An Intramolecular Cycloaddition Approach to the Kauranoid Family of Diterpene Metabolites

Lanthanide (Substituted-)Cyclopentadienyl Bis(phosphinimino)methanediide Complexes: Synthesis and Characterization

Design and synthesis of high-performance single-molecule magnets (SMMs) have long been a research focus. Inspired by the best dysprosium(III) metallocene SMMs and dysprosium(III) bis(methanediide) SMMs, we assumed dysprosium SMMs, which had electrical neutrality by combining the two types of ligands. As the Dy3+ center is coordinated by one (substituted-)cyclopentadienyl (CpR) ligand and one methanediide ({C(PPh2NSiMe3)2}2-) ligand on the axial sites, this ideal structure with linear Ccarbene-Dy-Cpcent would strengthen the magnetic anisotropy and exhibit excellent SMM properties. However, it is not easy to synthesize it in this configuration. We for the first time obtained [Y{C(PPh2NSiMe3)2}(CpR)(THF)] (CpR = Cp(1), Cp tBu(2), Cp*(3), Cp = C5H5, Cp tBu = C5H4 tBu, Cp* = C5Me5) through the reactions of NaCp/KCpR and [Y{C(PPh2NSiMe3)2}(I)(THF)2]. In the molecular structures of 1-3, except for the two expected ligands, one coordination tetrahydrofuran (THF) molecule was also found in each complex. The P-C-P values of 1-3 were 135.46(7), 136.421(8), and 131.43(10)°, respectively, which were far less than 180°. The Ccarbene-Y-Cpcent of 1-3 deviated significantly from the linear shape, which was 118.021, 129.459, and 118.331°, respectively. Such a coordination environment makes the dysprosium congener [Dy{C(PPh2NSiMe3)2}(Cp*)(THF)] (4) whose Ccarbene-Dy-Cpcent (118.295°) was too small to maintain axiality and which almost exhibited no SMM properties. Even though the first exploration of lanthanide (substituted-)cyclopentadienyl bis(phosphinimino)methanediide complexes did not live up to our expectation, it provided great experiences for the future success of high-performance lanthanide (substituted-)cyclopentadienyl methanediide SMMs.

Selective 1,1- and 1,2-dibromination of phenylethanes in the presence of NaBr/NaBrO3/H2SO4 as the bromination reagent

Selective 1,1- and 1,2-dibromination of phenylethanes by simply adjusting the reaction conditions has been developed. Mixtures of NaBr/NaBrO3/H2SO4 are employed as green bromination reagents, which can release Br2 or BrOH in situ as required without polluting the environment. Both the resulting 1,1- and 1,2-dibromoethyl arenes can be easily transformed to phenylacetylenes via elimination under basic conditions, demonstrating great potential for industrial applications.

Dithioallyl cation (3 + 2) cycloadditions under aprotic reaction conditions: rapid access to spiro-fused cyclopentane scaffolds

We report a general method to effect all-carbon (3 + 2) cycloadditions that can elaborate cyclopentenes from a range of olefins. The required dithioallyl cation reagents can be generated in a newly developed mild protocol starting from 2-allyloxypyridine precursors, thus avoiding the use of strong Brønsted acids. The novel method significantly expands the substrate scope, which now also includes acid-sensitive olefins, and thus enables the preparation of previously inaccessible spiro-fused scaffold types from simple and readily available starting materials.

1,4-Dithianes: attractive C2-building blocks for the synthesis of complex molecular architectures

This review covers the synthetic applications of 1,4-dithianes, as well as derivatives thereof at various oxidation states. The selected examples show how the specific heterocyclic reactivity can be harnessed for the controlled synthesis of carbon-carbon bonds. The reactivity is compared to and put into context with more common synthetic building blocks, such as 1,3-dithianes and (hetero)aromatic building blocks. 1,4-Dithianes have as yet not been investigated to the same extent as their well-known 1,3-dithiane counterparts, but they do offer attractive transformations that can find good use in the assembly of a wide array of complex molecular architectures, ranging from lipids and carbohydrates to various carbocyclic scaffolds. This versatility arises from the possibility to chemoselectively cleave or reduce the sulfur-heterocycle to reveal a versatile C2-synthon.

Dearomative (3 + 2) Cycloadditions of Unprotected Indoles

The (3 + 2) cycloaddition of various indoles with a dithioallyl cation affords dearomatized cyclopentannulated adducts, with complete control of regioselectivity and excellent chemo- and diastereoselectivity. The success of the reaction critically relies on the use of an excess of very strong Brønsted acid, which paradoxically prevents carbocationic side reactions. The reaction tolerates sensitive functionalities such as basic amines or free hydroxyls, and we demonstrate its use in late stage derivatization of highly functionalized, unprotected indoles.

Recent Advances in Transition-Metal-Free (4+3)-Annulations

(4+3)-Annulations are incredibly versatile reactions which combine a 4-atom synthon and a 3-atom synthon to form both 7-membered carbocycles as well as heterocycles. We have previously reviewed transition-metal-catalyzed (4+3)-annulations. In this review, we will cover examples involving bases, NHCs, phosphines, Lewis and Brønsted acids as well as some rare examples of boronic acid catalysis and photocatalysis. In analogy to our previous review, we exclude annulations involving cyclic dienes like furan, pyrrole, cyclohexadiene or cyclopentadiene, as Chiu, Harmata, Fernándes and others have recently published reviews encompassing such substrates. We will however discuss the recent additions (2010–2020) to the literature on (4+3)-annulations involving other types of 4-atom-synthons.

1 Introduction

2 Bases

3 Annulations Using N-Heterocyclic Carbenes

3.1 N-Heterocyclic Carbenes (NHCs)

3.2 N-Heterocyclic Carbenes and Base Dual-Activation

4 Phosphines

5 Acids

5.1 Lewis Acids

5.2 Brønsted Acids

6 Boronic Acid Catalysis and Photocatalysis

7 Conclusion

1,1,2-Tribromoethyl arenes: novel and highly efficient precursors for the synthesis of 1-bromoalkynes and α-bromoketones

A novel tribromination method to prepare versatile intermediate 1,1,2-tribromoethyl arenes, which can not only be transformed to synthetically valuable 1-bromoalkynes via elimination but also be hydrolyzed to a variety of α-bromoketones, was developed.