Acetylenic compounds, formation

J. Nakayama, R. Yomoda and M. Hoshino, Reactions of elemental sulfur and selenium with some acetylenic compounds. Formation of thiophenes and selenophenes. Heterocycles, 26, 2215-2222 (1987). 

Compounds containing a double or triple bond, usually activated by additional unsaturation (carbonyl, cyano, nitro, phenyl, etc.) In the ap position, add to the I 4-positions of a conjugated (buta-1 3-diene) system with the formation of a ax-membered ring. The ethylenic or acetylenic compound is known as the dieTwphile and the second reactant as the diene the product is the adduct. The addition is generally termed the Diels-Alder reaction or the diene synthesis. The product in the case of an ethylenic dienophile is a cyctohexene and in that of an acetylenic dienophile is a cyctohexa-1 4-diene. The active unsaturated portion of the dienophile, or that of the diene, or those in both, may be involved in rings the adduct is then polycyclic. 

Acetylenic Grignard compounds or the corresponding organoalkali metal derivatives are important intermediates in many syntheses of acetylenic compounds. The various methods for their formation in organic solvents and in liquid ammonia have been discussed extensively and we here give only a brief summary. 

Acetylene compounds Dicobaltoctacarbonyl Formation of colored complexes. After the reagent excess has been washed out, reaction with bromine vapor yields cobalt bromide, which reacts with a-nitroso-P-naphthol to yield red chromatogram zones on an almost colorless background. [11]... 

Besides the weak bonds listed in the previous table, there are other multiple bonds that endow the molecules in which they are situated with a positive enthalpy of formation. Such compounds are termed endothermic compounds. The danger they represent does not necessarily come from the fact that they are unstable, but is related to the exothermicity of their decomposition reaction. The most convincing examples are the acetylenic compounds, and in particular, acetylene. It is also the case for ethylene, aromatic compounds, imines and nitriles. 

If propynol and similar acetylenic compounds are dried with alkali before distillation, the residue may explode (probably owing to acetylenic salt formation). Sodium sulfate is recommended as a suitable desiccant. 

FIGURE 8.20 Peptides activated at an IV-methylamino-acid residue are postulated to epimer-ize because of the formation of the oxazolonium ion. Evidence for the latter resides in spectroscopic studies,96 and the isolation of a substituted pyrrole that was formed when methyl propiolate was added to a solution of Z-Ala-MeLeu-OH in tetrahydrofuran 10 minutes after dicyclohexylcarbodiimide had been added.95 The acetylenic compound effected a 1,3-dipolar cycloaddition reaction (B), with release of carbon dioxide, with the zwitter-ion that was generated (A) by loss of a proton by the oxazolonium ion. 

Acetylenic compounds with replaceable acetylenically bound hydrogen atoms must be kept out of contact with copper, silver, magnesium, mercury or alloys containing them, to avoid formation of explosive metal acetylides. 

Photolysis of 1,2,3,4- and 1,2,4,5-benzenetetracarboxylic dianhydride with different wavelengths invariably results in formation of hexatriyne 125. Attempts to isolate 1,2,5,6- and 2,3,6,7-tetradehydronaphthalenes (naphthdiynes) by photolysis of the corresponding naphthalenetetracarboxylic dianhydrides yield only acetylenic compounds, which suggests that naphthdiynes are formed, but rapidly decompose under photochemical conditions. 

Mo and W hexacarbonyls, Mo(CO)6 and W(CO)6, alone do not induce polymerization of acetylenic compounds. However, UV irradiation toward these catalysts in the presence of halogenated compounds can form active species for polymerization of various substituted acetylenes. Carbon tetrachloride, CCI4, when used as the solvent for the polymerization, plays a very important role for the formation of active species, and thus cannot be replaced by toluene that is often used for metal chloride-based catalysts. Although these metal carbonyl-type catalysts are less active compared to the metal halide-based counterparts, they can provide high MW polymers. It is a great advantage that the metal carbonyl catalysts are very stable under air and thus handling is much easier. 

For many elements, the atomization efficiency (the ratio of the number of atoms to the total number of analyte species, atoms, ions and molecules in the flame) is 1, but for others it is less than 1, even for the nitrous oxide-acetylene flame (for example, it is very low for the lanthanides). Even when atoms have been formed they may be lost by compound formation and ionization. The latter is a particular problem for elements on the left of the Periodic Table (e.g. Na Na + e the ion has a noble gas configuration, is difficult to excite and so is lost analytically). Ionization increases exponentially with increase in temperature, such that it must be considered a problem for the alkali, alkaline earth, and rare earth elements and also some others (e g. Al, Ga, In, Sc, Ti, Tl) in the nitrous oxide-acetylene flame. Thus, we observe some self-suppression of ionization at higher concentrations. For trace analysis, an ionization suppressor or buffer consisting of a large excess of an easily ionizable element (e g. caesium or potassium) is added. The excess caesium ionizes in the flame, suppressing ionization (e g. of sodium) by a simple, mass action effect ... 

Scheme 43 shows the details of the different steps involved in the equilibrium. The nucleophilic attack of the P(III) derivative on the acetylenic bond yields a 1,3-dipole which, after a fast protonation, frees aZ ion. If the subsequent addition of this ion occurs on the P atom (reaction a), a P(V) phosphorane is formed, but the addition of Z on the ethylenic C atom (reaction b) results in the formation of an ylide. Both of these reactions occur under kinetic control and, in both cases, X is always an OR group from the initial acetylene dicarboxylic ester. When the acetylenic compound is a diketone and X is an alkyl or aryl moiety, the C=0 group is much more electrophilic and the attack by the Z ion produces an alcoholate (reaction c), a new intermediate which can cyclize on to the P+ to form a phosphorane, or attack the a-C atom to form an ylide as in Scheme 42. Hence, reactions a and c can coexist, and are strongly dependent on the nature of the trapping reagent and of the P compound, but reaction b is blocked, whatever the reagent. This is well illustrated by the reaction of the 2-methoxytetramethylphospholane 147 on diben-zoylacetylene in the presence of methanol as trapping reagent. The proportions of the vinylphosphorane 157 and spirophosphorane 158 formed (Figure 24) are 13% and 84%, respectively.

Often, the basic group that is responsible for the proton abstraction is also the nucleophilic group in the Michael addition. Thus, most of the suicide inhibitors made so far have been aimed at enzymes that catalyze the formation of carban-ions or carbanion-like intermediates. Suicide inhibitors are typically based on acetylenic compounds (as in equation 9.8), /3, y-unsaturated compounds (as in equation 9.9), or /3-halo compounds (as in equation 9.10). (The a protons in such compounds are acidic because the negative charge in the carbanion is delocalized by the conjugation with X.)... 

In about 2 hours the beads adhere together somewhat then paracet-aldehyde begins to collect at the bottom of the bottle. Water is added, 2—3 c.cs. at a time, at intervals during the formation. The yield is good, and there is practically no escape of acetylene or acetaldehyde from the apparatus. The action consists in the formation of a mercuric sulphate acetylene compound and its subsequent decomposition giving paracet-aldehyde. The passage of acetylene should be continued for about 2 days. The contents of the bottle are finally shaken up with ether, the ethereal solution separated, dried over anhydrous sodium sulphate, and distilled. Paracetaldehyde passes over as a colourless liquid, boiling point 124°. 

The coupling of terminal acetylenic compounds with the reactive allylic bromides in the presence of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and copper(i) iodide appears to be a useful route to the non-conjugated enyne system, e.g. the formation of hex-5-en-2-yn-l-ol.46... 

We have shown without any doubt the formation of carbanions obtained when trivalent phosphorus compounds react with an acetylenic compound. Trapping of these carbanionic species with protic reagents, alcohol for instance,leads to an ylid A. An alternative pathway involves reaction on the phosphorus atom leading to a phosphorane JB. 

One of the first transition metal-catalyzed ring-expansion reactions of SCBs with the formation of new C-C bonds involved the insertion of acetylenes catalyzed by Pd-complexes to furnish silacyclohexenes (Scheme 46) <1975CL891, 1991BCJ1461>. In addition to the acetylene-insertion products (silacyclohexenes), ring-opened allyl-vinylsilane products that also incorporate the acetylene moieties were observed. The ratio of the two types of the products depends heavily on the nature of acetylenic compounds. 

The reaction of a Co(I) nucleophile with an appropriate alkyl donor is used most frequently for the formation of a Co-C bond, which also can be formed readily by addition of a Co(I) complex to an acetylenic compound or an electron-deficient olefin (5). The nu-cleophilicity of Co(I) in Co(I)(BDHC) is expected to be similar to that in the corrinoid complex, as indicated by their redox potentials. The formation of Co-C a-bond is the attractive criterion for vitamin Bi2 models. Sodium hydroborate (NaBH4) was used for the reduction of Co(III)(CN)2(BDHC) in tetrahydrofuran-water (1 1 or 2 1 v/v). The univalent cobalt complex thus obtained, Co(I)(BDHC), was converted readily to an organometallic derivative in which the axial position of cobalt was alkylated on treatment with an alkyl iodide or bromide. As expected for organo-cobalt derivatives, the resulting alkylated complexes were photolabile (17). 

The researches of Zal kind were concerned with the catalytic hydrogenation of acetylenic compounds acetylenic glycols add only two hydrogen atoms over palladium and divinylacetylenes add only six hydrogen atoms, the hydrogenation being limited to the formation of olefins (431). 

The synthesis of 86 in reaction (54) implies that preparation of 87 requires a reagent which adds to the acetylenic compound 126 and then can be substituted with formation of a CH group. It is known from the pure carbon acetylenes that R2A1H adds to the C C bond and that cis-olefines are formed after hydrolysis. This reaction was tried using trimethylsilylphenyl acetylene40. ... 

Similar irradiation of 34 and p-tolylpentamethyldisilane with 1-hexyne or trimethylsilylacetylene in benzene leads to the formation of ( )-olefins in moderate yields (89). In no case are (Z)-isomers detected. The quenching capacity of the acetylenic compounds, however, seems lower than... 

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