Dendrimers with 1,3,4-oxadiazole units, 1. Synthesis and characterization

The synthesis and characterization of two unsymmetrically substituted star-shaped oxadiazoles and of a dendrimer of the first generation are described. The key step in the synthesis is the intramolecular ring transformation of aromatic tetrazoles. The well-defined compounds form stable amorphous glasses with high glass transition temperatures up to 222°C. They are promising candidates for organic light emitting diodes.     

Organoaluminum-mediated selective 1,2-rearrangement of γ,γ-disubstituted γ-amino α,β-unsaturated carbonyl compounds leading to unsymmetrically substituted pyrroles

γ,γ-Dialkyl-γ-amino-α,β-unsaturated carbonyl compounds were found to undergo selective skeletal rearrangement under the influence of modified organoaluminum Lewis acid to give unsymmetrically substituted pyrroles through the rapid Paal-Knorr type cyclization upon acidic hydrolysis. This new structural reorganization of amino carbonyl compounds triggered by the 1,2-alkyl shift provides a unique entry to the synthesis of pyrroles, an important class of heterocycles with diverse biological activities.     

Unsymmetrically N,N'-substituted saturated carbenes: synthesis, reactivity and preparation of a rhodium(I) carbene complex

A versatile synthesis of unsymmetrically N,N'-substituted saturated carbenes is described. The novel racemic imidazolidin-2-ylidenes rac-5 have been synthesized by reductive desulfurization of the corresponding imidazolidin-2-thiones rac-4. The thiones were prepared in two reaction steps from aldimines and secondary amines. Three different substituents at N1, N3 and C4 of the five-membered N-heterocyclic ring can be introduced by choice of suitable aldimines and secondary amines. The dimerization behaviour (diaminocarbene/enetetramine equilibrium) for the unsymmetrically N,N'-substituted imidazolidin-2-ylidenes has been investigated by NMR spectroscopy. Unsymmetrically N-iPr and N-iBu substituted N-heterocyclic carbenes undergo a slow dimerization, whereas N-tBu substituted derivatives are stable as monomeric carbenes indefinitely. The carbene ligand rac-5d has been coordinated to rhodium(I) to give the square-planar rhodium carbene complex [Cl(cod)Rh(rac-5d)]rac-6d which has been characterized by an X-ray diffraction analysis.     

Desymmetrization of Dicationic Diboranes by Isomerization Catalyzed by a Nucleophile

Cationic monoboranes exhibit a rich chemistry. By constrast, only a few cationic diboranes are known, that all are symmetrically substituted. In this work, the first unsymmetrically substituted dicationic diboranes, featuring sp²–sp²‐hybridized boron atoms, are reported. The compounds are formed by intramolecular rearrangement from preceding isomeric symmetrically substituted dicationic diboranes, a process that is catalyzed by nucleophiles. From the temperature‐dependence of the isomerization rate, activation parameters for this unprecedented rearrangement are derived. The difference in fluoride ion affinity between the two boron atoms and the bonding situation in these unique unsymmetrical dicationic diboranes are evaluated.     

Synthesis and properties of 2,7-phenylethenyl- and benzoxazol-2-ylethenyl <Emphasis Type="Italic">N</Emphasis>-ethylcarbazole derivatives

A number of new symmetrically and unsymmetrically substituted 2,7-phenylethenyl and benzoxazol-2-ylethenyl N-ethylcarbazole derivatives were synthesized by successive Wittig and Knoevenagel olefinations of 9-ethylcarbazole-2,7-dicarbaldehyde. The resulting compounds showed strong photoluminescence in the blue region. The spectral parameters of unsymmetrically substituted 9-ethylcarbazoles with a donor-donor-acceptor conjugation type are determined by intramolecular interaction in the donor-acceptor fragment.     

Combining Electronic and Steric Effects for Highly Stable Unsymmetric Pentacenes

This paper describes the reactivity of unsymmetrically substituted pentacenes to photochemical oxidation. Acenes in general, and pentacenes in particular, are a key family of compounds for a variety of organic electronics applications. The instability of many pentacene derivatives, particularly to oxidation, is an important restriction in their applicability. Several substitution strategies for decreasing the reactivity of pentacene exist, but these almost always involve symmetrically substituted derivatives, restricting the chemical space of structures from which to choose. In this paper, we demonstrate that combining electronic and steric effects yields highly stable unsymmetrically substituted pentacenes.     

Synthesis and heteroelectrocyclization of unsymmetrically substituted diazomalonamides

A selective procedure was developed for the synthesis of 1,2,3-triazoles and unsymmetrically substituted diazomalonamides. Cyclization of unsymmetrically substituted diazomalonamides to 1,2,3-triazoles was studied by the method of intramolecular competitive reactions. The kinetic and thermodynamic characteristics of the process were determined. Quantum-chemical calculations for the monorotatory electrocyclic and nonrotatory heteroelectrocyclic mechanisms of cyclization were carried out. N-Aryldiazomalonamides undergo cyclization according to the heteroelectrocyclic mechanism, whereas cyclization of N-alkyldiazomalonamides proceeds by the monorotatory mechanism. The experimental constant of competition between these processes is (1.3—8.3)·10³ (DMSO-d₆) and (45.2—72.4)·10³ (CD₃OD).     

Rhodium‐Catalyzed CH Annulation of Nitrones with Alkynes: A Regiospecific Route to Unsymmetrical 2,3‐Diaryl‐Substituted Indoles

The direct C? H annulation of anilines or related compounds with internal alkynes provides straightforward access to 2,3‐disubstituted indole products. However, this transformation proceeds with poor regioselectivity in the synthesis of unsymmetrically 2,3‐diaryl substituted indoles. Herein, we report the rhodium(III)‐catalyzed C? H annulation of nitrones with symmetrical diaryl alkynes as an alternative method to prepare 2,3‐diaryl‐substituted N‐unprotected indoles with two different aryl groups. One of the aryl substituents is derived from N?C‐aryl ring of the nitrone and the other from the alkyne substrate, thus providing the indole products with exclusive regioselectivity.     

Rhodium‐Catalyzed C?H Annulation of Nitrones with Alkynes: A Regiospecific Route to Unsymmetrical 2,3‐Diaryl‐Substituted Indoles

The direct C H annulation of anilines or related compounds with internal alkynes provides straightforward access to 2,3‐disubstituted indole products. However, this transformation proceeds with poor regioselectivity in the synthesis of unsymmetrically 2,3‐diaryl substituted indoles. Herein, we report the rhodium(III)‐catalyzed C H annulation of nitrones with symmetrical diaryl alkynes as an alternative method to prepare 2,3‐diaryl‐substituted N‐unprotected indoles with two different aryl groups. One of the aryl substituents is derived from NC‐aryl ring of the nitrone and the other from the alkyne substrate, thus providing the indole products with exclusive regioselectivity.     

Synthesis of 1,2,4,5-tetrazines, symmetrically and unsymmetrically 3,6-disubstituted by N-nucleophiles

6-R-3-(3,5-Dimethylpyrazol-1-yl)-1,2,4,5-tetrazines with aliphatic, cycloaliphatic, and aromatic amines, and also with NH-heterocycles undergo a nucleophilic substitution of the dimethylpyrazole moiety yielding symmetrically and unsymmetrically substituted 1,2,4,5-tetrazines. In the 3,6-diimidazolyl-and 3,6-dibenzotriazolyl derivatives reactions of nucleophilic substitution of the heterocyclic moiety also occur. In some cases an ipsosubstitution of amino, hydrazino, and azido groups is observed.     

Synthesis of unsymmetrically substituted subphthalocyanines

A series of new unsymmetrically substituted subphthalocyanines containing iodo or octylthioether substituents on the outer aromatic rings have been synthesized. The statistical reaction of one equivalent of 1,2-dicyano-3-iodobenzene whether with two equivalents of 1,2-dicyano-4-octylthiobenzene or with two equivalents of 1,2dicyano-4,5-dioctylthiobenzene in the presence of boron trichloride in 1-chloronaphthalene yielded in both cases all the possible expected unsymmetrically substituted subphthalocyanines, which were separated by column chromatography on silica gel. All compounds were identified by FAB mass spectrometry and then characterized by HR-LSIMS spectrometry. The unambiguous characterization of each constitutional isomer was made possible by careful examination of the symmetry environment experienced by each proton on their 1H-NMR spectra.     

Mehrstufige Redoxsysteme. XLIX. Mitteilung. Mit π-Systemen substituierte 1,3-Dimethylidencyclobutane: Synthese und Eigenschaften

Multistep Redox Systems. 1,3-Dimethylidenecyclobutanes Substituted by π-Syntheses and Properties As potential redox systems 1,3-dimethylidenecyclobutanes are synthesized in which the ‘exo’-methylidene group participates in a cyclopentadienyl (type C4 )or cycloheptatrienyl (type A5 ) system. Benzo-annellated systems are also described. The formation of the C?C bond by the reaction of thioketones ( 3 and 31 ) with diazo compounds is extended to extended to 1,6,7,21 , and 24 , which so far had not been employed. In some cases, intermediate thiiranes can be isolated (e.g. 4 ) which are smoothly transformed into the target compounds of types C4 and A5 by extrusion of sulfur. The new C?C bonds may be introduced stepwise, thereby allowing the synthesis of unsymmetrically substituted 1,3-dimethylidenecyclobutanes (e.g. 20 and 35 ). Reduction of by Na does not produce bicyclobutanes but compounds 43 and 44 with one dihydrofulvene unit.     

Double elimination protocol for access to unsymmetrically substituted aromatic polyynes starting from sulfones and aldehydes

A double elimination protocol has been established for access to unsymmetrically substituted aromatic polyynes by use of dialdehyde and substituted benzylic or propargylic sulfones as starting compounds. For this protocol, various substituted sulfones and phthalaldehydes were usable giving rise to the formation of the desired substituted aromatic polyynes.     

Domino Heck-C-H activation reaction of unsymmetrically substituted [3]cumulene

The Heck reaction of an unsymmetrically substituted [3]cumulene has been investigated. Although a carbonyl conjugated alkene is present, the arylpalladium species selectively inserts into the C3-4 double bond, and a subsequent C-H activation reaction with a neighboring phenyl group gives the indene derivatives with a tetrasubstituted olefin moiety.     

Homolytic alkylations of substituted pyridazines [1]

The unsymmetrically substituted pyridazines 1 (X ? Ph, CH₃) have been found to undergo homolytic heteroaromatic alkylation in moderate to good yields with high regioselectivity. The site of substitution was established by conversion to 5 and measurement of the pyridazine proton coupling constant. In addition, homolytic alkylations of 1 (X ? CH₃) afforded the N-alkylated products 6 and 7.     

Observation of filamentary growth patterns at the isotropic–SmCPA phase transition in new unsymmetrical compounds composed of bent-core molecules

The synthesis and mesomorphic properties of several compounds composed of bent-core molecules, unsymmetrically substituted about the central phenyl ring, are discussed. These compounds contain substituents such as fluoro, chloro and trifluoromethyl at the terminal position of one of the arms of the bent-core molecules. Many of the compounds containing a chloro terminal substituent show filamentary growth patterns at the transition from isotropic to polar antiferroelectric smectic C phase. The liquid crystalline phase exhibited by these compounds has been characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies.     

Diastereoselective Formation of Methylene-Bridged Glycoluril Dimers

The acid-catalyzed formation of methylene-bridged glycoluril dimers yields the C(2)(v)()-diastereomer selectively. Product resubmission experiments establish that the selectivity is the result of thermodynamic control. A modified synthetic route is presented that allows for the preparation of unsymmetrically substituted dimers. We present the X-ray crystal structures of both diastereomers. This class of compounds is useful for studies of self-assembly in aqueous solution.     

Observation of filamentary growth patterns at the isotropic-SmCPA phase transition in new unsymmetrical compounds composed of bent-core molecules

The synthesis and mesomorphic properties of several compounds composed of bent-core molecules, unsymmetrically substituted about the central phenyl ring, are discussed. These compounds contain substituents such as fluoro, chloro and trifluoromethyl at the terminal position of one of the arms of the bent-core molecules. Many of the compounds containing a chloro terminal substituent show filamentary growth patterns at the transition from isotropic to polar antiferroelectric smectic C phase. The liquid crystalline phase exhibited by these compounds has been characterized using a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies.     

Synthetic transformations of higher terpenoids: XXVIII. Diels-Alder reactions of 16-(trimethylsiloxybutadienyl) labdanoids

15,16-Epoxy-16-(3-trimethylsiloxybuta-1,3-dien-1-yl)labdanoids were synthesized, and their reactions with cyclic dienophiles (1,4-benzoquinone, 1,4-toluquinone, 2-bromo-6-methyl-1,4-benzoquinone, and N-methylmaleimide) were studied. The reactions with unsymmetrically substituted benzoquinones were not selective. Methods for the preparation of hybrid compounds containing furan diterpenoid and substituted naphthoquinone, octahydroisoindoletrione, or hexahydroisoindoledione fragments were proposed.     

Identification of some isomeric 3,5-unsymmetrically substituted 1-aroyl-pyrazole derivatives by NMR spectroscopy

The ¹H and ¹³C nmr chemical shifts are used for the structural assignment of isomeric 1-aroyl-4,5-dihydro-5-hydroxy-4,4-dimethyl-1H-pyrazoles 1 unsymmetrically substituted with phenyl or methyl in the 3,5-positions of the pyrazole ring. The ¹H nmr spectra of 1-aroyl- or 1-acetyl-4-methyl-1H-pyrazoles 2 are useful in structure elucidation of unsymmetrically 3- or 5-methyl substituted derivatives.