ESR. Spectra of Radical Anions in the Cyclazine Series

The ESR. spectra of the radical anions of three cyclazines are described. Their π-spin distributions are discussed in terms of simple MO theory and compared with those of structurally related species.     

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

Dibenzo[a,e]pentalene (DBP) is a non-alternant conjugated hydrocarbon with antiaromatic character and ambipolar electrochemical behavior. Upon both reduction and oxidation, it becomes aromatic. We herein study the chemical oxidation and reduction of a planar DBP derivative and a bent DBP-phane. The molecular structures of its planar dication, cation radical and anion radical in the solid state demonstrate the gained aromaticity through bond length equalization, which is supported by nucleus independent chemical shift-calculations. EPR spectra on the cation radical confirm the spin delocalization over the DBP framework. A similar delocalization was not possible in the reduced bent DBP-phane, which stabilized itself by proton abstraction from a solvent molecule upon reduction. This is the first report on structures of a DBP cation radical and dication in the solid state and of a reduced bent DBP derivative. Our study provides valuable insight into the charged species of DBP for its application as semiconductor.     

ESR. Spectra of the Radical Anions of Cycloalkylbenzenes

Radical anions of five cycloalkylbenzenes (alkyl = propyl, butyl, pentyl, hexyl, or heptyl) have been studied by ESR. spectroscopy at ?90°C. In the case of cyclopropylbenzene, no reliable experimental data could be obtained, because of the instability of its radical anion. The spectra of the radical anions of the four higher homologues have been analysed by means of a computer program. The assignment of the coupling constants to the α-protons in the para position of the benzene ring and to the cycloalkyl ß-protons has been based on the spectra of specifically deuterated derivatives. The experimental data of the radical anion of cycloalkylbenzenes have been compared with those of the radical anions of five alkylbenzenes (alkyl = methyl, ethyl, n-propyl, isopropyl, or t-butyl), the spectra of the latter being reexamined at ?90°C. In the cycloalkyl series the degeneracy of the two benzene-type lowest antibonding orbitals is the more effectively removed the larger the substituent, whereas the reverse relation holds for the non-cyclic series. The preferred conformation of the substituents is that in which the alkyl or cycloalkyl ß-proton is near to the nodal plane of the benzene π-system, and this preference is accentuated with the increasing size of the substituent group.     

Effects of Non-planarity in the Radical Anions of Tribenzo[a,c,e]cyclooctenes

Reduction of tribenzo[a,c,e]cyclooctene ( 2 ) and its 2,3- and 1,4-dimethyl derivatives ( 4 and 5 ), as well as of 1,1-dimethyl-10,11-propane-2,2-diylidene-1H-benzo[5,6]cycloocta[1,2,3,4-def]fluorene ( 6 ) and its 5,6-didehydro derivative ( 7 ) was followed by cyclic voltammetry. The radical anions of these compounds and those of their derivatives (D) 2 , (D) 5 , and (D) 6 , deuteratsd at C(9) in 1 and 5 or in the corresponding position of 6, have been characterized with the use of ESR, ENDOR, and TRIPLE-resonance spectroscopy. The cyclic-voltammetric and proton-hyperfine data are consistent with the increasing deviations of the radical anions from planarity in the order These deviations, due to steric or interferences in the peri-positions 1?14 and 4?5, are removed in and by the introduction of bridging groups. The non-plalarity affects the thermodynamic and kinetic stabilities of the radical anions and causes a shift in the π-spin distribution away from that benzene ring which is linked to the two others by the essential single bonds C(4a)? C(4b) and C(14a)? C(14b). This finding suggests that the steric hindrance in , and is alleviated by twisting this ring out of coplanarity with the remaining (Z)-stilbene-like π-system.     

ESR. Spectra of the Radical Ions of Indeno[1, 2, 3-cd]fluoranthene

The ESR. spectra of the radical anion and the radical cation of indeno [1, 2, 3-cd]-fluoranthene have been recorded. Their hyperfine structures are consistent with those expected on the basis of simple MO models.     

A stable derivative of cyclooctatrienyne: Synthesis and crystal structures of 1,4,7,10-tetramethyl-5,6-didehydrodibenzo[a,e]cyclooctene and 1,4,7,10-tetramethyldibenzo[a,e]cyclooctene

A stable derivative of cyclooctatrienyne (1), namely 1,4,7,10-tetramethyl-5,6-didehydrodibenzo-[a,e]cyclooctene (3), has been synthesized and fully characterized. X-Ray crystallographic analyses indicate that 3 adopts a “butterfly” conformation intermediate between those of virtually planar 5,6-didehydrodibenzo-[a,e]cyclooctene (2) and tub-shaped 1,4,7,10-tetramethyldibenzo[a,e]cyclooctene(16). The methyl substituents effectively shield the otherwise rather exposed strained triple bond of 3 and kinetically stabilize it against dimerization and/or air oxidation.     

Syntheses and Crystal Structures of Pyridyl Substituted Dibenzo[b,e]-[1,4]diazepin-1-one Derivatives

A new series °f 11-pyridyl substituted dibenz°-diazepin-1-°ne derivatives 1 was synthesized by a tw° step cycl°c°ndensati°n reacti°n. All the c°mp°unds were characterized by IR,MS, and 1H NMR. And crystals °f 1a and 1b were °btained and determined by X-ray single-crystal diffracti°n. Crystal data °f 1a: C48H52N6O7, m°n°clinic system, space gr°up C2/c, a = 28.39(4), b =9.275(12), c = 16.84(2) A, β = 104.98(3)°, V = 4284(10) A3, Z = 4, F(000) = 1752, Dc = 1.279 g/cm3,R = 0.0603, and wR = 0.1314. Crystal data °f 1b: C24H21N3 O, m°n°clinic system, space gr°up P21/c,a = 11.625(5), b = 19.055(9), c = 8.741(4) A, β = 95.547(6)°, V = 1927.2(15) A3, Z = 4, F(000) =776, Dc = 1.266 g/cm3, R = 0.0494, and wR = 0.1287.     

Gas-phase conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene

The conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene (THDC) have been studied using resonance-enhanced two-photon ionization spectroscopy in a supersonic jet expansion. It has been estimated from the spectral analysis that 90% of THDC exists in the twist-boat (TB) conformation; the chair (C) conformer constitutes the remaining 10%. Most of the vibronic activity in the spectrum of THDC is associated with the symmetric flapping of the aromatic rings of the TB conformer. The observed S₁/S₂ exciton splitting of the TB conformer is 100 cm^?1. The S₁/S₂ transition of the C conformer is found to be forbidden. The exciton splittings of the C and TB conformers were estimated by the spectral analysis of two deuterated isotopomers of THDC. The estimated exciton splittings of the C and TB conformers are 14.7 and 101.9 cm^?1, respectively. The supramolecular model of bichromophores with identical chromophores at the CIS/6-31+G(d)//HF/6-31+G(d) level of theory predicted electronic coupling energies that are very close to the experimental exciton coupling energies.     

Radical Ions in the Pentalene Series [1]. Part II. Dibenzo [b,f]pentalene and its 5, 10-dimethyl derivative

Proton hyperfine data have been determined for the radical anions and cations of dibenzo [b,f]pentalene (III) and its 5, 10-dimethyl derivative (IV) . The assignment of the coupling constants to pairs of equivalent protons follows from a simple MO model, the use of which enables one to reproduce satisfactorily the experimental values. The proton hyperfine data, a_Hμ

1 The meaning of ^aH_μis ^aH? C(x),H? C(y),whereas only x and y are given in the particular cases.

, for the radical anion III?. correlate fairly well with the π-charge populations ?_μ derived from ¹H-NMR. spectra for the carbon centres μ in the dianion III^2?. The analogous correlation is less good in the case of the radical cation III⊕. and the dication III^2⊕., presumably due to the rough approximations involved in the evaluation of the numbers ?_μ for the latter species. The coupling constants a_H5,10 for III?. and III⊕. are very close to the corresponding values a_H4,6 for the radical ions of 1,3,5-tri-t-butylpentalene (II), in accord with the prediction of the MO model. A similarity is also found between the proton hyperfine data for III?. and those for the radical anions of structurally related 1,4-diphenyl-1,3-butadiene (V). On the other hand, there are striking changes in the coupling constants of the analogous protons on passing from III?. to the radical anions of dibenzo [a,e] cyclooctene (VI) and [16] annulene (VII), as a consequence of raising the symmetry from C_2h to D_2h and D_4h, respectively.     

Double elimination protocol for synthesis of 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene

A new method for constructing 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene is described on the basis of one-pot double elimination protocol. The target molecule, which is the smallest cyclophane with alternate arylene-ethynylene linkage, is synthesized in 61 % yield through oxidative dimerization of ortho-(phenylsulfonylmethyl)benzaldehyde. The initial carbon-carbon bond formation between sp(3) carbons followed by stepwise conversion to sp(2) and finally sp carbons bypasses the difficulty encountered in direct coupling of the sp carbon in the terminal acetylene. The mechanism of this process is discussed. The Wittig-Horner-type coupling is a key reaction employed for the carbon-carbon bond formation. Generation of (E)-vinylsulfone moiety in the first coupling between alpha-sulfonyl anion and aldehyde functions is crucial for the effective second coupling to complete the cyclization. The syn-elimination of the (E)-vinylsulfone moieties in the cyclized intermediate furnishes the acetylenic bonds.     

The Influence of the Axial Chirality of Dibenzo[a,c]cyclooctene on the Configuration of Its Photo-Diels-Alder Adducts

Triazole-diones and naphthoquinone are shown to add in a photochemical [4+2] reaction to the strongly twisted title diene 1 . With 1, 4-naphthoquione, the process is also accompanied by a [2+2] cycloaddition. When the pure atropisomer (?)- 1 is irradiated in presence of 2, 3-dichloro- 1, 4-naphthoquinone ( 9 ), the axial chirality of the diene is preserved. Moreover, it is found to exert complete control over the chirality induced in the resulting spiro-dihydropyrane 10 . Absolute configuration are determined by X-ray crystallography. In absence of a photo-dienophile, the axially chiral, dextrorotatory 6-phenyldibenzo[a,ccyclootene] ((++)- 11 ) undergoes a stereospecific electrocyclization to give levorotatory 4b,6a-dihydro-5-phenylcyclobuta[l]phenanthrene ((?)- 13 ). Thus, only one out of two possible, disrotatory modes of ring closure is preferred.     

ESR. Spectra of the Radical Anion of Dimethyl-phenyl-phosphine

ESR. spectra of the radical anion (I^?) produced from dimethyl-phenyl-phosphine (I) both by electrolysis and reaction with alkali metals have been studied upon variation of temperature. The coupling constant assigned to the ³¹P nucleus depends strongly on temperature, whereas the coupling constants attributed to protons do not exhibit such a dependence. The π-spin populations at the benzene ring of I^? give evidence - in accordance with other experimental data [1] [2] – that the dimethylphosphino substituent is electron-attracting. This effect is thought to be due mainly to P ← C_π delocalization, which is analogous to the Si? C_π interaction in trimethylsilyl-substituted π-systems [3]. The ESR. spectrum previously [4] ascribed to I^? is shown to arise from a secondary radical. The formation and structure of this radical are briefly discussed.