Tamio Hayashi’s research while affiliated with National Taiwan Normal University and other places

Carbon-silicon-switch strategy, replacing one specific carbon atom in organic molecules with a silicon, has garnered significant interest for developing new functional molecules. However, the influence of a reaction regarding its selectivity and reactivity by carbon-silicon-switch strategy has far less been investigated. Here we discover an unusual carbon-silicon-switch effect in the enantioselective construction of silicon-stereogenic center. It is found that there has been a significant change in the desymmetrization reaction of silacyclohexadienones using asymmetric conjugate addition or oxidative Heck reaction with aryl/alkyl nucleophiles when compared with their carbon analogues cyclohexadienones. Specifically, the carbon-silicon-switch leads to a reversal in enantioselectivity with arylzinc as the nucleophile by the same chiral catalyst, and results in totally different reactivity with arylboronic acid as the nucleophile. Control experiments and density functional theory (DFT) calculations have shown that the unusual carbon-silicon-switch effect comes from the unique stereoelectronic feature of silicon.

Transition metal-catalyzed addition of organometallics to aryl(alkyl)alkynes has been well known to proceed with the regioselectivity in forming a carbon–carbon bond at the alkyl-substituted carbon (β-addition). Herein, the reverse regiochemistry with high selectivity in giving 1,1-diarylalkenes (α-addition) was realized in the reaction of arylboronic acids with aryl(alkyl)alkynes by use of a rhodium catalyst coordinated with a chiral diene ligand, whereas the arylation of the same alkynes proceeded with the usual regioselectivity (β-addition) in the presence of a rhodium/DM-BINAP catalyst. The regioselectivity can be switched by the choice of ligands on the rhodium catalysts. This reverse regioselectivity also enabled the catalytic asymmetric synthesis of phoenix-like axially chiral alkylidene dihydroanthracenes with high enantioselectivity through an α-addition/1,4-migration/cyclization sequence.

Asymmetric catalysis has emerged as a general and powerful approach for constructing chiral compounds in an enantioselective manner. Hence, developing novel chiral ligands and catalysts that can effectively induce asymmetry in reactions is crucial in modern chemical synthesis. Among such chiral ligands and catalysts, chiral dienes and their metal complexes have received increased attention, and a great progress has been made over the past two decades. This review provides comprehensive and critical information on the essential aspects of chiral diene ligands and their importance in asymmetric catalysis. The literature covered ranges from August 2003 (when the first effective chiral diene ligand for asymmetric catalysis was reported) to October 2021. This review is divided into two parts. In the first part, the chiral diene ligands are categorized according to their structures, and their preparation methods are summarized. In the second part, their applications in asymmetric transformations are presented according to the reaction types.

The reaction of readily available E-allyl difluorides (R1CH = CHCF2COR2) containing a CF2COR2 moiety with arylboronic acids catalyzed by a chiral diene-rhodium complex gives high yields of chiral monofluoroalkenes (R1ArC*HCH = CFCOR2) with perfect enantioselectivity (enantiomeric excess (ee) of > 99% in all cases) and high level of Z/E selectivity (up to 20:1). The reaction is proposed to proceed through enantioselective arylrhodation of the E-allyl difluorides forming a β,β-difluoroalkylrhodium intermediate, followed by stereoselective β-fluoride elimination. The CF2COR2 moiety plays a key role in activating the E-allyl difluorides toward arylrhodation/defluorination in executing the catalytic cycle, and it serves as a handle for further functionalization.

The rhodium-catalyzed controllable diverse arylation of 2,5-dihydrofuran with arylboronic acids is reported. By fine-tuning of the reaction conditions, four different ring-opening or oxidative arylation pathways are controlled in the rhodium-catalyzed arylation of 2,5-dihydrofuran, granting selective access to 2-aryl or 3-aryl homoallylic alcohols and 3-aryl or 4-aryl-2,3-dihydrofurans. The catalytic asymmetric ring-opening arylation of 2,5-dihydrofuran was also realized. Four plausible reaction pathways were proposed based on the experimental results.

Enantioselective addition of boronic acids to N‐unsubstituted isatin‐derived ketimines was realized using rhodium(I)/chiral diene catalysts. The reactions can be performed in the presence of catalytic amounts of a base to give adducts in high yield with high enantioselectivity. Preliminary mechanistic information including a computational model to explain the observed enantioselectivity is also provided.

New route to practical intermediates: The direct synthesis of arylacetaldehydes through a carbon–carbon bond‐forming reaction has been realized by the iridium‐catalyzed hydroarylation of readily available vinylene carbonate with arylboronic acids. Tetrahydroxydiboron serves as a cocatalyst in this reaction. image

The one‐step synthesis of arylacetaldehydes by carbon–carbon bond formation between formylmethyl and aryl groups has been realized by the reaction of vinylene carbonate with arylboronic acids in the presence of an iridium/bisphosphine catalyst and a catalytic amount of tetrahydroxydiboron. Der direkte Weg zum Erfolg: Die direkte Synthese von Arylacetaldehyden über eine C‐C‐Kupplung gelang durch Ir‐katalysierte Hydroarylierung von leicht verfügbarem Vinylencarbonat mit Arylboronsäuren. Tetrahydroxydiboron wirkt als Cokatalysator bei dieser Reaktion.