Higher cyclic polyenes

In competition experiments, using Re20v as catalyst, CDT is considerably less reactive than CHT, CEP or cyclooeta-1,5-diene (COD) (Saito 1979). It appears that, like cw,cw-cyclodeca-l,6-diene (see Section 12.8), this particular ring is more stable than the others. Yet it can act as a cocatalyst, when used in small quantities with WClfi, for the ROMP of COD (Kupper 1979). 

Ring monomer Catalyst system Solvent / temperature (°C) Reference 

9-Phenyl-1,5-cyclododecadiene undergoes ROMP in the presence of WCU(OC6H3-Cl2-2,6)2/Et4Pb in PhCl at 80°C 98% yield in 2 h (Cho, I. 1995). 

---

Let us now examine tlie properties of the next higher cyclic polyene analog of benzene, 1,3,5,7-cyclooctatetraene, another 4n tt cycle (n = 2). Is it antiaromatic, like 1,3-cyclobutadiene First prepared in 1911 by Willstatter, this substance is now readily available from a special reaction, the nickel-catalyzed cyclotetramerization of ethyne. It is a yellow liquid (b.p. 152 °C) that is stable if kept cold but that polymerizes when heated. It is oxidized by air, eatalytically hydrogenated to cyclooctane, and snbject to electrophilic additions and to cycloaddition reactions. This chemical reactivity is diagnostic of a normal polyene (Section 14-7). 

Cyclic polyenes absorb at higher wavelengths than do acyclic polyenes. 

Note the signs of the coefficients. We can conclude from what was said above that the higher or lower stability of a cyclic polyene as compared to an acyclic one depends on the combination of signs of the coefficients at the ends of the demethylized compound. If the signs are identical, the even AS is aromatic due to cyclic stabilization if the signs are different, the system is anti-aromatic due to cyclic destabilization. Hence, the Hiickel aromaticity... 

According to Hiickel s rule, annulenes with 4n tt electrons are not aromatic. Cyclobutadiene and cyclooctatetraene are [4n]-annulenes, and their properties are more in accord with their classification as cyclic polyenes than as aromatic hydrocarbons. Among higher [4n]-annulenes, [16]-annulene has been prepared. [16]-Annulene is not planar and shows a pattern of alternating short (average 134 pm) and long (average 146 pm) bonds typical of a nonaromatic cyclic polyene. 

Here calculations and experiments indicate that a planar cyclooctatetraene would have higher rr-electron energy than the open-chain octatetraene. Therefore, a planar form of cyclooctatetraene would, if it existed, be antiaromatic. As we saw earlier, cyclooctatetraene is not planar and behaves like a simple cyclic polyene. 

Hoecker and Musch [32] elaborated in more detail the methods of chromatographic and spectrometric analysis of these macrocyclic compounds and studied the influence of the reaction parameters on their formation. Interestingly, through metathesis reaction of some cyclic polyenes such as 1,5-cyclooctadiene and 1,5,9-cyclododecatriene, Scott and Calderon [29] obtained macrocycles with similar structures. It is noteworthy that the higher degree of unsaturation led in this case to a greater fragmentation of the reaction products. 

UnUke cyclobutadiene and cyclooctatetraene, certain higher cyclic conjugated polyenes are aromatic. All of them have two properties in common They contain (4n + 2) tt electrons, and they are sufQciently planar to allow for delocahzation. 

Cyclic polyenes are expected to show similar behavior thus, cyclopentadiene has El I of 9.00 e.v. rather similar to the methyl-butadienes and lower than that of butadiene itself. Cycloheptatriene, however, has El / = 8.55 e.v., somewhat higher than that of hex-atriene even allowing for the difference between UV and El determinations. The value is intermediate between that which would normally be expected for a cycloheptatriene structure ( 7.89 e.v. (47)] and that of toluene, El / = 9.23 e.v. (46), and suggests that tropylidene or tropylidene cation actually has r-bonding between the 2- and 7-positions, VI. 

For an even cyclopolyene (alternating hydrocarbon) the 7r-electron density is equal to unity at each atom. Therefore, from (12), an increase in /r-bond energy always follows the introduction of a heteroatom. But at the same time the cyclic delocalization energy, which is the difference in 7r-bond energy between the cyclic and the corresponding linear polyenes, will be decreased because the maximum atom self-polarizability of a linear polyene chain always has a higher value than that of the corresponding cyclopolyene 12°). 

---