Enantioselective Synthesis of Planar-Chiral Carba-Paracyclophanes: Rhodium-Catalyzed [2+2+2] Cycloaddition of Cyclic Diynes with Terminal Monoynes

Catalytic Enantioselective Synthesis of Helicenes

Helicenes and helicene-like molecules, usually containing multiple ortho-fused aromatic rings, possess unique helical chirality. These compounds have found a wide range of important applications in many research fields, such as asymmetric catalysis, molecular recognition, sensors and responsive switches, circularly polarized luminescence materials and others. However, the catalytic enantioselective synthesis of helicenes was largely underexplored, when compared with the enantioselective synthesis of molecules bearing other stereogenic elements (e.g. central chirality and axial chirality). Since the pioneer work of asymmetric synthesis of helicenes via enantioselective [2+2+2] cycloaddition of triynes by Stará and Starý, last two decades have witnessed the tremendous development in the catalytic enantioselective synthesis of helicenes. In this review, we comprehensively summarized the advances in this field, which include methods enabled by both transition metal catalysis and organocatalysis, and provide our perspective on its future development.

Planar Chirality: A mine for catalysis and structure discovery

Planar chirality is one of the most fascinating expressions of chirality, exploited by Nature to lock three-dimensional chiral conformations and, more recently, by chemists to create new chiral reagents, catalysts and functional organic materials. Nevertheless, the shortage of protocols able to induce and secure asymmetry during the generation of these unique chiral entities has dissuaded chemists to exploit their structural properties. This Minireview intends to illustrate the limited but remarkable catalytic methodologies reported for the production of planar chirality in strained molecules and serve as source of inspiration for the development of new unconventional protocols that are expected to come in the near future.

Enantioselective Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)-Based Planar Chiral Bent Cyclophanes by Rhodium-Catalyzed [2+2+2] Cycloaddition

The enantioselective synthesis of polycyclic aromatic hydrocarbon (PAH)-based planar chiral cyclophanes was achieved for the first time by the rhodium-catalyzed intramolecular regio- and enantioselective [2+2+2] cycloaddition of tethered diyne-benzofulvenes followed by stepwise oxidative transformations. The thus synthesized planar chiral bent cyclophanes, that possess bent p-terphenyl- and 9-fluorenone-cores, were converted to 9-fluorenol-based ones with excellent ee values of >99 % by diastereoselective 1,2-reduction. These 9-fluorenol-based cyclophanes exhibited high fluorescence quantum yields, which were significantly higher than that of an acyclic reference molecule (78-82 % vs. 48 %). The bending effect on the chiroptical property was also examined, which revealed that the anisotropy factors (g_abs values) for electronic circular dichroism (ECD) of these 9-fluorenol-based planar chiral bent cyclophanes increase as the tether length becomes shorter.

Insight into the Activation of In Situ Generated Chiral RhI Catalysts and Their Application in Cyclotrimerizations

We report a detailed study concerning the efficient generation of highly active chiral rhodium complexes of the general structure [Rh(diphosphine)(solvent)₂ ]⁺ as well as their exemplary successful utilization as catalysts for cyclotrimerizations. Such solvent complexes could likewise be prepared from novel ammonia complexes of the type [Rh(diphosphine)(NH₃ )₂ ]⁺ . A valuable, feasible approach to generate novel chiral Rh^I complexes was found by in situ generation from Wilkinson's catalyst [RhCl(PPh₃ )₃ ] with chiral P,N ligands. The generated catalysts led to moderate to good enantioselectivities and excellent yields in the cyclotrimerizations of triynes, showcasing their usefulness in the synthesis of axially chiral benzene derivatives.

Unusual nitrile-nitrile and nitrile-alkyne coupling of Fc-C≡N and FC-C≡C-C≡N

The reactions of the Group 4 metallocene alkyne complexes, [Cp*2M(η2-Me3SiC2SiMe3)] (1 a: M=Ti, 1 b: M=Zr, Cp*=η5-pentamethylcyclopentadienyl), with the ferrocenyl nitriles, Fc-CN and Fc-C≡C-C≡N (Fc=Fe(η5-C5H5)(η5-C5H4)), is described. In case of Fc-C≡N an unusual nitrile–nitrile C-C homocoupling was observed and 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (3 a, b) were obtained. As the first step of the reaction with 1 b, the nitrile was coordinated to give [Cp*2Zr(η2-Me3SiC2SiMe3)(N≡C-Fc)] (2 b). The reactions with the 3-ferrocenyl-2-propyne-nitrile FcC≡C-C≡N lead to an alkyne–nitrile C-C coupling of two substrates and the formation of 1-metalla-2-aza-cyclopenta-2,4-dienes (4 a, b). For M=Zr, the compound is stabilized by dimerization as evidenced by single-crystal X-ray structure analysis. The electrochemical behavior of 3 a, b and 4 a, b was investigated, showing decomposition after oxidation, leading to different redox-active products.

Total synthesis of the putative structure of xylarinol B

The total synthesis of the putative structure of xylarinol B is described and the need to revise its structure is demonstrated. The central benzoxepine skeleton was constructed by employing a cobalt-mediated bimolecular [2+2+2] Reppe-Vollhardt alkyne cycloaddition reaction.