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

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

Recent development of synthetic preparation methods for guanidines via transition metal catalysis

Triazole-phosphine-copper complexes (TAP−Cu) have been synthesized and applied as tunable and efficient catalysts for the selective synthesis of fluoro-substituted 2-aryl-1H-benzo[d]imidazole and 1-benzyl-2-aryl-1H-benzo[d]imidazole derivatives from simple alcohols in only one step. TAP−Cu exhibited excellent and tunable catalytic activity for both dehydrogenation and borrowing hydrogen reactions with more than 80 examples being demonstrated for the first time. It was observed that the ligand played a critical role in catalyst activity. Mechanistic studies and deuterium labeling experiments indicated that the reactions proceeded by an initial and reversible alcohol dehydrogenation resulting in a copper hydride intermediate. This was also supported by the direct observation of a diagnostic copper hydride signal by solid-state infrared spectroscopy. The TAP−Cu-H complex showed absorptions at 912 cm−1 that could be assigned to copper−hydride stretches. Furthermore, the direct trapping of an intermediate bisimine was also successfully performed.

Tunable Triazole-Phosphine-Copper Catalysts for the Synthesis of 2-Aryl-1H-benzo[d]imidazoles from Benzyl Alcohols and Diamines by Acceptorless Dehydrogenation and Borrowing Hydrogen Reactions

A growing number of organopnictogen redox catalytic methods have emerged-especially within the past 10 years-that leverage the plentiful reversible two-electron redox chemistry within Group 15. The goal of this Perspective is to provide readers the context to understand the dramatic developments in organopnictogen catalysis over the past decade with an eye toward future development. An exposition of the fundamental differences in the atomic structure and bonding of the pnictogens, and thus the molecular electronic structure of organopnictogen compounds, is presented to establish the backdrop against which organopnictogen redox reactivity-and ultimately catalysis-is framed. A deep appreciation of these underlying periodic principles informs an understanding of the differing modes of organopnictogen redox catalysis and evokes the key challenges to the field moving forward. We close by addressing forward-looking directions likely to animate this area in the years to come. What new catalytic manifolds can be developed through creative catalyst and reaction design that take advantage of the intrinsic redox reactivity of the pnictogens to drive new discoveries in catalysis?

Main Group Redox Catalysis of Organopnictogens: Vertical Periodic Trends and Emerging Opportunities in Group 15.

Transition metal-catalyzed cyclization reactions of unsaturated substrates have become one of the most important and useful methodologies for the preparation of heterocycles. To this end, the association between alkynes and nitrogen compounds results in versatile substrates for the synthesis of N-heterocycles, including indole derivatives. In order to demonstrate the growth in this area, this review highlights ten years of success in the synthesis of indoles using alkynes and nitrogen sources as substrates in the cyclization reactions catalyzed by metals. We believe that summarizing these methods would be very useful for the chemists who are interested in the synthesis of N-heterocycles.

https://pubs.rsc.org/en/content/articlepdf/2020/qo/c9qo01315f

Recent advances in the synthesis of indoles from alkynes and nitrogen sources

Compared to the aza-Wittig reaction of aldehydes, ketones, amides, and esters, the aza-Wittig reaction of acid anhydride was always overlooked, which should be an important part of Wittig reactions. Here, the aza-Wittig reaction of anhydride and catalytic aza-Wittig reaction of anhydride were both developed with high yields, which provides an efficient method to synthesize of 4H-benzo[d][1,3]oxazin-4-ones and 4-benzylidene-2-aryloxazol-5(4H)-ones. The strategy of copper-catalyzed reduction of phosphine oxide was used and found effective for this transformation. Additionally, the one-pot catalytic aza-Wittig reaction of carboxylic acids was achieved. Furthermore, NMR experiments and Hammett plot recorded the process of catalytic aza-Wittig reaction of anhydride, which provides direct proof that the copper-catalyzed reduction of waste phosphine oxide is the key step in this transformation.

Catalytic aza-Wittig Reaction of Acid Anhydride for the Synthesis of 4H-Benzo[d][1,3]oxazin-4-ones and 4-Benzylidene-2-aryloxazol-5(4H)-ones

This paper reports the tandem reaction strategy of the Passerini/Staudinger/aza-Wittig reaction based on the in situ capture of isocyanides. According to this strategy, isocyanides are synthesized in situ and immediately work as the substrate for the Passerini reaction and postmodification tandem reaction in one pot. In addition, two types of new compounds, 5-oxo-3,5-dihydrobenzo[e][1,4]oxazepines and 6-oxo-5,6-dihydro-2H-1,4-oxazines, were synthesized using the tandem reaction strategy that includes five-step transformations in one pot.

Odorless Isocyanide Chemistry: One-Pot Synthesis of Heterocycles via the Passerini and Postmodification Tandem Reaction Based on the in Situ Capture of Isocyanides

Compared to N, O and P as directing functional atoms, the application of directing group containing sulfur atoms for transition metal-catalyzed C–H activation is much more difficult for the known reason (some metal catalysts are easily poisoned by sulfur). Here, the disulfide-directed C–H activation for thew synthesis of sulfonyl diphenyl sulfides was realized for the first time by a copper-catalyzed, tandem, one-step C–S coupling/hydroxylation of disulfides and arylboronic acids with oxygen as the oxidant. This method provides a mild and easy method for the synthesis of sulfonyl diphenyl sulfides and 2-(phenylthio)phenol derivatives and avoids producing sulfoxides and sulfones under air conditions and elevated temperatures.

Disulfide-Directed C–H Hydroxylation for Synthesis of Sulfonyl Diphenyl Sulfides and 2-(Phenylthio)phenols with Oxygen as Oxidant

An efficient Ag(I)-catalyzed regioselective cyclization of (2-alkynylphenyl)guanidine or (2-alkynylphenyl)isourea to indole N-carboximidamides or N-carboximidoates has been developed. The approach has the advantages of high regioselectivity, mild reaction conditions, easily accessible starting materials, and good yields.

Efficient regioselective synthesis of indole N-carboximidamides and N-carboximidoates by a sequential Aza-Wittig/Ag(I)-catalyzed cyclization.

Direct N-glycosylation between glycals and amides/amines was achieved with exclusive stereoselectivity in moderate to high yields. Various amides, amines, and 3,4-O-carbonate-glycals were tolerated, and unique β-N-glycosides were obtained. The strategy was based on palladium-catalyzed decarboxylative allylation, and the high 1,4-cis-selectivity was proposed because of the hydrogen bonding effect. Notably, all the synthesized products were subjected to preliminary bioactivity studies, revealing that three compounds were cytotoxic to tumor cells and nontoxic to normal human cells.

Ming-Guo Liu

Papers

Catalytic aza-Wittig Reaction of Acid Anhydride for the Synthesis of 4H-Benzo[d][1,3]oxazin-4-ones and 4-Benzylidene-2-aryloxazol-5(4H)-ones

Odorless Isocyanide Chemistry: One-Pot Synthesis of Heterocycles via the Passerini and Postmodification Tandem Reaction Based on the in Situ Capture of Isocyanides

Disulfide-Directed C–H Hydroxylation for Synthesis of Sulfonyl Diphenyl Sulfides and 2-(Phenylthio)phenols with Oxygen as Oxidant

Efficient regioselective synthesis of indole N-carboximidamides and N-carboximidoates by a sequential Aza-Wittig/Ag(I)-catalyzed cyclization.

Direct N-Glycosylation of Amides/Amines with Glycal Donors