LongLarf

The first general example of regio‐ and enantioselective palladium catalyzed allylic sulfonylation of vinyl cyclic carbonates provides an efficient and direct way to construct sulfone‐bearing quaternary carbon stereocenters. A three steps formal total synthesis of (+)‐agelasidine A is achieved which exemplifies the practical use of this protocol in a synthetic setting.

Abstract

Chiral sulfones are of great importance in medicinal chemistry and chemical synthesis. Efficient methods for preparing enantiomerically enriched sulfone‐containing molecules can therefore be of significant value; such methods, however, are uncommon. Herein, we report the first general palladium‐catalyzed sulfonylation of vinyl cyclic carbonates with sodium sulfinates. A series of enantiomerically enriched tertiary allylic sulfones were synthesized in good yields with excellent enantiomeric ratios. Both aliphatic‐ and aryl‐substituted vinyl cyclic carbonates are suitable reactants with excellent results. This reaction features broad substrates scope, readily available starting materials, excellent regio‐ and enantioselectivity, and synthesis of sulfone‐bearing quaternary carbon stereocenters. Through the sulfonylation of geranyl derived cyclic carbonate 1 h, we achieve the formal total synthesis of (+)‐agelasidine A.

Tune the pincer: A solid‐phase synthesis approach is described to prepare a diverse library of nonsymmetrical PNP pincer ligands. The heterogenized library is applied in ruthenium‐catalyzed hydrogenation of esters and lactones under mild conditions. Catalyst recovery and recyclability are facilitated by covalent attachment to solid supports.

Abstract

In contrast to their symmetrical analogues, nonsymmetrical PNP‐type ligand motifs have been less investigated despite the modular pincer structure. However, the introduction of mixed phosphorus donor moieties provides access to a larger variety of PNP ligands. Herein, a facile solid‐phase synthesis approach towards a diverse PNP‐pincer ligand library of 14 members is reported. Contrary to often challenging workup procedures in solution‐phase, only simple workup steps are required. The corresponding supported ruthenium‐PNP catalysts are screened in ester hydrogenation. Usually, industrially applied heterogeneous catalysts require harsh conditions in this reaction (250–350 °C at 100–200 bar) often leading to reduced selectivities. Heterogenized reusable Ru‐PNP catalysts are capable of reducing esters and lactones selectively under mild conditions.

Driven by electricity, the dehydrogenative coupling reaction of phenols carrying electron‐withdrawing groups to give 2,2′‐biphenols or the corresponding polycyclic derivatives is facilitated for the first time. 1,1,1,3,3,3‐Hexafluoroisopropanol and a small amount of base in the electrolyte make additional supporting electrolyte superfluous. The electrolysis is scalable, can be conducted with very simple equipment, and is much more concise than previous conventional syntheses.

Abstract

We herein present a metal‐free, electrosynthetic method that enables the direct dehydrogenative coupling reactions of phenols carrying electron‐withdrawing groups for the first time. The reactions are easy to conduct and scalable, as they are carried out in undivided cells and obviate the necessity for additional supporting electrolyte. As such, this conversion is efficient, practical, and thereby environmentally friendly, as production of waste is minimized. The method features a broad substrate scope, and a variety of functional groups are tolerated, providing easy access to precursors for novel polydentate ligands and even heterocycles such as dibenzofurans.

Light induces carbon–heteroatom bond formations through different modes of action. Photochemical strategies to activate inert substrates or intermediates either with or without photocatalysts are discussed.

Photochemistry enables new synthetic means to form carbon–heteroatom bonds. Photocatalysts can catalyze carbon–heteroatom cross‐couplings by electron or energy transfer either alone or in combination with a second catalyst. Photocatalyst‐free methods are possible using photolabile substrates or by generating photoactive electron donor‐acceptor complexes. This review summarizes and discusses the strategies used in light‐mediated carbon–heteroatom bond formations based on the proposed mechanisms.

Publication date: 4 October 2019

Source: Tetrahedron, Volume 75, Issue 40

Author(s): Tran Quang Hung, Do Trung Hieu, Dinh Van Tinh, Ha Nam Do, Tuan Anh Nguyen Tien, Dang Van Do, Le Thanh Son, Ngoc Han Tran, Nguyen Van Tuyen, Vu Minh Tan, Peter Ehlers, Tuan Thanh Dang, Peter Langer

Graphical abstract

Publication date: 4 October 2019

Source: Tetrahedron, Volume 75, Issue 40

Author(s): Rodisnel Perdomo Rivera, Peter Ehlers, Lars Ohlendorf, Marian Blanco Ponce, Eugenio Torres Rodríguez, Peter Langer

Graphical abstract

Triplet curious carbenes: The cyclic (aryl)(amido)carbenes (CArAmCs) constitute a class of NHCs that react in a manner akin to triplet carbenes. Examining the electronic properties of the carbenes revealed that they are among the most highly π‐accepting and electrophilic carbenes to date. Such features enabled the CArAmCs to undergo C−H insertions, engage in [2+1] cycloadditions, and oxidize spontaneously upon exposure to oxygen.

Abstract

The synthesis and study of a library of cyclic (aryl)(amido)carbenes (CArAmCs), which represent a class of electrophilic NHCs that feature low calculated singlet‐triplet gaps (ΔE _ST=19.9 kcal mol⁻¹; B3LYP/def2‐TZVP) and exhibit reactivity profiles expected from triplet carbenes, are described. The electrophilic properties of the CArAmCs were quantified by analyzing their respective selenium adducts, which exhibited the largest downfield ⁷⁷Se NMR chemical shifts (up to 1645 ppm) measured for any NHC derivative known to date, as well as their Ir carbonyl complexes, from which large Tolman electronic parameter (TEP) values (up to 2064 cm⁻¹) were ascertained. The CArAmCs were found to engage in reactions that are typically observed with triplet carbenes, including C−H insertions, [2+1] cycloadditions with alkenes as well as alkynes, and spontaneous oxidation upon exposure to oxygen.

Nucleophilic substitution reactions of alcohols are among the most fundamental and strategically important transformations in organic chemistry. For over half a century, these reactions have been achieved by using stoichiometric, and often hazardous, reagents to activate the otherwise unreactive alcohols. Here, we demonstrate that a specially designed phosphine oxide promotes nucleophilic substitution reactions of primary and secondary alcohols in a redox-neutral catalysis manifold that produces water as the sole by-product. The scope of the catalytic coupling process encompasses a range of acidic pronucleophiles that allow stereospecific construction of carbon-oxygen and carbon-nitrogen bonds.