Abstract:[reaction: see text] Low temperature (-100 degrees C) dehydrohalogenation of 1,2,5,6,9,10-hexabromocyclododecane (a common fire retardant) with potassium tert-butoxide in THF followed by one-electron reduction yields the anion radical of the di-trans form of [12]annulene. This system yields a well-resolved EPR signal that reveals that most of the spin density resides on one side (the planar side) of the anion radical. Five of the carbons in this [12]annulene system are twisted from the plane of the remaining s… Show more
“…Annulenes have attracted much attention since the first report on [8]annulene in 1911 . Many efforts have been devoted to probing the (anti)aromaticity of [4 n ]annulenes by using NMR spectroscopy and computational methods. − The addition of π-electrons to the [4 n ]annulenes that are antiaromatic in the Hückel sense has been carried out to alter their aromaticity and magnetic properties. − Among the [4 n ]annulenes, studies on [12]annulenes and hetero[12]annulenes are very limited due to their decreased thermal stability. ,− Stevenson reported that [12]annulenes show diamagnetic properties 10a…”
N-Substituted diaza[12]annulenes are obtained by one -pot reaction of N-(2,4-dinitrophenyl)pyridinium chloride with amines in moderate to high yields. The 1H NMR spectrum reveals that diamagnetic ring current is generated in the diaza[12]annulene ring. The N-substituted diaza[12]annulenes are electrochemically active in solution.
“…Annulenes have attracted much attention since the first report on [8]annulene in 1911 . Many efforts have been devoted to probing the (anti)aromaticity of [4 n ]annulenes by using NMR spectroscopy and computational methods. − The addition of π-electrons to the [4 n ]annulenes that are antiaromatic in the Hückel sense has been carried out to alter their aromaticity and magnetic properties. − Among the [4 n ]annulenes, studies on [12]annulenes and hetero[12]annulenes are very limited due to their decreased thermal stability. ,− Stevenson reported that [12]annulenes show diamagnetic properties 10a…”
N-Substituted diaza[12]annulenes are obtained by one -pot reaction of N-(2,4-dinitrophenyl)pyridinium chloride with amines in moderate to high yields. The 1H NMR spectrum reveals that diamagnetic ring current is generated in the diaza[12]annulene ring. The N-substituted diaza[12]annulenes are electrochemically active in solution.
“…Tri-trans isomer 1a (Scheme ) was reportedly synthesized over 30 years ago by Oth and co-workers, although one [4n]annulene review casts doubt on the matter, whereas another review devotes only a few sentences to [12]annulene, stating that “there has been very little recent work” on it 1b. Indeed, at the time of both reviews, the most recent experimental work was the report of Stevenson and co-workers, who repeated Oth's synthesis of 1a to make the corresponding radical anion. , 1 …”
[reaction: see text] Automerization in tri-trans-[12]annulene (1) was investigated by DFT, MP2, and coupled-cluster methods. Using the highest level of theory employed here, CCSD(T)/cc-pVDZ//BHandHLYP/6-311+G(d,p), we located two low-energy pathways for degenerate conformational change from the lowest-energy conformer of 1 (1a): one with E(a) = 4.5 kcal/mol that interconverts the three inner trans hydrogens with the three outer trans hydrogens and one with E(a) = 2.7 kcal/mol that interconverts the three inner hydrogens with each other. These results are consistent with the experimental results of Oth and co-workers on [12]annulene 1a (Oth, J. F. M.; Röttele, H.; Schröder, G. Tetrahedron Lett. 1970, 61). The conformational exchange of the inner trans hydrogens with the outer ones is predicted to occur via a one-step process involving a C(2)-symmetric transition state and not via the D(3)-symmetric transition state (1b) that was postulated earlier. Conformer 1b was found to be a shallow minimum 6.7 kcal/mol above 1a with a barrier of 0.4 kcal/mol for conversion to 1a. Finally, GIAO-B3LYP/6-311+G(d,p) and BHandHLYP/6-311+G(d,p) computed (1)H NMR chemical shifts of 1a and three other low-lying isomers support Oth's original assignment of observed (1)H NMR peaks to 1a at both low and high temperature.
“…The reduction of the di-trans isomer (4b) with K was recently reported [76]. Its radical anion has a different EPR spectrum (13.5 G) from that of the tri-trans isomer.…”
Section: [12]annulenementioning
confidence: 90%
“…C 78 and C 84 were reduced as a mixture of isomers, and in some of the measurements the different isomers could be distinguished. The EPR measurements by Pénicaud [152] showed that the g values of all of the monoanions were higher than that of C 60 À (2.0023, 2.0018 and 2.0008 for C 76 À , C 78 À , C 84 À , respectively, in benzonitrile). In tetrachloroethane the two isomers of C 78 À could be distinguished by their different EPR characteristics [152].…”
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