Persistance of the Cyclopnane Radical Anions and its Relation to Structure

Reactions of [2_N]cyclophanes (N = 2, ?6) with solvated electrons in 1,2-di-methoxyethane at 193 K have been studied by ESR. and ENDOR. spectroscopy. All but the two most highly bridged cyclophanes (N = 5 and 6) are reduced to paramagnetic species under these conditions. Whereas the radical anions of [2.2]-paracyclophane and [2₃](1,2,4)- and [2₄](l,2,4,5)cyclophanes are sufficiently persistent to be characterized by their hyperfine data, those of the remaining five cyclophanes undergo a rapid cyclization to the radical anions of 4,5,9,10-tetrahydropyrenes. These have been identified as the unsubstituted tetrahydropyrene (from [2.2]-metacyclophane and [2₃](l,2,3)cyclophane), the 2,7-dimethyl-derivative (from [2₃](1,3,5)- and [2₄](l,2,3,5)cyclophanes) and the 1,8-dimethyl-derivative (from (2₄l,2,3,4)cyclophane). The persistence of the cyclophane radical anions seems to depend on the numbers, n_meta and n_para, of the meta-and para-positions of the bridging ethano groups in the two benzene rings. The prerequisite for the radical anion to be persistent is n_meta?n_para.     

The Chemical Behavior of Multibridged [2n] Cyclophanes

Multibridged [2_n] cyclophanes comprise a novel class of aromatic compounds, in which two benzene rings are connected by three up to a maximum of six ethano bridges. Compared to classical nonannelated arenes, their most distinct chemical property is the ease with which, particularly the higher-bridged homologs, take part in addition reactions (Diels-Alder additions, hydrogenations, and ionic additions). On the other hand, the typical regenerative behavior of aromatic molecules is not fully suppressed in the [2_n]cyclophanes as evidenced by such typical electrophilic aromatic substitution reactions as bromination, Friedel-Crafts acylation, and nitration. Besides these reactions in which the benzene rings are either destroyed or retained, reactions at and with the ethano bridges, e.g. their cleavage, isomerization, and functionalization can also occur.     

Synthesis of meta- and paracyclophanes containing unsaturated amino acid residues

[7.7], [8.8], [9.9] and [13.13] Paracyclophanes and [9.9] and [13.13] metacyclophanes containing two unsaturated amino acid residues have been synthesised. An X-ray crystallographic study of three of the paracyclophanes and molecular modelling of two paracyclophanes and two metacyclophanes revealed two main structural types. The ‘staggered’ structure appears to be favoured by longer hydrocarbon chains, whilst the ‘barrel’ structure appears to be more accessible to compounds containing shorter hydrocarbon chains. The [9.9] paracyclophane has been hydrogenated and deprotected to give a saturated amino acid, and an alternative approach to key aldehydes is reported.     

Phenylene‐bridged Porphyrin meso‐Oxy Radical Dimers

Stable meta‐ and para‐phenylene bridged porphyrin meso‐oxy radical dimers and their Ni^II and Zn^II complexes were synthesized. All the dimers exhibited optical and electrochemical properties similar to the corresponding porphyrin meso‐oxy radical monomers, indicating small electronic interaction between the two spins. Intramolecular spin‐spin interaction through the π‐spacer was determined to be J/k_B=?15.9 K for m‐phenylene bridged Zn^II porphyrin dimer. The observed weak antiferromagnetic interaction has been attributed to less effective conjugation between the porphyrin radical and linking π‐spacer due to large dihedral angle. In the case of Zn^II complexes, both para‐ and meta‐phenylene bridged dimers formed 1D‐chain in solutions and in the solid states through Zn‐O coordination.     

Heterobi‐ and ‐trinuclear Complexes with an Amino(ethynyl)carbene as Bridging Ligand

The sequential reaction of the amino(trimethylsilyl)carbene complex [(CO)₅W=C(NH₂)C≡CSiMe₃] ( 1 ) with nBuLi and [I‐Fe(CO)₂Cp] affords the C(carbene)‐N bridged heterobinuclear complex [(CO)₅W=C{NHFe(CO)₂Cp}C≡CSiMe₃] ( 2 ). Desilylation of 1 is achieved by treatment with KF in THF/MeOH. From the reaction of the resulting complex [(CO)₅W=C(NH₂)C≡CH] ( 3 ) with nBuLi and [I‐Fe(CO)₂Cp] two binuclear WFe compounds in a ratio of approximately 1:1 are obtained: the C(carbene)‐C≡C bridged complex 4 and the C(carbene)‐N bridged complex 5 . Repetition of the deprotonation/metallation sequence yields the trinuclear WFe₂ complex 6 . One Fe(CO)₂Cp fragment in 6 is bonded to the amino group and the other one to the terminal carbon atom of the ethynyl substituent. The analogous reaction of 3 with nBuLi and [Br‐Ni(PMe₂Ph)₂Mes] gives a ca. 1:1 mixture of two heterobinuclear complexes ( 7 and 8 ). Complex 7 is bridged by the C(carbene)‐C≡C and complex 8 by the C(carbene)‐N fragment. Subsequent reaction of 7 with BuLi and [Br‐Ni(PMe₂Ph)₂Mes] finally affords the trinuclear WNi₂ complex 9 related to 6 . The solid‐state structure of 2 is established by an X‐ray diffraction analysis. The spectroscopic data of the bi‐ and trinuclear complexes indicate electronic communication between the metal centers through the bridging group.     

A ‘meta effect’ in the fragmentation reactions of ionised alkyl phenols and alkyl anisoles

The competition between benzylic cleavage (simple bond fission [SBF]) and retro‐ene rearrangement (RER) from ionised ortho, meta and para RC₆H₄OH and RC₆H₄OCH₃ (R = n‐C₃H₇, n‐C₄H₉, n‐C₅H₁₁, n‐C₇H₁₅, n‐C₉H₁₉, n‐C₁₅H₃₁) is examined. It is observed that the SBF/RER ratio is significantly influenced by the position of the substituent on the aromatic ring. As a rule, phenols and anisoles substituted by an alkyl group in meta position lead to more abundant methylene‐2,4‐cyclohexadiene cations (RER fragmentation) than their ortho and para homologues. This ‘meta effect’ is explained on the basis of energetic and kinetic of the two reaction channels. Quantum chemistry computations have been used to provide estimate of the thermochemistry associated with these two fragmentation routes. G3B3 calculation shows that a hydroxy or a methoxy group in the meta position destabilises the SBF and stabilises the RER product ions. Modelling of the SBF/RER intensities ratio has been performed assuming two single reaction rates for both fragmentation processes and computing them within the statistical RRKM formalism in the case of ortho, meta and para butyl phenols. It is clearly demonstrated that, combining thermochemistry and kinetics, the inequality (SBF/RER)_meta < (SBF/RER)_ortho < (SBF/RER)_para holds for the butyl phenols series. It is expected that the ‘meta effect’ described in this study enables unequivocal identification of meta isomers from ortho and para isomers not only of alkyl phenols and alkyl anisoles but also in other alkyl benzene series. Copyright © 2012 John Wiley & Sons, Ltd.     

Indeno[1,2‐b]fluorene‐Based [2,2]Cyclophanes with 4n/4n and 4n/[4n+2] π Electrons: Syntheses,Structural Analyses,and Excitonic Coupling Properties

Indeno[1,2‐b]fluorene‐based [2,2]cyclophanes with 4n/4n and 4n/[4n+2] π‐electron systems were prepared, and their structures were identified by X‐ray crystallography. With short π–π distances around 3.0 Å, [2.2](5,11)indeno[1,2‐b]fluorenophane and its precursor [2.2](5,11)indeno[1,2‐b]fluorene‐6,12‐dionophane exhibit remarkable transannular interactions, leading to their unusual electrochemical and photophysical properties. With the aid of femtosecond transient absorption spectroscopy, the transition from the monomeric excited state to the redshifted H‐type dimeric state was first observed, correlating to the calculated excitonic energy splitting and the steady‐state absorption spectra induced by charge‐transfer‐mediated superexchange interaction.     

Synthesis,characterization and catalytic activity in Suzuki–Miyaura coupling of palladacycle complexes with n‐butyl‐substituted N‐heterocyclic carbene ligands

A series of monomeric palladacycle complexes bearing n‐butyl‐substituted N‐heterocyclic carbenes, namely [Pd(NHC)X(dmba)] (dmba: dimethylbenzylamine and [Pd(NHC)X(ppy)]; NHC: 1‐n‐butyl‐3‐substituted benzylimidazol‐2‐ylidene; ppy: 2‐phenylpyridine), were prepared either by transmetallation from the corresponding silver carbene complexes or by the reaction of the corresponding acetate‐bridged palladacycle dimer with N‐heterocyclic carbene ligands in high yields. The palladium(II) complexes were characterized using elemental analyses, APCI‐MS, ¹H NMR and ¹³C NMR spectroscopies. These complexes are efficient in the Suzuki–Miyaura coupling reaction between phenylboronic acid and aryl bromides.     

Indeno[1,2‐b]fluorene‐Based [2,2]Cyclophanes with 4n/4n and 4n/[4n+2] π Electrons: Syntheses,Structural Analyses,and Excitonic Coupling Properties

Indeno[1,2‐b]fluorene‐based [2,2]cyclophanes with 4n/4n and 4n/[4n+2] π‐electron systems were prepared, and their structures were identified by X‐ray crystallography. With short π–π distances around 3.0 Å, [2.2](5,11)indeno[1,2‐b]fluorenophane and its precursor [2.2](5,11)indeno[1,2‐b]fluorene‐6,12‐dionophane exhibit remarkable transannular interactions, leading to their unusual electrochemical and photophysical properties. With the aid of femtosecond transient absorption spectroscopy, the transition from the monomeric excited state to the redshifted H‐type dimeric state was first observed, correlating to the calculated excitonic energy splitting and the steady‐state absorption spectra induced by charge‐transfer‐mediated superexchange interaction.     

Binuclear meta‐xylyl‐linked Ag(I)‐N‐heterocyclic carbene complexes of N‐alkyl/aryl‐alkyl‐substituted bis‐benzimidazolium salts: synthesis,crystal structures and in vitro anticancer studies

Salts of meta‐xylyl‐linked N‐ethyl/n‐butyl/benzyl‐substituted bis‐benzimidazolium having hexafluorophosphate counterions have been synthesized. The corresponding binuclear Ag(I)‐N‐heterocyclic carbene complexes were prepared by the reaction of Ag₂O. The N‐heterocyclic carbene (NHC) ligand precursor 7 and Ag(I)–NHC complexes 10 and 11 have been structurally characterized by single‐crystal X‐ray diffraction technique. All of the reported compounds have been tested for their anticancer activity using human colorectal (HCT 116) cancer cell lines. Sterically varied benzimidazolium salts displayed significant activity against HCT 116 cell line, yielding IC₅₀ values in the range 0.1–19.4 µ m , while Ag(I)–carbene complexes showed exceptionally good activity (0.2–1.3 µ m ) against tested cancer cell lines. Copyright © 2013 John Wiley & Sons, Ltd.     

Site‐Selective Linear Alkylation of Anilides by Cooperative Nickel/Aluminum Catalysis

We report meta‐ and para‐selective linear alkylation reactions of anilides with alkenes by nickel/N‐heterocyclic carbene (NHC) and aluminum catalysis. With a less bulky NHC, the alkylation reaction of N‐methyl‐N‐phenylcyclohexanecarboxamides proceeded mainly at the meta position. In contrast, a bulky NHC ligand led to the para‐selective alkylation of N‐sec‐alkyl anilides.     

NMR studies of conformations and dynamic processes—III : Cyclophanes with unsaturated bridges

Three cyclophanes, each displaying a different type of dynamic process, have been studied by NMR methods. The barriers to these processes are attributed mainly to the decrease in π-electron overlap between the benzene rings and adjacent double bonds which occurs in the transition state for each process. In [5₂] paracyclophanetetraene, two successive flippings of the benzene rings interconvert the two hydrogens in the methylene groups (Scheme 1). In tetramethyl [2₄] paracyclophanetetraene, the passage of one methyl group through the central cavity of the molecule interconverts two conformations of similar, but not equal, free energy (Scheme 2). In [2₆] orthoparacyclophanehexaene, the orthosubstituted rings change sides by passing through the centre of the cyclophane (Scheme 3).     

meta‐ and para‐Phenylenediamine‐Fused Porphyrin Dimers: Synthesis and Magnetic Interactions of Their Dication Diradicals

meta‐ and para‐Phenylenediamine‐fused nickel(II) porphyrin dimers were synthesized by S_NAr reaction of meso,β,β‐trichloro nickel(II) porphyrin with meta‐ and para‐phenylenediamines and subsequent Pd‐catalyzed intramolecular C?H arylation. Their tetrachlorinated dication diradicals are very stable, allowing SQUID magnetometry and revealing clear open‐shell characters for both meta and para isomers with ferro‐ and anti‐ferromagnetic interactions, respectively. The nitrogen analogue of Thiele's hydrocarbon usually displays predominant closed‐shell nature but its hidden diradical characters increase either in a twisted conformation or upon insertion of an additional phenylene spacer. The observed distinct diradical nature of the para‐congener indicates that diradical properties can be enhanced also by efficient spin delocalization.     

How the Substitution Faraway from NHCs Affects the Structural Features and Catalytic Activity of Dicarbene Dipalladium Complexes

A series of [PdPyCl₂]₂(di‐NHC) complexes were prepared (di‐NHC are two 1‐(2,6‐dimethylphenyl)imidazolidene molecules bridged by an aliphatic –(CH₂)_n– linker (n = 3, 4, 5, 6, and 10)). All complexes were fully characterized by NMR spectroscopy and elemental analyses. The crystal structures of four complexes (n = 3, 4, 5 and 6) were determined by X‐ray diffraction. The influence of the distant methyl group on the structural features and catalytic activity with increasing of length of linker was investigated by comparing the results of these 2,6‐dimethylphenyl palladium complexes with those of their known mesityl analogues. X‐ray studies show the distant methyl substitution has big impact on the structure feature of the complexes with the shorter linker between two NHC (ethylene and propylene), but has a little or no effect on that of the complexes with longer linker (butylene and hexylene). Catalytic results of the arylation of styrene show that the remote substitute has big effect on the regioselectivity of the product in all complexes with shorter and longer linkers, but has a limited effect on the yield.     

Periodic Mesoporous Organosilica Nanoparticles with Controlled Morphologies and High Drug/Dye Loadings for Multicargo Delivery in Cancer Cells

Despite the worldwide interest generated by periodic mesoporous organosilica (PMO) bulk materials, the design of PMO nanomaterials with controlled morphology remains largely unexplored and their properties unknown. In this work, we describe the first study of PMO nanoparticles (NPs) based on meta‐phenylene bridges, and we conducted a comparative structure–property relationship investigation with para‐phenylene‐bridged PMO NPs. Our findings indicate that the change of the isomer drastically affects the structure, morphology, size, porosity and thermal stability of PMO materials. We observed a much higher porosity and thermal stability of the para‐based PMO which was likely due to a higher molecular periodicity. Additionally, the para isomer could generate multipodal NPs at very low stirring speed and upon this discovery we designed a phenylene–ethylene bridged PMO with a controlled Janus morphology. Unprecedentedly high payloads could be obtained from 40 to 110 wt % regardless of the organic bridge of PMOs. Finally, we demonstrate for the first time the co‐delivery of two cargos by PMO NPs. Importantly, the cargo stability in PMOs did not require the capping of the pores, unlike pure silica, and the delivery could be autonomously triggered in cancer cells by acidic pH with nearly 70 % cell killing.     

Aromaticity of ortho and meta 8-Cycloparaphenylene and Their Dications: Induced Magnetic Field Analysis with Localized and Delocalized Orbitals in Strained Nanohoops

Dications of cycloparaphenyles ([n]CPPs) are known to exhibit in-plane global aromaticity, contained in a nanobelt structure. Recently synthesized ortho and meta isomers of [n]CPPs break the radial symmetry of π structure incorporating perpendicular oriented π orbitals. Herein we set to explore the aromaticity of neutral and dicationic ortho and meta isomers of [8]CPP by dissecting the induced magnetic field to contributions of the twofold radial/perpendicular π system using delocalized canonical molecular orbitals (CMO), and introducing the natural localized molecular orbitals (NLMO) analysis with DFT methods. The dications sustain a reduced global aromatic character of the radial π system under a perpendicular orientation of the external field which declines from ortho to meta isomer and reinforces local aromaticity of ortho ring while it destroys aromaticity of meta ring. Aromaticity variations are determined by symmetry governed rotational excitations of frontier π orbitals. The parallel orientation reveals a substantial reduction of local aromaticity verified with NICS_π analysis and electron delocalization indices.     

ortho‐, meta‐, and para‐Dihydroindenofluorene Derivatives as Host Materials for Phosphorescent OLEDs

This work reports the first structure–properties relationship study of ortho [2,1‐c]‐, meta [1,2‐a]‐, and para [1,2‐b]dihydroindenofluorenes, highlighting the influence of bridge rigidification on the electronic properties. This study has made it possible to devise an extended π‐conjugated molecule with both a high triplet state energy level and excellent thermal and morphological stability. As a proof of concept, dihydroindenofluorenes were used as the host in sky‐blue phosphorescent organic light‐emitting diodes (PhOLEDs) w ith high performance.     

Electronic Coupling and Electron Transfer between Two Mo2 Units through meta- and para-Phenylene Bridges

A series of three Mo₂ dimers bridged by a meta-phenylene group has been studied in terms of electronic coupling (EC) and electron transfer (ET) in comparison with the para isomers. Optical analyses on the mixed-valence complexes indicate that by replacing a para-phenylene bridge with a meta one, the EC between the two Mo₂ centers is dramatically weakened; consequently, the ET rates (k_et) are lowered by two to three orders of magnitude. In the para series, the EC parameters (H_ab) and ET rates (k_et) are greatly affected by O/S atomic alternation of the bridging ligand. However, for the meta analogues, similar EC and ET parameters are obtained, that is, H_ab=300–400 cm⁻¹ and k_et≈10⁹ s⁻¹. These results suggest that through-σ-bond and/or through-space coupling channels become operative as the π conjugation is disabled. DFT calculations reveal that destructive quantum interference features seen for the meta series arise from the cancellation of two π-conjugated coupling pathways.     

Regioselectivity of the Diels-Alder Additions of 2-Substituted 5,6 Dimethylidenenorbornanes and-bicyclo[2.2.2]octanes

The cycloadditions of methyl propynoate and methyl vinyl ketone to 5,6-dimethylidene-2-norbornanone ( 6 ) are para′-regioselective

1 “Para” (p) designs in this paper the 4, 9-disubstituted tricclo[6.2.1.0^{2, 7}] undecane- and 4, 9-disubsstituted tricycle[6.2.20^2,7] dodecane derivatives, “meta” (m) design the corresponding 4, 10-disubstituted compounds.

. A smaller para -regioselectivity is observed for the addition of methyl propynoate to 5,6-dimethylidene-2bicyclo[2.2.2]octanone ( 10 ). No regioselectivity is observed with 5,6-dimethylidene-2exo-norbornyl alcohol ( 3 ), acetate ( 5 ) and 5,6-dimethylidene-2exo-bicyclo[2.2.2]octanol ( 9 ). PMO arguments based on the shape of the HOMO's and subHOMO's of the dienes allow to rationalize these observations. Unpredictable para′- or ‘meta’regioselectivities are found for the Diels,-Alder additions of 5,6-dimethylidene-2endo-norbornyl alcohol ( 2 ), acetate ( 4 ) and 5,6-dimethylidene-2endo-bicyclo[2.2.2]octanol ( 8 ). The carbonyl group of β,γ unsaturated ketones such as 6 and 10 can act as an electron donating homoconjugated substituent. The n(CO) ? σ[C(1), C(2)] ? π[C(5), C(6)] hyperconjugative interaction can override the usual electron-withdrawing effect of this function.     

A New Class of Remote N‐Heterocyclic Carbenes with Exceptionally Strong σ‐Donor Properties: Introducing Benzo[c]quinolin‐6‐ylidene

We make the case for benzo[c]quinolin‐6‐ylidene ( 1 ) as a strongly electron‐donating carbene ligand. The facile synthesis of 6‐trifluoromethanesulfonylbenzo[c]quinolizinium trifluoromethanesulfonate ( 2 ) gives straightforward access to a useful precursor for oxidative addition to low‐valent metals, to yield the desired carbene complexes. This concept has been achieved in the case of [Mn(benzo[c]quinolin‐6‐ylidene)(CO)₅]⁺ ( 15 ) and [Pd(benzo[c]quinolin‐6‐ylidene)(PPh₃)₂(L)]²⁺ L=THF ( 21 ), OTf ( 22 ) or pyridine ( 23 ). Attempts to coordinate to nickel result in coupling products from two carbene precursor fragments. The CO IR‐stretching‐frequency data for the manganese compound suggests benzo[c]quinolin‐6‐ylidene is at least as strong a donor as any heteroatom‐stabilised carbene ligand reported.