Study of polarisation ratios in mixed molecular crystals

The perturbation theory of Craig and Thirunamachandran is used to obtain the polarisation ratio (b/a) of guest transitions in an anthracene host crystal as a function of trap depth. Results from several substituted anthracenes as dopant are compared with the calculated curve. Substantial agreement is obtained over a wide range of trap depths confirming the applicability of the simple theory in this energy region. Degree of misorientation and possible deviation of transition moment from the short molecular axis (M) are briefly discussed.     

Regioselective `One‐Pot' Synthesis of Antipodal Bis‐adducts by Heating of Solid [5,6]Fullerene‐C60‐Ih and Anthracenes,Preliminary Communication

Antipodal (`trans‐1') Diels‐Alder bis‐adducts 3 and 7 – 9 of [5,6]fullerene‐C₆₀‐I_h ( 1 ) with some anthracenes were prepared highly regioselectively by heating mixtures of the solid 1 and anthracene or of (one of) three alkyl‐substituted anthracenes in the absence of solvents (Scheme 2). Other bis‐cycloadducts were not detected, but lesser amounts of mono‐cycloadducts 2 and 4 – 6 , respectively, were also formed. Heating of solvent‐free mixtures of 1 and three other alkyl‐substituted anthracenes did not result in a detectable amount of (antipodal) bis‐cycloadducts. The antipodal bis‐adduct 7 of 1 and of 1‐methylanthracene was analyzed by X‐ray crystallography. The preparative outcome of heating of anthracenes and solid 1 parallels the result of the heating of the corresponding crystalline mono‐adducts of anthracenes and 1 . Both approaches reveal a remarkably consistent dependence of the reaction upon the presence and position of alkyl substituents at the anthracene unit. The regioselective assembly of antipodal bis‐adducts from anthracene(s) and 1 cannot be rationalized by their (inherent molecular) stability, but it indicates the crucial control of the lattice.     

Supramolecular-induced regiocontrol over the photochemical [4 + 4] cyclodimerization of NHC- or azole-substituted anthracenes

Thanks to the impressive control that microenvironments within enzymes can have over substrates, many biological reactions occur with high regio- and stereoselectivity. However, comparable regio- and stereoselectivity is extremely difficult to achieve for many types of reactions, particularly photochemical cycloaddition reactions in homogeneous solutions. Here, we describe a supramolecular templating strategy that enables photochemical [4 + 4] cycloaddition of 2,6-difunctionalized anthracenes with unique regio- and stereoselectivity and reactivity using a concept known as the supramolecular approach. The reaction of 2,6-azolium substituted anthracenes H₄-L(PF₆)₂ (L = 1a–1c) with Ag₂O yielded complexes anti-[Ag₂L₂](PF₆)₄ featuring an antiparallel orientation of the anthracene groups. Irradiation of complexes anti-[Ag₂L₂](PF₆)₄ proceeded under [4 + 4] cycloaddition linking the two anthracene moieties to give cyclodimers anti-[Ag₂(2)](PF₆)₂. Reaction of 2,6-azole substituted anthracenes with a dinuclear complex [Cl-Au-NHC–NHC-Au-Cl] yields tetranuclear assemblies with the anthracene moieties oriented in syn-fashion. Irradiation and demetallation gives a [4 + 4] syn-photodimer of two anthracenes. The stereoselectivity of the [4 + 4] cycloaddition between two anthracene moieties is determined by their orientation in the metallosupramolecular assemblies.

A supramolecular templating strategy that enables the photochemical [4 + 4] cycloaddition of 2,6-difunctionalized anthracene derivatives with unique stereoselectivity has been developed based on metal-NHC units.     

Mono-Aryl/Alkylthio-Substituted (Hetero)acenes of Exceptional Thermal and Photochemical Stability by the Thio-Friedel–Crafts/Bradsher Cyclization Reaction

The highly substituted mono-aryl/alkylthio-(hetero)acenes prepared in this study have been found to be thermally more stable (T_decomp.=331–354 °C) than the known di-aryl/alkylthio-substituted acenes by an average of 25 °C. They are also much more photostable at 254 and 365 nm (in both argon and air) than the parent anthracene and other reported anthracenes. The most photostable aryl/alkylthio-anthracenes at 254 nm were found to be 60–70 (in air) and 130 (in argon) times more stable in solution than the unsubstituted anthracene, and much more stable than known EDG/EWG-substituted anthracenes (EDG=electron-donating group, EWG=electron-withdrawing group) with an extended aromatic core. Furthermore, the acenes showed significantly higher photostability at 365 nm in both air and argon. The anthracenes were obtained by the novel thio-Friedel–Crafts/Bradsher cyclization reaction of hitherto unknown [o-(1,3-dithian-2-yl)aryl](aryl)methyl thioethers. The developed approach provides a general access to mono-aryl/alkylthio-substituted (hetero)acene frameworks containing at least three fused (hetero)aromatic rings. The characteristic feature of this approach, which leads to highly substituted acenes, is that the substituents, unlike in other methods, may be introduced at an early stage of the synthesis. DFT and TD-DFT calculations confirmed the stabilizing role of the aryl/alkylthio substituent in the mono-aryl/alkylthio-substituted anthracenes, which are the most stable anthracenes prepared to date. Their high photostability is mainly due to the quenching of singlet oxygen by the acene and the quenching of the acene S₁ state by molecular oxygen.     

Kinetic Stabilization of Blue-Emissive Anthracenes: Phenylene Bridging Works Best

In attempts at kinetically stabilizing blue-emissive anthracenes, a series of 9,10-diaryl substituted derivatives were tested for their photochemical and photooxidative persistence. A major breakthrough in light fastness comes from a new bis-meta-terphenylyl substituted anthracene which is much superior to industrially relevant 9,10-biarylated anthracenes. The key issue is the steric shielding of the anthracene core. Further, intramolecular ring closure via Yamamoto coupling furnished a doubly bridged anthracene as a “self-encapsulated” sky-blue emitter which is most resistant to photodegradation. The improved stabilization was corroborated by time-resolved irradiation experiments and rationalized by X-ray crystallography.     

The stark effect in condensed aromatic hydrocarbons

The changes in the static electric polarizability upon transition to excited singlet states are measured by perturbing the absorption spectrum with an intense (10⁶ V/cm) electric field. Observations of the Stark effect in a polystyrene matrix are reported for a series of substituted anthracenes, a group of condensed aromatic hydrocarbons and a substituted hexatriene.     

Synthesis of 2,6-diaryl-1,2-dihydropyridines through a 6π-electrocyclization of N-sulfonylazatrienes

The development of an efficient synthetic method toward substituted 1,2-dihydropyridines from cinnamylideneacetophenones is reported. The key intermediates N-sulfonylazatrienes were synthesized through a TiCl₄-mediated direct condensation of primary sulfonamides with the substituted (E,E)-cinnamylideneacetophenones. The 6π-electrocyclization of these intermediates, catalyzed by a Lewis acid, selectively afforded the desired products in good yields.     

Stereoselective synthesis of substituted dienes by the double ortho ester Claisen rearrangement

This letter shows the highly stereoselective synthesis of substituted (E)-1,3-dienes from substituted propargylic diols via the double ortho ester Claisen rearrangement. The cyclohexyl-substituted diene undergoes thermal Diels-Alder cycloaddition with maleic anhydride to produce the corresponding bicyclic diester in highly stereoselective manner.     

An easy preparation of pyridinium N-heteroarylaminides

Differently substituted pyridinium N-heteroarylaminides have been prepared in one step with good yield from N-aminopyridinium iodide and the corresponding heteroaryl chloride.     

Reaction of difluorocarbene with anions from amides and oximes: Synthesis of N-difluoromethyl substituted amides, O-difluoromethylimidates or O-difluoromethyl substituted oximes under phase-transfer catalysis conditions

N-Aryl substituted amides react with chlorodifluoromethane in the presence of concentrated aqueous sodium hydroxide and benzyltriethylammonium chloride (TEBAC) as a catalyst in benzene (phase-transfer catalysis, PTC), affording mixtures of N- and O-difluoromethyl substituted derivatives. Amide anions are involved in this process. The reaction carried out with oximes gives O-difluoromethyl oxime ethers.     

Synthesis and reactivity of N-aryl substituted N-heterocyclic silylenes

The synthesis of two N-aryl substituted 2-silaimidazolidenes 9a, b by metal-reduction of the appropriate silicon(IV) heterocycles is reported. Structural as well as spectroscopic data obtained for the N-aryl substituted N-heterocyclic silylenes (NHSi) are very close to those obtained previously for their N-alkyl substituted counterparts. NHSis 9a, b are used as starting materials for the synthesis of a series of dichalcogenadisiletanes 19-24 and for of a mono silylene tungsten complex 29. The reactivity studies revealed only marginally differences between the N-aryl substituted NHSis 9a, b and previously described N-alkyl substituted silylenes.     

Pd2dba3/P(i-BuNCH2CH2)3N: a highly efficient catalyst for the one-pot synthesis of trans-4-N,N-diarylaminostilbenes and N,N-diarylaminostyrenes

A Pd₂dba₃/P(i-BuNCH₂CH₂)₃N catalyzed one-pot synthesis of unsymmetrically substituted trans-4-N,N-diarylaminostilbenes and both symmetrically and unsymmetrically substituted N,N-diarylaminostyrene derivatives is reported. The procedure involves two or more palladium catalyzed sequential coupling reactions (an amination and an inter-molecular Heck reaction) in one-pot using the same catalyst system with two different aryl halides, including aryl chlorides and hetero aryl halides as the coupling partners.     

Diels-Alder reactions of pyridinone o-quinodimethanes generated from substituted sulfolene[3,4-c]pyridin-4(1H)-ones

1-Benzyl-3-(bromomethyl)-2(1H)-pyrazinone was converted to [3,4-c] sulfolene pyridinone 8a and further (1- or 3-) substituted derivatives having a dienophilic side chain on the sulfolene ring. Thermolytic extrusion of sulfur dioxide from o-QDM precursor 8 led to generation of 3,4-dimethylene-2(1H)-pyrazinone 9, which was reacted in situ with various dienophiles. Thermolysis of the substituted precursors resulted in intramolecular cycloaddition of the corresponding o-QDM intermediates     

A novel method for the synthesis of 5,6-dihydro-4H-oxocin-4-ones: 6-endo-dig versus 8-endo-dig cyclizations

The condensation of substituted α-keto alkynes with p-nitrobenzaldehyde in the presence of lithium diisopropylamide (LDA) affords highly substituted 5,6-dihydro-4H-oxocin-4-ones in good yields. Surprisingly, no six-membered carbocycles were formed in this 8-endo-dig cyclization to the oxocinone system.     

A facile approach to spirocyclic butenolides through cascade cyclization/oxidative cleavage reactions of (Z)-enynols catalyzed by gold under dioxygen atmosphere

A facile approach for the syntheses of spirocyclic butenolides through cascade cyclization/oxidative cleavage reactions of (Z)-enynols bearing cyclic substituents at the C-1 position catalyzed by gold under dioxygen atmosphere has been developed. A variety of substituted butenolides was constructed in a regioselective manner from suitably substituted (Z)-2-en-4-yn-1-ols. (Z)-Enynols substituted both at C2 and C3-position afforded the spirocyclic butenolides in moderate to good yields, C-2 unsubstituted (Z)-enynols afforded the products in moderate yields, and the C-3 unsubstituted (Z)-enynols afforded the desired products in low yields.     

Dehalogenation of o-dihalogen substituted arenes and α,α′-dihalogen substituted o-xylenes with lanthanum metal

It was found that lanthanum metal caused the dehalogenation of o-dihalogen substituted arenes and α,α′-dihalogen substituted o-xylenes to generate the corresponding benzynes and o-quinodimethanes. When o-dihalogen substituted arenes were allowed to react with lanthanum metal in the presence of dienes, the Diels-Alder products between benzyne and dienes were formed in moderate to good yields. Similarly, the Diels-Alder adducts of o-quinodimethane with dienophiles were obtained, in the reaction of α,α′-dibromo-o-xylenes with lanthanum metal in the presence of dienophiles.     

Thermally assisted and heavy-atom assisted intersystem crossing in meso-substituted anthracenes

A single linear correlation between the activation energy of S₁—T intersystem crossing and the logarithm of the bimolecular fluorescence quenching rate constant by haloalkanes for meso-substituted anthracenes in fluid solutions is established. It is concluded that both the monomolecular and the heavy-atom-assisted deactivation of S₁ depend in the same way on the change of S₁ energy by substitution or solvent relative to the energy of the accepting triplet T_n. The anomalous increase of fluorescence of Br-substituted anthracenes in heavy-atom containing solvents is consistent with a widening of the S₁—T_n energy gap due to the changing solvent.     

Efficient N-arylation of pyridazin-3(2H)-ones

A variety of substituted pyridazin-3(2H)-ones are directly N-arylated in good yield using lead tetraacetate/zinc chloride in benzene or in substituted benzenes including chloro- and bromobenzene.     

Efficient regioselective heterocyclisation leading to fluorescent fused pyrazine derivatives

The regioselective syntheses of substituted pyrrolo[2,3-b]quinoxaline, pyrido[2,3-b]pyrrolo[2,3-e]pyrazine, pyrido[2,3-b]pyrrolo[3,2-e]pyrazine and pyrido[3,4-b]pyrrolo[3,2-e]pyrazine are reported. Differential reactivity between two amino groups in ortho-diaminopyridine can be exploited to obtain new regio-defined unsymmetrical pyridopyrrolopyrazine derivatives. Weak electron-donating methyl or moderately electron-withdrawing carboxylic groups attached to the aromatic ortho-diamines reduce the regioselectivity of obtaining unsymmetrical substituted pyrrolo[2,3-b]quinoxaline. The fluorescence properties of the resultant 1-alkyl pyridopyrrolopyrazine and substituted pyrrolo[2,3-b]quinoxaline derivatives are presented.     

Enhancement of the Fluorimetric Properties of Some Carboxyl Substituted Anthracenes by Complexation with Bovine Serum Albumin

Abstract

A study has been conducted of the fluorimetric properties of substituted anthracenes, following complexation with bovine serum albuinin(BSA). It was found that although protein binding of the ligands enhances their fluorimetric properties, it fails to do so for their phosphorimetric properties. BSA would serve well in post column reactors for HPLC detectors, but not in pre-column systems.