Pillar[5]arene-Based Dual Chiral Organoboranes with Allowed Host-Guest Chemistry and Circularly Polarized Luminescence

We first describe two examples of highly luminescent organoboranes (NP5BN1 and NP5BN2) with dual chirality that were achieved by molecular functionalization of planar chiral pillar[5]arenes with naphthyls. Sufficiently strong steric effects are imposed by triarylamine (Ar₃N) and triarylborane (Ar₃B) moieties and further enhanced by the proximity of the chiral building blocks, leading to the isolation of multiple enantiomers via chiral high-performance liquid chromatography. The intramolecular charge transfer from N-donor to B-acceptor across both chiral subunits enabled the circularly polarized luminescence and thermally robust colorimetric responses in their emissions. Furthermore, their remarkable host-guest chemistry was allowed at no expense in the pursuit of advanced chiroptical properties using pillar[5]arene-based supramolecular scaffolds.

Origins of stereoselectivity in uncatalyzed and ZnBr2-catalyzed Diels-Alder reactions of a chiral sulfinylquinone

Density functional theory calculations are reported which explore how a chiral sulfinyl substituent controls the stereoselectivities of the Diels-Alder reactions of a 2-p-tolylsulfinylbenzoquinone. The π-facial stereoselectivities vary depending on the diene (cyclopentadiene or trans-piperylene) and on the presence or absence of a ZnBr₂ catalyst. The stereoselectivities are shown to be controlled by steric effects and non-covalent (CH-π) interactions. The calculations reveal that the active dienophile in the ZnBr₂-catalyzed reactions is likely to be a 1 : 2 complex of the dienophile and catalyst, not a 1 : 1 complex as commonly assumed.

Understanding the axial chirality control of quinidine-derived ammonium cation-directed O-alkylation: a computational study

As a privileged chiral scaffold, cinchona alkaloid and its derivatives have reached remarkable success in catalytic asymmetric organic synthesis. In addition to the wide applications of point chirality control, Smith and co-workers recently discovered a quinidine-derived ammonium cation-catalyzed O-alkylation of tetralones, providing an effective approach for the synthesis of axially chiral biaryls. Using density functional theory (DFT) calculations, we studied the reaction mechanism and origins of enantioselectivity of this novel transformation. A stepwise strategy is adopted to ensure efficient and thorough exploration of the massive conformational space of transition state. Our computations suggested that enolate oxygen forms two hydrogen bonds with the chiral ammonium catalyst, and the non-covalent interactions between the cationic benzylic fragment and the methoxy group of enolate plays a critical role in determining the enantioselectivity.

Dynamic kinetic resolution of biaryl atropisomers by chiral dialkylaminopyridine catalysts

Dynamic kinetic resolutions of atropisomers using chiral DMAPs with fluxional chirality.

Stereoselective arene formation

The common perception of arenes as flat and symmetric entities is juxtaposed to the counterintuitive circumstance that stereoselective arene formation offers a means to prepare an exceptional range of chiral aromatic structures in isomerically enriched form.

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.

Recent advances and new concepts for the synthesis of axially stereoenriched biaryls

Over the past decade the field of the synthesis of axially chiral compounds has been rapidly expanding. Not only key advances have been achieved concerning the already established strategies but also new synthetic routes have been devised. This review showcases the recent developments in this domain.

Asymmetric synthesis of heteroaryl atropisomers via a gold-catalyzed cycloisomerization–amination cascade reaction

The chiral gold(i) complex enables enantioselective cycloisomerization–amination of 2-(alkynyl)phenyl boronic acids and diazenes in high yields.

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.

Enantioselective [2+2+2] cycloisomerisation of alkynes in the synthesis of helicenes: The search for effective chiral ligands

The enantioselective [2+2+2] cycloisomerisation of the aromatic triynes under nickel(0) catalysis to afford nonracemic [6]- and [7]helicene derivatives has been systematically studied. A collection of mono- and bidentate phosphines, phosphites, phosphinites and phosphinous amides possessing stereogenic units such as chiral centre, axis or plane (or their combinations) has been tested and axially chiral binaphthyl-derived monodentate MOP-type phosphine ligands were the optimal class of ligands. Nickel complexes of these ligands afforded nonracemic tetrahydro[6]helicene in up to 64% ee in a model reaction.

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.

Stereoselective synthesis of the C15-C26 fragment of the antitumor agent (-)-dictyostatin

The synthesis of the C15-C26 fragment of (-)-dictyostatin is reported in 10 steps and 28% overall yield. The key steps are the two stereoselective sulfoxide-directed processes: a Reformatsky-type reaction and a β-keto sulfoxide reduction.