Cyclooctatetraenyl dianion

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. 

It is also possible to add two electrons to the non-planar, non-aromatic (cf. p. 17) cyclooctatetrane (17) by treating it with potassium, thereby converting it into the isolable, crystalline salt of the cyclooctatetraenyl dianion (18) ... 

Cyclooctatetraene was reduced electrochemically to cyclooctatetraenyl dianion. In DMF the product is mostly (92%) 1,3,5-cyclooctatriene at —1.2 V. If the potential is lowered the main product is 1,3,6-cyclooctatriene. Previous experiments, in which the anion radical was found to be disproportionated, were explained on the basis of reactions of the cyclooctatetraene dianion with alkali metal ions to form tightly bound complexes, or with water to form cyclooctatrienes. The first electron transfer to cyclooctatetraene is slow and proceeds via a transition state which resembles planar cyclooctatetraene102. 

Problem 10.30 Design a table showing the structure, number of tt electrons, energy levels of tt MO s and electron distribution, and state of aromaticity of (a) cyclopropienyl cation, b) cyclopropenyl anion, (c) cyclobutadiene, (d) cyclobutadienyl dication, (c) cyclopentadienyl anion, (/) cyclopentadienyl cation, (g) benzene, (h) cycloheptatrienyl anion, (/) cyclooctatetraene, (/ ) cyclooctatetraenyl dianion. ... 

A simple chemical example of a photodriven one-to-two electron exchange was encountered earlier in the excited state chemistry of cyclooctatetraenyl dianion, eq. 98 (243-246) ... 

The cyclooctatetraenyl dianion 19 (COT-2) is an aromatic 10a electron system and hence can be readily prepared from cyclooctatetraene by reduction (alkali metals or electrochemically). An early report [36] of the photochemistry of COT-2 in the presence of weak acids such as amines and terminal alkynes showed that it is protonated in the excited state (to give the monoanion) COT-2 is more basic in the excited state than in the ground state. However, in the absence of such weak acids, photolysis of COT-2 results in electron photoejection [37-39]. The electron photoejection process and subsequent chemistry has been studied in... 

Compared to the cyclooctatetraenyl dianion 19, other cyclic anions (besides cyclopentadienyl anions discussed in Sect. 1.5) have received considerably less attention. Of those that have been studied, not all of them display electron photoejection as a reaction pathway. For example, the 8,8-dimethyl-2,4,6-cyclooctatrienyl anion 22 undergoes cyclization to give 8,8-dimethylbicy-clo[5.1.0]octa-3,4-dienyl anion 23 on photolysis as the exclusive photochemical pathway [42] (Eq. 6). Photolysis of the cyclononatetraenyl anion 24 results in protonation of the more basic excited state anion, to give transient cis, cis, cis, cis-1,3,5,7-cyclononatetraene 25 (Eq. 7), which subsequently undergoes intramole-... 

When cyclooctatetraene accepts two electrons, it becomes a An + 2) n electron aromatic ion. Cyclooctatetraenyl dianion is planar with a carbon-carbon bond angle of 135° (a regular octagon). 

Since K- is a strong reductant and no H2 is evolved, two K s supply two electrons to form a cyclooctatetraenyl dianion (Fig. 10-5). This planar conjugated unsaturated monocycle has 10 electrons, conforms to the Hiickel rule ( = 2) and is aromatic. 

Instead, the dipotassium salt of the cyclooctatetraenyl dianion 162 2K was detected exclusively. This result was confirmed by Goldstein and Wenzel " reduction of semibullvalene (160) with potassium or sodium/potassium alloy even at -78°C resulted only in 162" 2K. ... 

Although the elements of the lanthanide and actinide series have long been known to exhibit a quite extensive organometallic chemistry, it is only within the last decade that typical sandwich species have been prepared and studied. These systems however, although resembling the familiar metallocene and bis-arene compounds of the d-block elements, are not strictly their analogues since in both f-orbital series the known sandwich complexes are derived only from the cyclooctatetraenyl dianion. 

For the crystal field model one assumes (see Fig. 4) that each carbon atom of the two cyclooctatetraenyl dianion rings will bear a negative charge, zj, of 1/4 with... 

Reduction of semibpllvalene 71 could lead to the destabilized [3.3.0] dianion 72. A symmetry-allowed and thermodynamically attractive rearrangement to the cyclooctatetraenyl dianion 73, however, would seem likely. 

As expected, dilithium semibullvalenide 74 rearranges at 0 °C with an apparent first-order rate constant k = 9.0(1) 10 5 s-1 to the cyclooctatetraenyl dianion 73. 

The dipotassium compound of 72 (or its dimer 74) could not be prepared by deprotonation of a mixture of tetrahydropentalenes (e.g. 75) with a 1 1 n-butyllithium/ potassium t-pentoxide mixture 58). Instead, only the dipotassium salt of the cyclooctatetraenyl dianion 73 was detected. 

Addition of two electrons to cyclooctatetraene would lead to a 10ji-electron system, the cyclooctatetraenyl dianion, Cyclooctatetraene, which is non-planar, reacts with potassium in tetrahydrofuran to give dipotassium cyclooctatetraenide, which is planar. The C-C bond lengths are all the same (141 pm). At low temperatures, a dication is formed when cyclooctatetraene reacts with SbF.. This also has some aromatic characteristics in keeping with a 6jt-electron system. 

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