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Introduction of heterocycles into the helical skeleton of helicenes allows modulation of their redox, chiroptical, and photophysical properties. This paper describes the straightforward preparation and structural characterization by single-crystal X-ray diffraction of thiadiazole-[7]helicene, which was resolved into M and P enantiomers by chiral HPLC, together with its S-shaped double [4]helicene isomer, as well as the smaller congeners thiadiazole-[5]helicene and benzothiadiazole-anthracene. A copper(II) complex with two thiadiazole-[5]helicene ligands was structurally characterized, and it shows the presence of both M and P isomers coordinated to the metal center. The emission properties of the heterohelicenes are highly dependent on the helical turn, as the [7]- and [5]helicene are poorly emissive, whereas their isomers, that is, the S-shaped double [4]helicene and thiadiazole-benzanthracene, are luminescent, with quantum efficiencies of 5.4 and 6.5 %, respectively. DFT calculations suggest quenching of the luminescence of enantiopure [7]helicenes through an intersystem-crossing mechanism arising from the relaxed excited S1 state. [ABSTRACT FROM AUTHOR]

The determination of molecular stereochemistry and absolute configuration is an important part of modern chemistry, pharmacology, and biology. Electronic circular dichroism (ECD) spectroscopy is a widely used tool for chirality assignment, especially with porphyrin macrocycles employed as reporter chromophores. However, the mechanisms of induced ECD in porphyrin complexes are yet to be comprehensively rationalized. In this work, the ECD spectra of a sterically hindered hexa‐cationic porphyrin with two camphorsulfonic acids in dichloromethane and chloroform were experimentally measured and computationally analyzed. The influence of geometric factors such as the position of chiral guest molecules, distortion of the porphyrin macrocycle, and orientation of aromatic and non‐aromatic peripheral substituents on the ECD spectra was theoretically studied. Various potential pitfalls, such as a lack of significant conformations and accidental agreement of experimental and simulated spectra, are considered and discussed. [ABSTRACT FROM AUTHOR]

A rare, perfect case of a highly selective regiodivergent reaction of a racemic mixture is presented. Upon interaction with the Pd – (S,S) ‐ NHC catalyst (NHC=N ‐ heterocyclic carbene), the R enantiomer takes the right fork and the S enantiomer the left one. Thus, the racemic mixture reacts to give two regioisomeric enantiopure products. In this reaction a methyl CH activation competes with an asymmetric CH ‐ bond activation in a methylene unit to give single enantiomers of substituted indolines. DFT was the tool to map out the course of the reaction and to extend and rationalize the outcome. For more details see the Full Paper by E. P. Kündig et al. on page 15021 ff. [ABSTRACT FROM AUTHOR]

With our new home-built circularly polarized luminescence (CPL) instrument, we measured fluorescence and CPL spectra of the enantiomeric pairs of two quasi-isomeric BODIPY DYEmers 1 and 2, endowed with axial chirality. The electronic circular dichroism (ECD) and CPL spectra of these atropisomeric dimers are dominated by the exciton coupling between the main π-π* transitions (550-560 nm) of the two BODIPY rings. Compound 1 has strong ECD and CPL spectra ( glum=4×10−3) well reproduced by TD-DFT and SCS-CC2 (spin-component scaled second-order approximate coupled-cluster) calculations using DFT-optimized ground- and excited-state structures. Compound 2 has weaker ECD and CPL spectra ( glum=4×10−4), partly due to the mutual cancellation of electric-electric and electric-magnetic exciton couplings, and partly to its conformational freedom. This compound is computationally very challenging. Starting from the optimized excited-state geometries, we predicted the wrong sign for the CPL band of 2 using TD-DFT with the most recommended hybrid and range-separated functionals, whereas SCS-CC2 or a DFT functional with full exact exchange provided the correct sign. [ABSTRACT FROM AUTHOR]

Small chiral organic molecules with CD properties are in high demanded due to their potential use in promising electronic and biological applications. Herein, we reveal a system in which the oxidation of a phosphino group to the corresponding phosphine oxide on the inner rim of a helicene derivative induces a CPL response. Laterally π‐extended 7,8‐dihydro[5]helicenes bearing phosphine and phosphine oxide groups on their inner helical rims (i. e. the C1 position) were synthesized, and their helical structures were unambiguously determined by X‐ray crystallography. The photophysical (UV/visible and emission) and chiroptical properties of these compounds were investigated in various solvents. Despite their structural similarities, phosphine oxide showed a significantly better CPL response than phosphine, with a high dissymmetry factor for emission (|glum|=(1.3–1.9)×10−3) that can be attributed to structural changes in the interior of the helicene helix. [ABSTRACT FROM AUTHOR]

Alkyllithium compounds are not generally stable at room temperature, therefore strategies were established to overcome this limitation. We present a systematic approach to obtain a stereochemically enriched benzyllithium compound, starting with the simple addition of a chiral auxiliary and ending by incorporation of the auxiliary in the substrate. Thereby, a very unusual dimer of a lithiated benzylsilane was obtained, which could be split into stereochemically enriched monomers by the addition of Lewis bases. Furthermore, we were able to understand the factors responsible for this stereochemical enrichment by using quantum chemical calculations and clarify the configuration of the lithiated compound and the corresponding trapping product by crystallisation. This enabled us to determine the stereochemical course of the deprotonation and the subsequent metathesis reaction. [ABSTRACT FROM AUTHOR]

Axially chiral BODIPY DYEmers display intense fluorescence and circularly polarized luminescence (CPL) spectra. Due to exciton coupling between the BODIPY rings the π* states split into two energy levels, giving strong ECD couplets. The strongly emitting excited state is the low ‐ lying exciton ‐ coupled level. TD ‐ DFT and CC2 calculations elucidated the photophysics behind the CPL of these systems. More information can be found in the Full Paper by M. Bröring, L. Di Bari, G. Pescitelli et al. on page 16089 ff. [ABSTRACT FROM AUTHOR]

A series of peptides that contain homo- and heterochiral Ala−Pro sequences attached to the turn-inducing ferrocene-1,1′-diamine scaffold were synthesized. The effects of the backbone chirality and the N-terminal group (Boc/Ac) on the conformational properties of the novel peptidomimetics were thoroughly explored by IR, NMR, and CD spectroscopy and the experimental observations were corroborated by DFT studies in solution. The most stable conformers of the homochiral peptides adopted the interstrand hydrogen-bond patterns, realized through ten- and thirteen-membered rings. The common feature of the most stable conformers of the heterochiral peptides was the adoption of the turn-like structures that feature the simultaneous intra- (seven-membered) and interstrand (sixteen-membered) hydrogen-bonded rings. An exchange of two N-terminal groups had a somewhat larger influence on the distribution of the hydrogen-bond patterns in homochiral than in heterochiral derivatives. The homochiral peptides that contain pyridine moieties as metal coordination sites formed 1:1 complexes with divalent metal ions, which included Zn2+, Cd2+, Cu2+ and Fe2+. [ABSTRACT FROM AUTHOR]

The linkage of two P2-phosphazenyl groups through a C2-symmetric ( R, R)-1,2-diaminocyclohexane (DACH) backbone yielded the new chiral superbases DACH-P2NMe2 and DACH-P2Pyr (Pyr=pyrrolidinyl). These bases were prepared by a Kirsanov reaction and studied with respect to their spectroscopic and structural characteristics. Theoretical calculations concerning their basicity properties revealed remarkable p KBH+ values of 38.1 and 39.9 on the acetonitrile scale; this makes them the strongest nonionic chiral superbases known to date. [ABSTRACT FROM AUTHOR]

The oxidative metalation method, involving the interaction of free-base meso-triarylcorroles and W(CO)6 in refluxing decalin, led to a set of three tungsten(VI) biscorroles, the first homoleptic sandwich compounds involving corroles. Single-crystal X-ray structures of two of the complexes revealed square-antiprismatic coordination and strongly domed corroles with long W−N distances of 2.15-2.22 Å and a substantial displacement of ∼1.17 Å of the metal relative to the mean N4 planes of the ligands. The structures correspond to approximate C2 symmetry and are thus chiral. DFT calculations strongly indicate that the enantiomers are configurationally stable and hence amenable to chiral resolution. Their other notable properties include a strongly blueshifted Soret band at (357±2) nm, a relatively intense π→W(d [ABSTRACT FROM AUTHOR]

A combined experimental-computational analysis of the one- and two-photon absorption (OPA, TPA) and the electronic circular dichroism (ECD) of complex chiral shape-persistent (2,6)-pyrido[142]allenoacetylenic cyclophane, ( P, P, P, P) -MC[142] enantiomers, sheds light on the origin of their peculiar spectral signatures. The study on MC[142], which is represented in solution by three possible conformers, indicates that two of them (chair and twist) are the prevalent conformers at room temperature; the population ratio depending on the solvent. The complex shape of the red-edge of the UV and ECD spectra is qualitatively reproduced by using vibronic calculations and assigned to progressions on the ethynyl stretchings observable only in the chair and twist conformers. The picture arising from the analysis of the linear spectra is confirmed by two-photon spectroscopies, showing that they constitute valuable tools for the exploration of high-energy electronic transitions in pyridoallenoacetylenic cyclophanes. Discrepancies among computed and experimental spectra point out that an accurate description of such complex systems with a large delocalized π framework and a dense manifold of electronic excited state remains a challenge for theoretical models. [ABSTRACT FROM AUTHOR]

A highly enantioselective b-isocupreidine (b-ICD) catalyzed synthesis of dihydropyran-fused benzofurans through [4+2] cycloaddition of allenoates and benzofuranone alkenes was developed. Switchable chirality inversion of cycloaddition products was achieved by replacing the phenolic proton of the catalyst with a methyl, demonstrating an amazing effect of minimal structural variation on inverting enantioselectivity. DFT calculations were utilized to elucidate the origin of the observed phenomena. Computation also provided a clue for a rational design in which the multi-hydrogen bond with the alcohol additive was found to improve the enantioselectivity of the cycloaddition. Finally, the substrate scope was examined, in which a number of functionalized dihydropyran-fused benzofurans could be obtained in high yields (up to 97%) with very good regio- (>20:1) and enantioselectivities (up to 98:2 e.r.). [ABSTRACT FROM AUTHOR]

The hydroxyphenyl chiral ketone, ( S)- 3, reacts with D-amino acids bearing hydrophobic side chains exclusively over the L-amino acids in a two-phase liquid-liquid extraction, and thus acts as a highly stereoselective extractant. Calculations for the energy-minimized structures for the imine diastereomers and the comparison of the selectivities with other phenyl ketones, ( S)- 4 and ( S)- 5, demonstrate that the hydrogen bond between the carboxylate group and the phenolic hydroxyl group contributes to the remarkable enantioselectivities. The multiple hydrogen bonds present in the imine of ( S)- 3 reinforce the rigidity, and results in the difference between the stabilities of the imine diastereomers. The imine could be hydrolyzed in methanolic HCl solution, and the extraction of the evaporated residues revived the organic layer of ( S)- 3, which could enter into a new extractive cycle and leaves the D-amino acid with enantiomeric excess ( ee) values of over 97 % in the aqueous layer. [ABSTRACT FROM AUTHOR]

In this article, we describe our efforts on the total synthesis of bolivianine ( 1) and isobolivianine ( 2), involving the synthesis of onoseriolide ( 3). The first generation synthesis of bolivianine was completed in 21 steps by following a chiral resolution strategy. Based on the potential biogenetic relationship between bolivianine ( 1), onoseriolide ( 3), and β-( E)-ocimene ( 8), the second generation synthesis of bolivianine was biomimetically achieved from commercially available (+)-verbenone in 14 steps. The improved total synthesis features an unprecedented palladium-catalyzed intramolecular cyclopropanation through an allylic metal carbene, for the construction of the ABC tricyclic system, and a Diels-Alder/intramolecular hetero-Diels-Alder (DA/IMHDA) cascade for installation of the EFG tricyclic skeleton with the correct stereochemistry. Transformation from bolivianine to isobolivianine was facilitated in the presence of acid. The biosynthetic mechanism and the excellent regio- and endo selectivities in the cascade are well supported by theoretical chemistry based on the DFT calculations. [ABSTRACT FROM AUTHOR]

A series of tris-aryl phosphanes, structurally designed to exist as residual enantiomers or diastereoisomers, bearing substituents differing in size and electronic properties on the aryl rings, were synthesized and characterized. Their electronic properties were evaluated on the basis of their electrochemical oxidation potential determined by voltammetry. The configurational stability of residual phosphanes, evaluated by dynamic HPLC on a chiral stationary phase or/and by dynamic 1H and 31P NMR spectroscopy, was found to be rather modest (barriers of about 18-20 kcal mol−1), much lower than that shown by the corresponding phosphane oxides (barriers of about 25-29 kcal mol−1). For the first time, the residual antipodes of a tris-aryl phosphane were isolated in enantiopure state and the absolute configuration assigned to them by single-crystal anomalous X-ray diffraction analysis. In this case, the racemization barrier could be calculated also by CD signal decay kinetics. A detailed computational investigation was carried out to clarify the helix reversal mechanism. Calculations indicated that the low configurational stability of tris-aryl phosphanes can be attributed to an unexpectedly easy phosphorus pyramidal inversion which, depending upon the substituents present on the blades, can occur even on the most stable of the four conformers constituting a single residual stereoisomer. [ABSTRACT FROM AUTHOR]

The mechanism of ring-chain-ring tautomerization and the prominent effect of the solvent environment have been computationally investigated in an effort to explain the enantiomeric interconversion observed in 2-oxazolidinone derivatives, heterocyclic analogues of biphenyl atropisomers, which were isolated as single stable enantiomers and have the potential to be used as axially chiral catalysts. This study has shed light on the identity of the intermediate species involved in the ring-chain-ring tautomerization process as well as the catalytic effect of polar protic solvents. These mechanistic details will prove very useful in predicting and understanding ring-chain tautomeric equilibria in similar heterocyclic systems and will further enable experimentalists to devise appropriate experimental conditions in which axially chiral catalysts remain stable as single enantiomers. [ABSTRACT FROM AUTHOR]

What accounts for a particular chiral selection in the case of a few sugars of prebiotic relevance, thereby mirroring the asymmetry observed in nature? By using first-principles calculations, the generation of pentoses from glycolaldehyde (the initial product of the autocatalytic formose reaction), which has been detected in outer space), has been modeled by using L-Val- L-Val as a primeval catalyst. Our theoretical study provides insight into the mechanism of this reaction and satisfactorily explains a few key molecular events. Our rationale agrees with the reported experimental data and shows that the D-configuration is only favored for ribose. L-pentoses are usually favored in the presence of L-configured dipeptides, as observed experimentally, although no chiral selection could be observed in the case of xylose. These results confirm that a prebiotic sugar soup could be fine-tuned in the presence of shorter peptides as catalysts and that D-ribose would have also resulted in an advantageous imbalance for further amplification and chemical evolution. [ABSTRACT FROM AUTHOR]