Ligand-Controlled Selective Synthesis of Indoles and Benzofurans from Secondary Anilines

A ligand-controlled method for the selective synthesis of indoles and benzofurans from secondary anilines has been discovered. A six-ring palladacycle intermediate may be involved in this process by olefins as a transient directing mediator to give indoles. The indole/benzofuran ratio can be easily tuned by the MPAA ligand. Various substituted secondary anilines were well-tolerated, affording the corresponding products in moderate to good yields. Indole-derived drugs such as JWH-081, BB-22, and ML-098 could be obtained using our new method to achieve their precursors. Preliminary mechanistic studies indicated that BQ is the key factor in avoiding the β-H elimination of insertion species.

DFT Study on the Mechanism of the Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides via Benzylic and meta-C-H Bond Activation: Role of the External Ligand Ac-Gly-OH

The mechanism of the Ac-Gly-OH-assisted palladium-catalyzed [3 + 2] annulation of aromatic amides with maleimides is investigated using density functional theory calculations. The results show that the reaction undergoes the sequential steps of N-H bond deprotonation, first benzylic C-H bond activation, maleimide insertion, second meta-C-H bond activation, reductive elimination, and oxidation. The external ligand Ac-Gly-OH acts as the internal base for hydrogen abstraction in the first benzylic C-H bond activation. The maleimide insertion step is found to be the rate-determining step. Based on the nearly same energetic span of the two pathways to generate the enantio products, the computational results are consistent with the experimental observation that the terminal [3 + 2] annulation products are racemic when using an achiral ligand. These calculation results disclose the detailed reaction mechanism and shed light on some experimental ambiguities.

Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides through Dual C-H Activation

A palladium-catalyzed [3 + 2] annulation of substituted aromatic amides with maleimides providing tricyclic heterocyclic molecules in good to moderate yields through weak carbonyl chelation is reported. The reaction proceeds via a dual C-H bond activation where the first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position to afford a five-membered cyclic ring. An external ligand Ac-Gly-OH has been used to succeed in this protocol. A plausible reaction mechanism has been proposed for the [3 + 2] annulation reaction.

Modular Synthesis of Benzocyclobutenes via Pd(II)-Catalyzed Oxidative [2+2] Annulation of Arylboronic Acids with Alkenes

Benzocyclobutenes (BCBs) are highly valuable compounds in organic synthesis, medicinal chemistry, and materials science. However, catalytic modular synthesis of functionalized BCBs from easily accessible starting materials remains limited. We report herein an efficient synthesis of diversely functionalized BCBs by a Pd(II)-catalyzed formal [2+2] annulation between arylboronic acids and alkenes in the presence of N-fluorobenzenesulfonimide (NFSI). An intermolecular carbopalladation followed by palladium oxidation, intramolecular C(sp²)-H activation by a transient C(sp³)-Pd(IV) species, and selective carbon-carbon (C-C) bond-forming reductive elimination from a high-valent five-membered palladacycle is proposed to account for the reaction outcome. Kinetically competent oxidation of alkylPd(II) to alkylPd(IV) species is important to avoid the formation of a Heck adduct. The reaction forges two C-C bonds of the cyclobutene core and is compatible with a wide range of functional groups. No chelating bidentate directing group in the alkene part is needed for this transformation.

Synthesis of Highly Substituted Phenols and Benzenes with Complete Regiochemical Control

Substituted phenols are requisite molecules for human health, agriculture, and diverse synthetic materials. We report a chemical synthesis of phenols, including penta-substituted phenols, that accommodates programmable substitution at any position. This method uses a one-step conversion of readily available hydroxypyrone and nitroalkene starting materials to give phenols with complete regiochemical control and in high chemical yield. Additionally, the phenols can be converted into highly and even fully substituted benzenes.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Palladium Catalyzed meta-C-H Functionalization of Masked Aromatic Aldehydes

Palladium catalyzed meta-C-H functionalization enabled by transient mediators has the potential to extend the utility of directed ortho-C-H functionalization to remote positions. However, there have been no reports of palladium catalyzed meta-C-H functionalization of aromatic aldehyde derivatives, which are highly versatile intermediates in organic synthesis. Herein we report the development of a directing group that, in the presence of a norbornene derived mediator and an appropriate pyridone ligand, allows palladium catalyzed meta-C-H functionalization of masked aromatic aldehydes. Mechanistic insight regarding the impact of the directing group length on this catalysis is also discussed.