Synthesis of Thiourea−Oxazolines, a New Class of Chiral S,N-Heterobidentate Ligands: Application in Pd-Catalyzed Asymmetric Bis(methoxycarbonylation) of Terminal Olefins

Palladium-catalyzed enantioselective carbonylation reactions

Carbonylation, one of the most powerful approaches to the preparation of carbonylated compounds, has received significant attention from researchers active in various fields. Indeed, impressive progress has been made on this subject over the past few decades. Among the various types of carbonylation reactions, asymmetric carbonylation is a straightforward methodology for constructing chiral compounds. Although rhodium-catalyzed enantioselective hydroformylations have been discussed in several elegant reviews, a general review on palladium-catalyzed asymmetric carbonylations is still missing. In this review, we summarize and discuss recent achievements in palladium-catalyzed asymmetric carbonylation reactions. Notably, this review’s contents are categorized by reaction type.

Chiral Thioureas-Preparation and Significance in Asymmetric Synthesis and Medicinal Chemistry

For almost 20 years, thioureas have been experiencing a renaissance of interest with the emerged development of asymmetric organocatalysts. Due to their relatively high acidity and strong hydrogen bond donor capability, they differ significantly from ureas and offer, appropriately modified, great potential as organocatalysts, chelators, drug candidates, etc. The review focuses on the family of chiral thioureas, presenting an overview of the current state of knowledge on their synthesis and selected applications in stereoselective synthesis and drug development.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Transition Metal-Catalyzed Dicarbofunctionalization of Unactivated Olefins

Transition metal (TM)-catalyzed difunctionalization of unactivated olefins with two carbon-based entities is a powerful method to construct complex molecular architectures rapidly from simple and readily available feedstock chemicals. While dicarbofunctionalization of unactivated olefins has a long history typically with the use of either carbon monoxide to intercept C(sp³ )-[M] (alkyl-TM) species or substrates lacking in β-hydrogen (β-Hs), development of this class of reaction still remains seriously limited due to complications of β-H elimination arising from the in situ-generated C(sp³ )-[M] intermediates. Over the years, different approaches have been harnessed to suppress β-H elimination, which have led to the development of various types of olefin dicarbofunctionalization reactions even in substrates that generate C(sp³ )-[M] intermediates bearing β-Hs with a wide range of electrophiles and nucleophiles. In this review, these developments will be discussed both through the lens of historical perspectives as well as the strategies scrutinized over the years to address the issue of β-H elimination. However, this review article by no means is designed to be exhaustive in the field, and is merely presented to provide the readers an overview of the key reaction developments.

Pd-catalyzed Heck cyclization and in situ hydrocarboxylation or hydromethenylation via a hydrogen borrowing strategy

A palladium-catalyzed Heck cyclization and in situ hydrocarboxylation reaction was developed using chloroform as a one-carbon elongation reagent.

Palladium-catalyzed oxidative carbonylation reactions

Palladium-catalyzed coupling reactions have become a powerful tool for advanced organic synthesis. This type of reaction is of significant value for the preparation of pharmaceuticals, agrochemicals, as well as advanced materials. Both, academic as well as industrial laboratories continuously investigate new applications of the different methodologies. Clearly, this area constitutes one of the major topics in homogeneous catalysis and organic synthesis. Among the different palladium-catalyzed coupling reactions, several carbonylations have been developed and widely used in organic syntheses and are even applied in the pharmaceutical industry on ton-scale. Furthermore, methodologies such as the carbonylative Suzuki and Sonogashira reactions allow for the preparation of interesting building blocks, which can be easily refined further on. Although carbonylative coupling reactions of aryl halides have been well established, palladium-catalyzed oxidative carbonylation reactions are also interesting. Compared with the reactions of aryl halides, oxidative carbonylation reactions offer an interesting pathway. The oxidative addition step could be potentially avoided in oxidative reactions, but only few reviews exist in this area. In this Minireview, we summarize the recent development in the oxidative carbonylation reactions.

Diastereoselective total synthesis of (±)-schindilactone A, Part 1: Construction of the ABC and FGH ring systems and initial attempts to construct the CDEF ring system

First-generation synthetic strategies for the diastereoselective total synthesis of schindilactone A (1) are presented and methods for the synthesis of the ABC, FGH, and CDEF moieties are explored. We have established a method for the synthesis of the ABC moiety, which included both a Diels-Alder reaction and a ring-closing metathesis as the key steps. We have also developed a method for the synthesis of the FGH moiety, which involved the use of a Pauson-Khand reaction and a carbonylative annulation reaction as the key steps. Furthermore, we have achieved the construction of the central 7-8 bicyclic ring system by using a [3,3]-rearrangement as the key step. However, unfortunately, when this rearrangement reaction was applied to the construction of the more advanced CDEF moiety, the anticipated annulation reaction did not occur and the development of an alternative synthetic strategy would be required for the construction of this central core.

Copper(II)-catalyzed room temperature aerobic oxidation of hydroxamic acids and hydrazides to acyl-nitroso and azo intermediates, and their Diels-Alder trapping

CuCl(2), in the presence of a 2-ethyl-2-oxazoline ligand, is an effective catalyst for the room temperature, aerobic oxidation of hydroxamic acids and hydrazides, to acyl-nitroso and azo dienophiles respectively, which are efficiently trapped in situ via both inter- and intramolecular hetero-Diels-Alder reactions with dienes. Both inter- and intramolecular variants of the Diels-Alder reaction are suitable under the reaction conditions using a variety of solvents. Under the same conditions, an acyl hydrazide was also oxidized to give an acyl-azo dienophile which was trapped intramolecularly by a diene.

Asymmetric palladium-catalyzed hydroarylation of styrenes and dienes

Alkenes are desirable and highly versatile starting materials for organic transformations, and well-known substrates for palladium catalysis. Typically, these reactions result in the formation of a new alkene product via β-hydride elimination. In contrast to this scenario, our laboratory has been involved in the development of alkene hydro- and difunctionalization reactions, where β-hydride elimination can be controlled. We report herein the development of an asymmetric palladium-catalyzed hydroarylation, which yields diarylmethine products in up to 75% ee. Interestingly, a linear free energy relationship is observed between the steric bulk of the ligand within a certain range and the enantiomeric excess of the reaction.

Investigations into Competitive Cycloaddition/Cyclization or Elimination from 1,1-Dimethyl-propargylcarbamates of Anilines

The copper-catalyzed reaction of 1,1-dimethyl-O-propargyl aniline carbamates was studied and revealed the unexpected formation of oxazolidin-2-ones and alkylamines. An in-depth study of the reaction conditions showed that the formation of these products was highly dependent on the solvent, copper catalyst and aniline substituents. The reaction can be oriented towards oxazolidinones in pyridine and alkylamines in ethanol, whereas cycloaddition can be achieved in dry tetrahydrofuran.