David Šaman

Transition metal catalysed synthesis of tetrahydro derivatives of [5]-, [6]- and [7]helicene

Abstract Tetrahydro analogues of [5]-, [6]- and [7]helicene have been easily prepared by intramolecular [2+2+2] cycloisomerization of appropriate triynes under CpCo(CO) 2 /PPh 3 or Ni(cod) 2 /PPh 3 catalysis. This nonphotochemical methodology allows enantioselective synthesis of a helical skeleton employing the Ni(cod) 2 /(S)-(−)-MOP catalytic system. On reaction with DDQ, tetrahydro[5]helicene was transformed to [5]helicene.

An organometallic route to long helicenes

Along with the recent progress in the development of advanced synthetic methods, the chemical community has witnessed an increasing interest in promising carbon-rich materials. Among them, helicenes are unique 3D aromatic systems that are inherently chiral and attractive for asymmetric catalysis, chiral recognition and material science. However, there have been only limited attempts at synthesizing long helicenes, which represent challenging targets. Here, we report on an organometallic approach to the derivatives of undecacyclic helicene, which is based on intramolecular [2 + 2 + 2] cycloisomerization of aromatic hexaynes under metal catalysis closing 6 new cycles of a helicene backbone in a single operation. The preparation of nonracemic compounds relied on racemate resolution or diastereoselective synthesis supported by quantum chemical (density functional theory) calculations. The fully aromatic [11]helicene was studied in detail including the measurement and theoretical calculation of its racemization barrier and its organization on the InSb (001) surface by STM. This research provides a strategy for the synthesis of long helical aromatics that inherently comprise 2 possible channels for charge transport: Along a π-conjugated pathway and across an intramolecularly π-π stacked aromatic scaffold.

Helquats: A Facile, Modular, Scalable Route to Novel Helical Dications

The synthesis and properties of helical extended diquat (helquat), and derivatives that bear resemblance to diquat and azoniahelicene, was reported. Triyne with elongated tethers connecting the heterocyclic moiety with the pendant alkyne functionalities undergoing cycloisomerization give helquat featuring two seven-membered rings. The seven helquats reported are accessed uniformly in three steps from commercially available starting materials, entailing a Sonogashira coupling, bisquaternization, cycloisomerization, and 2+2+2 cycloisomerization. The evidence for the reversible electrochemical Weiz-type manifold and regular columnar stacks in crystal structures suggest the potential of helquats as electroactive functional elements.

A General Approach to Optically Pure [5]-, [6]-, and [7]Heterohelicenes†

The lack of a general method for the effective synthesis of nonracemic helicenes and their analogues has been a major hurdle that has limited a wider exploitation of these helically chiral aromatic systems in enantioselective catalysis, molecular recognition, self-assembly, surface science, chiral materials, and other branches of science. Ideally, a practical asymmetric synthesis would be independent of both the length of the helical backbone and the presence of functional groups. Since the pioneering studies by Martin et al. and Katz et al. , who successfully used diastereoselective photodehydrocyclization of stilbene-type precursors, various concepts of the asymmetric synthesis of helicenes have been explored but no general procedure for obtaining optically pure helicenes or their analogues with a wide structural diversity has yet been reported. Recently, we demonstrated diastereoselective [2+2+2] cycloisomerization of centrally chiral triynes to receive nonracemic helicene-like compounds with incorporated dihydrooxepine or dihydroazepine ring(s). This promising approach, however, has not yet reached the merit of being general and practical. Herein, we present fundamental progress in this endeavor to receive optically pure and functionalized [5]-, [6]-, and [7]heterohelicenes by means of asymmetric synthesis. With the aim of keeping the molecular shape of the helicene analogues as close as possible to that of the parent helicenes, such as 1 (Figure 1), we proposed embedding two

Steroid-porphyrin conjugate for saccharide sensing in protic media.

A new saccharide receptor in protic media has been designed and synthesized. The receptor combines advantages of steroids, which are responsible for saccharide binding, and of the porphyrin moiety acting as a signalling component of the molecule due to changes in UV-vis electronic spectra. The synthesis is based on condensation of steroid aldehyde with pyrrole to form the porphyrin unit with four protected steroid moieties. After deprotection, meso-substituted porphyrin contains 12-hydroxy groups on the steroidal part. The receptor is soluble in aqueous solutions and exhibits high complexation affinity towards saccharides. Because the receptor extensively aggregates in water, most of the experiments were performed in 50% aqueous 2-propanol where aggregation is significantly eliminated. Binding is evidenced by spectral changes in the Soret region of the receptor in UV-vis absorption spectra allowing the evaluation of the binding constants. Additional confirmation of binding is obtained using 1H NMR, Raman and IR spectroscopies and the surface plasmon resonance technique. The receptor exhibits higher selectivity for oligosaccharides over monosaccharide. The results point to the importance of a combination of multiple binding via H-bonding and hydrophobic interactions.

An Ultimate Stereocontrol in Asymmetric Synthesis of Optically Pure Fully Aromatic Helicenes.

The role of the helicity of small molecules in enantioselective catalysis, molecular recognition, self-assembly, material science, biology, and nanoscience is much less understood than that of point-, axial-, or planar-chiral molecules. To uncover the envisaged potential of helically chiral polyaromatics represented by iconic helicenes, their availability in an optically pure form through asymmetric synthesis is urgently needed. We provide a solution to this problem present since the birth of helicene chemistry in 1956 by developing a general synthetic methodology for the preparation of uniformly enantiopure fully aromatic [5]-, [6]-, and [7]helicenes and their functionalized derivatives. [2 + 2 + 2] Cycloisomerization of chiral triynes combined with asymmetric transformation of the first kind (ultimately controlled by the 1,3-allylic-type strain) is central to this endeavor. The point-to-helical chirality transfer utilizing a traceless chiral auxiliary features a remarkable resistance to diverse structural perturbations.

Metal coordination as a tool for controlling the self-assembling and gelation properties of novel type cholic amide–phenanthroline gelating agent

Abstract (3α,7α,12α)-Trihydroxy-N-[1,10-phenanthrolin-5-yl]-5β-cholan-24-amide was found to be a powerful gelating agent for methanol–water in gelator to solvent ratio starting from 0.1% in absence of metal ion. Formation of phenanthroline–zinc (II) 2:1 complex changes dramatically gelating properties; when stored, it dissolves into clear solution without Tyndall effect and this solution, when heated to ca. 70°C reversibly forms a gel again, without chemical change as proven by NMR spectrometry. Structures of the gels of cholic amide–phenanthroline and its Zn2+ complex were studied by SEM.

Helicity control in the synthesis of helicenes and related compounds

Asymmetric synthesis of helicenes and their congeners has been demonstrated to rely either on enantioselective Ni0/PR3*-catalyzed [2+2+2] cycloisomerization of triynes or on diastereoselective CoI-catalyzed [2+2+2] cycloisomerization of chiral triynes. The former approach providing tetrahydrohelicenes in a nonracemic form requires further development as moderate enantioselectivities (up to 54 % ee) have so far been achieved under kinetic control. The latter approach affording helicene-like structures in a diastereomerically enriched form allows for reaching good to excellent diastereoselectivities (up to 100:0) under thermodynamic control.

Resolution of a configurationally stable [5]helquat: enantiocomposition analysis of a helicene congener by capillary electrophoresis

Racemic [5]helquat as a triflate salt has been synthesized using a robust, three-step procedure. Subsequent exchange of triflate anions for inexpensive (R,R)-dibenzoyltartrate anions via an ion exchange resin afforded two diastereoisomeric salts. Crystallization led to the resolution of the helquat (ee > 98%). This is the first time that a non-racemic helquat has been obtained; its helicity having been assigned and its racemization barrier determined. Capillary electrophoresis with a sulfated β-cyclodextrin chiral selector is introduced for the first time as a straightforward method to analyze the enantiocomposition of charged, helicene-like species.

COUPLING REACTIONS OF ORTHO-SUBSTITUTED ARYL HALIDES WITH ALKYNES. THE SYNTHESIS OF FUNCTIONALIZED 1-NAPHTHYL-, 1-(1-NAPHTHYL)-2-PHENYL-, AND 1,2-BIS( 1-NAPHTHYL))ACETYLENES

Abstract Coupling of 2-functionalized 1-naphthyl halides with gaseous acetylene, (trialkylsilyl)acetylenes, and aryl acetylenes under Pd(PPh3)4 or Pd(PPh 3 ) 4 CuI catalysis has been investigated to prepare 1-naphthyl-, 1-(1-naphthyl)-2-phenyl-, and 1,2-bis(1-naphthyl)acetylenes with various ortho substituents, i.e., the -CH3, -CH2OH, -CO2Me, and -CH2OCH2CCCH3 groups. The reaction conditions have been optimized (yields up to 96 %) by changing halogen atom in aryl halides, solvent, alkyl in (trialkylsilyl)acetylenes, and catalyst (Pd(O) vs. Pd(O) Cu(I) ). In case of 1-naphthyl iodide with a tethered alkyne unit, coupling has been observed to compete with a cascade of intramolecular Heck-type reactions. The mechanism of β-elimination of a hydridopalladium species has been discussed. 1-Naphthyl bromide bearing the -CO2Me group has been found to be susceptible to nucleophilic aromatic substitution with a solvent. The successful synthesis of an unsymmetrical 1-(1-naphthyl)-2-phenylacetylene derivative has been shown to depend critically on combination of aryl halide/aryl acetylene.