Annulene System Research Articles

Unexpected one-pot formation of the 1H-6a,8a-epiminotricyclopenta[a,c,e][8]annulene system from cyclopentanone, ammonia and dimethyl fumarate. Synthesis of highly strained polycyclic nitroxide and EPR study.

The unexpected formation of a highly strained polycyclic amine was observed in a one-pot synthesis from cyclopentanone, dimethyl fumarate and ammonium acetate. This multistep reaction includes 1,3-dipolar cycloaddition of dimethyl fumarate to the cyclic azomethine ylide formed in situ from cyclopentanone and ammonia. The polycyclic amine product was easily converted into a sterically shielded polycyclic nitroxide. The EPR spectra and spin relaxation behavior of the nitroxide were studied in solution. The spin relaxation seems well suited for the use as a biological spin label and are comparable with those of cyclic nitroxides with two spirocyclic moieties adjacent to the N–O· group.

Explanation of the sign inversion in the magnetic circular dichroism of Möbius [28] hexaphyrin

Meso-phenyl hexaphyrin is an extended annulene system which can adopt both Hückel and Möbius topologies based on the oxidation state. In this contribution we analyze the remarkable sign inversion of the MCD spectrum which accompanies this topological switch. The treatment is based on the identification of the circular wave assignments of the relevant excited states; the corresponding magnetic moments are calculated using the CASCI (4,4) approach. In this approach wavefunctions are obtained from configuration interaction in the active space of the four Kohn–Sham frontier orbitals of the chromophore. The results reproduce the signs of the MCD spectra. The anomalous sign in the Möbius conformation is attributed to the quenching of the excited state orbital moment by the ring twist.

ChemInform Abstract: Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

AbstractReview: 331 refs.

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]- and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties.

The Geometry and Electronic Topology of Higher-Order Charged Möbius Annulenes

Higher-order aromatic charged Möbius-type annulenes have been L(k) realized computationally. These charged species are based on strips with more than one electronic half-twist, as defined by their linking numbers. The B3LYP/6-311+G(d,p) optimized structures and properties of annulene rings with such multiple half-twists (C(12)H(12)(2+), C(12)H(12)(2-), C(14)H(14), C(18)H(18)(2+), C(18)H(18)(2-), C(21)H(21)(+), C(24)H(24)(2-), C(28)H(28)(2+), and C(28)H(28)(2-)) have the nearly equal C-C bond lengths, small dihedral angles around the circuits, stabilization energies, and nucleus-independent chemical shift values associated with aromaticity. The topology and nature of Möbius annulene systems are analyzed in terms of the torus curves defined by electron density functions (rho(r)(pi), ELF(pi)) constructed using only the occupied pi-MOs. The pi-torus subdivides into a torus knot for annulenes defined by an odd linking number (L(k) = 1, 3pi) and a torus link for those with an even linking number (L(k) = 2, 4pi). The torus topology is shown to map onto single canonical pi-MOs only for even values of L(k). Incomplete and misleading descriptions of the topology of pi-electronic Möbius systems with an odd number of half twists result when only signed orbital diagrams are considered, as is often done for the iconic single half twist system.

Donors and Acceptors Based on Triangular Dehydrobenzo[12]annulenes: Formation of a Triple-Layered Rosette Structure by a Charge-Transfer Complex

We present here the results of studies of the synthesis and properties of donors and acceptors based on triangular dehydrobenzo[12]annulene ([12]DBA) system as a pi core. These studies were aimed at controlling the supramolecular crystal structure. Toward this end, the tricyano[12]DBA 2 and dodecafluoro[12]DBA ( 3) were synthesized as acceptors (A) and the tris(dialkylamino)[12]DBAs 4a-d as donors (D), and their electronic properties were determined by electronic absorption spectroscopy and electrochemical measurements. The main focus, though, was the formation of supramolecular structures in crystals. These compounds form distinct packing patterns as a result of the different intermolecular interactions. Tricyano[12]DBA 2 forms a two-dimensional (2D) sheet structure via hydrogen-bonding interactions, whereas a tilted-stack structure was found for 3 because of the lack of significant intermolecular interactions. Tris(dibutylamino)[12]DBA 4b exhibits a ladder-type 2D structure, probably because of van der Waals interactions between the butyl groups. The most significant finding is that charge-transfer interactions between donor 4a and acceptor 3 combined with their triangular molecular shapes and lateral CH...F hydrogen bonding result in the formation of a 2D rosette structure consisting of two different trimeric (DAD- and ADA-type) sandwich structures with 1:2 and 2:1 A/D ratios, respectively.

Synthesis of Annulenes via Multiple Wittig Reactions; In‐Situ Dimerization of 1,2;5,6‐Dibenzocyclooctatetraene

AbstractA simple synthetic route to tetrabenzo[16]‐ and hexabenzo[24]annulene systems using one‐pot multiple Wittig reactions is presented. We report the characterization of the all‐trans‐isomer of tetrabenzo[16]annulene by NMR spectroscopy and X‐ray diffraction. Several isomers of an unexpected, tetrabenzocyclooctatetraene dimer product are also isolated and identified by NMR‐ and X‐ray diffraction studies.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

12]Annulene Gemini Surfactants: Structure and Self-Assembly

The authors of this Communication wish to alter the proposed structure of annulene 1, a system that had previously been reported by Yamaguchi et al.1 After one of the authors (F.M.M.) had originally inspected the then available analytical data (1H and 13C NMR spectra, elemental analysis, and ESI-MS data), he affirmed that they were (and still are) consistent with the annulene system. In particular, parent mass spectral signals at m/z 531.44353, 587.50648, and 643.56867 for the R=C12H25, C14H29, and C16H33 derivatives, respectively, all correspond to the mass of [1−Cl−]+. Recently, however, Prof. M. Cristl suggested2 that the pyridinium salt 2 would be an alternative and more likely possibility. There exists an intriguing ambiguity in this case because 1 and 2 have indistinguishable NMR spectra and elemental analyses and because our [1−2 Cl−]2+ base peak and the [2−Cl−]+ parent peak happen to have identical masses. We are now able to differentiate the two structures through weak 13C-containing MS signals. These signals have a shift one mass unit higher than the all-12C signal (consistent with [2−Cl−]+) as opposed to 0.5 units higher (consistent with [1−2 Cl−]2+). In view of these new data, our peaks at m/z>500 must, we surmise, arise from dimers of 2 in the gas phase. Fortunately, the altered identity of the compound in no way affects our high-level calculations on the annulene structure. Moreover, our conclusion based on the NMR data, namely, that the terminal methyl groups of the chains loop within a micelle so as to contact the micelle surface, remains valid, although the micelles are now more classical in nature than we had previously envisioned.1

A Theoretical Model of a Magnetic Polymer

We report a theoretical model of a magnetic polymer consisting of benzenoid structures incorporated in a simple linear polyethylene-like polymer chain. The benzenoid structures are double ring annulenes having 12, 16, 20 and 24 outer ring π electrons and the benzene ring inside the main cyclic π system. Either six or three covalent bonds of benzene linked to the outer ring carbon atoms at regular intervals will bring planarity to the annulene systems. Hückel molecular orbital theory of quantum chemistry predicts that a 4n (n = 0, 1, 2,….) π alternating cyclic system has a triplet (2s + 1 = 3) stable ground state molecular orbital, provided the cyclic system is planar. Although cyclobutadiene is planar, it is quasi-stable. The 8, 12, 16, 20 and 24 annulenes are not planar but they are stable. If a benzene ring is introduced into the annulene ring and six or three carbon atoms of benzene are covalently linked to the carbon atoms of the outer ring annulene at regular intervals, the annulene systems might become planar. The presence of a benzene ring inside the annulene ring has little influence on the symmetry, electron distribution, energy level and molecular orbitals of the 4n system and as a result it is expected that these modified annulenes would have quite a stable triplet ground state. These predictions are here confirmed by DFT (density functional theory) geometrical optimisation of one of the systems at the B3LYP/6-31+G*level. Calculation of the nucleus independent chemical shift (NICS) at the centre of the ring produces strongly positive values for the triplet 4n annulene structure. Furthermore, the resulting systems do not undergo cyclo-addition (Diels Alder) reactions either at room temperature or in presence of UV radiation. If these planar star-like annulene molecules are incorporated into the polyethylene chain at regular intervals, the polymer is expected to be magneto- active.

Optical Excitations in Carbon Architectures Based on Dodecadehydrotribenzo[18]annulene

The origin of excitations in multi-chromophore carbon network substructures based on dodecadehydrotribenzo[18]annulene has been investigated by steady-state and photon echo spectroscopy, configuration interaction (CIS and CIS(D)), and time-dependent density functional theory (TD-DFT). 1,4-diphenylbutadiyne, the simplest structural subunit within the annulene, was used in modeling the spectroscopic studies to explain the origin of excitations in the macrocycles. The optical excitations in longer linear systems were found to be similar to its diphenylacetylene analogue. However, the results from dodecadehydrotribenzo[18]annulene and other multichromophore networks systems illustrate the possibility of strong intramolecular interactions and the formation of delocalized excited states. Calculations were carried out to explain the basic similarities and differences in excitations of the model compounds such as diphenylbutadiyne and the macrocycles. The fundamental excitation in these systems can be primarily described as a pi --> pi* transition. Two low-energy resonances were observed from experiment for the annulene systems, and possible explanations for these low-energy resonances in the macrocycles are explored. The significant difference found in the calculated oscillator strength of the two low-energy bands for the macrocycles as well as the dynamics of solvent interactions was further investigated by three-pulse photon echo measurements. A simple exciton model was developed to discuss the excitations in the larger macrocycles. The results from this model were found to be in good agreement with the TD-DFT calculations.

1,2-Didehydro[10]annulenes: Structures, Aromaticity, and Cyclizations

The conformational space of C(10)H(8) 1,2-didehydro[10]annulenes, along with their unimolecular conversion to isonaphthalenes (cyclic allenes), has been studied computationally using DFT (B3LYP), single-reference [CCSD(T)], and multireference (MCQDPT2) post-HF methods. The introduction of the linear alkynyl moiety releases enough angle strain to make a nearly planar "heart" aromatic form the preferred conformer by more than 6 kcal/mol [CCSD(T)] over a localized C(2) "twist" structure, as opposed to the closely related C(10)H(10) [10]annulene system. Computations also show that electrocyclic ring-opening of isonaphthalenes to the heart C(10)H(8) annulene takes place through a low barrier of 15 kcal/mol, and this should be considered the working mechanism for the reported isomerizations during dehydro Diels-Alder reactions of phenylacetylenes.

Anion radicals of di-trans-[12]annulene and heptalene in a one-pot synthesis from a common fire retardant.

[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 seven carbons, and the rate of rearrangement between the degenerate conformations is on the EPR time scale (k = 10(6)-10(7) s(-1)). Warming of the solution results in the formation of a sigma-bond between the two internal carbons, loss of molecular hydrogen, and consequent generation of the anion radical of heptalene. Tractable quantities of neutral heptalene can be obtained via the reoxidation of this anion radical with iodine.

In Pursuit of the Elusive Bond-Stretch Isomers by ab Initio MethodsBenzocyclobutene, Benzo[1,2:4,5]dicyclobutadiene, and Some Related Substituted Systems

The problem of bond-stretch isomerism in benzocyclobutene (1) and benzo[1,2:4,5]- dicyclobutadiene is addressed by single-state (SS-CASSCF and SS-CASPT2) as well by two-state (TWS-CASSCF and TWS-CASPT2) theoretical formalisms. It is shown beyond reasonable doubt that benzocyclobutene 1 does not exhibit bond-stretch isomerism. The situation is more complex and interesting in the case of benzo[1,2:4,5]-dicyclobutadiene. The TWS-CASPT2 ( π ) + σ approach, based on the TWS-CASSCF(10,10) π zeroth-order wave function, yields two bond-stretch isomers 2 and 3. Their total energies, including the ZPVE contributions, indicate that the structure 3 is slightly favored. Because they are separated by an almost vanishing potential energy barrier (0.4-1.0 kcal/mol), it is concluded that 3 prevails and that bond-stretch isomerism probably does not occur in the case of benzo[1,2:4,5]dicyclobutadiene either. However, the final asnwer will be given only by high level multireference (MR) ab initio calculations. It is important to stress that 3 exhibits a pattern of 10π electrons fully delocalized over the CC bonds of the molecular perimeter and thus represents a planar pseudo-[10]annulene system par excellence. Finally, it is argued that strategic substitution of benzo[1,2:4,5]-dicyclobutadiene might lead to a discrimination of the isomers 2 and 3 and to the isolation of some judiciously selected derivatives in the laboratory. In particular, it is shown that a number of substituted pseudo-[10]annulene systems are possible.

Stabilized and destabilized carbocations in the 1,6-methano[10]annulene series.

2-Chloromethyl and 3-chloromethyl-1,6-methano[10]annulene systems solvolyze in methanol to give simple substitution products. Solvent effect studies and the special salt effect support the involvement of cationic intermediates stabilized by the 1,6-methano[10]annulene group. Rate data indicate that the degree of cation stabilization greatly exceeds that of naphthyl groups. B3LYP/6-31G computational studies also suggest that the cationic intermediates are greatly stabilized by the 1,6-methano[10]annulene. By way of contrast to these findings, solvolytic and computational studies indicate that the 11-(1,6-methano[10]annulenyl) cation is a destabilized analogue of the cycloheptatrienyl cation. There are no favorable interactions with the annulene ring. Distortions from planarity prevent charge delocalization as in the analogous aromatic cycloheptatrienyl cation.

Vibrational spectroscopy of macrocyclic oligo(Phenyldiacetylenes)—I. A theoretical and experimental study of octadehydrodibenzo[12]annulene and dodecadehydrotribenzo[18]annulene

The infrared and Raman spectra of butyl and decyl symmetrically substituted octadehydrodibenzo[12]annulene and dodecadehydrotribenzo[18]annulene have been recorded in solid phase and solution. A vibrational assignment in internal coordinates is given based on the DFT calculations at the B3LYP/6-31G* level of theory for the methyl derivatives of the named annulene systems. An analysis of the stretching and bending vibrations of the diacetylene bridges revealed that they are all found in the same regions be the annulene ring strained or not. Some spectral characteristics could only be explained as deviations from the ideal symmetry of annulene rings imposed by substituents.

‘Oligomere' Kondensationsprodukte von (1E,3E,5E)‐1,6‐Di(2‐furyl)hexa‐1,3,5‐trien mit Acetaldehyd: Tetrahydro‐tetramethyl‐octaepoxy[60]annulen(6.1.6.1.6.1.6.1)

AbstractOligomeric Condensation Products of (1E,3E,5E)‐1,6‐Di(2‐furyl)hexa‐1,3,5‐triene with Acetaldehyde: Tetrahydro‐tetramethyl‐octaepoxy[60]annulene(6.1.6.1.6.1.6.1)The Ca(NO3)2‐induced condensation of (1E,3E,5E)‐1,6‐di(2‐furyl)hexa‐1,3,5‐triene (6) with acetaldehyde yields the linear ‘oligomers' 7–11 with 2–6 1,6‐di(2‐furyl)hexa‐1,3,5‐triene units and 1–4 acetaldehyde units, besides a cyclic condensation product 12 obtained from 4 equiv. of 6 with 4 equiv. of acetaldehyde. According to spectroscopic studies, 12 is the tetrahydro‐tetramethyl‐octaepoxy[60]annulene(6.1.6.1.6.1.6.1) as the most expanded annulene system known so far. While the dehydrogenation of 12 to give the tetramethyl‐octaepoxy[60]annulene(6.1.6.1.6.1.6.1) cannot be achieved, the oxidation of 12 with Br2 yields a black, in all organic solvents nearly insoluble solid 14, which possibly is the tetramethyl‐octaepoxy[58]annulene(6.1.6.1.6.1.6.1) dication. Because of the insolubility of 14, unfortunately most of its spectroscopic data are not available. However, the λmax values in the UV/VIS/NIR spectrum of 14 (Soret and Q bands) are in line with the values of the tetraepoxy[26]annulene(4.2.4.2) dication, the tetraepoxy[30]annulene(4.4.4.4) dication, and the tetraepoxy[34]annulene(6.4.6.4) dication.

Synthesis of linearly-fused benzodehydro[12]annulenes

This communication describes the preparation of the first linearly-fused benzodehydro[12]annulene systems, which we have called acediynes. These stable antiaromatic materials exhibit absorption properties that closely resemble the spectra of the corresponding acenes.

“アセチレン‐クムレン”デヒドロアヌレンの研究

Our studies on the synthesis and properties of dehydroannulenes containing acetylenic and cumulenic linkages in their cyclic systems (“acetylene-cumulene” dehydroannulenes) are briefly described. The “acetylene-cumulene” dehydroannulenes hold conformational stability owing to the presence of rigid and straight sp-sp hybridized linkages in the cyclic system, thus affording reliable information on the structure and the π-electron character of the annulene system.

Solution Electron Affinity Perturbation Due to the Deuteration of [16]Annulene

X-Band (9.8 GHz) EPR measurements show that the free energy change controlling the electron transfer from the anion radical of perdeuterated [16]annulene to neutral [16]annulene (C16D16•- + C16H16 ⇋ C16H16•- + C16D16) is ΔG° = −0.65 kcal/mol. When supplied with vibrational frequencies from B3LYP/6-31+G* calculations, the QUIVER program determines a ΔG° value of −0.142 kcal/mol. Thus, the experimental and theoretical values are in qualitative agreement. This result is the opposite of that obtained for the [8]annulene system, where the solution electron affinity of C8D8 proved to be slightly greater that that for C8H8. 1H NMR (400 MHz) experiments reveal that the barrier to ring flattening is greater in the C16D16 system than in the C16H16 system. Coupled with the DFT prediction that the C16D16•- is nearly planar, this accounts for the equilibrium isotope effect observed in the electron transfer. The W-Band (94 GHz) EPR spectra, showing that the isotropic g-factor of the [16]annulene anion radical is not al...

Novel, strained 10b,10c-dihydropyrenes bearing bulky TMSCH2 groups at the internal positions

Two novel, strained trans-10b,10c-dihydropyrenes, 9a–b, with internal, bulky trimethylsilylmethyl substituent(s) were synthesized. Their 14π-electron annulene systems were found to be distorted, as shown by X-ray crystallographic analyses and 1H-NMR and UV spectroscopy.