetic methodology based on organotransition metal complexes has revolutionized the field [2]. Specifically, palladium-catalyzed cross-coupling reactions between alkyne sp-carbon atoms and sp -carbon atoms of arenes and alkenes have allowed for rapid assembly of relatively complex structures [3]. In particular, the preparation of alkyne-rich macrocycles, the subject of this report, has benefited enormously from these recent advances. For the purpose of this review, we Emit the discussion to cychc systems which contain benzene and acetylene moieties only, henceforth referred to as phenylacetylene and phenyldiacetylene macrocycles (PAMs and PDMs, respectively). Not only have a wide... 

The degradation of alkynes has been the subject of sporadic interest during many years, and the pathway has been clearly delineated. It is quite distinct from those used for alkanes and alkenes, and is a reflection of the enhanced nucleophilic character of the alkyne C C bond. The initial step is hydration of the triple bond followed by ketonization of the initially formed enol. This reaction operates during the degradation of acetylene itself (de Bont and Peck 1980), acetylene carboxylic acids (Yamada and Jakoby 1959), and more complex alkynes (Figure 7.18) (Van den Tweel and de Bont 1985). It is also appropriate to note that the degradation of acetylene by anaerobic bacteria proceeds by the same pathway (Schink 1985b). 

A complexation-initiated reaction was realized for the first time as depicted below. Thus, the octacarbonyldicobalt complex of furan is subjected to silica gel and gives rise to the adduct with a seven-membered ring owing to bending of the triple bond to a structure with an angle of around 140° when the alkyne is allowed to react with Co2(CO)8 at room temperature . 

Since the first step of all of these reactions is dinitrogen coordination to either the surface of the catalyst or transition metal center of the complex, let us briefly discuss the nature and importance of the M-N2 interaction, and the possible coordination modes ofN2 to transition metal centers. These issues were the subjects of many discussions in the literature [10, 11] and it is commonly agreed that the interaction of the N2 molecule with transition metal centers facilitates the activation of the N=N triple bond the stronger the M-N2 interaction, the easier to break the N=N triple bond. 

One further characteristic unique to carbon is important and needs to be covered before leaving the subject of valences bonds. A few paragraphs ago, you saw that carbon could link up to itself and three other atoms. In fact, carbon can also link up to itself with double bonds or triple bonds to satisfy its valence requirements of four. For example, in Figure 1—3. two carbon atoms are linked together with single, double, or triple bonds filled out with hydrogens, forming three different compounds ethane, ethylene, and ethyne, or as it s more commonly known, acetylene. 

We also discovered the ability of 2-azadienes of this sort to cycloadd to unactivated carbon—carbon double and triple bonds in an intramolecular fashion (89CC267) (Scheme 50) such a process appears to be one of the first examples of intramolecular [4 + 2] cycloadditions of simple 2-azadienes. Azadiene 216 was made from O-allyl salicylaldehyde 215 (R = allyl) and heated at 120°C in toluene to furnish the trans-fused tricyclic adduct 217 in excellent yield further dehydrogenation of 217 with DDQ afforded 5H-[ 1 ]-benzopyran[4,3-6]pyridine 218. On the other hand, when 0-(2-butynyl) salicylaldehyde 215 (R = 2-butynyl) was transformed into azadiene 219 and subjected to heating in a sealed tube at 150°C, pyridine 222 was isolated in very high yield. Its formation can be rationalized to occur via the expected Diels-Alder intermediate 220 thus, [1,5]-H shift in 220 would give rise to tautomer 221, which would suffer electro-cyclic ring-opening and aromatization to pyridine derivative 222. 

The transition metal catalysed formation of five membered heterocycles through the insertion of a triple bond has also been explored. o-Halophenyl-alkynylamines, propargylamines and propargyl-ethers have been subjected to ring closure reactions. These processes, however also require the presence of a second, anionic reagent, which converts the palladium complex formed in the insertion step to the product. 

It seems obvious that electron-withdrawing groups enhance nucleophilic addition and inhibit electrophilic addition because they lower the electron density of the double bond. This is probably true, and yet similar reasoning does not always apply to a comparison between double and triple bonds.70 There is a higher concentration of electrons between the carbons of a triple bond than in a double bond, and yet triple bonds are less subject to electrophilic attack and more subject to nucleophilic attack than double bonds.71 This statement is not universally true, but it does hold in most cases. In compounds containing both double and triple bonds (nonconjugated), bromine, an electrophilic reagent, always adds... 

The Os(III) dimers, [Os2X8]2- (X = Cl", Br, or I") each have an Os=Os triple bond and terminal halogens. They have been characterized by X-ray diffraction, IR and UV/Vis spectroscopies, and electrochemistry (232, 520-523). The complexes have also been subjected to SCF-Xa-SW calculations of the energies of their electronic states (521). 

This suggests that the more the contribution from this interaction, the more bare the carbon nuclei [cf. structures (A) and (B) ] and the more the triple bond would be subjected to nucleophilic attack. 

As regards the protecting effect, the complex is stable to Lewis acids. Also, no addition of BH3 occurs. As Co2(CO)6 can not coordinate to alkene bonds, selective protection of the triple bond in enyne 137 is possible, and hydroboration or diimide reduction of the double bond can be carried out without attacking the protected alkyne bond to give 138 and 139 [32], Although diphenylacetylene cannot be subjected to smooth Friedcl Crafts reaction on benzene rings, facile /7-acylation of the protected diphenylacetylene 140 can be carried out to give 141 [33], The deprotection can be effected easily by oxidation of coordinated low-valent Co to Co(III), which has no ability to coordinate to alkynes, with CAN, Fe(III) salts, amine /V-oxidc or iodine. 

The stability of the chromium nitride triple bond makes it possible to subject preformed CrVI nitrides to reductive conditions that produce the corresponding Crv nitride. A recent example of this process was reported in which treatment of 40 with Na/Hg afforded nitride 41. X-ray crystallographic analysis revealed a dimeric complex with M2(p-N)2 core (Eq. (12)) [20]. 

In another example, the hexaacetylene 109 - after deprotection with potassium carbonate in methanol - is subjected to typical Bergman trapping conditions, resulting in the formation of the anthracene derivative 110 [61]. As a third, more complex illustration, the aroma-tization of the triacetylene 111 may be considered. Here, the 1,4-diradical intermediate faces another triple bond as an internal trap, and, after hydrogen transfer from 1,4-cyclohexadiene, the tricyclic allylic alcohol 112 is produced [61]. 

The relevant studies on the subject are summarized in Table 3. In all cases shown, conditions were such as to exclude solvent addition to the triple bond. 

Hydrogenation of the carbon-carbon triple bonds, particularly to the olefinic bonds, has been the subject of numerous investigations since the very early stage of the study on catalytic hydrogenation, not only in terms of its synthetic utility but also with respect to the selectivity of catalytic metals for the semihydrogenation.1-6... 

Nucleophilic additions to carbon-carbon triple bonds are also subject to solvent influence, as shown by the example given in Eq. (5-32) [88],... 

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