Cycloocta-l,3,5,7-tetraene

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

Photoaddition of perfluorobut-2-yne to benzene has been accomplished in the vapor phase. The product is l,2-bis(trifluoromethyl)cycloocta-l,3,5,7-tetraene [71]. In solution, this product undergoes a Diels-Alder reaction with perfluorobut-2-yne. 

Using a relative rate method, rate constants for the gas-phase reactions of O3 with 1- and 3-methylcyclopentene, 1-, 3- and 4-methylcyclohexene, 1-methylcycloheptene, cw-cyclooctene, 1- and 3-methylcyclooctene, cycloocta-1,3- and 1,5-diene, and cyclo-octa-l,3,5,7-tetraene have been measured at 296 2 K and atmospheric pressure. The rate constants obtained (in units of 10-18 cm3 molecule-1 s-1) are as follows 1-methylcyclopentene, 832 24 3-methylcyclopentene, 334 12 1-methylcyclohex-ene, 146 10 3-methylcyclohexene, 55.3 2.6 4-methylcyclohexene, 73.1 3.6 1-methylcycloheptene, 930 24 d.s-cyclooclcnc, 386 23 1-methylcyclooctene, 1420 100 3-methylcyclooctene, 139 9 d.v.d.v-cycloocta-1,3-diene, 20.0 1.4 cycloocta- 1,5-diene, 152 10 and cycloocta-l,3,5,7-tetraene, 2.60 0.19 the indicated errors are two least-squares standard deviations and do not include the uncertainties in the rate constants for the reference alkenes (propene, but-l-ene, d.s-but-2-ene, trans-but-2-ene, 2-methylbut-2-ene, and terpinolene). These rate data were compared with the few available literature data, and the effects of methyl substitution have been discussed.50... 

In Figure 21, attention is directed to a molecule for which identical modules are bound to non-overlapping halves of a ring. Cahn and Dermer [45], name this compound trans-p-(l-4-r 5-8-r -cyclo-octatetraene)- bis-(tricarbonyliron). Godwin [46], on the other hand, recognizes that there is the possibility for connections to only some of the atoms and thus elects to include all relevant locant numbers namely p-(l,2,3,4-r - 5,6,7,8-T -cycloocta-l,3,5,7-tetraene)- bis-(tricarbonyliron). In the nomenclature being developed in this... 

With respect to the carbene, arylcyclopropane formation occurs with rather low stereospecificity. In most cases the ewt/o-isomer predominates, which is in accordance with the general observation made when arylmethylenes from other sources are added to alkenes. Thus, photolysis of phenyldiazomethane dissolved in cyclohexene gave 7-phenylbicyclo[4.1.0]heptane (2) in 34% yield with a endojexo ratio of 1.1. Preference for the e (/o-isomer is also observed when monoarylcarbenes are reacted with cyclohepta-l,3,5-triene, but when 1-naphthylcar-bene, 2-naphthylcarbene and 9-anthrylcarbene are reacted with cycloocta-l,3,5,7-tetraene the exo-isomer predominates. ... 

A solution of 9-diazo-9//-fluorene (1.0 g, 5.2 mmol) in freshly distilled cycloocta-l,3,5,7-tetraene (10 g, 96 mmol) was sealed under Nj in a Pyrex tube and irradiated with a medium-pressure Hg arc for 48 h. After removal of the cycloocta-l,3,5,7-tetraene by vacuum distillation, chromatography on alumina eluting with hexane gave ca. 0.7 g ( 50%) of product mp 145-148 C. 

When a mixture of benzene and perfluorobutyne was irradiated in a Vycor tube with 253.7 nm light, slow formation of at least eight compounds was detected by gas chromatography. The major compound was l,2-bis(trifluoromethyl)cycloocta-l,3,5,7-tetraene (10, 40%), accompanied by three isomeric bis(trifluoromethyl)tricyclo[3.3.0.0 ]octa-3,6-dienes (semibullvalenes) 11-13 in 25, 12 and 5% yield, respectively. ... 

Cycloocta-l,3,5,7-tetraene readily reacts with potassium metal to form the stable cyclooctatetraene dianion, CsHg ". Why do you suppose this reaction occurs so easily What geometry do you think the cyclooctatetraene dianion might have ... 

Cycloocta-l,3,5,7-tetraene was first synthesized in 1911 by a route that involved the following transformation. How would you accomplish this reaction ... 

---