Magnetic susceptibilities and resonance energies of annelated [14] - and [18] annulenes

Calculations of magnetic susceptibilities and topological resonance energies of annelated [14] - and [18] annulenes are reported. For several compounds the magnetic susceptibilities are also measured. In these cases a good agreement between calculated and experimental values of magnetic susceptibilities is obtained. All compounds studied are found to be diamagnetic. According to the magnetic exaltation criterion all compounds studied are predicted to be aromatic. This result is supported in the case of annelated annulenes by the TRE criterion. However, the parent structures, [14] annulene and [18] annulene, are found by the TRE to be non-aromatic. Finally, it is found that the aromatic stability does not parallel inert reactivity, i.e., some highly aromatic annulenoids are predicted by the low HOMO—LUMO separation to be rather reactive and some low aromatic structures are predicted by the high HOMO—LUMO separation to exhibit a considerable reactive inertness.     

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 .     

Fluorescence from the second excited singlet state of [18] annulenes

The emission spectra of the second excited singlet state of [18] annulene and of monofluoro [18] annulene (in a 3-methylpentane glass at 4 K) are reported. The large energy gap between the first and second excited singlet states inhibits fast internal conversion and favours the appearance of S₂ → S₀ emission. In addition, fluorescence from the S₁ state can be observed in monofluoro [13] annulene by exciting into the S₂ or directly into the S₁ absorption.     

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 .     

7,10,14-Tri-t-butyl-3-amino-1,8-bisdehydro[14]annulene

Tri-t-butylcarboxybisdehydro[14]annulene was converted by the Curtius reaction into unstable aminoannulene, which could be characterized as N-acetyl derivative. The pK_a-value of the aminoannulene reflects aromatic nature of the annulene nucleus.     

Benzene Oxide-Oxepin Valence Tautomerism

Benzene oxide and the potential 8π-electron system oxepin exist in valence-tautomeric equilibrium with each other, to which both components contribute to approximately the same extent. NMR spectroscopic measurements show that the equilibrium is rapidly established (activation energies of the forward and reverse reactions 9.1 and 7.2 kcal mole^?1, respectively). The present knowledge of the properties of oxepin justifies its classification as a “heterotropilidene”. Benzene oxide-oxepin represents a system having fluctuating bonds, the equilibrium of which can be displaced from one extreme to the other by means of suitable substituents. The oxide component determines the reactions of the system with most agents. With 1,6-oxido[10]annulene, which is formally a 2,7-bridged oxepin, the oxepin character is completely suppressed by the formation of a delocalized 10π-electron system extending over the C₁₀ perimeter. The existence and aromatic character of 1,6-oxido[10]-annulene give rise to the conception of a homologous series of oxygen bridged annulenes (1,6; 8,13-bisoxido[14]annulene, 1,6; 8,17; 10,15-trisoxido[18]annulene etc.), which, like the parent acenes, possess a (4n + 2)π-electron system. Molecular models demonstrate that a considerable flattening of the C_4n+2 perimeter is achievable in the case of a syn or all-syn arrangement of the oxygen bridges, and that the requirement for aromaticity is thus satisfied. This is confirmed in a striking manner by the synthesis and properties of syn-1,6; 8,13-bisoxido[14]annulene.     

Syntheses and Properties of Donor/Acceptor Arylethynyl-Substituted 1, 6-Methano[10]annulenes

The stepwise palladium-catalyzed coupling of dibromo-1,6-methano[10]annulene derivatives to a phenyl-acetylene substituted by an electron-withdrawing group followed by coupling with a phenylacetylene possessing an electron-donating group allows the preparation of donor/acceptor systems which contain three aromatic moieties linked by ethynediyl bridges. The same type of cross-coupling starting from 2-ethynyl-1,6-methano[10]annulene and bromo-substituted azobenzene derivatives provides easy access to a new class of azo dyes. The properties of these compounds are compared to those of analogous compounds containing only benzene rings.     

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.     

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.     

Synthesen und chemische Reaktionen neuer Chinonderivate mit 1,6-Methano[10]annulenpartialstruktur oder ihrer Valenztautomeren

Syntheses and Chemical Reactions of New Quinone Derivatives with 1,6-Methano[10]annulene Fragments or their Valencetautomers The coupling of 2-Methoxy-1,6-methano[10]annulene with aryldiazonium salts leads to the corresponding quinone hydrazones. These compounds undergo reduction with LiAlH₄, condensation with hydrazines and hydrazides and alkylation at the carbonyl O-atom or the hydrazone N-atom.     

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.     

Cyclopenta Ring Fused Bisanthene and Its Charged Species with Open‐Shell Singlet Diradical Character and Global Aromaticity/ Anti‐Aromaticity

Cyclopenta ring fused bisanthene and its charged species were synthesized. The neutral compound has an open‐shell singlet ground state and displays global anti‐aromaticity. The dication also exhibits singlet diradical character but has a unique [10]annulene‐within‐[18]annulene global aromatic structure. The dianion is closed‐shell singlet in the ground state and shows global aromaticity with 22 π electrons delocalized on the periphery. These findings prrovide new insight into the design and properties of global aromatic/anti‐aromatic systems based on π‐conjugated polycyclic hydrocarbons.     

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.     

Recommendations for the evaluation of aromatic stabilization energies

[reaction: see text] Perturbing influences, such as strain and the presence of heteroatoms, complicate the evaluation of aromatic stabilization energies (ASE). These complications are overcome easily by employing the "isomerization method". This is based on the differences (ISE) between total energies computed for only two species: a methyl derivative of the aromatic system and its nonaromatic exocyclic methylene isomer. The ISEs of benzene, pyridine, and naphthalene as well as the strained [5]paracyclophane, methano[10]annulene, and octadehydro[14]annulene are evaluated.     

Tetrakisdehydro[16]annuleno[18]annulene

Tetrakisdehydro[16]annuleno consisting of trisdehydro[16]annulene and bisdehydro[18] annulene has been synthesized. Induction of para- and diamagnetic ring currents in 16- and 18-membered rings, respectively, was clearly recognized by the ¹H NMR spectroscopy. A marked suppresion of the diatropicity in the 18π moiety was observed in the same trend as observed in tetrakisdehydro[14]annuleno[16]annulene.     

On the reaction of 4-substituted trimethyltin aromatics with perchlorylfluoride

To evaluate the suitability of [¹⁸F]perchorylfluoride [¹⁸F]FClO₃ as an electrophilic fluorination agent for the preparation of radiopharmaceuticals, the reactivity non-radioactive FClO₃ towards 4-substituted trimethyltin aromatic compounds was studied. Contrary to the expectation, an electrophilic fluorination of the aromatic nucleus did not occur. The reaction of perchlorylfluoride with aromatic trimethylstannyl compounds resulted in the formation of trimethyltin fluoride and the respective destannylated aromatics in variable yields.     

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.     

Synthesis and structures of cross-conjugated bis-dehydroannulenes with a Y-enediyne motif and different pi topologies

A synthesis of cross-conjugated bis-dehydroannulenes with different topologies of the pi electrons by Cu(II)-mediated oxidative coupling of the corresponding terminal acetylenic precursors is reported. In general, of the two possible modes of cyclization, which would yield either a [13]annulene or an [18]annulene, the precursors yielded exclusively the bis-dehydro[13]annulenes. However, one example of the formation of a bis-dehydro[18]annulene is also reported. The mode of cyclization to form either the [13]annulene or the [18]annulene is explained on the basis of the conformational preference of the core unit bearing the Y-enediyne moieties. The structures of the two types of bis-annulenes have been unequivocally established by means of single-crystal X-ray crystallographic analysis.     

Möbius aromaticity in [12]annulene: cis-trans isomerization via twist-coupled bond shifting

Density functional and coupled cluster calculations show that facile thermal configuration change in [12]annulene occurs via a twist-coupled bond-shifting mechanism. The transition state for this process is highly aromatic with M?bius topology. At the CCSD(T)/cc-pVDZ//BH&HLYP/6-311+G** level, the isomerization of tri-trans-[12]annulene 1a (CTCTCT) to its di-trans isomer 2 (CCCTCT) via such a mechanism has a barrier of 18.0 kcal/mol, in good agreement with earlier experiments. Two other aromatic M?bius bond-shifting transition states were located that result in configuration change for other [12]annulene conformers. This mechanism contrasts sharply with diradical configuration change for acyclic polyenes and with planar bond-shifting mechanisms generally assumed for annulenes. This constitutes evidence that neutral M?bius aromatic annulenes play a role in the dynamic processes of neutral [4n]annulenes.     

Aromatic pathways of porphins, chlorins, and bacteriochlorins

Magnetically induced current densities have been calculated for free-base porphynoids using the gauge including magnetically induce current (GIMIC) method. Numerical integration of the current density passing selected chemical bonds yields current pathways and the degree of aromaticity according to the magnetic criterion. The ring-current strengths of the porphins, chlorins, and bacteriochlorins are 1.5-2.5 times stronger than for benzene. The calculations show that the 18π [16]annulene inner cross is not the correct picture of the aromatic pathway for porphyrins. All conjugated chemical bonds participate in the current transport independently of the formal number of π electrons. The ring current branches at the pyrrolic rings taking both the outer and the inner route. The NH unit of the pyrrolic rings has a larger resistance and a weaker current strength than the pyrroles without inner hydrogens. The traditional 18π [18]annulene with inactive NH bridges is not how the ring-current flows around the macroring. The porphins have the strongest ring current of ca. 27 nA/T among the investigated porphynoids. The current strengths of the chlorins and bacteriochlorins are 19-24 nA/T depending on whether the ring current is forced to pass an NH unit or not. The current strengths of the 3-fold and 4-fold β-saturated porphynoids are 13-17 nA/T, showing that the inner-cross 18π [16]annulene pathway is not a preferred current route.