Formation of a novel arenium ion from the radical cation of a twisted triphenylene fully annelated with bicyclo[2.2.2]octene units

[reaction: see text]. Triphenylene 1 was synthesized by palladium-catalyzed cyclotrimerization of a benzyne annelated with two bicyclo[2.2.2]octene (BCO) units. Theoretical calculations indicated that 1 is constrained to a twisted conformation with a C2 symmetry as a result of steric repulsion between the BCO units. The one-electron oxidation of 1 with SbCl5 gave the corresponding radical cation 1*+, which abstracted a chlorine atom in the medium with the concomitant rearrangement to form novel arenium ion 2+.     

Radical cation of dibenzothiophene fully annelated with bicyclo[2.2.2]octene units: X-ray crystal structure and electronic properties

New dibenzothiophene 2 fully annelated with bicyclo[2.2.2]octene units was synthesized and oxidized to stable radical cation salt 2(*+)SbCl(6)(-), whose structure was determined by X-ray crystallography. Although the intrinsic electronic structure of 2(*+) is predicted to be close to structure A, an interaction with the counteranion makes structure B contribute significantly. A part of the salt 2(*+)SbCl(6)(-) underwent rearrangement to arenium ion 6(+), whose structure was also clarified by X-ray crystallography.     

Manifestations of the Alder-Rickert reaction in the structures of bicyclo[2.2.2]octadiene and bicyclo[2.2.2]octene derivatives

The Alder-Rickert ethylene extrusion reaction manifests in the ground state structures of compounds 9-12 which contain the bicyclo[2.2.2]octadiene moiety and compounds 13, 14, and 17-20 which contain the bicyclo[2.2.2]octene moiety. A significant decrease of the 13C-13C one-bond coupling constants for the C-C bonds, which break in this fragmentation reaction, suggests lengthening, and hence weakening of these bonds. In the unsymmetrical systems these effects are also shown to be associated with strengthening of the CH2-CH2 bond, which is ultimately lost from the molecule as ethylene. Low-temperature crystal structures of compounds 9-12 and 16 provide evidence for similar crystal packing requirements of the CH2-CH2 and CH=CH moieties.     

1,2-dithiin annelated with bicyclo[2.2.2]octene frameworks. One-electron and two-electron oxidations and formation of a novel 2,3,5,6-tetrathiabicyclo[2.2.2]oct-7-ene radical cation with remarkable stability owing to a strong transannular interaction

A stable derivative of 1,2-dithiin annelated with bicyclo[2.2.2]octene frameworks 4 was synthesized as red crystals by the reaction of a dilithiated dimer of bicyclo[2.2.2]octene with elemental sulfur in 59% yield. The cyclic voltammetry of 4 in CH(2)Cl(2) at -78 degrees C showed two reversible oxidation waves at E(1/2) +0.18 V and +0.72 V versus Fc/Fc(+), indicating that the radical cation and dication of 4 are stable under these conditions. Upon chemical one-electron oxidation of 4 in a rather low concentration (4.0 x 10(-4) M) with a 1.5 equiv of SbCl(5) in CH(2)Cl(2), a radical cation 4.+ was formed, whose spin distribution was determined by ESR spectroscopy and by the results of theoretical calculations (UB3LYP/6-31G). The electronic absorption spectrum of 4.+ in CH(2)Cl(2) exhibited a maximum absorption at 428 nm (epsilon = 2.3 x 10(3)), which was hypsochromically shifted from that of neutral 4 (469 nm). When the radical cation 4.+ was produced in higher concentration (0.06 M) in CH(2)Cl(2), a disproportionation was found to take place to give a SbCl(6)(-) salt of remarkably stable radical cation 5.+ having a novel 2,3,5,6-tetrathiabicyclo[2.2.2]oct-7-ene structure. In the X-ray structure of 5.+SbCl(6)(-), the transannular distance (2.794(3) A) between the sulfur atoms was found to be less than the sum of the van der Waals radii of a sulfur atom (3.70 A), suggesting the existence of a bonding interaction between the two disulfide linkages. The theoretical calculations (UB3LYP/6-31G) suggested that this transannular interaction could be described as the resonance between the limiting structures, each of them having a two-center three-electron bond between two sulfur atoms belonging to two different disulfide linkages: thus, both the spin and positive charge are equally delocalized to the four sulfur atoms, causing a great stabilization of 5.+. On the other hand, the 1,2-dithiin radical cation 4.+ was found to readily react with triplet oxygen with subsequent rearrangement to give the 1,2-dithiolium derivative 6+ having a carboxyl group. Finally, the reaction of 4 with an excess amount of SbF(5) gave the corresponding dication 4(2+), which was found to be a 6pi aromatic system on the basis of the results of NMR measurement and theoretical calculations.     

Synthesis of a pericosine analogue with a bicyclo[2.2.2]octene skeleton

A new analogue of the antitumor pericosines possessing a bicyclo[2.2.2]octene skeleton has been synthesized from methyl gallate using oxidative dearomatization and regio- and diastereoselective Diels-Alder reaction as the key steps.     

Autocatalytic domino Michael reaction leading to bicyclo[2.2.2]octane-2,5-dione derivatives

In the presence of a catalytic amount of lithium amide, the reaction of oxophorone with electron-deficient olefins afforded bicyclo[2.2.2]octane-2,5-dione derivatives in high yields. The reaction proceeds autocatalytically by an enolate of bicyclo[2.2.2]octane-2,5-dione, generated by an initial domino Michael reaction.     

Radical cation and dication of fluorene fully annelated with bicyclo[2.2.2]octene units: importance of the quinoidal resonance structure in the cationic fluorene

Fluorene 1 fully annelated with bicyclo[2.2.2]octene units was newly synthesized and oxidized to stable cationic species. The structure of radical cation salt 1(.+)SbCl(6)(-) was determined by X-ray crystallography, while the first fluorene dication 1(2+) was characterized by (1)H and (13)C NMR at -80 degrees C. Combined with the results of theoretical calculations, an important contribution of a quinoidal structure to the resonance hybrid was demonstrated in both 1(.+) and 1(2+). [structure: see text]     

Ionized bicyclo[2.2.2]oct-2-ene: a twisted olefinic radical cation showing vibronic coupling

The bicyclo[2.2.2]oct-2-ene radical cation (1(.+)) exhibits matrix ESR spectra that have two very different types of gamma-exo hydrogens (those hydrogens formally in a W-plan with the alkene pi bond), a(2H) about 16.9 G and a(2H) about 1.9 G, instead of the four equivalent hydrogens as would be the case in an untwisted C(2v) structure. Moreover, deuterium substitution showed that the vinyl ESR splitting is not resolved (and under about 3.5 G); this is also a result of the twist. Enantiomerization of the C(2) structures is rapid on the ESR timescale above 110 K (barrier estimated at 2.0 kcalmol(-1)). Density functional theory calculations estimate the twist angle at the double bond to be 11-12 degrees and the barrier as 1.2-2.0 kcalmol(-1). Single-configuration restricted Hartree-Fock (RHF) calculations at all levels that were tried give untwisted C(2v) structures for 1(.+), while RHF calculations that include configuration interactions (CI) demonstrate that this system undergoes twisting because of a pseudo Jahn-Teller effect (PJTE). Significantly, twisting does not occur until the sigma-orbital of the predicted symmetry is included in the CI active space. UHF calculations at all levels that include electron correlation (even semiempirical) predict twisting at the alkene pi bond because they allow the filled alpha and the beta hole of the SOMO to have different geometries. The 2,3-dimethylbicyclo[2.2.2]oct-2-ene radical cation (2(.+)) is twisted significantly less than 1(.+), but has a similar temperature for maximum line broadening. Neither the 2,3-dioxabicyclo[2.2.2]octane radical cation (3(.+)) nor its 2,3-dimethyl-2,3-diaza analogue (5(.+)) shows any evidence of twisting. Calculations show that the orbital energy gap between the SOMO and PJTE-active orbitals for 3(.+) is too large for significant PJTE stabilization to occur.     

Novel bicyclo[2.2.2]octanyl-1,4-benzodiazepinones. Their syntheses and rearrangement to bicyclooct-2-enylbenzimidazoles

The synthesis of the novel bicyclo[2.2.2]octanyl[1,4]benzodiazepinone ring system (IV) and its facile acid catalysed rearrangement to the corresponding bicyclo[2.2.2]oct-2-enylbenzirnid-azole system (IX) is described.     

Terthiophene radical cations end-capped by bicyclo[2.2.2]octene units: formation of bent pi-dimers mutually attracted at the central position

A terthiophene fused with bicyclo[2.2.2]octene units only at both ends was newly synthesized. Since there is no steric hindrance at the central position, this terthiophene has a possibility to interact only at the central position. One-electron oxidation of this terthiophene afforded a highly stable radical-cation salt as deep blue crystals. The result of X-ray crystal structural analysis demonstrated a characteristically bent pi-dimereric structure, which is formed by mutual attraction of single radical-cation species at the central position to minimize the steric repulsion. Remarkably short intermolecular distances between the central thiophene rings of each unit of the dimeric pair, that is, 2.976(10) A for Cbeta-Cbeta, 3.091(10) A for Calpha-Calpha, and 3.779(3) A for S-S, are good indication of the existence of attracting interaction, which was confirmed by theoretical calculations. This interaction was experimentally demonstrated by the reversible formation of the pi-dimer in CH2Cl2 solution using ESR and UV-vis-NIR spectroscopy. The crystal of the pi-dimer is in its singlet state and ESR silent in the solid state at 300 K, but the signal of a triplet state of the pi-dimer was observed by heating the solid at 400 K. This indicates that this pi-dimer has a quite small triplet-singlet enegy gap and the triplet state is thermally accessible.     

Structure and dynamic stability of cyclodextrin inclusion complexes with 1,4-disubstituted bicyclo[2.2.2]octanes

The properties of inclusion complexes of 1,4-di-R-bicyclo[2.2.2]octaves (R = H (1), Me (2), Cl (3), Br (4), and OH (5)) with cyclodextrins have been studied by NMR, microcalorimetry, and force-field computations. The compounds2 and3 (but not the other compounds) give dynamically stable 1:2 guest-host complexes with -cyclodextrin. Microcalorimetry of5 in water indicates a moderately strong 1:1 complex with - but weak complexes with - or -cyclodextrin. The behaviour depends on the subtle interplay size, polarity, hydrophobicity and type of solvent.     

Stereoselectivity of the reaction of mercuric acetate with dimethyl bicyclo[2.2.2]oct-2-en-5,6-cis-endo-dicarboxylate

The stereoselelectivity of the reaction of mercuric acetate with dimethyl bicyclo[2.2.2]oct-2-en-5, 6-cis-endo-carboxylate in acetic acid was studied. One of the reaction products is formed when the reagent attacks the double bond from the sterically less accessible side. The nature of the orientation effect of the carboxylate groups is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1646–1648, September, 1993.     

Diazabicycloalkanes with nitrogen atoms in the nodal positions. 21. Heteroatom-promoted lithiation of benzo[b]-1,4-diazabicyclo[2.2.2]octene and introduction of substituents into the annelated benzene ring

The action of n-butyllithium on benzo[b]-1,4-diazabicyclo[2.2.2]octene leads to lithiation in the 3 position of the aromatic ring. The reaction of this lithium derivative with electrophilic reagents was used to synthesize 3- and 3,6-substituted derivatives of benzo[b]-1,4-diazabicyclo[2.2.2]octene.See [1] for Communication 20.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 798–803, June, 1991.     

Crystal structures and spectroscopic characterization of radical cations and dications of oligothiophenes stabilized by annelation with bicyclo[2.2.2]octene units: sterically segregated cationic oligothiophenes

The remarkably stable SbF(6)(-) salts of the radical cations of bithiophene 1(2T) and terthiophene 1(3T), completely surrounded by bicyclo[2.2.2]octene (BCO) units, were obtained by one-electron oxidation of the neutral precursors with NO(+)SbF(6)(-), and their solid-state structures were determined by X-ray crystallography. In these radical cations, the presence of quinoidal character was apparent, as shown by the increased planarity and by comparison of the bond lengths with those of the neutral precursors. The shortest intermolecular pi-pi distances in the crystal structure of 1(2T)(*)(+)SbF(6)(-) (distance between two sp(2) carbon atoms, 4.89 A) and 1(3T)(*)(+)SbF(6)(-) (distance between an sp(2) carbon and a sulfur atom, 3.58 A) were found to be longer than the sums of the van der Waals radii of the corresponding atoms. In accord with this, no apparent change was observed in ESR and UV-vis-NIR spectra of solutions of 1(2T)(*)(+) and 1(3T)(*)(+) upon lowering the temperature, indicating that the pi- (or sigma-) dimer formation is inhibited in solution as well as in the solid state. The dications 1(2T)(2+) and 1(3T)(2+) were generated with the stronger oxidant SbF(5) in CH(2)Cl(2) at -40 degrees C and characterized by NMR spectroscopy. In the (1)H NMR spectra, two conformers were observed for each dication of both 1(2T)(2+) (transoid (t) and cisoid (c)) and 1(3T)(2+) (t,t and c,t) at room temperature due to the high rotational barrier around the inter-ring bond(s) between thiophene rings, which was caused by the enhanced double bond character of these bonds following two-electron oxidation. This is supported by DFT calculations (B3LYP/6-31G(d)), which predicted the rotational barriers in the dications of unsubstituted bithiophene and terthiophene to be 27.6 and 22.9 kcal mol(-)(1), respectively. In the case of quaterthiophene and sexithiophene surrounded by BCO frameworks 1(4T) and 1(6T), oxidation with even one molar equivalent of NO(+)SbF(6)(-) afforded the dication salts 1(4T)(2+)2SbF(6)(-) and 1(6T)(2+)2SbF(6)(-), which were isolated as stable single crystals and allowed the X-ray crystallography. In their crystal structures, the cationic pi-systems became planar again due to the great contribution of quinoidal resonance structures, and the pi-systems, which were arrayed in a parallel geometry, were also segregated by the steric effect of BCO units. The degree and tendency of changes in the bond lengths of thiophene rings of 1(4T)(2+) and 1(6T)(2+) as compared with neutral precursors were similar to those of 1(2T)(*)(+)SbF(6)(-) and 1(3T)(*)(+)SbF(6)(-), respectively, implying that the contribution of quinoidal character is modulated by the amount of positive charge per thiophene unit.     

The reaction of mercuric salts with dimethyl bicyclo[2.2.2]octa-2,5-diene-5,6-dicarboxylate

The regioselectivity and stereochemistry of the reaction of mercuric salts with dimethyl bicyclo[2.2.2.]octa-2,5-diene-5,6-dicarboxylate were studied in different solvents. The factors determining the reaction regioselectivity were considered.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1642–1645, September, 1993.     

Diazabicycloalkanes with nitrogen atoms in bridgehead positions 20. Synthesis of naphtho[2,3-b]-1,4-diazabicyclo[2.2.2]octene

Reaction of 2-acetamido-3-bis(2-hydroxyethyl)aminonaphthalene with phosphorus oxychloride leads to the formation of 2-methyl-1-(2-chloroethyl)naphtho[2,3-d]imidazole, while reaction with hydrobromic acid gives naphtho[23-b]-1,4-diozabicyclo[2.2.2]octane in 13% yield. The yield of the latter can be increased to 45% by exchange of the hydroxyl groups in the starting material by chlorine and by deacetylation.For Communication 19, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 97–100, January, 1991.     

The synthesis of bicyclo[2.2.2]octan-2-ones by sequential michael reactions

The reaction of cross-conjugated dienolate anions derived from substituted cyclohexenones with methyl acrylate and vinyl ketones has been studied. Bicyclo[2.2.2]octan-2-ones are formed by a sequential Michael mechanism; however, the reactions of vinyl ketones must be conducted under amine-free conditions. Unexpecedly, the conjugate base from 2,3-dimethylcyclohex-2-enone gives only a single Michael adduct with vinyl ketones, and if forced to react further undergoes an intramolecular aldol condensation. An alternative Diels-Alder approach to the bicyclo[2.2.2]octanone products is shown to be effective.     

Sustainable domino Michael reaction catalyzed by a Brønsted base on silica gel: synthesis of bicyclo[2.2.2]octane-2,5-dione derivatives

Domino Michael reactions of oxophorone and its derivatives 4 with Michael partners 3 have been effected to give bicyclo[2.2.2]octane-2,5-dione derivatives 7 by a catalytic amount of a Brønsted base (NMAP-Li) generated from N-methylaminopropylated silica gel (NMAP) and n-BuLi. The NMAP pellets were efficiently reused up to six times in 86% average yield.