Triple bonds in rings

A similar problem with angles exists for the cycloalkenes, but it is less severe. In practice, the smallest ring in which an alkyne is stable under normal conditions is cyclooctyne. Thus, it and larger cycloalkynes (cyclononyne, etc.) are known, but the smaller cycloalkynes are either unknown or have been observed only as fleeting intermediates (Fig. 3.62). 

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Preuent formation of triple bonds in rings <= 7 members... 

The bonds indicated can be single or double Prevents disconnection of core bonds Prevents formation of triple bonds in rings with less than 8 members... 

For a review of strained double bonds, see Zefirov Sokolov Russ. Chem. Rev. 1967, 36, 87-100. For a review of double and triple bonds in rings, see Johnson Mol. Struct. Energ. 1986, 3. 85-140. 

For reviews of triple bonds in rings, see Meier Adv. Strain Org. Chem. 1991, 1. 215-272 Krebs Wilke Top. Curr. Chem. 1983,109, 189-233 Nakagawa in Patai The Chemistry of the C—C Triple Bond. pt. 2 Wiley New York, 1978, pp. 635-712 Krebs, in Viehe Acetylenes-, Marcel Dekker New York. 1969. pp. 987-1062. For a list of strained cycloalkynes that also have double bonds, see Meier Hanold Molz Bissinger Kolshorn Zountsas Tetrahedron 1986, 42. 1711. 

UNS At least one double or triple bond in ring, or in the case of CYC, in a carbon chain attached to the ring. 

Palladium catalyzed cycloisomerizations of 6-cn-l-ynes lead most readily to five-membered rings. Palladium binds exclusively to terminal C = C triple bonds in the presence of internal ones and induces cyclizations with high chemoselectivity. Synthetically useful bis-exocyclic 1,3-dienes have been obtained in high yields, which can, for example, be applied in Diels-Alder reactions (B.M. Trost, 1989). 

Apparently only one hetaryne containing oxygen as a hetero atom, 2,3-dehydrocoumarone (98), has been described thus far. It was formulated by Stoermer and Kahlert as an intermediate containing a triple bond in its hetero ring system as eaHy as 1902 and was probably the first aryne mentioned in the hterature of organic... 

Intramolecular addition of the amide group to the triple bond in pyrazoles is more difficult, and results in closure of the 5-lactam rather than the y-lactam ring. The reaction time of the 4-phenylethynylpyrazole-3-carboxylic acid amide under the same conditions is extended to 42 h (Scheme 129) (Table XXVII). The cyclization of l-methyl-4-phenylethynyl-l//-pyrazole-3-carboxylic acid amide, in which the acetylene substituent is located in the 7r-electron-rich position of the heterocycle, is the only one complete after 107 h (Scheme 130) (90IZV2089). 

Cycloalkynes. These structures have one triple bond in the carbon ring, as shown in the following example ... 

Benzyne (Section 16.8) An unstable compound having a triple bond in a benzene ring. 

The application of these methods to unsaturated hydrocarbons involves certain complications. Unsaturated hydrocarbons show an additional polarizability19 of 0.58 x 10 24 cm3 per double bond and 0.86 x 10 24 cm3 per triple bond in the molecule. Similarly the polarizability of a molecule containing a benzene ring exceeds that computed for the atoms present by about 1.28 x 10 24 cm3. These results are most readily explained on the basis that oscillations of charge from atom to atom are significant when double bonds are present. 

The smallest unstrained cyclic triple bond is found in cyclononyne. Cyclooctyne has been isolated," but its heat of hydrogenation shows that it is considerably strained. There have been a few compounds isolated with triple bonds in seven-membered rings. 3,3,7,7-Tetramethylcycloheptyne (lOS) dimerizes within 1 h at room temperature,but the thia derivative 106, in which the C—S bonds are longer than the corresponding C—C bonds in 105, is indefinitely stable even at 140°C. ° Cycloheptyne itself has not been isolated, though its transient existence... 

Recrystallization of 76 (R=H) from CH2CI2 provided crystals adequate for X-ray structural determination. The molecule was found to be saddle shaped with a phenyl ring at each vertex and nadir. The alkynyl bonds were found to be essentially linear and to possess a mean length of 1.194 A, typical for the length of triple bonds in free butadiyne. Although 76 is a dehydrobenzoannulene possessing a 4n TT-electron circuit, the nonplanarity of the macrocycle alleviated much of the strain associated with a flat structure and thus precluded the possibility of anti-aromatic ring currents. 

It is proposed that the electrophilic iodo ion first reacts with one triple bond in 1-126 to give the relative stable vinyl cation 1-128. Ring closure leads to a seven-mem-bered ring 1-129 containing another vinyl cation moiety, which then cydizes to produce the final tricycle 1-127. 

An aromatic ring and a double or triple bond in the a-position relative to the C—H bond weaken this bond by virtue of the delocalization of the unpaired electron in its interaction with the iT-bond. The weakening of the C—H bond is very considerable for example, D(C—H) is 422 kJ mol-1 in ethane [27], 368 kJ mol-1 in the methyl group of propene [27] (AD = 54 kJ mol-1), and 375 kJ mol-1 in the methyl group of toluene [27] (AD = 47 kJ mol-1). Such decrease in the strength of the C—H bond diminishes the enthalpy of the radical abstraction reaction and, hence, its activation energy. This effect is illustrated below for the reactions of the ethylperoxyl radical with hydrocarbons ... 

The investigations on acetylenic compounds in the thiophene series added to the evidence that in most cases thiophene resembles benzene (78). It has been shown in our laboratories that reactions with sodium amide and sodium acetylide in liquid ammonia could be readily utilized in this series. The introduction of a triple bond in the side chain of a thiophene ring was possible only by employing halogenatcd thiophene derivatives such as ... 

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