Alkynes triple bond

The electronic structure of benzyne, shown in Figure 16.19, is that of a highly distorted alkyne. Although a typical alkyne triple bond uses sp-hybridized carbon atoms, the benzyne triple bond uses sp2-hybridized carbons. Furthermore, a typical alkyne triple bond has two mutually perpendicular it bonds formed bv p-p overlap, but the benzyne triple bond has one tt bond formed by p-p overlap and one tt bond formed by sp2 sp2 overlap. The latter tt bond is in the plane of the ring and is very weak. 

After alkylation, the alkyne triple bond can be used in other reactions ... 

Allylzirconation of alkynes with allylzirconocene chloride reagents (obtained by hydrozir-conation of allenes) takes place in the presence of a catalytic amount of methylaluminox-ane (MAO) [67,68]. MAO presumably abstracts chloride to form an allylzirconocene cation, which coordinates to the alkyne triple bond. The subsequent migratory insertion is regioselective, as it is found that the new bond is mainly formed between the a-carbon of the allylzirconium species and the internal carbon of a terminal alkyne (Scheme 8.33). 

The polymerization of the alkyne triple bond (Secs. 5-7d and 8-6c) and ring-opening metathesis polymerization of a cycloalkene (Secs. 7-8 and 8-6a) yield polymers containing double bonds in the polymer chain. Cis-trans isomerism is possible analogous to the 1,4-polymer-ization of 1,3-dienes. 

Alkyne metathesis is a curious reaction in view of the fact that two alkyne triple bonds are cleaved and reconstructed simultaneously leading to different triple bonds. The first reported effective catalyst is a heterogeneous mixture of tungsten oxide and silica. Then Mortreux found that a catalytic system that consisted of Mo(CO)6 and resorcinol was effective for alkyne metathesis. As reported, the added alkynes come into equilibrium with different product... 

Oligocyclizations Succeeding Inter- and Intramolecular Carbopaliadations of Alkyne Triple Bonds... 

Treatment with HC1 converts cw-[PdCl2(Ph2PCsCCF3)2] into trans-[PdCl2 Ph2PCH=C(a)CF3 2]. An X-ray examination of this complex shows that the HC1 adds trans across the alkynic triple bond.104... 

Ans. (a) alcohol (hydroxyl) (b) amine (tertiary amine) (c) ester (d) chloride (e) ketone (f) carboxylic acid (g) ester (triglyceride) (h) amide (i) ether (/ ) aldehyde (k) bromide (dibromide) (Z) alkyne (triple bond)... 

Many of the reactions of alkynes are similar to the corresponding reactions of alkenes because both involve pi bonds between two carbon atoms. Like the pi bond of an alkene, the pi bonds of an alkyne are electron-rich, and they readily undergo addition reactions. Table 9-4 shows how the energy differences between the kinds of carbon-carbon bonds can be used to estimate how much energy it takes to break a particular bond. The bond energy of the alkyne triple bond is only about 226 kJ (54 kcal) more than the bond energy of an alkene double bond. This is the energy needed to break one of the pi bonds of an alkyne. 

Nitrile triple bond stretching absorptions are at slightly higher frequencies (and usually more intense) than those of alkyne triple bonds. Compare this spectrum of butyronitrile with that of oct-l-yne in Figure 12-8. 

Alkyne triple bonds result from the overlap of two sp-hybridized carbon atoms. 

Carbon is tetravalent (forming four bonds) and can form single bonds, double bonds, and triple bonds. As seen in Table 12.1, the four types of hydrocarbons are alkanes (single bonds), alkenes (double bonds), alkynes (triple bonds), and aromatic. Aromatics are unsaturated hydrocarbons that have cyclic structures. A common and representative compound for aromatic is benzene. 

Highly strained cyclic compounds containing an alkyne group can be prepared upon complexation to Co2(CO)6. The success of these reactions relies on the severely bent nature of the alkyne triple bond when complexed to cobalt. Cyclization of (137) to form bicyclo[7.3.0]tridecane (138), a calichiamycinone derivative has been reported (Scheme 204). The nine-membered enyne complex (140) was prepared by ring-closing metathesis of (139) (Scheme 205). A related reaction was used to give complex (141) (Scheme 206). It should be noted that no cyclization occurs without prior complexation to cobalt in the latter case. A complexation-initiated... 

Perhaps another litmus test of the ability of the alkynic C—H group to donate a proton in a H-bond arises when a molecule of this type is paired with a hydrogen halide, HX. One then has two distinct possibilities. The X atom, although a weak proton acceptor by nature, can form a complex of the C—H—XH type. An alternative would have the XH acting as the proton donor, with the electron-rich alkyne triple bond acting as the acceptor. Experimental measurements indicate the latter is the more stable of the two alternatives. Indeed, a similar sort of geometry is adopted when HE approaches the n system of ethylene , even though the electron source in this double bond is less rich than in the triple bond of an alkyne. 

New, Unsaturated Three- and Four-Membered Rings Formal Addition of CH2, SiH2, GeH2, or SnH2 to Phospha-alkyne Triple Bonds. ... 

Trimethylsilyl groups at triple bonds are easily removed by mild bases such as alkali metal carbonates to yield 35 with a terminal triple bond. The following reaction is a Pd-catalyzed reductive addition of tributyl stannane, HSnBu3, to the triple bond, which forms the -configured vinyl stannane 19. One possible rationalization of the outcome is the mechanism shown below First, the catalytically active species 36 inserts into the tin-hydrogen bond to form 37. Then, cis addition takes place after coordination to the alkyne triple bond, generating species 39. Reductive elimination affords the vinyl stannane 19 and regenerates the catalyst 36. ... 

Unsaturated hydrocarbons that contain one or more triple bonds between carbon atoms in a chain are called alkynes. Triple bonds involve the sharing of three pairs of electrons. The simplest and most commonly used alkyne is ethyne (C2H2), which is widely known by its common name acetylene. Study the models of ethyne in the following diagram. 

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