Acetylenes cyclic alkynes

Hydrocarbons that contain a carbon-carbon triple bond are called alkynes Non cyclic alkynes have the molecular formula C H2 -2 Acetylene (HC=CH) is the simplest alkyne We call compounds that have their triple bond at the end of a carbon chain (RC=CH) monosubstituted or terminal alkynes Disubstituted alkynes (RC=CR ) have internal triple bonds You will see m this chapter that a carbon-carbon triple bond is a functional group reacting with many of the same reagents that react with the double bonds of alkenes... 

Cyclic alkynes (C9, Ci0, or Cn) also rearrange in the presence of bases to an equilibrium mixture containing cyclic allene [43]. The bases used were NaNH2-liq. NH3 at -33.4°C, KOH-C2H5OH at 131°-134°C (sealed tubes), KO-f-Bu-f-BuOH at 79.4°-l 20.0°C (sealed tubes) [44]. A solution of sodamide in liquid ammonia gave the most rapid ( to 3 hr) allene-acetylene interconversions of all systems examined. 

Coupling by copper(II) acetate in pyridine is also a useful procedure for active alkynes such as phenyl-acetylene and conjugated polyynes. This method is also a useful procedure for making cyclic alkynes. 

In linear acetylenes the two n-MOs of the triple bonds are degenerate. This degeneracy is lifted when the triple bond is incorporated in a ring system. Thus, in cyclic alkynes, three different sides of the triple bond, which correspond to three different modes of reactivity, can be distinguished (see Fig. 8-1) ... 

The Fritsch-Buttenberg-Wiechell rearrangement provides a method to expand a cyclic ketone to a cyclic acetylene with an additional carbon atom in the ring [34]. The ketone is first converted to a vinyl bromide or a vinylidene dibromide, from which the corresponding vinylidene carbene is produced with butyllithium. The vinylidene carbene then rearranges to a cyclic alkyne (Scheme 8-10). 

Since an allenic system is built of only three collinear carbon atoms, whereas an acetylenic unit requires two bond angles of 180°, it is expected that the allene isomers of cyclic alkynes will become increasingly more stable than their acetylenic analogs with decreasing ring size. This has been demonstrated for the ring sizes 9-11 by Moore and Ward [61] (Table 8-5). In their experiments, the equilibrium compositions of cyclic allene/cyclic acetylene mixtures were... 

The triple bonds in cyclic alkynes can, of course, be subjected to all known addition reactions of acetylenes. Here, we will discuss examples which either lead to particularly interesting addition products or demonstrate unusual reactivity of bent triple bonds. 

Hong FE, Chang H et al (2001) Preparation and characterization of a cyclic compound consisting of bis(diphenylphosphino)acetylene joined alkyne-bridged dicobalt fragments. Chem Lett 30 1130-1131... 

The above cycloaddition process consists of two separate [3-1-2] cycloaddition steps and represents a 1,3-2,4 addition of a multiple bond system to a hetero-1,3-diene [7S7]. The structure ot the azomethine imine intermediate has been proved unequivocally by X-ray analysis [195] Ethylene [194], acetylene [/iS2] . many alkyl- and aryl- as well sgemmal dialkyl- and diaryl-substituted alkenes [196,197, 198, 199], dienes [200], and alkynes [182, 201], certain cyclic alkenes [198, 199,... 

The Pauson-Khand reaction was originally developed using strained cyclic alkenes, and gives good yields with such substrates. Alkenes with sterically demanding substituents and acyclic as well as unstrained cyclic alkenes often are less suitable substrates. An exception to this is ethylene, which reacts well. Acetylene as well as simple terminal alkynes and aryl acetylenes can be used as triple-bond component. 

Similar reactions have been carried out on acetylene. Aldehydes add to alkynes in the presence of a rhodium catalyst to give conjugated ketones. In a cyclic version of the addition of aldehydes, 4-pentenal was converted to cyclopen-tanone with a rhodium-complex catalyst. In the presence of a palladium catalyst, a tosylamide group added to an alkene unit to generate A-tosylpyrrolidine derivatives. ... 

The intramolecular addition of the O-H bond to alkynes catalyzed by palladium complexes has been developed by K. Utimoto et al. (Eq. 6.59) [104]. An alkynyl alcohol can be converted to a cyclic alkenyl ether in the presence of a catalytic amount of [PdCl2(PhCN)2 or [PdCl2(MeCN)2] in ether or THE at room temperature. When the reaction was carried out in MeCN-H20 under reflux in the presence of a catalytic amount of PdCl2, hydration of the acetylenic alcohol occurred and the ketoalcohol was obtained in good yield instead. 

Other Methods of Synthesis Intramolecular cyclic hydroxyamino-alkynes (188) and (189), depending on the relative position between the hydroxy-lamino group and acetylenic fragment, leads to five-(190), six-(191), and seven-(192) membered cyclic nitrones (Scheme 2.67) (348-350). 

Oligomerization of alkynes can lead to linear enyne dimers or afford cyclic oligomers. Dimerization of terminal acetylenes induced by Ru complexes yields stereoisomeric conjugated head-to-head enynes depending on the substituents 532... 

Silylene extrusion from siliranes in the presence of alkynes, notably bis(trimethyl-siiyl)acetylene, gives the silirene (35) in good yield (Scheme 41) (76JA6382). Compound (35) is more stable thermally than hexamethylsilirane and shows 2 Si NMR absorptions for the ring atom at 5 = 106.2 p.p.m., some 50 p.p.m. downfieid from those of silacyclopropanes, and about 100 p.p.m. downfieid from normal cyclic and acyclic tetraalkylsilanes. Notable reactions include alcoholysis and the insertion of aldehydes and ketones, dimethylsilylene... 

Under high concentration of alkynes, the cooligomerization of alkyne and butadiene in 2 2 ratio takes place using the Ni(0)catalyst, coordinated by (PhO)3P, to give the 12-membered cyclic tetraenes 48 [13], Acetylene and butadiene react in 2 1 ratio to give 5-vinyl-l,3-cyclohexadiene (49) using the Ni complex of Bu3P [14]. 

It is possible to depict an RC = CR metal binding by several resonance structures including the metallocyclopropane form, — 111 form, and almost noncontributing form with two M = C bonds. It is noted that alkynes are usually nonlinear when coordinated [184e]. An interesting consequence of the distorted coordination geometry is the fact that small cyclic acetylenes, which are unstable as free molecules, are stabilized by coordination to a metal, for example CpTa(Me)2(benzene) or (cycloheptyne)Pt(PPh3)2 [21b]. 

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