On aromatic stability of oxygen- imino-, and sulfur-bridged [18]annulenes

Oxygen-, imino-. and/or sulfur-bridged [18]annulenes are studied using the A_s-index of aromatic stabilization. Theoretical predictions, which indicate that these compounds should differ among themselves in their aromatic properties, correlate nicely with the available experimental findings. Unknown compounds of this class: [18]annulene imino-dioxide and [18]annulene oxide-imino-sulfide are predicted to be aromatic and good candidates for preparation. The same prediction is also reached for 1,10-diaza[18]annulene.     

Aromaticity in bridged heteroannulenes

The aromatic stabilities of bridged hetero[n]annulenes (n = even, odd) and the parent structures of hetero-[n]annulenes and hetero[n]polycycles have been predicted by the topological resonance energy, TRE, approach. The TRE criterion places bridged structures in all cases studied between the hetero[n]annulenes and hetero[n]polycyclic systems. The extra aromatic stability of the bridged heteroannulenes compared to heteroannulenes is attributed to homoaromatic interaction across the ring at the bridge sites. Available experimental evidence supports the theoretical results.     

Aromaticity and Homoaromaticity in Methano[10]annulenes

Aromaticity and neutral homoaromaticity have been evaluated in methano[10]annulenes systems, 1,4-methano[10]annulene (1), 1,5-methano[10]annulene (2), and 1,6-methano[10]annulene (3). C-C bond lengths indicate that 1 presents higher bond alternation than 2 and 3. The relative energies were determined at the B3LYP/6-311+G(d,p) level, and they pointed out that 3 is the most stable isomer. Strain energies, evaluated employing homodesmotic reactions, show the same order as the relative energies. Through a decomposition of strain energies, it could be concluded that the rings absorb more tension than the bridges. The changes in aromaticity were evaluated by magnetic susceptibilities, chiM, HOMA, NICS, and resonance energies, RE. HOMA, RE, and chiM indicate that 2 and 3 are strongly, and 1 is fairly, aromatic. NICS does not provide reliable results, due to interference of ring and bridge atoms. NBO analysis presents some interactions that suggest the existence of neutral homoaromaticity. GPA indices (evaluated at the B3LYP/6-31G* level) point out that homoaromaticity plays a relevant role only in 3. Moreover, this work is the first in the current literature that studies 1,4-methano[10]annulene (1).     

Structure-property investigations of conjugated thiophenes fused onto a dehydro[14]annulene scaffold

A series of 12 thieno-fused macrocycles based on the dehydro[14]annulene framework have been prepared. Studies have focused on the optical and electronic properties of the dehydrobenzothieno[14]annulenes (DBTAs) and dehydrothieno[14]annulenes (DTAs) utilizing NMR spectroscopy, UV-vis spectrophotometry, electrochemistry, and DFT computations. X-ray crystal structures were also obtained for two of the macrocycles. The structure-property relationships were found to vary significantly based on the relative orientation of the thiophenes. The stability, properties, and reactivity of these macrocycles were found to be more typical of dehydroannulenes rather than oligothiophenes.     

Calculation of 1H chemical shifts of [n]annuleno[m]annulenes and [n]annulenyl[m]annulenes

The ring current and local anisotropic contributions to the ¹H chemical shifts of [n]annuleno[m]annulenes and [n]annulenyl[m]annulenes with n, m = 12, 14, 18 and n, m = 13, 15 are calculated. The agreement between experimental and calculated shifts for the few known compounds is very good so that the predictions for the as yet unknown compounds are reliable. The effect of an annulene ring on the shifts of the protons at the other ring in these bicyclic compounds is discussed for several types of fusion of the two constituent annulenes.     

Perimeter-Type Structures in Charged [4n + 2] Annuleno [4n + 2] annulenes. The Importance of the Frontier-Orbitals

The tetra(tert-butyl)[14]annuleno[14]annulene 1 is transformed, via alkali metal reduction, into its radical monoanion, dianion, radical trianion, and tetraanion. The paramagnetic species are characterized by ESR and ENDOR spectroscopy and the diamagnetic species by ¹H- and ¹³C-NMR spectroscopy. Well-resolved ESR and NMR spectra can only be obtained, if the reduction is carefully monitored. For the interpretation of the data, the ions derived from the structurally related [14]- and [22]annulenes 3 and 5 as well from the tetra(tert-butyl)[14]annuleno[18]annulene 2 serve as suitable model compounds. While the behavior of the neutral annulenoannulene 1 is governed by that of the [14]annulene-subunit, the corresponding ionic systems can best be described as macrocyclic perimeters. This outcome can be rationalized by the nodal properties of the frontier MO's.     

Trisdehydro[14]annuleno[16]annulene

Trisdehydro[14]annuleno[16]annulene consisting of an aromatic bisdehydro[14]annulene and an antiaromatic bisdehydro[16]annulene has been synthesized. The strong paratropicity was observed in the [16]annulene moiety being comparable with that of extremely unstable parent bisdehydro[16]annulene.     

Diepoxy[18]annulene(10.0) : (Z,E,Z,E,Z)‐Diepoxy[18]annulen(10.0) – ein hochdynamisches Annulen

Diepoxy[18]annulenes(10.0): ( Z , E , Z , E , Z )‐Diepoxy[18]annulene(10.0) – a Highly Dynamic Annulene The McMurry reaction of (all‐E)‐5,5′‐([2,2′‐bifuran]‐5,5′‐diyl)bis[penta‐2,4‐dienal] ( 13 ) only occurs intramolecularly to give a mixture of the diepoxy[18]annulenes(10.0) 6 and 7 . Tetraepoxy[36]annulene(10.0.10.0) resulting from an intermolecular McMurry reaction is not formed. According to spectroscopic data, 6 is (Z,E,Z,E,Z)‐ and 7 (Z,E,E,Z,E)‐configured. The ¹H‐NMR data confirm that in 6 the (E)‐ethene‐1,2‐diyl bonds (C(11)=C(12) and C(15)=C(16)) rotate around the adjacent σ‐bonds. Beginning at −70°, this rotation freezes, and 6 is becoming a diatropic aromatic ring system. Beside [18]annulene itself, (Z,E,Z,E,Z)‐diepoxy[18]annulene(10.0) 6 is the only hitherto known [18]annulene derivative with dynamic properties.     

Tetrakisdehydro[18]annuleno[20]annulene

Tetrakisdehydro[18]annuleno[20]annulene has been synthesized. The ¹H NMR spectra clearly indicate the induction of dia- and paramagnetic ring currents in 18- and 20-rings, respectively. A marked suppression of the diatropicity in the 18π moiety was observed being in the same trend in other tetrakisdehydro[4n]annuleno[4n′+2]annulenes. The ¹H NMR spectroscopic behavior of the annulenoannulenes is consistent with theoretical conclusion.     

Synthesis and properties of the first Möbius annulenes

Heilbronner in 1964 predicted that annulenes with ".. a planar perimeter of N=4r AO's, which would yield an open shell configuration when occupied by 4r electrons, can be twisted into a closed shell M?bius strip perimeter without loss in pi electron energy". We have been able to synthesize the first [4n]annulene with such a M?bius topology and now present further M?bius isomers and the details of their preparation as stable compounds. To address the question whether the twist in the pi system has an effect on the properties we systematically investigate energy, geometry and magnetic parameters of a large number isomers of [16]annulenes. The M?bius twisted annulenes are consistently more aromatic than the non-twisted isomers. This is true for the parent as well as our benzoannelated systems. Our results are in contrast to those published recently by C. Castro, W. L. Karney, P. von R. Schleyer et al.     

Mono- and bis(tetrathiafulvaleno)hexadehydro[12]annulenes

Hexadehydro[12]annulenes annelated with one or two TTF units have been synthesized to investigate their pi-amphoteric properties based on the TTF and [12]annulene moieties; these compounds show multi-redox potentials, solvatochromism and the formation of large sandwich complexes.     

Synthesis of dicyclopenta[a,e]pentalenes via a molybdenum carbonyl mediated tandem allenic Pauson-Khand reaction and the X-ray crystal structure of a planar dicyclopenta[a,e]pentalene

Two 14 pi cross-linked annulenes which belong to the family of dicyclopenta[a,e]pentalenes have been synthesized, 14 pi bis enol triflate ester 27 and the 3,7-diisopropylsilyl substituted 14 pi dicyclopenta[a,e]pentalene 30. The new allenic tandem Pauson-Khand reaction mediated by Mo(CO)(6) was employed as the key process to construct the core of the tetracycles. The two linear dicyclopenta[a,e]pentalenes 27 and 30 underwent significant electronic delocalization, perhaps even aromaticity, as revealed by the X-ray structure of 27. The tetracyclic rings in 27 assumed a flat geometry (Figure 4); the bond lengths of the tetracycle in 27 also fit well into the criteria for aromatic compounds. A comparison of the NMR and UV spectra of both 27 and 30 demonstrated that they both exhibited similar electronic properties, therefore, the purple colored 14 pi cross linked annulene 30 is planar as well as delocalized.     

Tetrakisdehydro[14]annuleno[20]annulene

Tetrakisdehydro[14]annuleno[20]annulene consisting of an aromatic bisdehydro[14]annulene and an antiaromatic trisdehydro[20]annulene has been synthesized. The ¹H NHR spectra clearly indicate the induction of para- and diamagnetic ring currents in each of the [4n]- and [4n+2]-rings, respectively.     

On the polyenic character of anti-bridged[14]annulene. The structure of anti-1,6:8,13-bis(difluoromethano) [14]annulene at 200K

The X-ray crystal structure determination ofanti-1,6:8,13-bis(difluoromethano) [14]annulene (1) at 200 K is described. The structure has been solved by direct methods, and least-squares refinement based on 1716 reflections of non-zero weight yielded a finalR index of 0.046. The molecular structure of1 appears to be polyenic, as expected from a comparison with the structures of previously studiedanti-bridged[14]annulenes, and in particular confirms conclusions previously reported from a study of the stucture ofanti-1,6:8,13-biscarbonyl [14]annulene.     

One-pot synthesis of N-substituted diaza[12]annulenes

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.     

Tetrakisdehydro[14]annuleno[16]annulene

Tetrakisdehydro[14]annuleno[16]annulene consisting of an aromatic bisdehydro[14]-annulene and an antiaromatic trisdehydro[16]annulene has been synthesized. The ¹H NMR parameters clearly indicate the strong paratropicity and diatropicity of the 16-membered and 14-membered rings, respectively.     

Macrocyclic Tetraethynylethene Molecular Scaffolding: Perethynylated aromatic dodecadehydro[18]annulenes,antiaromatic octadehydro[12]annulenes,and expanded radialenes

Tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne) molecular scaffolding provided access to novel macrocyclic nanometer-sized C-rich molecules with unusual structural and electronic properties. Starting from cis-bis-deprotected cis-bis(trialkylsilyl)protected tetraethynylethenes, the per(silylethynyl)ated octadehydro[12]annulenes 1 and 2 and the corresponding dodecadehydro[18]annulenes 4 and 5 were prepared by oxidative Hay coupling. X-Ray crystal-structure analyses of (i-Pr)₃Si-protected 2 and Me₃Si-protected 4 showed that both annulene perimeters are perfectly planar. Electronic absorption spectral comparisons provided strong evidence that the macro rings in the deep-purple-colored 1 and 2 are antiaromatic (4n π-electrons), whereas those in yellow 4 and 5 are aromatic ((4n + 2) π-electrons). Although unstable in solution, the antiaromatic compound 2 gave high-melting crystals in which the individual octadehydro[12]annulene chromophores are isolated and stabilized in a matrix-type environment formed by the bulky (i-Pr)₃Si groups. Electrochemical studies demonstrated that the antiaromatic octadehydro[12]annulene 2 undergoes two stepwise one-electron reductions more readily that the aromatic chromophore 5 . This redox behavior is best explained by the formation of an aromatic (4n + 2) π-electron dianion from 2 , whereas 5 loses its aromaticity upon reduction. The Me₃Si derivative 4 was deprotected with borax in MeOH/THF to give the highly unstable hexaethynyl-dodecadehydro[18]annulene 6 , a C₃₀H₆ isomer and macrocyclic precursor to a two-dimensional all-C-network. Deprotection of 2 did not give isolable amounts of tetraethynyl-octadehydro[12]annulene 3 due to the extreme instability of the latter. Starting from dimeric and trimeric acyclic tetraethynylethene oligomers, a series of expanded radialenes were obtained. They possess large C-cores with silylethynyl-protected peripheral valences and can be viewed as persilylated C₄₀ ( 7 ), C₅₀ ( 8 ), and C₆₀ ( 9 ) isomers. These expanded C-sheets are high-melting, highly stable, soluble materials which were readily characterized by laser-desorption time-of-flight (LD-TOF) mass spectrometry. Due to inefficient macrocyclic cross-conjugation and/or non-planarity, the extent of π-electron delocalization in 7 – 9 is limited to the longest linearly conjugated π-electron fragment. In agreement with these properties, all three expanded radialenes exhibited similar redox behavior; they are difficult to oxidize but undergo several reversible one-electron reductions in similar potential ranges. Presumably, the reduced π-electron delocalization is also at the origin of the particularly high stability of 7 – 9 .     

Neue 1,6-Methano[10]annulen-Derivate durch Umlagerungsreaktionen der Trimethylsilyl-Gruppe

New Derivatives of 1,6-Methano[10]annulene by Rearrangement of Trimethylsilyl Groups Oxidation of derivatives of trimethylsilyl-substituted bicyclo[4.4.1]undeca-1,3,5,7-tetraenes 1–3 leads to 1,6-methano[10]annulenes. In the case of 2 and 3a , rearrangement of the trimethylsilyl group takes place. This rearrangement can be used to prepare 2,9-disubstituted 1,6-methano[10]annulenes.     

ESR-Spectroscopic Investigation of Radical Cations,Radical Anions,and Radical Trianions of Didehydro[n]annulenes

The didehydro[n]annulenes 1 (n = 14), 2 (n = 18), and 3 (n = 22) are oxidized to radical cations. Reduction of the title compounds leads to radical anions and, in the case of 2 and 3 , to radical trianions. The hyperfine data of the paramagnetic derivatives are rationalized in terms of the occupation of n-membered (n = 14, 18, 22) π-perimeter MO's (HOMO, LUMO, NLUMO). The contact-ion pairs of the radical anions show unexpectedly large alkali-metal coupling constants.     

8,19-Dimethyl-tetraepoxy[22]annulen(2.1.2.1): ein erstes Tetraepoxy-Verbrücktes aromatisches [22]Annulen

8,19-Dimethyl-tetraepoxy[22]annulen(2.1.2.1): The First Tetraepoxy-Bridged Aromatic[22]Annulene By McMurry reaction of 5,5′-ethylidenebis[furan-2-carbaldehyde] ( 15 ), a syn/anti mixture 16 of (E,E)- and (Z,Z)-8,19-dihydro-8,19-dimethyl-tetraepoxy[22]annulene is obtained. The (E/E)-isomers 16 are the first rotation- ally active noncyclic conjugated macrocycles, where the (E)-ethenediyl moieties rotate around the connecting single bonds. The dihydro-tetraepoxy[22]annulenes 16 are dehydrogenated by (Ph₃C)BF₄ as well as by O₂ to give the tetraepoxy[22]annulene 11 . The spectroscopic data support the character of 11 as an aromatic, diatropic ring system, which is rather sensitive towards O₂. In the oxidation mixture obtained from 11 , beside polymeric products, two compounds 19 and 20 can be isolated, carrying one and two CHO groups, respectively, resulting by oxidation of one or both Me-groups but having retained the aromatic 22π system of 11 .