Cyclooctatrienes

These selectivity issues were solved with the intramolecular version using temporary silicon-tethered partners of the cycloaddition [52], 

A-lsopropylpyridine-imine Co(I) complexes were efficient catalysts to promote the intermolecular [4+2+2] cycloaddition, which combines two alkynes with butadiene to generate cyclooctatrienes in a highly regioselective manner. The addition of iron powder was found to reduce by-product formation and increase the yield 

This catalytic system tolerates terminal alkynes bearing various functionalities such as ketone, sulfone, ester, ketal, ether, alcohol, imide, or nitrile, which are useful for further transformations. The reaction with ethynyltrimethylsilane allowed the formation of the expected [6-1-2] cycloadduct in 92% yield. Such a result could not be obtained with the use of [(CHT)Co(CO)3] as catalyst [41]. Later, Hilt expanded the scope of the [6-t-2] cycloaddition to internal alkynes with modified pre-catalysts such as CoBr2[P(0/Pr)3]2 [55]. 

The absolute configuration of the cycloadducts was established with vibrational circular dichroism (VCD) studies. 

General Procedure for the Enantioselective Co(I)-Catalyzed [6+2] Cycloadditions In a 10-mL, flame-dried Schlenck, phosphoramidite (0.10 equiv) was added to a solution of C0I2 (13 mg, 0.042 mmol, 5 mol%) in 1,2-dichloethane (1 mL) and stirred for 10 min. Then, powdered zinc (8.3 mg, 0.127mmol, 0.15 equiv), a solution of 1,3,5-cycloheptatriene (93 mg, Immol, 1.20 equiv) in 1,2-dichloroethane (ImL), a solution of acetylenic compound (0.85 mmol, 1 equiv) in 1,2-dichoroethane (1 mL), and zinc iodide (27 mg, 0.085 mmol, 0.1 equiv) were added. The resulting mixture was heated at 40 C for 20 h. After cooling to room temperature, the reaction 

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The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

Is cyclooctatetraene aromatic To tell, compare the first and second hydrogenation energies, leading to 1,3,5-cyclooctatriene and then to 1,3-cyclooctadiene. (The energy for hydrogen is provided at left.)... 

How might you use the Hofmann elimination to accomplish this reaction How would you finish the synthesis by converting cyclooctatriene into cyclooctatetraene ... 

Finally, an electrochemical reduction of bis-a-bromobenzyl sulfone to stilbene195 and a spectacular, so-called bis-homoconjugative, version of the Ramberg-Backlund reaction, which converts the a-chlorosulfone 100 into the bridged cyclooctatriene derivative 101 (equation 63)196-197, have also been published. 

Cyclooctatetraen kann elektrochemisch zum Cyclooctatetraenyl-Dianion reduziert werden. In waBrigem Alkohol bzw. DMF bildet sich mit bis zu 92% Umsatz (bei -2,1 V) hauptsachlich Cyclooctatrien-(l,3,5). Dagegen kann bei niedrigeren Kathodenpotentialen (auBer in reinem DMF) der Anteil des isomeren Cyclooctatriem-(1,3,6) auf bis zu 67% steigen4. 

Similarly, the cyclobutane-fused bicyclic peroxide 19 was prepared by diimide reduction of the corresponding bicyclic endoperoxide derived from 1,3,5-cyclooctatriene (Eq. 14)31a). 

Like the [3.2.2]-peroxide, the parent system 23 could be prepared in two ways, either from 1,3-cyclooctadiene 28 345 or from 1,3,5-cyclooctatriene 31 a> (Eq. 18). 

The diimide reduction again proceeds sluggishly and several recycles are essential to achieve complete conversion. The doubly unsaturated endoperoxide is the major product in the singlet oxygenation of 1,3,5-cyclooctatriene (Eq. 14). 

Finally, an improved synthesis of (rj6-l, 3,5-cyclooctatriene)-(i74-l,5-cyclooctadiene)ruthenium(0) has been reported which utilizes a cleaning bath to hasten the Zn reduction of RuCl3 in the presence of 1,5-cyclooctadiene (240). The use of ultrasound in simple reductions using Zn are a likely area for further development. 

The RuCl2(PPh3)3 complex has now been used to selectively hydrogenate 1,5,9-cyclooctatriene to the monoene (101), dienes to monoenes... 

Relatedly, one would have expected 1,3,5-cyclooctatriene to have a more negative enthalpy of formation than tropilidene by the same —20.6 kJmol-1. By contrast, the difference for these enthalpies of formation of species 86 and 87 as derived from experimentally measured enthalpies of formation is ca +12 kJ mol-1. From this we may deduce that tropilidene enjoys considerable stabilization due to homoaromatic interactions. While this conclusion is not new64, nonetheless we find it encouraging to see it corroborated. 

We suspect fewer problems would have arisen had Oth and coworkers (see Reference 97) decided to perform enthalpy of hydrogenation measurements on [18]annulene. Nonetheless, we note that Oth s suggested value for the enthalpy of formation of benzo-l,3,5-cyclooctatriene is within 2 kJ mol of that estimated summing Roth s enthalpy of formation of 1,3,5-cyclooctatriene and Liebman s (cited in Reference 68) benzoannelation constant. 

The predicted conrotatory cyclization of octatetraenes was confirmed for the case of the methyl-substituted compounds, which above 16 °C readily formed the cyclooctatrienes shown in equations 13 and 14)14. We conclude this section with an electrocyclic reaction involving ten TT-electrons, that is, the formation of azulene (17) when the fulvene 16 is heated (equation 15)15,16. 

Radialene 89, a dimer of 1,2,3-cyclooctatriene derivative 88, was isolated when the [10](9,10)anlhracenophanc-4,6-diyne 87 was exposed to sunlight62. In this case, the intermediate occurrence of 88 could not only be substantiated by isolation of (4 + 2) cycloadducts in the presence of furan or cyclopentadiene, but also by a IJV/Vis spectrum obtained at 77 K in an organic glass. 

Photochemical [2+2]cycloaddition between benzo[b]furan and 3-cyano-2-alkoxy-pyridines in benzene has been reported to follow a very interesting mechanism supported also by Frontier-MO calculations using the PM3 Hamiltonian. It is believed that the singlet excited state of the pyridine and the ground state benzofuran react to form a [2+2] adduct and is followed by ring opening to the cyclooctatriene, which cyclizes to the secondary endo- and exo-isomers shown below <00CC1201>. 

In addition to cycloheptatriene, cyclooctatriene and cyclooctatetraene systems can be used as six-carbon components in the [6 + 2]-cycloaddition with 7t-systems (Scheme 36). Interestingly, in these cases, dienes react exclusively... 

Catalyst [Ru(H)(cyclooctatriene)(ligand)]BF4. SCR Substrate catalyst ratio. 

FIGURE 7 Pericyclic reactions in the biosynthesis of giffordene and 7-methyl-cyclooctatriene. (a) The [1.7]-hydrogen shift of the thermolabile undeca-(1,3Z,5Z,8Z)-tetraene generates undeca-(2Z,4Z,6E,8Z)-tetraene (giffordene), the major product of the brown alga G, mitchellae. (b) The thermolabile nona-(l,3Z,5Z,8E)-tetraene cyclizes at ambient temperature rapidly to 7-methylcy-cloocta-l,3,5-triene. At ambient temperature, the bicyclic isomer does not contribute to the equilibrium (requires s=80°C). 

Although a few other acyclic examples of stereospecific isomerisation of hexatrienes are known, specially in the field of natural product like steroid chemistry, the commonest reactions of this type are in cyclic hexatrienes. Cyclooctatriene and cyclooctatetraene are systems in which the electrocyclic reaction goes very readily and they show an interesting trend. 

The reverse reaction is more favourable for the cyclooctatriene equilibrium. 

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