▼Highly Active Chiral Dilithium(I) Binaphthyldisulfonate Catalysts for Enantio- and Chemoselective Strecker-Type Reactions
Manabu Hatano, Kosuke Nishio, Takuya Mochizuki, Keisuke Nishikawa, Kazuaki Ishihara*
ACS Catal. 2019, 9(xxx), 8178-8186.
Publication Date:July 30, 2019
An enantioselective Strecker-type reaction of aldimines and ketimines was developed by using a chiral dilithium(I) binaphthyldisulfonate as a chiral acid–base cooperative catalyst. The present catalytic system features an extremely short reaction time (10 min to 4 h), unlike conventional catalytic systems. Along with the design of stronger chiral Li(I) Lewis acid catalysts, a highly reactive pentacoordinate silicate generated in situ could promote the reactions. In particular, instead of unstable N-Bn Strecker products, more stable N-CH2(9-anthryl) and N-CH2(1-naphthyl) Strecker products could be obtained in high yields with high enantioselectivities. By a switch of the present and previous catalyst systems, chemoselective cyanation to a ketoaldimine could be performed, respectively. Moreover, mechanistic investigations provided useful information regarding the active catalysts, catalytic cycles, and possible transition states.
▼Tris(pentafluoropenyl)borane-Assisted Chiral Phosphoric Acid Catalysts for Enantioselective Inverse-Electron-Demand Hetero-Diels–Alder Reaction of α,β‐Substituted Acroleins
Manabu Hatano, Tatsuhiro Sakamoto, Takuya Mochizuki, Kazuaki Ishihara*
Asian J. Org. Chem. 2019, 8(7), 1061-1066.
First published: 06 March 2019
In the enantioselective inverse‐electron‐demand hetero‐Diels‐Alder reaction, simple α,β‐unsaturated aldehydes (i. e., acroleins) are still challenging substrates, unlike α,β‐unsaturated carbonyl compounds containing electron‐withdrawing groups. In the present study, the reaction of α‐aryl‐β‐alkyl‐substituted acroleins with ethyl vinyl sulfide was developed with the use of bulky chiral supramolecular Brønsted acid catalysts, such as tris(pentafluorophenyl)borane‐assisted chiral phosphoric acid catalysts. As a result, cis‐cycloadducts as optically active 3,4‐dihydro‐2H‐pyrans were exclusively obtained in high yields with high enantioselectivities via the favored endo orbital approach. An obtained optically active cis‐isomer could be transformed into the corresponding trans‐isomer without a loss of enantiopurity by O,S‐acetal epimerization. Moreover, transformations to synthetically useful optically active epoxide and δ‐valerolactone are also demonstrated.
Front Cover: Tris(pentafluorophenyl)borane‐Assisted Chiral Phosphoric Acid Catalysts for Enantioselective Inverse‐Electron‐Demand Hetero‐Diels‐Alder Reaction
of α,β‐Substituted Acroleins (Asian J. Org. Chem. 7/2019)
First published: 26 June 2019
▼Structure and Reactivity of Aromatic Radical Cations Generated by FeCl3
Takahiro Horibe, Shuhei Ohmura, Kazuaki Ishihara*
J. Am. Chem. Soc. 2019, 141(5), 1877-1881
First published: 06 March 2019
This paper describes the isolation and characterization of an aromatic radical cation generated by FeCl3. X-ray crystallographic analysis and kinetic studies reveal the mechanism of the generation of aromatic radical cation. In the solid state, a tight ion-pair of a radical cation with FeCl4– is observed. Leveraging the efficient generation of the radical cation–FeCl4– ion pair, we explore a radical cation-induced cycloaddition of trans-anethole initiated by catalytic amount of FeCl3. Both [4+2] cycloaddition and [2+2] cycloaddition with a broad substrate scope are also described. Moreover, a 100 g-scale reaction is demonstrated with the use of 1 mol % of FeCl3 as a simple and a highly active initiator.
▼Ammonium Hypoiodite-catalyzed Oxidative Dearomatizative Azidation of Arenols
Muhammet Uyanik, Kohei Nishioka, and Kazuaki Ishihara*
Chem. Lett. 2019, 48(4), 353–356.
The first transition metal-free catalytic oxidative dearomatizative azidation of arenols has been developed using hypoiodite catalysis with aqueous hydrogen peroxide and trimethylsilyl azide as an oxidant and azide source, respectively, under mild conditions. The corresponding quaternary azides can be obtained in high to excellent yields.
▼Enantioselective [1,3] O-to-C Rearrangement: Dearomatization of Alkyl 2-Allyloxy/benzyloxy-1/3-naphthoates Catalyzed by a Chiral π–Cu(II) Complex
Lu Yao, Kazuaki Ishihara*
Chem. Sci. 2019, 10(8), 2259–2263.
The article was first published on 10 Jan 2019
An unprecedented catalytic asymmetric [1,3] O-to-C rearrangement of alkyl 2-allyloxy/benzyloxy-1/3-naphthoates was realized under the catalysis of a chiral π–Cu(II) complex (1–10 mol%). This dearomatization strategy provides facile access to highly functionalized β-naphthalenone derivatives bearing an all-carbon quaternary stereogenic center in high yield with excellent enantioselectivity. The π–cation interaction between the aromatic substituent of the ligand and the Cu(II) center was proved by X-ray diffraction analysis and shown to be crucial for enantioselective control. Further preliminary mechanistic studies suggest that this intramolecular reaction proceeds through a contact ion pair intermediate.
▼Inside Front Cover Picture (Chem. Sci. 2019, 10(8))
▼Regioselective Oxidative Chlorination of Arenols Using NaCl and Oxone
Muhammet Uyanik, Naoto Sahara, and Kazuaki Ishihara*
Eur. J. Org. Chem. 2019, (1), 27–31.
A practical and efficient chlorination of naphthols and phenols was developed using transient chlorinating species generated in situ from inexpensive sodium chloride and Oxone as a Cl source and oxidant, respectively, under mild conditions.
▼Front Cover Picture (Eur. J. Org. Chem. 2019, No. 1.)