Rhenium-catalyzed insertion of aldehyde into a C-H bond: synthesis of isobenzofuran derivatives

A rhenium complex, [ReBr(CO)3(thf)]2, catalyzed reactions of aromatic ketimines with aldehydes to give isobenzofuran derivatives in good to excellent yields. In contrast to ruthenium and rhodium catalysts, aldehydes, which are polar unsaturated molecules, inserted into the C-H bond after activation by the rhenium complex.     

Rhenium-catalyzed insertion of terminal acetylenes into a C-H bond of active methylene compounds

[reaction: see text] A rhenium complex, [ReBr(CO)(3)(thf)](2), catalyzed the intermolecular reactions of 1,3-dicarbonyl compounds with terminal acetylenes and gave the corresponding alkenyl derivatives in excellent yields. These reactions could apply to an intramolecular version and gave the corresponding cyclic compounds quantitatively.     

Catalytic diboration of aldehydes via insertion into the copper-boron bond

Mesitaldehyde reacts cleanly with (IPr)CuB(pin) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene); pin = 2,3-dimethyl-2,3-butanediolate] to afford the product complex 1, the first well-defined product of carbonyl group insertion into a metal-boron bond. Analysis of 1 by NMR spectroscopy and single-crystal X-ray diffraction indicates the formation of a copper-carbon and a boron-oxygen bond. A copper(I) precatalyst supported by the less sterically demanding ligand ICy (1,3-dicyclohexylimidazol-2-ylidene) achieves the efficient 1,2-diboration of aryl-, heteroaryl-, and alkyl-substituted aldehydes at room temperature.     

Generation of a carbene-stabilized bora-borylene and its insertion into a C-H bond

A novel NHC adduct of a dihalodiborane(4), 1, is reduced by KC(8) with formation of the five-membered boracycle 2. The reaction most likely proceeds via C-H insertion of an intermediate NHC-stabilized free bora-borylene species.     

Platinum-catalyzed aromatization of enediynes via a C-H bond insertion of tethered alkanes

[STRUCTURE: SEE TEXT] PtCl2 (5 mol%) is an effective catalyst for aromatization of enediynes via a C-H bond insertion of tethered alkanes. The reaction mechanism of this cyclization is proposed to involve platinum-pi-alkyne intermediates. This cyclization works not only for terminal alkynes but also for internal alkynes.     

A convergent and enantioselective synthesis of (+)-amurensinine via selective C-H and C-C bond insertion reactions

A convergent and enantioselective synthesis of the natural product amurensinine is described. The synthetic strategy takes advantage of mild and selective C-H and C-C bond insertion reactions, in addition to the palladium-catalyzed aerobic oxidative kinetic resolution recently developed in these laboratories.     

Rhenium-catalyzed insertion of terminal alkenes into a C(sp)-H bond and successive transfer hydrogenation

Treatment of aromatic aldimines with terminal alkenes in the presence of a rhenium catalyst, [HRe(CO)₄]_n, gives 2-alkenylbenzylamines in good to excellent yields. This reaction proceeds via the insertion of the alkene into a C-H bond at the ortho-position of the imino group of the aromatic aldimine followed by sequential β-hydride elimination from the formed alkyl rhenium intermediate and then by hydrogenation of the imino group of the aldimine.     

Enantioselective synthesis of a PKC inhibitor via catalytic C-H bond activation

[reaction: see text] The syntheses of two biologically active molecules possessing dihydropyrroloindole cores (1 and 2) were completed using rhodium-catalyzed imine-directed C-H bond functionalization, with the second of these molecules containing a stereocenter that can be set with 90% ee during cyclization using chiral nonracemic phosphoramidite ligands. Catalytic decarbonylation and direct indole/maleimide coupling provide efficient access to 2.     

Cobalt-catalyzed chemoselective insertion of alkene into the ortho C-H bond of benzamide

Insertion of 1-alkene, 2-alkene, and styrene into the ortho C-H bond of benzamide in the presence of an inexpensive cobalt catalyst, DMPU as a crucial ligand, and cyclohexylmagnesium chloride proceeds smoothly at 25 °C to selectively give the ortho-alkylated product. Notable features of this reaction include the structural variety of the alkene and the amide substrate and the tolerance of functional groups such as halide, olefin, ester, and amide groups.     

A facile enantioselective synthesis of (+)-rolipram via C-H insertion process

Rolipram, (±)-4-(3-cyclopentyloxyl-4-methoxylphenyl)-2-pyrrolidinone,is known as a potent and selective inhibitor of phosphodiesterase type IV(PDE IV)1.In fact, although both enantiomers are active, the levorotatory isomer, for which several synthesis has been recently reported2, has proved to be the most active one. Hashimoto and Jung have repored rolipram,s synthesis via intramolecular C-H insertion reaction of α-methoxylcarbonyl-α-diazoacetamide and α-phenylsulfonyl-α-diazoacetamid…     

Palladium-catalyzed asymmetric synthesis of silicon-stereogenic dibenzosiloles via enantioselective C-H bond functionalization

A Pd-catalyzed asymmetric synthesis of Si-stereogenic dibenzosiloles is developed through enantioselective C-H bond functionalization of prochiral 2-(arylsilyl)aryl triflates. High chemo- and enantioselectivities are achieved by employing a Josiphos-type ligand under mild conditions.     

Rhenium-catalyzed formation of indene frameworks via C-H bond activation: [3+2] annulation of aromatic aldimines and acetylenes

A rhenium complex, [ReBr(CO)3(thf)]2, catalyzes the reaction of an aromatic aldimine with an acetylene to give an indene derivative in a quantitative yield. The reaction proceeds via C-H bond activation, insertion of the acetylene, intramolecular nucleophilic cyclization, and reductive elimination. In contrast to ruthenium and rhodium catalysts, which are usually employed in this type of reaction, the rhenium catalyst promotes the intramolecular nucleophilic cyclization of the alkenylmetal species generated by insertion of the acetylene.     

Rearrangement of biaryl monoaminocarbenes via concerted asynchronous insertion into aromatic C-H bonds

The biphenyl and binaphthyl diisopropylaminocarbenes were found to be only transient species that spontaneously and quantitatively rearrange into the corresponding aminofluorenes. DFT calculations confirm that these insertion reactions of aminocarbenes into proximal aromatic C-H bonds require only a moderate energy barrier and support a concerted, strongly asynchronous, mechanism dominated by C arom-->C carbene proton transfer.     

Palladium-catalyzed C-N bond formation via direct C-H bond functionalization. Recent developments in heterocyclic synthesis

The formation of carbon-nitrogen bonds by reaction between a nitrogen atom and an unactivated carbon-hydrogen bond is a highly atom-economical process that attracted the attention of the chemists in the last two decades. The widely useful amination and hydroamination reactions, which furnish acyclic or cyclic products, give access to various nitrogen-containing basic and fine chemicals. This review highlights recent progress in the development of palladium-catalyzed reactions that occur by direct functionalization of simple carbon-hydrogen bonds to give heterocyclic products. Pd(0)- and Pd(II)-catalyzed reactions are described separately, emphasizing the different behavior of the metal in these two oxidation states.     

C-C bond formation via C-H bond activation: synthesis of the core of teleocidin B4

The core of teleocidin B4, a complex fragment of a natural product containing two quaternary stereocenters and a penta-substituted benzene ring, was synthesized in four C-C bond-forming steps starting from tert-butyl derivative 1. The first step involved alkenylation of the tert-butyl group with a vinyl boronic acid, followed by the successful annulation of the cyclohexane ring to the benzene nucleus via an intramolecular Friedel-Crafts reaction. The third step required a diastereoselective oxidative carbonylation of the geminal dimethyl group, followed at last by indole assembly via the alkenylation of the phenol nucleus, to afford the teleocidin B4 core. Noteworthy is the fact that steps 1 and 3 critically depended on the directing role of the aniline nitrogen (directed C-H bond functionalization).     

Palladium-catalyzed synthesis of cyclopentane-fused benzocyclobutenes via tandem directed carbopalladation/C-H bond functionalization

A new Pd-catalyzed reaction for the stereoselective synthesis of cyclopentane-fused benzocyclobutenes is described. These transformations likely proceed via carbamate-directed carbopalladation followed by intramolecular C-H activation of an alkylpalladium intermediate. The mechanistic relationship between these transformations and Pd-catalyzed reactions of gamma-(n-Boc-amino)alkenes with aryl bromides that afford pyrrolidines is discussed. Differences in reactivity between Pd-amino and Pd-amido complexes appear to play a key role in the outcome of these transformations.