Catalytic synthesis of nonracemic azaproline derivatives by cyclization of β-alkynyl hydrazines under kinetic resolution conditions

Hydroamination of Triisopropylsilyl Acetylene Sulfur Pentafluoride - a Bench-top Route to Pentafluorosulfanylated Enamines

Synthetic access to a variety of aliphatic and vinylic pentafluorosulfanylated building blocks remains a major challenge in contemporary organofluorine chemistry hampering its investigation in the context of medicinal chemistry, agrochemistry and functional materials. Herein, we report a bench-top protocol to access the virtually unknown class of α-SF5 -enamines under mild reaction conditions in good to excellent yields (up to 95 %). This reaction combines the protodesilylation of the commercially available precursor TASP with the in situ hydroamination of HC≡C-SF5 . The on-site use of highly toxic gases or corrosive reagents is avoided, making access to this motif applicable to a wide chemical audience. The excellent E-diastereoselectivity of this two-step cascade reaction is suggested to be the result of the convergence of the fast Z-/E- isomerization of a vinyl anion as well as the isomerization of the iminium ion. The remarkable thermal stability of these SF5 -enamines encourages further studies of their synthetic utility.

Allenoate Prenucleophiles: A Triply Diastereoselective Approach to β-Hydroxy Esters Containing All-Carbon α-Quaternary Centers

Allenyl esters activated by titanium(IV) underwent additions to a wide range of aldehydes in high regio- and diastereoselectivities leading to products containing an all-carbon quaternary center bearing an α-vinyl group that was installed with high selectivity for the Z-geometry. An aldol product was also converted to an indanone offering a new route to this important compound class. Product triple diastereoselectivity has been rationalized using a concerted transition-state model.

Onium Ion-assisted Organic Reactions Through Cation–π Interactions

The cation–π interaction is an attractive noncovalent interaction between a cation and a π-face. Owing to the stronger interaction energy than those of the other π interactions, such as π–π and CH–π interactions, the cation–π interaction has recently been recognized as a new tool for controlling the regio- and stereoselectivities in various types of organic reactions. This chapter attempts to cover a variety of organic reactions assisted by interactions between unreactive onium ions and π-faces, which will provide comprehensive knowledge on the role of cation–π interactions in organic synthesis.

Diversification reactions of γ-silyl allenyl esters: selective conversion to all-carbon quaternary centers and γ-allene dicarbinols

The unique reactivity of γ-silyl allenyl esters is described. Taking advantage of the silyl group as a fluoride acceptor, these allenoates readily underwent addition to a variety of electrophiles to selectively yield products with all-carbon quaternary centers or allenoate dicarbinols. These dicarbinols were subsequently converted to novel highly substituted 6-hydro-2-pyrones.

Alkyne Cycloadditions Mediated by Tetrabutylammonium Fluoride: A Unified and Diversifiable Route to Isoxazolines and Pyrazolines

A versatile approach to isoxazolines and pyrazolines by the cyclization of alkyne substrates using tetrabutylammonium fluoride (TBAF) is described. The reported diheteroatom cycles were produced under mild reaction conditions and with broad product scope. Evidence is also provided for a vinyl anion intermediate produced under unusually mild conditions, which was trapped in situ as part of a tandem cyclization/aldehyde addition sequence. Finally, a deprotection/functionalization method is described, leading to a substituted pyrazoline in good diastereoselectivity.

A Teaching Experiment to Elucidate a Cation-π Effect in an Alkyne Cycloaddition Reaction and Illustrate Hypothesis Driven Design of Experiments

An organic chemistry experiment is described that is based on recent research to elucidate a novel cation-π interaction between tetraalkammonium cations and propargyl hydrazines. This non-bonded interaction is a key component of the mechanism of ammonium-catalyzed intramolecular cycloaddition of nitrogen to the terminal carbon of a C-C triple bond of the propargyl substrate. In this teaching experiment, reactions and control experiments are employed to demonstrate the testing of two alternative mechanistic hypotheses. Specifically, cyclization reactions are performed with a soluble base (sodium phenoxide) with and without tetrabutylammonium bromide under homogeneous conditions. Students observe that the added ammonium salt accelerates the reaction. They are then encouraged to develop a testable hypothesis for the role of the ammonium salt in the cyclization mechanism: typical phase transfer or other. IR spectroscopy is then used to directly observe a dose dependent shift of the alkyne stretching mode due to a cation-π interaction. In this experiment, undergraduate "researchers" were able to practice the scientific method on a contemporary system and see how data are generated and interpreted to adjudicate between rival hypotheses in a way that emulates authentic and current research in a lab setting. This experimental design was tested on students enrolled in the introductory undergraduate Organic Chemistry Lab.

A facile synthesis of 3,4-dimercaptofurans via sulfenylation of (E)-β-chlorovinyl ketones and 1,2-sulfur migration

A facile access to 3,4-dimercaptofurans was developed by the sulfenylation of β-chlorovinyl ketones followed by a novel 1,2-sulfur migration.

Ammonium catalyzed cyclitive additions: evidence for a cation-π interaction with alkynes

The addition of carbamate nitrogen to a non-conjugated carbon-carbon triple bond is catalyzed by an ammonium salt leading to a cyclic product. Studies in homogeneous systems suggest that the ammonium agent facilitates nitrogen-carbon bond formation through a cation-π interaction with the alkyne unit that, for the first time, is directly observed by Raman spectroscopy.

Facile Alder-Ene Reactions of Silylallenes Involving an Allenic C(sp(2) )-H Bond

Facile and selective Alder-ene reactions of silylallenes involving the activation of an allenic C(sp(2) )H over an allylic C(sp(3) )H bond is described. In this ene reaction, the presence of a silyl substituent was found to be critical for the observed reactivity and selectivity since the corresponding alkyl-substituted allenes show different reaction profiles. Computational studies show that the origin of this unusual reactivity is the lower bond dissociation energy of the α-C(sp(2) )H bond in silylallenes compared to the corresponding nonsilylated allenes.

Recent developments in asymmetric phase-transfer reactions

Phase-transfer catalysis has been recognized as a powerful method for establishing practical protocols for organic synthesis, because it offers several advantages, such as operational simplicity, mild reaction conditions, suitability for large-scale synthesis, and the environmentally benign nature of the reaction system. Since the pioneering studies on highly enantioselective alkylations promoted by chiral phase-transfer catalysts, this research field has served as an attractive area for the pursuit of "green" sustainable chemistry. A wide variety of asymmetric transformations catalyzed by chiral onium salts and crown ethers have been developed for the synthesis of valuable organic compounds in the past several decades, especially in recent years.