Abstract:Electron paramagnetic resonance (EPR) studies have revealed that the steric interaction between the methyl hydrogens on a tert-butoxy substituent and the cyclooctatetraene (COT) ring system sterically induces a puckering of the eight-membered ring in the anion radical of tert-butoxy-COT. The induced nonplanarity of the COT ring system causes a large attenuation of the EPR coupling constants. Since the C-D bond length is slightly shorter than is the C-H bond length, replacement of the tert-butyl group with a te… Show more
“…The synthesis of the pseudo-ortho protiated ring deuterated menthoxy-COT ( IIIa ) was also based on our synthesis of “pseudo-ortho” deuterated tert -butoxy-COT …”
The anion radicals of alkoxy-substituted cyclooctatetraenes in hexamethylphosphoramide spontaneously dimerize to form the dianions of dialkoxy-[16]annulenes. The dianions reveal the expected high-field NMR resonance for the internal protons. After electron transfer, the EPR spectra of the corresponding anion radicals reveal that only the 1,5-dialkoxy systems are formed. Further, the measured proton and (13)C spin densities show that the odd electron resides in a molecular orbital with six hydrogens in "deep" nodal positions that completely hide them from EPR detection. This MO corresponds to the nonbonding (singly occupied) MO of higher energy after splitting of the degenerate nonbonding MOs by the two-electron-withdrawing substituents. The surprising electron-withdrawing nature of the alkoxy substituents is attributed to a rather strong mixing of the sigma and pi systems in [16]annulene.
“…The synthesis of the pseudo-ortho protiated ring deuterated menthoxy-COT ( IIIa ) was also based on our synthesis of “pseudo-ortho” deuterated tert -butoxy-COT …”
The anion radicals of alkoxy-substituted cyclooctatetraenes in hexamethylphosphoramide spontaneously dimerize to form the dianions of dialkoxy-[16]annulenes. The dianions reveal the expected high-field NMR resonance for the internal protons. After electron transfer, the EPR spectra of the corresponding anion radicals reveal that only the 1,5-dialkoxy systems are formed. Further, the measured proton and (13)C spin densities show that the odd electron resides in a molecular orbital with six hydrogens in "deep" nodal positions that completely hide them from EPR detection. This MO corresponds to the nonbonding (singly occupied) MO of higher energy after splitting of the degenerate nonbonding MOs by the two-electron-withdrawing substituents. The surprising electron-withdrawing nature of the alkoxy substituents is attributed to a rather strong mixing of the sigma and pi systems in [16]annulene.
“…The COT moiety in the anion radicals of primary and secondary alkoxy substituted COTs have planar D 4 h structures (Scheme ). , However, when the substituent is a bulky tert -butoxy group, perfect planarity is prohibited due to the steric interactions between the methyl and ring protons .…”
mentioning
confidence: 99%
“…Reduction of a wide variety of primary or secondary alkoxy-COT systems in HMPA with alkali metal results in solutions exhibiting a quartet of pentets upon EPR analysis with a H 's of about 5.6 G for protons on positions 3, 5, and 7 and around 0.7 G due to the protons on positions 2, 4, 6, and 8 . The appearance of the EPR spectrum of tert -butoxy-COT •- is significantly different due to the altered magnitudes of both splittings (3 Hs with a H = 4.89 G and 4 Hs with a H = 1.30 G). , When [K + ] ≪ [ 3 ] total , the EPR pattern for 3 B • - (3 Hs with a H = 5.4 G and 4 Hs with a H = 1.04 G) reflects a system with a degree of ring puckering between that for tert -butoxy-COT •- and a planar secondary alkoxy-COT anion radical. 3 …”
mentioning
confidence: 99%
“…However one proton is actually closer to the ipso carbon. The apparent equivalency has been attributed to quantum mechanical tunneling between the two D 4 h structures, Scheme . Apparently, the presence of the second reduced ring renders positions 2 and 8 nonequivalent on the primary side of 3 2( • - ) .…”
[reaction: see text] The room-temperature potassium reduction of 1,2-bis-cyclooctatetraeneoxypropane yields two different regio-spin isomer anion radicals in equilibrium (COT-O-C(Me)HCH(2)CH(2)-O-COT(*)(-) = (*)(-)COT-O-C(Me)HCH(2)CH(2)-O-COT) that is shifted far to the right. The presence of the unreduced ring perturbs the spin density on the reduced ring. Addition of more electrons generates the diradical dianion ((*)(-)COT-O-C(Me)HCH(2)CH(2)-O-COT(*)(-)), and the anion radical on the secondary side splits the degeneracy of the psuedo-ortho protons of the anion radical on the primary side.
“…These were reduced by K, and all the formed dianions were characterized as planar or essentially planar. The steric interactions and planarity of the radical anions of tert-butoxy-3 were also studied [71]. Another interesting derivative of 3 is 1,5,22,26-tetraoxa-[5,5]-(2,8)-dibenzo[a,e]cyclooctatetraenophane 10 [72], which is the first macrocycle incorporating two 3 units.…”
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