Towards Configurationally Stable [4]Helicenes: Enantioselective Synthesis of 12-Substituted 7,8-Dihydro[4]helicene Quinones

Padlocking dihydrofurannulation for the control of small degree of helicity built on a fused-tetracyclic core

Enantioselective construction of small molecules displaying a configurationally stable helical shape built on a fused-tetracyclic core is a daunting synthetic challenge even more pronounced when five-membered rings are incorporated in the structure. The resulting higher configurational lability strongly hampers their access, and therefore the development of new efficient methodologies is timely and highly desirable. In this context, we describe a padlocking approach via the enantioselective organocatalytic domino furannulation of appropriately designed achiral fused-tricyclic precursors resulting in the synthesis of configurationally locked helically chiral tetracyclic scaffolds featuring one or two five-membered rings with the simultaneous control of central and helical chiralities.

Investigating the diastereoselective synthesis of a macrocycle under Curtin-Hammett control

This work sheds new light on the stereoselective synthesis of chiral macrocycles containing twisted aromatic units, valuable π-conjugated materials for recognition, sensing, and optoelectronics. For the first time, we use the Curtin-Hammett principle to investigate a chiral macrocyclisation reaction, revealing the potential for supramolecular π-π interactions to direct the outcome of a dynamic kinetic resolution, favouring the opposite macrocyclic product to that expected under reversible, thermodynamically controlled conditions. Specifically, a dynamic, racemic perylene diimide dye (1 : 1 P : M) is strapped with an enantiopure (S)-1,1'-bi-2-naphthol group (P-BINOL) to form two diastereomeric macrocyclic products, the homochiral macrocycle (PP) and the heterochiral species (PM). We find there is notable selectivity for the PM macrocycle (dr = 4 : 1), which is rationalised by kinetic templation from intramolecular aromatic non-covalent interactions between the P-BINOL π-donor and the M-PDI π-acceptor during the macrocyclisation reaction.

Enantioselective synthesis of [4]helicenes by organocatalyzed intermolecular C-H amination

Catalytic asymmetric synthesis of helically chiral molecules has remained an outstanding challenge and witnessed fairly limited progress in the past decades. Current methods to construct such compounds almost entirely rely on catalytic enantiocontrolled fused-ring system extension. Herein, we report a direct terminal peri-functionalization strategy, which allows for efficient assembling of 1,12-disubstituted [4]carbohelicenes via an organocatalyzed enantioselective amination reaction of 2-hydroxybenzo[c]phenanthrene derivates with diazodicarboxamides. The key feature of this approach is that the stereochemical information of the catalyst could be transferred into not only the helix sense but also the remote C-N axial chirality of the products, thus enabling the synthesis of [4]- and [5]helicenes with both structural diversity and stereochemical complexity in good efficiency and excellent enantiocontrol. Besides, the large-scale preparations and representative transformations of the helical products further demonstrate the practicality of this protocol. Moreover, DFT calculations reveal that both the hydrogen bonds and the C-H---π interactions between the substrates and catalyst contribute to the ideal stereochemical control.

A C-H activation-based enantioselective synthesis of lower carbo[n]helicenes

The three-dimensional structure of carbohelicenes has fascinated generations of molecular chemists and has been exploited in a wide range of applications. Their strong circularly polarized luminescence has attracted considerable attention in recent years due to promising applications in new optical materials. Although the enantioselective synthesis of fused carbo- and heterohelicenes has been achieved, a direct catalytic enantioselective method allowing the synthesis of lower, non-fused carbo[n]helicenes (n = 4-6) is still lacking. We report here that Pd-catalysed enantioselective C-H arylation in the presence of a unique bifunctional phosphine-carboxylate ligand provides a simple and general access to these lower carbo[n]helicenes. Computational mechanistic studies indicate that both the C-H activation and reductive elimination steps contribute to the overall enantioselectivity. The observed enantio-induction seems to arise from a combination of non-covalent interactions and steric repulsion between the substrate and ligand during the two key reductive elimination steps. The photophysical and chiroptical properties of the synthesized scalemic [n]helicenes have been systematically studied.

Enantioenriched Ruthenium-Tris-Bipyridine Complexes Bearing One Helical Bipyridine Ligand: Access to Fused Multihelicenic Systems and Chiroptical Redox Switches

The synthesis and photophysical and chiroptical properties of novel aza[n]helicenes (6a-d, 10a,b, n = 4-7) substituted with one or two 2-pyridyl groups are described. The preparation was performed via an adapted Mallory reaction using aromatic imines as precursors. The obtained novel class of helical 2,2'-bipyridine ligands was then coordinated to Ru(bipy)₂²⁺ units, thus affording the first diastereomerically and enantiomerically pure [RuL(bipy)₂]²⁺ (11a,c, L = 6a,c) or [Ru₂L'(bipy)₄]⁴⁺ (12, L' = 10b) complexes. The topology and stereochemistry of these novel metal-based helical architectures were studied in detail, notably using X-ray crystallography. Interestingly, the coordination to ruthenium(II) enabled the preparation of fused multihelical systems incorporating aza- and ruthena-helicenes within the same scaffold. The photophysical, chiroptical, and redox properties of these complexes were examined in detail, and efficient redox-triggered chiroptical switching activity was evidenced.

Asymmetric systematic synthesis, structures, and (chir)optical properties of a series of dihetero[8]helicenes

A series of dihetero[8]helicenes have been systematically synthesized in enantiomerically enriched forms by utilizing the characteristic transformations of the organosulfur functionality. The synthetic route begins with assembling a ternaphthyl common synthetic intermediate from 2-naphthol and bissulfinylnaphthalene through an extended Pummerer reaction followed by facile multi-gram-scale resolution. The subsequent cyclization reactions into dioxa- and dithia[8]helicenes take place with excellent axial-to-helical chirality conversion. Dithia[8]helicene is further transformed into the nitrogen and the carbon analogs by replacing the two endocyclic sulfur atoms via SNAr-based skeletal reconstruction. The efficient systematic synthesis has enabled comprehensive evaluation of physical properties, which has clarified the effect of the endocyclic atoms on their structures and (chir)optical properties as well as the unexpected conformational stability of the common helical framework.

Enantioselective Synthesis of 1,12-Disubstituted [4]Helicenes

A highly enantioselective synthesis of 1,12-disubstituted [4]carbohelicenes is reported. The key step for the developed synthetic route is a Au-catalyzed intramolecular alkyne hydroarylation, which is achieved with good to excellent regio- and enantioselectivity by employing TADDOL-derived (TADDOL=α,α,α,α-tetraaryl-1,3-dioxolane-4,5-dimethanol) α-cationic phosphonites as ancillary ligands. Moreover, an appropriate design of the substrate makes the assembly of [4]helicenes of different substitution patterns possible, thus demonstrating the synthetic utility of the method. The absolute stereochemistry of the newly prepared structures was determined by X-ray crystallography and characterization of their photophysical properties is also reported.

Frontier molecular orbital effects control the hole-catalyzed racemization of atropisomeric biaryls

Biaryl atropisomerization is dramatically accelerated by the removal of an electron. The planar transition state is preferentially stabilized from depopulation of the highest occupied molecular orbital.

Atropisomeric biaryl systems are privileged architectures used in asymmetric synthesis and pharmaceutical structures. We report that by simply removing a single-electron, the resistance of biaryls towards racemization is reduced dramatically. Even though the steric properties are unaltered, biaryl oxidation changes atropisomerization into a two step mechanism with considerably smaller activation barriers than closed-shell biaryls. The effect is general for a series of biaryls and helicenes studied and results from the dependence of frontier molecular orbital energies on biaryl conformation.

Enantioselective Synthesis of Four Stereoisomers of Sulfinyl Ferrocenyl Quinones with Central, Planar, and Helical Chirality

Four stereoisomers of sulfinyl ferrocenyl-substituted helicenequinones having central, planar, and helical elements of chirality were stereoselectively formed, in one step, from reaction between enantiopure sulfinyl ferrocenyl dienes and a sulfinyl quinone. Asymmetric synthesis, kinetic resolution, or chemical resolution processes occurred in sequential cycloaddition, sulfoxide elimination, and partial aromatization steps.

The Use of (-)-Sparteine/Organolithium Reagents for the Enantioselective Lithiation of 7,8-Dipropyltetrathia[7]helicene: Single and Double Kinetic Resolution Procedures

The effect of organolithium reagent (RLi: R=nBu, iPr, sBu, tBu), solvent (diethyl ether, diethyl ether/THF and MTBE), and stoichiometry on the (-)-sparteine-mediated silylation of 7,8-dipropyltetrathia[7]helicene shows that, unusually, substantially more than 0.5 equivalent of RLi (R=iPr, sBu, tBu) and a large excess of (-)-sparteine (R=nBu, sBu) is often needed to achieve substantial conversions and good ee values. With nBuLi, however, just one equivalent of the organolithium reagent is sufficient to obtain high conversions. Our best results were obtained using the convenient tBuLi/(-)-sparteine adduct with which the need for a high (-)-sparteine/RLi ratio can be avoided. Single- and double-kinetic resolution (KR) procedures give enantiopure samples of 2-trimethylsilyl- and 2,13-di(trimethylsilyl)-7,8-dipropyltetrathia[7]helicene and two-step double-KR combining (-)-sparteine/sBuLi and chiral formamides affords the synthetically valuable 2-formyl-7,8-dipropyltetrathia[7]helicene. This is the first use of (-)-sparteine for the enantioselective lithiation of helicenes and the first report of tBuLi outperforming sBuLi in a (-)-sparteine-mediated procedure.

Facile, efficient, and diastereoselective synthesis of heterohelicene-like molecules

A concise and efficient route to new and interesting heterohelicene-like molecules has been developed through the one-pot, cascade reductive coupling reaction of o-hydroxydiimines or o-nitrodiimines and triphosgene in the presence of TiCl4/Sm. Purification of the final products only required a single recrystallization leading to high purity. High diastereoselectivity was also achieved, and two structures of the final products were confirmed by X-ray diffraction analysis.

Synthesis of chiral bis-oxazines: a preliminary assessment of helical conformational framework

A series of novel naphthalene attached bis-oxazines were synthesized and characterized. The bis-oxazines were studied by VT-NMR analysis to assess the possibility of conformational twist. The bis-oxazine prepared from (l)-methylvalinate show a helical conformational twist in the single crystal X-ray analysis. Three isomers of bis-oxazines were prepared from chiral α-methylbenzyl amines, the meso isomer showed small optical rotation probably indicating the helical conformational twist in the molecule.

Domino Friedel-Crafts-type cyclizations of difluoroalkenes promoted by the α-cation-stabilizing effect of fluorine: an efficient method for synthesizing angular PAHs

In order to synthesize polycyclic aromatic hydrocarbons with nonlinear arrangements (angular PAHs), acid-promoted domino cyclizations of 1,1-difluoroalk-1-enes and 1,1-difluoroalka-1,3-dienes were studied. 1,1-Difluoroalkenes, each bearing two aryl substituents, were regioselectively protonated with FSO(3)H⋅SbF(5) to generate fluorine-stabilized carbocations, which readily underwent domino Friedel-Crafts-type cyclizations to give carbocycles based on 6/n/m/6 ring systems (n,m=5-7) in good to high yields. Protonation of 1,1-difluoroalka-1,3-dienes took place at their electron-rich methylene (CH(2)) carbon atoms in the presence of milder acids such as camphorsulfonic acid and trifluoromethanesulfonic acid. Domino cyclizations of the resulting fluorine-stabilized allylic carbocations afford carbocycles based on 6/6/6/6 or 6/6/5/6 ring systems in high yields.

Synthesis and chiroptical properties of ferrocene-[4]-helicenequinones: kinetic resolution of a planar-chiral diene

Both enantiomers of ferrocene [4]-helicenequinone 6, showing planar and helical chiralities, have been synthesized with very high optical purities using, as the key step, a kinetic resolution process between planar-chiral racemic ferrocene diene 2 and enantiopure sulfinyl benzoquinone (S)-3.

Chiral amplification in a cyanobiphenyl nematic liquid crystal doped with helicene-like derivatives

The addition of a chiral non-racemic dopant to a nematic liquid crystal (LC) has the effect of transferring the molecular chirality to the phase organization and a chiral nematic phase is formed. This molecular chirality amplification in the LC provides a unique possibility for investigating the relationship between molecular structure, intermolecular interactions, and mesoscale organization. It is known that axially chiral or helical-shaped molecules with reduced conformational disorder are good candidates for high helical twisting power derivatives. In particular, biaryl derivatives are known to be efficient chiral inducers in biaryl nematic mesophases. In this paper, we focus on a new series of helicene-like molecules of known absolute configuration. We have integrated cholesteric pitch measurements with geometry optimization by DFT calculations and analysis of the twisting ability by the Surface Chirality model to shed light on the structural features responsible for the analogies and differences exhibited by these derivatives. The investigation of these dopants with well-defined geometry, by virtue of the low conformational freedom, and the substituents variously distributed around the core, allows us to extend our knowledge of the molecular origin of the chirality amplification in liquid crystals and to confirm the simple relationship “molecular P-helicity” → “cholesteric P-handedness” for helical-shaped helicene-like derivatives.