Synthesis and properties of 1,6-methano[10]annuleno[3,4-c]thiophene and its 1,3-dicyano derivative

New thiophene-annulated 1,6-methano[10]annulene 1 and 2 were synthesized. The anisotropic deshielding effect from the π-electron system, based on the chemical shift values of the bridged methylene protons, is reduced compared with that of 3, and their crystal structures show clear bond alternation.     

Modulating chemical reactivity using a photoresponsive molecular switch

The kinetics of an alkylation reaction are used to probe the effect of an electron-withdrawing pyridinium group on the nucleophilicity of a free pyridine in a photoresponsive dithienylcyclopentene (DTCP) derivative in order to demonstrate effective and reversible control over chemical reactivity. The kinetic data support the hypothesis that the ring-open isomer of the DTCP (1o) is more reactive than its ring-closed counterpart (1c) due to electronic communication between the two pyridine groups existing only in the latter isomer. The rates of the alkylation reactions of bis(pyridine) versions of the photochromic compounds are also evaluated to provide a better understanding of the through-bond and the through-space effects of the groups located at the ends of the linearly conjugated π-electron backbone. The use of the two DTCP isomers (1o and 1c) as nucleophilic catalysts is also suggested in preliminary investigations.     

Syntheses of aryl- and arylethynyl-substituted N-confused porphyrins

Palladium-catalyzed cross-coupling reactions under Suzuki, Sonogashira, and Stille conditions afford 3-aryl (9-12) and 3-arylethynyl N-confused porphyrin (NCP) silver(III) complexes (13-15) from the 3-bromo NCP complex (4) in ca. 70% yields along with the transmetalated products, 3-substituted NCP palladium(II) complexes (11-Pd to 15-Pd), in 10-30% yields. Substitution at 3-position was confirmed by the single crystal X-ray structures of 9, 13-Ag, and 13-Pd. The arylethynyl groups or five-membered heterocyclic aromatic rings at 3-position largely affected the optical properties of N-confused porphyrin, in which the longest absorption maxima of the Q-bands are shifted bathochromically by 30-120 nm. The electronic effect of substituent differs largely between palladium and silver complexes reflecting the different π-electron delocalization pathway of NCP cores. 3-Aryl- and 3-arylethynyl NCP silver(III) complexes were easily demetalated to afford the corresponding free base porphyrins by the treatment of sodium borohydride.     

Reductive ring opening reactions of diphenyldihydrofullerenylpyrroles

The reductive ring opening reaction conditions for the simple [60]fullerenyldihydropyrrole 1 have been optimized to include acetic acid in the reaction mixture to rapidly protonate the anionic intermediate. Under these conditions, the ring opened dihydrofullerene 2 was obtained in 68% yield. Under slightly modified conditions and at −78 °C, the reductive bis-ring opening of the tethered trans-4 isomer 3 provided the novel racemic bis-dihydrofullerenyl derivative 7.     

Aromaticity of anthranil and its isomers, 1,2-benzisoxazole and benzoxazole

Direct computation of the π-current density, that is, the ‘ring current’, of anthranil (1) and its isomers 1,2-benzisoxazole (2) and benzoxazole (3) reveals different patterns of current flow: isomers 2 and 3 sustain strong benzene-like currents in the six-membered and bifurcated flow in the five-membered ring, whereas, in keeping with its lower thermodynamic stability, 1 has only a perimeter circulation without separate monocycle currents. Although the ring current criterion does not offer a sharp distinction between 2 and 3, their difference in thermodynamic stability is identical to that between isoxazole (4) and oxazole (5) suggesting an aromaticity order 1 < 2 ≈ 3.     

Two new Ni(II) Schiff base complexes: X-ray absolute structure determination,synthesis of a N-labelled complex and full assignment of its H NMR and C NMR spectra

The Ni(II) complex of the Schiff base of (S)-N-(2-benzoyl-4-chlorophenyl)-1-benzylpyrrolidine-2-carboxamide and glycine (1) [GKCl] and the hemihydrate of the Ni(II) complex of the Schiff base of (S)-N-(2-benzoylphenyl)-1-benzylpyrrolidine-2-carboxamide and 2-aminoisobutiric acid (2) [Me₂GK] were prepared and their absolute structures determined. The conformations of the complexes and their hydrogen bonding are discussed in detail. In complex 2, the repulsion between the benzyl group and an equatorial methyl group should affect the conformation of the benzyl group, distribution of the π-electron density in this group and distortion of the internal coordination sphere, while for complex 1, only minor conformational changes were expected. ¹H and ¹³C NMR data for the ¹⁵N-labelled complex 2 were acquired and fully assigned in order to study the influence of π-electron density of the benzyl group to the long-range ¹³C–¹⁵N and ¹³C–¹³C spin–spin interactions.     

Quantification of the (anti)aromaticity of fulvenes subject to ring size

Tria-, penta-, hepta- and nonafulvenes (1-4) have been studied theoretically at the MP2 ab initio level of theory. For the global minimum structures, the occupation of the bonding π_CC orbital of the exocyclic CC double bond, obtained by NBO analysis, quantitatively proves π-electron delocalization which can reveal partial 2-, 6- and 10-π-electron aromaticity, and 4-, 8- and 12-π-electron antiaromaticity of the ring moieties. Beside the corresponding occupation number, this conjugation was quantified by the length of the exocyclic CC double bond whilst the (anti)aromaticity of the ring moieties of 1-4 was visualized and quantified by through space NMR shielding surfaces (TSNMRS).     

Efficient and regioselective chromium(0)-catalyzed reaction of 2-substituted furans with diazo compounds: stereoselective synthesis of (2E,4Z)-2-aryl-hexadienedioic acid diesters

Pentacarbonyl(η²-cis-cyclooctene)chromium(0) (1) catalyzes efficiently reactions of diazo compounds with electron-rich furans. The reaction of 2-methoxyfuran (2) with alkyl α-diazoarylacetate (3a-g) furnishes the (2E,4Z)-2-aryl-hexadienedioic acid diesters (4a-g) in excellent yields. These reactions are highly regioselective. The cyclopropanation intermediates formed from 1 and diazo compounds 3a-g always arise from a carbene addition to the less substituted CC bond of 2. The resulting cyclopropanation product undergoes a ring opening reaction to form the corresponding (2E,4Z)-2-aryl-hexadienedioic acid diesters (4a-g). The pentacarbonylchromium(0)-catalyzed reactions of 2-alkylfuran (5a-b) with ethyl α-diazophenylacetate (3a) and 9-diazo-9H-fluorene (3h) produce the 1(E),3(E)-butadienes (6a-d) in very good yields.     

Donor-acceptor organic sensitizers assembled with isoxazole or its derivative 3-oxopropanenitrile

A series of new donor-acceptor organic sensitizers LJ5-LJ7 assembled with isoxazole or its derivative 3-oxopropanenitrile was synthesized and used in dye-sensitized solar cells to study the effect of both π-spacers and anchoring groups on device performance. X-ray crystal structure analyses show planar arrangement of dithiophene and isoxazole fragments, providing an excellent π conjugation and hence the large electronic coupling between the triphenylamine donor and electron acceptor units. For the DSSC applications, the derived photophysical and photovoltaic properties revealed a lower η value of 1.06% (LJ7) and 2.41% (LJ5), and 3.30% (LJ7) for the dyes with rhodanine-3-acetic acid, cyanoacetyl group, and vinyl cyanoacetic acid as the anchor. Electrochemical impedance spectroscopy was used to determine the interfacial charge transfer process occurring in solar cells that employed different dyes.     

Selective formation of heterodimeric resorcinarene capsules

C_2V-symmetrical resorcinarene tetraesters 1 form in CDCl₃ dimeric capsules encapsulating one Me₄N⁺ cation. The homodimeric capsules of the tetra(3,4,5-trimethoxybenzoate) 1d and tetrabenzoate 1b or tetrafuroylate 1c disproportionate in solution to give quantitatively the heterodimers. The higher stability of the heterodimer is, most probably, caused by π-π attractions between the π-basic trimethoxyphenyl rings of 1d and relatively more π-accepting phenyl or furyl fragments of 1b or 1c.     

The effect of fluorine atoms on gas transport properties of new polynorbornene dicarboximides

The new N-4-trifluoromethylphenyl-norbornene-5,6-dicarboximide (2a) and N-3,5-bis(trifluoromethyl)phenyl-norbornene-5,6-dicarboximide (2b) mixtures of exo and endo monomers were synthesized and polymerized via ring opening metathesis polymerization (ROMP) using bis(tricyclohexylphosphine) benzylidene ruthenium(IV) dichloride (I) and tricyclohexylphosphine [1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene] ruthenium dichloride (II) to produce the corresponding polynorbornene dicarboximides Poly-2a and Poly-2b, respectively. The transport of five gases He, N₂, O₂, CO₂ and CH₄ across membranes prepared from Poly-2a was determined at 35 °C using a constant volume permeation cell. The gas transport properties of the fluorine containing polymer Poly-2a were compared with those found for membranes from non-fluorinated poly(N-phenyl-exo-endo-norbornene-5,6-dicarboximide) (P-PhNDI). Gas permeability, diffusion and solubility coefficients of the fluorine containing polynorbornene Poly-2a were up to an order of magnitude larger than those of the non-fluorinated one. Poly-2a was found to have one of the largest gas transport coefficients reported to date in glassy polynorbornene dicarboximides.     

Formation of spiro ketolactones versus alkoxy radical fragmentation-promoted three-atom ring enlarged lactones from cyclic ketones

A dual pathway from readily available 2-allyl-2-carboethoxycycloalkanones 1 provides a new facile stereoselective synthesis either of functionalized spiro ketolactones 4 or of ring enlarged lactones 7 in one-step. Thus, iodination of 5-8-membered 2-allyl-2-carboethoxycycloalkanones 1a-d led, in excellent yields, to spiro ketolactones 4a-d, respectively, as single stereoisomers. On the other hand, iodination of 1a-d under alkoxy radical fragmentation conditions via incipient hemiketals produced the 8-, 9-, 10-, or 11-membered, three-atom ring enlarged, poly-functionalized lactones 7a-c as two stereoisomers and 8 as a single isomer.     

Effects of chemical variations on the mesophase behavior of new fluorinated poly(vinylcyclopropane)s

Several new structures of fluorinated polymers poly(1)-poly(9) were prepared by free radical ring opening polymerization of vinylcyclopropane monomers 1-9 containing different fluorinated side groups of the type (CH₂)_n(CF₂)_pF. While in poly(1)-poly(3) p varied from 6 to 10 for a fixed n=2, in poly(4)-poly(6) n increased from 3 to 5 at the given p=8. In poly(7) and poly(8) a phenyl ring was incorporated to elongate the mesogenic side group (n=2; p=6 and 8, respectively), that was further separated from the polymer backbone by a methylene spacer (m=11) in poly(9). Therefore, the effects of various chemical variations of the polymer structure on the mesophase behavior could be assessed. The polymers were in fact co-polymers comprising both 1,5-linear and cyclobutane-ring isomer units. In any case they formed smectic mesophase(s) owing to the special character of the perfluorinated chains. The order and the isotropization temperature (T_i) of the mesophase were enhanced by increasing p, but T_i lowered with increasing n. Extension of the side group by insertion of a phenyl ring improved T_i. Wide angle X-ray diffraction studies clarified the nature of the different smectic phases, the occurrence of which was discussed in terms of the ability of the fluorinated side groups to pack antiparallel in either a partly or fully interdigitated structure. Co-polymers of 3 with a non-mesogenic, not fluorinated co-monomer 10 were also prepared with different chemical compositions. Co-polymerization was found to be another effective means of modifying the mesophase behavior of the poly(vinylcyclopropane)s.     

Synthesis and molecular structures of dinuclear complexes with 1,2-dihydro-1,2-diphenyl-naphtho[1,8-c,d]1,2-diphosphole as a bridging ligand

A bisphosphine in which a PhP-PPh bond bridges 1,8-positions of naphthalene, 1,2-dihydro-1,2-diphenyl-naphtho[1,8-cd]-1,2-diphosphole (1), was used as a bridging ligand for the preparation of dinuclear group 6 metal complexes. Free trans-1, a more stable isomer having two phenyl groups on phosphorus centers mutually trans with respect to a naphthalene plane, was allowed to react with two equivalents of M(CO)₅(thf) (M = W, Mo, Cr) at room temperature to give dinuclear complexes (OC)₅M(μ-trans-1)M(CO)₅ (M = W (2a), Mo (2b), Cr (2c)). The preparation of the corresponding dinuclear complexes bridged by the cis isomer of 1 was also carried out starting from the free trans-1 in the following way. Mono-nuclear complexes M(trans-1)(CO)₅ (M = W (3a), Mo (3b), Cr (3c)) which had been prepared by a reaction of trans-1 with one equivalent of the corresponding M(CO)₅(thf) (M = W, Mo, Cr) complex, were heated in toluene, wherein a part of the trans-3a-c was converted to their respective cis isomer M(cis-1)(CO)₅. Each cis trans mixture of the mono-nuclear complexes 3a-c was treated with the corresponding M(CO)₅(thf) to give a cis trans mixture of the respective dinuclear complexes 2a-c. The cis isomer of the ditungsten complex 2a was isolated, and its molecular structure was confirmed by X-ray analysis, showing a shorter W?W distance of 5.1661(3) Å than that of 5.8317(2) Å in trans-2a.     

Quinoxaline-walled π-stacking molecules: synthesis,conformation, and luminescence properties

A series of semi-rigid N-phenylsuccinimides I (11Xy), N-arylmethylsuccinimides II (18Xy), and N,N′-(1,4-arylenebis(methylene))disuccinimides III (19XyX) walled by quinoxaline (QX), dimethylquinoxaline (diMQX), or benzoquinoxaline (BQX) ring were synthesized. Intramolecular π–π interactions in solution were detected by NMR and fluorescence spectroscopy, in terms of reciprocal anisotropic shielding and exciplex formation, respectively. The X-ray crystallographic analysis revealed the preferential conformations in solid state, which were compatible with the results of conformational studies by NMR and fluorescence spectroscopy. The results collectively indicated that 18Xy and 19XyX are driven by intramolecular π–π interactions in the mode of π-stacking (face-to-face) or T-shaped (edge-to-face) configuration to preferentially adopt the folded- and the double folded-conformation, respectively, where the spaced aromatic rings are oriented syn to each other in close proximity.     

An efficient substituent dependent synthesis of congested pyridines and pyrimidines

An efficient and versatile synthesis of various congested pyridines 3a-h, 6a,b, 8a-n, 10a-g, and 16a,b, and (pyrimidin-4-yl)acetonitriles 13a-g has been delineated by base catalyzed ring transformation of suitably functionalized 2H-pyran-2-ones 1a-h, 5, 7, and 15 by formamidine acetate 2a, acetamidine hydrochloride 2b, S-methylisothiourea 9a, pyrazol-1-yl-carboxamidine 9b, and arylamidine hydrochloride 12 separately in the presence of powdered KOH in dry DMF.     

Formation of new organoselenium heterocycles and ring reduction of 10-membered heterocycles into seven-membered heterocycles

Reacting 2,4-bis(phenyl)-1,3-diselenadiphosphetane-2,4-diselenide [{PhP(Se)(μ-Se)}₂], Woollins’ reagent, with an equivalent of aromatic diol in refluxing toluene afforded 10-membered phosphorus-selenium heterocycles 1 and 2 with an O-P(Se)-Se-Se-P(Se)-O linkage. Two equivalents of aromatic diol and Woollins’ reagent in refluxing toluene gave seven-membered phosphorus-selenium heterocycles 3 and 4 with an O-P(Se)-O linkage together with 10-membered phosphorus-selenium heterocycles 1 and 2. It was also found that the diphosphorus species O-P(Se)-Se-Se-P(Se)-O rings 1 and 2 could be readily ring contracted into the monophosphorus rings 3 and 4 in almost quantitative yields via further reaction with another equivalent of corresponding aromatic diol. One representative X-ray structure is reported.     

Total synthesis of benzo[c]phenanthridine alkaloids based on a microwave-assisted electrocyclic reaction of the aza 6π-electron system and structural revision of broussonpapyrine

Total syntheses of the des-N-methyl (nor) type of benzo[c]phenanthridine alkaloids 1a-f and 19 and benzo[c]phenanthridine alkaloids, chelerythrine (2d), and broussonpapyrine (2f) were achieved. The key step was the construction of tetracyclic 10,11-dihydrobenzo[c]phenanthridines using a microwave-assisted electrocyclic reaction of the 2-cycloalkenylbenzaldoxime methyl ether 4 as an aza 6π-electron system, which was derived in two steps from a Suzuki-Miyaura cross-coupling reaction of 2-bromobenzaldehyde 6 with 2-(3,4-dihydro-6,7-methylenedioxynaphthyl)boronic acid pinacol ester 7. In addition, the exact structure of broussonpapyrine (2f) (2,3,9,10-tetraoxygenated type) was determined to be chelerythrine (2d).     

Access to ring-fused azepino[3,4-b]indole-1,5-dione derivatives by ring-closing olefin metathesis

The new ring-fused azepino[5,6-b]indole derivatives 3, 4 and 5 were prepared from diene precursors 8, 9, 13 and 15 in fair yields via a final ring-closing olefin metathesis.     

Marked difference in singlet-chemiexcitation efficiency between syn-anti isomers of spiro[1,2-dioxetane-3,1′-dihydroisobenzofuran] for intramolecular charge-transfer-induced decomposition

Spiro[1,2-dioxetane-3,1′-dihydroisobenzofuran] syn-3 bearing a hydroxy group at the 6-position (as a model syn-rotamer of parent dioxetane 4 bearing a 3-hydroxyphenyl group) and its isomer anti-3 (as a model anti-rotamer of 4) were synthesized. When these spiro-dioxetanes were treated with tetrabutylammonium fluoride (TBAF) in DMSO, anti-3 emitted light with high efficiency (Φ^CL = 0.41), while the respective value for syn-3 was only 1/10 for anti-3. This significant difference in Φ^CL between syn-3 and anti-3 was attributed to the difference in their singlet-chemiexcitation efficiencies.