Catalytic C-H functionalization by metal carbenoid and nitrenoid insertion

Practical copper(I)-catalysed amidation of aldehydes

The direct synthesis of amides by insertion into the C-H bond of aldehydes is shown to be a practical procedure through application of cheap, readily available catalysts generated in situ from copper(i) halides and pyridine.

Intramolecular Ir(I)-catalyzed benzylic C-H bond amination of ortho-substituted aryl azides

Iridium(I) catalyzes the intramolecular benzylic C-H bond amination of ortho-homobenzyl-substituted aryl azides to produce indolines at 25 degrees C.

Iridium(III) catalyzed diastereo- and enantioselective C-H bond functionalization

Iridium(III)-salen complexes were found to efficiently catalyze the enantioselective C-H carbene insertion. The insertion reaction at the alpha-position of tetrahydrofuran or at the methylene of 1,4-cyclohexadiene proceeded with high enantio- (up to 99%) and diastereoselectivity (up to >20:1 dr; in the former reaction) by using an appropriate combination of complex (1, 2, or 3) and alpha-diazoacetate (alpha-aryl-alpha-diazoacetates or alpha-diazopropionate) as the carbene source.

Application of donor/acceptor-carbenoids to the synthesis of natural products

The metal catalyzed reactions of diazo compounds have been broadly used in organic synthesis. The resulting metal-carbenoid intermediates are capable of undergoing a range of unconventional reactions, and due to their high energy, they are ideal for initiating cascade sequences leading to the rapid generation of structural complexity. This tutorial review will give an overview of the most versatile reactions of donor/acceptor carbenoids, an exciting class of intermediates capable of highly selective reactions. This will include cyclopropanation, [4 + 3] cycloaddition, and C-H functionalization methodologies. The application of this chemistry to the synthesis of a range of natural products will be described.

Elisapterosin F: a polycyclic gorgonian-derived diterpene with a facially amphiphilic structure

Analysis of the terpene metabolites of Pseudopterogorgia elisabethae collected in San Andrés island, Colombia has resulted in the discovery of a novel metabolite, elisapterosin F (1). The tangled molecular structure of 1, which was elucidated after extensive spectroscopic data interpretation, possesses hydrophilic and hydrophobic groups located on two opposite faces, rather than at two ends as in the more conventional head/tail amphiphiles.

Examination of the mechanism of Rh(2)(II)-catalyzed carbazole formation using intramolecular competition experiments

The use of a rhodium(II) carboxylate catalyst enables the mild and stereoselective formation of carbazoles from biaryl azides. Intramolecular competition experiments of triaryl azides suggested the source of the selectivity. A primary intramolecular kinetic isotope effect was not observed, and correlation of the product ratios with Hammett sigma(+) values produced a plot with two intersecting lines with opposite rho values. These data suggest that electronic donation by the biaryl pi-system accelerates the formation of rhodium nitrenoid and that C-N bond formation occurs through a 4pi-electron-5-atom electrocyclization.

Asymmetric synthesis of neolignans (-)-epi-conocarpan and (+)-conocarpan via Rh(II)-catalyzed C-H insertion process and revision of the absolute configuration of (-)-epi-conocarpan

Catalytic asymmetric synthesis of neolignan natural products (-)-epi-conocarpan and (+)-conocarpan has been achieved by exploiting an enantio- and diastereoselective intramolecular C-H insertion reaction to construct a cis-2-aryl-2,3-dihydrobenzofuran ring system as a key step. The C-H insertion reaction of 5-bromoaryldiazoacetate catalyzed by Rh(2)(S-PTTEA)(4), a new dirhodium(II) carboxylate complex that incorporates N-phthaloyl-(S)-triethylalaninate as chiral bridging ligands, provided 2-aryl-5-bromo-3-methoxycarbonyl-2,3-dihydrobenzofuran with exceptionally high diastereoselectivity (cis/trans = 97:3) and high enantioselectivity for the cis isomer (84% ee).

Total synthesis of haouamine A: the indeno-tetrahydropyridine core

A full account of synthetic efforts towards the indeno-tetrahydropyridine core of haouamine A is presented. Initial failed strategies led to the unexpected discovery of a mild abnormal Chichibabin pyridine synthesis and provided knowledge and inspiration for the development of a cascade annulation that has enabled rapid and scalable access to the core in either racemic or enantiopure form.

Understanding the differential performance of Rh2(esp)2 as a catalyst for C-H amination

Catalytic amination of saturated C-H bonds is performed efficiently with the use of Rh(2)(esp)(2). Efforts to identify pathways for catalyst degradation and/or arrest have revealed a single-electron oxidation event that gives rise to a red-colored, mixed-valence dimer, [Rh(2)(esp)(2)](+). This species is fortuitously reduced by carboxylic acid, a byproduct generated in the reaction cycle with each turnover of the diacyloxyiodine oxidant. These findings have led to the conclusion that the high performance of Rh(2)(esp)(2) is due in part to the superior kinetic stability of its one-electron oxidized form relative to other dimeric Rh complexes.

Rhodium catalyzed enantioselective cyclization of substituted imidazoles via C-H bond activation

The enantioselective intramolecular alkylation of substituted imidazoles with enantiomeric excesses up to 98% has been accomplished by rhodium catalyzed C-H bond functionalization with (S,S',R,R')TangPhos as the chiral ligand.

Catalytic C-H amination: recent progress and future directions

Recent developments in catalytic C-H amination are discussed in this feature article. The careful design of reagents and catalysts now provides efficient conditions for exquisitely selective intramolecular as well as intermolecular nitrene C-H insertion. The parallel emergence of C-H activation/amination reactions opens new opportunities complementary to those offered by nitrenes.

The key intermediate in the amination of saturated C-H bonds: synthesis, X-ray characterization and catalytic activity of Ru(TPP)(NAr)(2) (Ar = 3,5-(CF(3))(2)C(6)H(3))

The complex Ru(TPP)(NAr)(2) inserts a nitrene group into allylic and benzylic C-H bonds and is the key intermediate in the ruthenium porphyrin-catalyzed amination of hydrocarbons by aryl azides.

Tunable Phosphoramidite Ligands for Asymmetric Hydrovinylation: Ligands par excellence for Generation of All-Carbon Quaternary Centers

1-Alkylstyrenes undergo efficient hydrovinylation (addition of ethylene) in the presence of a Ni-catalyst prepared from [(allyl)NiBr](2), Na(+) [BAr(4)](-) (Ar = 3,5-bis-trifluromethylphenyl), and a phosphoramidite ligand giving products in excellent yields and enantioselectivities. In many cases phosphoramidites derived from achiral 2,2'-biphenol are almost as good as ligands derived from the more expensive enantiopure 2,2'-binaphthols. The hydrovinylation products, which carry two versatile latent functionalities, an aryl and a vinyl group, are potentially useful for the synthesis of several important natural products containing benzylic all-carbon quaternary centers.

Total synthesis of eudesmane terpenes by site-selective C-H oxidations

From menthol to cholesterol to Taxol, terpenes are a ubiquitous group of molecules (over 55,000 members isolated so far) that have long provided humans with flavours, fragrances, hormones, medicines and even commercial products such as rubber. Although they possess a seemingly endless variety of architectural complexities, the biosynthesis of terpenes often occurs in a unified fashion as a 'two-phase' process. In the first phase (the cyclase phase), simple linear hydrocarbon phosphate building blocks are stitched together by means of 'prenyl coupling', followed by enzymatically controlled molecular cyclizations and rearrangements. In the second phase (the oxidase phase), oxidation of alkenes and carbon-hydrogen bonds results in a large array of structural diversity. Although organic chemists have made great progress in developing the logic needed for the cyclase phase of terpene synthesis, particularly in the area of polyene cyclizations, much remains to be learned if the oxidase phase is to be mimicked in the laboratory. Here we show how the logic of terpene biosynthesis has inspired the highly efficient and stereocontrolled syntheses of five oxidized members of the eudesmane family of terpenes in a modicum of steps by a series of simple carbocycle-forming reactions followed by multiple site-selective inter- and intramolecular carbon-hydrogen oxidations. This work establishes an intellectual framework in which to conceive the laboratory synthesis of other complex terpenes using a 'two-phase' approach.

Construction of carbo- and heterocycles using radical relay cyclizations initiated by alkoxy radicals

An efficient method for the rapid construction of carbo- and heterocycles has been developed using radical relay cyclizations initiated by alkoxy radicals. Linear substrates were cyclized to form a wide range of cyclopentane, pyrrolidine, tetrahydropyran, and tetrahydrofuran derivatives in excellent yields. This methodology was utilized as a key step in the synthesis of the tetrahydrofuran fragment in (-)-amphidinolide K.

Cationic Ir(I)-Catalyzed sp(3) C-H Bond Alkenylation of Amides with Alkynes

A cationic Ir(I)-BINAP catalyst cleaved sp(3) C-H bonds of arylamides rather than sp(2) C-H bonds, which was followed by alkenylation with alkynes to give allylamides. Several types of amides and alkynes were suitable as substrates, and the corresponding allylamides were obtained in moderate to good yield. We also demonstrated that carbonyl-directed sp(3) C-H bond cleavage would be an initial step in the present reaction by a deuterium-labeling experiment.