Synthesis of [5]-, [6]-, and [7]helicene via Ni(0)- or Co(I)-catalyzed isomerization of aromatic cis,cis-dienetriynes

An original approach to helicene frameworks exploiting atom economic isomerization of appropriate energy-rich aromatic cis,cis-dienetriynes has been developed. The new paradigm provides nonphotochemical syntheses of helicenes based on the easy, convergent, and modular assembly of key cis,cis-dienetriynes and their nickel(0)-catalyzed [2+2+2] cycloisomerization. The potential of the methodology is underlined by the syntheses of the parent [5]helicene (2), 7,8-dibutyl[5]helicene (23), [6]helicene (24), and [7]helicene (25). The approach can be adapted to prepare functionalized helicenes as exemplified by the eight-step synthesis of 7,8-dibutyl-2,3-dimethoxy[6]helicene (34). Density functional theory (DFT) calculations showed that bis[2-((1Z)-1-buten-3-ynyl)phenyl]acetylene (1) and isomeric [5]helicene (2) differ enormously in the Gibbs energy content (DeltaG = -136.6 kcal x mol(-1)) to favor highly the devised intramolecular simultaneous construction of three aromatic rings.     

Electronic Structures and Chiroptical Properties of Post‐functionalized Helicene Quinones

The synthesis of redox‐active p‐ and o‐quinones 2‐phenylamino‐4‐phenylimino[6]helicene‐1‐one 1 , 2‐phenylamino[6]‐helicene‐1,4‐dione 2 , and 4‐phenyl[6]helicene‐1,2‐dione 3 in their enantiopure forms by post‐functionalization of (P)‐ and (M)‐1,2‐dimethoxy[6]helicene is presented. Structural characterization in solution and in the solid state was accomplished by 2D NMR spectroscopy methods and X‐ray diffraction analysis, respectively. Interpretation of electrochemical redox data was accompanied by a detailed orbital picture, derived from DFT calculations. The electronic structures of compounds 1 – 3 were investigated by UV/Vis and electronic circular dichroism (ECD) spectroscopy, complemented by TD‐DFT calculations. Quinones 1 – 3 were chemically reduced to study the EPR signatures of their respective radical anions. DFT methods were used for the atom assignment of the hyperfine coupling constants. The results are discussed within the context of electrochromic chiral switches and molecular recognition.     

The synthesis of new azoniathiahelicenes

Synthesis of new azonia derivatives of thia[6]helicene ( 1 ) and thia[7]helicene ( 2 ) is described. The Knoevenagel condensation of 2‐methylbenzothieno[3,2‐a]quinolizinium salt ( 8 ) with appropriate arylaldehydes yielded 2‐(arylvinyl)benzothieno[3,2‐a]quinolizinium salts ( 9 and 10 ), respectively. Photocyclization of 2‐styrylbenzothieno[3,2‐a]quinolizinium salt ( 8 ) gave 7a‐azonia‐5‐thia[6]helicene ( 1 ) in 63% yield. Similarly, 2‐[2‐(2‐naphthyl)vinyl]benzothieno[3,2‐a]quinolizinium salt ( 10 ) afforded 7a‐azonia‐5‐thia[7]helicene ( 2 ) in 56% yield. The complete and unambiguous assignment of their ¹H‐ and ¹³C‐nmr spectra was performed by utilizing two‐dimensional nmr spectroscopic methods.     

Synthesis of Terminally Fluorinated [7]Helicenes and Their Application to Photochemical Domino Reactions

The intramolecular Diels−Alder reactions of helicenes deform their π-conjugated screw-shaped skeletons. In particular, terminally tetrafluorinated [7]helicene (F₄-[7]helicene) undergoes a photoinduced Diels−Alder reaction followed by a photoinduced double fluorine atom transfer. Herein, we thoroughly investigated this photochemical domino process by decreasing the level of fluorine substitution. F₃-[7]Helicenes bearing two fluorine atoms at the dienophile terminal underwent photoinduced Diels−Alder reactions, but the whole domino process became slow. F₂-[7]Helicene, which is difluorinated only at the dienophile terminal, was also photolabile. As a result, two fluorine atoms were sufficient for the photochemical domino reaction to occur. X-ray crystallographic analysis revealed that F₂-[7]helicene was less compressed than F₄-[7]helicene, indicating that terminal polyfluorination enhanced the intramolecular arene−fluoroarene stacking interactions and thus promoted the transformations.     

Synthesis of Long Oxahelicenes by Polycyclization in a Flow Reactor

A series of oxahelicenes composed of ortho/meta-annulated benzene/pyridine and 2H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene.     

Hélicènes pontés: Ethano-3,15- et oxa-2-propano-3,15-[7]hélicène. Synthèse,spectrographie 1H-RMN. et étude par diffraction aux rayons X

Bridged helicenes: 3,15-ethano- and 3,15-(2-oxapropano)-[7]helicene The title compounds ( 35 and 33 , see Scheme 4) have been synthesized from a common intermediate: 3,15-dimethoxycarbonyl-[7]helicene ( 26 ). The conformation of the bridged [7]helicenes (X-ray diffraction) and their ¹H-NMR. spectra have been compared to the conformation and ¹H-NMR. spectra of [7]helicene and 3,15-dimethyl-[7]helicene ( 10 ).     

Assembly of π‐Conjugated Phosphole Azahelicene Derivatives into Chiral Coordination Complexes: An Experimental and Theoretical Study

Aza[n]helicene phosphole derivatives have been prepared from aza[n]helicene diynes by the Fagan–Nugent route. Their photophysical properties (UV/Vis absorption and emission behavior) have been evaluated. Their behavior as P,N chelates towards coordination to Pd^II and Cu^I has been investigated: metal–bis(aza[n]helicene phosphole) assemblies are formed by a highly stereoselective coordination process, as demonstrated by X‐ray crystallography. An aza[6]helicene phosphole bearing an enantiopure helicene part has been obtained, which allows the preparation of enantiopure Pd^II and Cu^I complexes with original topologies and high molar rotation (MR) and circular dichroism (CD). The structure–property relationship established from the experimental data has been studied in detail by theoretical studies (TDDFT calculations of UV/Vis, CD, and MR). Aza[n]helicene phosphole derivatives show π conjugation extended over the entire molecule, and its influence on the MR of aza[6]helicene phosphole 5 c has been demonstrated. Finally, it has been shown that the nature of the metal (coordination geometry and electronic interaction) can have a great impact on the amplitude of the chiroptical properties in metal–bis(aza[n]helicene phosphole) assemblies.     

S-shaped para-Quinodimethane-Embedded Double [6]Helicene and Its Charged Species Showing Open-Shell Diradical Character

Helicenes and extended helical π-conjugated compounds have been widely studied, but most of the systems contain only aromatic benzene or heterocyclic rings, showing local aromatic character. Herein, new S-shaped double [6]helicene 1 , which has two embedded para-quinodimethane (p-QDM) units, is reported. Due to the existence of a proaromatic quinoidal substructure, it has open-shell diradical character. Its model compound, C-shaped single [6]helicene 2 containing one p-QDM unit, was also synthesized and compared. Their ground-state structures and electronic properties were systematically studied by a combination of various experimental methods assisted by theoretical calculations. Compound 1 has a double-helical structure in the crystal, with the two terminal [6]helicene units bent in opposite directions (i.e., anti form). However, an anti/syn isomerization process with a moderate interconversion energy barrier was observed on the NMR timescale. Compound 1 shows amphoteric redox behavior. It also exhibits open-shell diradical character (y₀=12.1 %) and a small singlet–triplet gap. On the other hand, compound 2 has a typical closed-shell nature. The dication and dianion of 1 also show open-shell diradical character. The dianion of 2 and the tetraanion of 1 exhibit similar electronic structures to their respective isoelectronic structures, that is, [6]helicene and a double [6]helicene. This work provides some insights into the design and synthesis of stable helical π systems with open-shell diradical character and magnetic activity.     

(BO)2-Doped Tetrathia[7]helicene: A Configurationally Stable Blue Emitter

Helicenes combine two central themes in chemistry: extended π-conjugation and chirality. Hetero-atom doping preserves both characteristics and allows modulation of the electronic structure of a helicene. Herein, we report the (BO)₂-doped tetrathia[7]helicene 1 , which was prepared from 2-methoxy-3,3′-bithiophene in four steps. 1 is formally derived by substituting two (Mes)B−O moieties in place of (H)C=C(H) fragments in two benzene rings of the parent tetrathia[7]helicene. X-ray crystallography revealed a dihedral angle of 50.26(9)° between the two terminal thiophene rings. The (P)-/(M)- 1 enantiomers were separated by chiral HPLC and are configurationally stable at room temperature. The experimentally determined enantiomerization barrier of 27.4±0.1 kcal mol⁻¹ is lower than that of tetrathia[7]helicene (39.4±0.1 kcal mol⁻¹). The circular dichroism spectra of (P)- and (M)- 1 show a perfect mirror-image relationship. 1 is a blue emitter (λ_em=411 nm) with a photoluminescence quantum efficiency of Φ_PL=6 % (cf. tetrathia[7]helicene: λ_em≈405 nm, Φ_PL=5 %).     

A Quintuple [6]Helicene with a Corannulene Core as a C5‐Symmetric Propeller‐Shaped π‐System

The synthesis and structural analysis of a quintuple [6]helicene with a corannulene core is reported. The compound was synthesized from corannulene in three steps including a five‐fold intramolecular direct arylation. X‐ray crystallographic analysis revealed a C₅‐symmetric propeller‐shaped structure and one‐dimensional alignment in the solid state. The enantiomers of the quintuple [6]helicene were successfully separated by HPLC, and the chirality of the two fractions was identified by CD spectroscopy. A kinetic study yielded a racemization barrier of 34.2 kcal mol^?1, which is slightly lower than that of pristine [6]helicene. DFT calculations indicate a rapid bowl‐to‐bowl inversion of the corannulene moiety and a step‐by‐step chiral inversion pathway for the five [6]helicene moieties.     

Synthesis of Optically Pure Helicene Metallocenes

Two types of ruthenocenes and a ferrocene coordinated by rac ‐9H ‐cyclopenta[1,2‐c :4,3‐c ′]diphenanthrenyl anion(s), a [7]helicene with a cyclopentadienyl moiety at the center of its skeleton, were successfully synthesized: mono‐helicene ruthenocene 1 and its iron analogue 1_Fe with one [7]helicene ligand bound to the central metal, and bis‐helicene ruthenocene 2 with two [7]helicenes. Starting from a racemic mixture of the ligand precursor, rac ‐ 2 and meso ‐ 2 were obtained in a 7:3 ratio. Since the [7]helicene has a high racemization barrier, enantiomers of the complexes were isolated in their pure forms; they showed large optical rotations and intense circular dichroism (CD) responses.     

Phosphorescence Resulting from Interaction between Two Non‐equivalent Metals on a Helical π‐Conjugated Surface

A [7]helicene bis‐ruthenium complex, with one ruthenium atom bound to the cyclopentadienyl (Cp) ring and the other coordinated to the arene ring at the edge of the helicene, was synthesized. This complex showed phosphorescence both in butyronitrile (Φ=31 %, 77 K) and in the solid state (Φ=18 %, 77 K). The two non‐equivalent ruthenium metal atoms, attached to the helicene ligand, interact with each other upon photoabsorption and emission.     

Preparation and structural properties of 7,8-dioxa[6]helicenes and 7a,14c-dihydro-7,8-dioxa[6]helicenes.

The potentially chiral 7,8-dioxa[6]helicenes 1-1c have been prepared by oxidation of their precursors the 7a,14c-dihydro-7,8-dioxa[6]helicenes 3. The crystal structure determination of 3b cis-7a,14c-dihydro-3,12-dibromo-7,8-dioxa[6]helicene unambiguously confirms the cis configuration of the 7a,-14c hydrogens in compounds 3 as previously implied from NMR measurements and also shows that 3b crystallizes in a chiral conformation in the solid state. Selective deuteration of the sterically crowded 1,14 positions of 7,8-dioxa[6]helicene 1 influenced the crystal structure. The deuterium labeled compound D2-1 exhibits a disordered structure, whereas 1 had been found to crystallize in a complex structure which can be described as an analogous partly ordered modulated superstructure. When dehydrogenation of compound 3 to obtain compound 1 was attempted, harsh synthetic conditions gave the unexpected halogenated compounds 5-chloro-7,8-dioxa[6]helicene 1c and cis-7a,14c-dibromo-7,8-dioxa[6]helicene 3c. Compounds 1d and 3b were identified by solving their crystal structure.     

Stereospecific Synthesis of Enantiopure [6]Helicene Containing a Seven-Membered Ring and [7]Helicene by Acid-Promoted Stepwise Alkyne Annulations of Doubly Axial-Chiral Precursors**

Enantiopure [6]helicene containing an embedded seven-membered ring and carbo[7]helicene (>99 % ee) with opposite helicity were simultaneously and quantitatively (>99 %) synthesized with a perfect stereospecificity through stepwise acid-promoted intramolecular alkyne annulations of doubly axial-chiral cyclization precursors. The helical handedness of the [6]- and [7]helicenes was fully stereocontrolled by the doubly axial chirality of the precursors as a result of complete axial-to-helical chirality transfer. The cyclizations proceeded in a stepwise manner; the first six-membered ring formation was followed by the kinetically controlled seven- or six-membered ring formation with or without helix-inversion of a [4]helicene intermediate generated during the first cyclization step, thus quantitatively producing enantiopure circularly polarized luminescent [6]- and [7]helicenes with opposite helicity.     

Synthesis and characterization of 1,8-naphthalimide with [6]helicene skeleton

A 1,8-naphthalimide with [6]helicene derivative scaffold has been designed and synthesized. The (P)- and (M)-enantiomers of the [6]helicene derivative were resolved by HPLC on a chiral column. The single crystal of the [6]helicene derivative exhibits an intermolecular interactions of the 1,8-naphthalimide units.     

Synthesis and Properties of Spiro-double Sila[7]helicene: The LUMO Spiro-conjugation

A double helicene with a spiro-Si linker ( 4 ) was synthesized by four successive nucleophilic substitutions on SiCl₄. Its (P,P), (M,M) and (P,M) isomers were isolated and characterized by single crystal X-ray analysis. Due to the central spirosilabi[fluorene] moiety, the two helicene units in 4 are symmetrically and nearly perpendicularly arranged. (P,P)- 4 and (M,M)- 4 exhibit unique optical properties attributable to the LUMO spiro-conjugation between the two sila[7]helicene units.     

Aza[6]helicene Platinum Complexes: Chirality Control of cis–trans Isomerism

It was serendipitously observed that cis‐[PtCl₂(NCEt)PPh₃] reacted differently with either racemic or enantiopure 4‐aza[6]helicene, giving respectively cis (racemic) and trans (enantiopure) [Pt^IICl₂(4‐aza[6]helicene)PPh₃] complexes. This unexpected reactivity is explained through a dynamic process (crystallization‐induced diastereoselective transformation) and enables a new aspect of reactivity in chiral transition‐metal complexes to be addressed.     

Closed Pentaaza[9]helicene and Hexathia[9]/[5]helicene: Oxidative Fusion Reactions of ortho‐Phenylene‐Bridged Cyclic Hexapyrroles and Hexathiophenes

Oxidative fusion reactions of ortho ‐phenylene‐bridged cyclic hexapyrroles and hexathiophenes furnished novel closed helicenes in a selective manner. X‐Ray diffraction analysis unambiguously revealed the structures to be a closed pentaaza[9]helicene, the longest azahelicene reported so far, and an unexpected double‐helical structure of hexathia[9]/[5]helicene, whose formation was assumed to result from multiple oxidative fusion along with a 1,2‐aryl shift. The pentaaza[9]helicene exhibited well‐defined emission with high fluorescence quantum yield (Φ _F=0.31) among the known [9]helicenes. Chiral resolution of the racemic pentaaza[9]helicene and hexathia[9]/[5]helicene were achieved by chiral‐phase HPLC and the enantiomers were characterized by circular dichroism spectra and DFT calculations.     

Enantioselective Synthesis of [9]‐ and [11]Helicene‐like Molecules: Double Intramolecular [2+2+2] Cycloaddition

The enantioselective synthesis of completely ortho‐fused [9]‐ and [11]helicene‐like molecules has been achieved through a rhodium‐mediated, intramolecular, double [2+2+2] cycloaddition of phenol‐ or 2‐naphthol‐linked hexaynes. Crystal structures and photophysical properties of these [9]‐ and [11]helicene‐like molecules have also been disclosed.     

Diastereo‐ and Enantioselective Synthesis of Organometallic Bis(helicene)s by a Combination of C?H Activation and Dynamic Isomerization

Homochiral and heterochiral cis‐bis‐cycloplatinated‐[6]helicene derivatives 1 b^{1, 2} , as representative examples of platina[6]helicenes that share a common platinum center, have been prepared. A diastereo‐ and enantioselective synthesis, which combines CH activation and dynamic isomerization from heterochiral structure 1 b² into homochiral structure 1 b¹ , is also described. Overall, this isomerization process results in the transfer of chiral information from one helicene moiety to the other one. The chiroptical properties of homochiral (P)‐ and (M)‐ 1 b¹ were greatly modified upon oxidation into their corresponding (P)‐ and (M)‐diiodo‐Pt^IV complexes ( 5 ). The changes were also analyzed by performing theoretical calculations. C? H activation in the synthesis of organometallic helicenes is further demonstrated by the preparation of cis‐bis‐cycloplatinated‐[8]helicene 1 c .