Iron(II)‐Catalyzed Direct Cyanation of Arenes with Aryl(cyano)iodonium Triflates

A direct oxidative cyanation of arenes under Fe^II catalysis with 3,5‐di(trifluoromethyl)phenyl(cyano)iodonium triflate (DFCT) as the cyanating agent has been developed. The reaction is applicable to wide range of aromatic substrates, including polycyclic structures and heteroaromatic compounds.     

Synthesis of 2‐Alkynoates by Palladium(II)‐Catalyzed Oxidative Carbonylation of Terminal Alkynes and Alcohols

A homogeneous Pd^II catalyst, utilizing a simple and inexpensive amine ligand (TMEDA), allows 2‐alkynoates to be prepared in high yields by an oxidative carbonylation of terminal alkynes and alcohols. The catalyst system overcomes many of the limitations of previous palladium carbonylation catalysts. It has an increased substrate scope, avoids large excesses of alcohol substrate and uses a desirable solvent. The catalyst employs oxygen as the terminal oxidant and can be operated under safer gas mixtures.     

Palladium(0)‐Catalyzed Intermolecular Arylative Dearomatization of β‐Naphthols

The first Pd⁰‐catalyzed intermolecular arylative dearomatization of β‐naphthols with aryl halides is described. It was found that Q‐Phos could facilitate the palladium‐catalyzed cross‐coupling‐type dearomatization of β‐naphthols, while avoiding O‐arylation, to construct 2‐naphthalenones in excellent yields and with high chemoselectivity.     

A Palladium(II)‐Catalyzed Synthesis of Spiroacetals through a One‐Pot Multicomponent Cascade Reaction

Functionalized spiroacetals have been easily prepared in a one‐pot three‐component coupling process that involves the reaction of pentynol derivatives, salicylaldehydes, and amines in the presence of catalytic amounts of a palladium(II) complex (see scheme). Alternatively, oxygen‐substituted spiroacetals can be obtained by using orthoesters as the third component.

     

Competition of Nucleophilic Aromatic Substitution, σ‐Bond Metathesis,and syn Hydrometalation in Titanium(III)‐Catalyzed Hydrodefluorination of Arenes

Several functionalized and non‐functionalized perfluoroarenes were catalytically transformed into their para‐hydrodefluorinated products by using catalytic amounts of titanocene difluoride and stoichiometric amounts diphenylsilane. Turnover numbers of up to 93 were observed. Solution density functional theory calculations at the M06‐2X/TZ(PCM)//M06‐2X/TZ(PCM) level of theory provided insight into the mechanism of Ti^III‐catalyzed aromatic hydrodefluorination. Two different substrate approaches, with a Ti–F interaction (pathway A) and without a Ti–F interaction (pathway B), are possible. Pathway A leads to a σ‐bond metathesis transition state, whereas pathway B proceeds by means of a two‐step mechanism through a syn‐hydrometalation intermediate or through a Meisenheimer intermediate. Both pathways are competitive over a broad range of substrates.     

Palladium‐Catalyzed para‐Selective Alkylation of Electron‐Deficient Arenes

Intermolecular alkylations of electron‐deficient arenes proceed with good para selectivity. Palladium catalysts were used to generate nucleophilic alkyl radicals from alkyl halides, which then directly add onto the arenes. The arene scope and the site of alkylation are opposite to those of classical Friedel–Crafts alkylations, which prefer electron‐rich systems.     

Palladium(II)‐Catalyzed Enantioselective Azidation of Unactivated Alkenes

The first Pd‐catalyzed enantioselective azidation of unactivated alkenes has been established by using readily accessible 1‐azido‐1,2‐benziodoxol‐3(1H)‐one (ABX) as an azidating reagent, which affords a wide variety of structurally diverse 3‐N₃‐substituted piperidines in good yields with excellent enantioselectivity. The reaction features good functional‐group compatibility and mild reaction conditions. Notably, both an electrophilic azidating reagent and the sterically bulky chiral pyridinyl‐oxazoline (Pyox) ligand are crucial to the successful reaction.     

An Efficient Zinc‐Catalyzed Dehydration of Primary Amides to Nitriles

In the present study, the zinc‐catalyzed dehydration of a variety of amides with N‐methyl‐N‐(trimethylsilyl)trifluoroacetamide (MSTFA) as a dehydration reagent into the corresponding nitriles has been examined in detail. With the straightforward and commercially available zinc(II)triflate as the precatalyst and MSTFA, an excellent system has been established to afford nitriles in excellent yields and chemoselectivities. After investigation of reaction conditions and the scope and limitations, several efforts were carried out to understand the reaction mechanism.     

Recent Developments on the Trifluoromethylation of (Hetero)Arenes

Aryl‐CF₃ as an extremely important family of fluorinated organic compounds holds wide applications in pharmaceuticals, agrochemicals, and advanced materials. Traditionally, such trifluoromethylated compounds have been synthesized from the corresponding aryl trichlorides via Cl exchange reactions (Scheme 1 ). This Focus review gives an overview over the recent development of trifluoromethylation of (hetero)arenes.     

A Palladium(II)‐Catalyzed CH Activation Cascade Sequence for Polyheterocycle Formation

Polyheterocycles are found in many natural products and are useful moieties in functional materials and drug design. As part of a program towards the synthesis of Stemona alkaloids, a novel palladium(II)‐catalyzed C? H activation strategy for the construction of such systems has been developed. Starting from simple 1,3‐dienyl‐substituted heterocycles, a large range of polycyclic systems containing pyrrole, indole, furan and thiophene moieties can be synthesized in a single step.     

Mechanistic Studies on the SCS‐Pincer Palladium(II)‐Catalyzed Tandem Stannylation/Electrophilic Allylic Substitution of Allyl Chlorides with Hexamethylditin and Benzaldehydes

This paper describes a mechanistic study of the SCS‐pincer Pd^II‐catalyzed auto‐tandem reaction consisting of the stannylation of cinnamyl chloride with hexamethylditin, followed by an electrophilic allylic substitution of the primary tandem‐reaction product with 4‐nitrobenzaldehyde to yield homoallylic alcohols as the secondary tandem products. As it turned out, the anticipated stannylation product, cinnamyl trimethylstannane, is not a substrate for the second part of the tandem reaction. These studies have provided insight in the catalytic behavior of SCS‐pincer Pd^II complexes in the auto‐tandem reaction and on the formation and possible involvement of Pd⁰ species during prolonged reaction times. This has led to optimized reaction conditions in which the overall tandem reaction proceeds through SCS‐pincer Pd^II‐mediated catalysis, that is, true auto‐tandem catalysis. Accordingly, this study has provided the appropriate reaction conditions that allow the pincer catalysts to be recycled and reused.     

Intermolecular Methoxycarbonylation of Terminal Alkynes Catalyzed by Palladium(II) Bis(oxazoline) Complexes

Boxing clever : Direct conversion of a terminal alkyne group into a β‐methoxyacrylate is realized with the aid of the bis(oxazoline) ligand (box). Acetyl and ketal protecting groups, free hydroxy groups, and acid‐sensitive glycosidic bonds are not affected under the reaction conditions. The one‐pot synthesis of (±)‐dihydrokawain from the homopropargyl alcohol is also achieved. tfa=trifluoroacetate