PdII‐Catalyzed Mild CH ortho Arylation and Intramolecular Amination Oriented by a Phosphinamide Group

A novel protocol for the Pd‐catalyzed ortho‐arylation of aryl phosphinamide with boronic acid is reported. By using phosphinamide as a new directing group, the reaction proceeds efficiently under mild conditions at 40 °C. Mechanistic studies reveal that the reaction proceeds via a Pd^II to Pd⁰ cycle. The phosphinamide group is also shown to be an effective orienting group for direct C?H amination.     

C−H Activation of Unbiased C(sp3)−H Bonds: Gold(I)-Catalyzed Cycloisomerization of 1-Bromoalkynes**

Selective functionalization of non-activated C(sp³)−H bonds is a major challenge in chemistry, so functional groups are often used to enhance reactivity. Here, we present a gold(I)-catalyzed C(sp³)−H activation of 1-bromoalkynes without any sort of electronic, or conformational bias. The reaction proceeds regiospecifically and stereospecifically to the corresponding bromocyclopentene derivatives. The latter can be readily modified, comprising an excellent library of diverse 3D scaffolds for medicinal chemistry. In addition, a mechanistic study has shown that the reaction proceeds via a so far unknown mechanism: a concerted [1,5]-H shift / C−C bond formation involving a gold-stabilized vinylcation-like transition state.     

Site‐Selective δ‐C(sp3)−H Alkylation of Amino Acids and Peptides with Maleimides via a Six‐Membered Palladacycle

The site‐selective functionalization of unactivated C(sp³)?H bonds remains one of the greatest challenges in organic synthesis. Herein, we report on the site‐selective δ‐C(sp³)?H alkylation of amino acids and peptides with maleimides via a kinetically less favored six‐membered palladacycle in the presence of more accessible γ‐C(sp³)?H bonds. Experimental studies revealed that C?H bond cleavage occurs reversibly and preferentially at γ‐methyl over δ‐methyl C?H bonds while the subsequent alkylation proceeds exclusively at the six‐membered palladacycle that is generated by δ‐C?H activation. The selectivity can be explained by the Curtin–Hammett principle. The exceptional compatibility of this alkylation with various oligopeptides renders this procedure valuable for late‐stage peptide modifications. Notably, this process is also the first palladium(II)‐catalyzed Michael‐type alkylation reaction that proceeds through C(sp³)?H activation.     

Alkenylation of C(sp3)−H Bonds by Zincation/Copper‐Catalyzed Cross‐Coupling with Iodonium Salts

α‐Vinylation of phosphonates, phosphine oxides, sulfones, sulfonamides, and sulfoxides has been achieved by selective C?H zincation and copper‐catalyzed C(sp³)?C(sp²) cross‐coupling reaction using vinylphenyliodonium salts. The vinylation transformation proceeds in high efficiency and stereospecificity under mild reaction conditions. This zincative cross‐coupling reaction represents a general alkenylation strategy, which is also applicable for α‐alkenylation of esters, amides, and nitriles in the synthesis of β,γ‐unsaturated carbonyl compounds.     

Dehydrogenative N‐Incorporation: A Direct Approach to Quinoxaline N‐Oxides under Mild Conditions

An efficient method for the synthesis of quinoxaline N‐oxides proceeds by the dehydrogenative N‐incorporation of simple imines by C(sp²)? H and C(sp³)? H bond functionalization. The overall transformation involves the cleavage of three C? H bonds. The reaction is easily handled and proceeds under mild conditions. Simple and readily available tert‐butyl nitrite (TBN) was employed as the NO source.     

Gold(I)‐Catalyzed Haloalkynylation of Aryl Alkynes: Two Pathways,One Goal

Haloalkynylation reactions provide an efficient method for the simultaneous introduction of a halogen atom and an acetylenic unit. For the first time, we report a gold(I)‐catalyzed haloalkynylation of aryl alkynes that delivers exclusively the cis addition product. This method enables the simple synthesis of conjugated and halogenated enynes in yields of up to 90 %. Notably, quantum chemical calculations reveal an exceptional interplay between the place of the attack at the chloroacetylene: No matter which C?C bond is formed, the same enyne product is always formed. This is only possible through rearrangement of the corresponding skeleton. Hereby, one reaction pathway proceeds via a chloronium ion with a subsequent aryl shift; in the second case the corresponding vinyl cation is stabilized by a 1,3‐chlorine shift. ¹³C‐labeling experiments confirmed that the reaction proceeds through both reaction pathways.     

The Reaction of (N-Isocyanimino)triphenylphosphorane with Anthranilic Acid Derivatives: One-Pot Synthesis of 2-Substituted 1,3,4-Oxadiazoles via Intramolecular Aza-Wittig Reaction

The reaction of anthranilic acid derivatives with (N-isocyanimino)triphenylphosphorane proceeds smoothly at room temperature to afford 2-substituted 1,3,4-oxadiazoles via an intramolecular aza-Wittig reaction in excellent yields under neutral conditions. The structures of the products were deduced from their IR, ¹H NMR, and ¹³C NMR spectra and mass spectrometry.     

Catalytic synthesis and stereostructure of 1,2,3-trisubstituted decahydroisoquinolines

Liquid-phase catalytic reduction of 1,2,3-substituted 5,6,7,8-tetrahydroisoquinolinium perchlorates was investigated. The reaction proceeds stereodirectionally affording cis-decahydroisoquinolines. The probable scheme of reaction products formation is suggested. The structure of compounds obtained was established by IR and ¹³C NMR spectroscopy.     

New Route for the Synthesis of Four‐Membered Cyclic Ketene Dithioacetals

Novel ketene dithioacetals have been synthesized by a one‐pot condensation reaction of active methylene compounds with CS₂ in the presence of alkyl acetylenecarboxylates. This reaction proceeds in a regioselective manner and provides products in good yields. The structures of the ketene dithioacetals were characterized by IR, ¹H‐ and ¹³C‐NMR, and MS data, and elemental analyses.     

Pd(II)‐Catalyzed CH Activation of Styrylindoles: Short,Efficient, and Regioselective Synthesis of Functionalized Carbazoles

A novel Pd^II‐catalyzed approach for the direct synthesis of highly functionalized carbazoles from unprotected styrylindoles has been developed. The reaction features a variety of olefin substrates, which are readily switchable by subtle tuning of the reaction conditions. Investigations of the mechanism suggest that the C?H activation proceeds via enamine formation.     

Metal formyl complexes. The reaction of sodium pentacarbonylmanganate with acetic[13C]formic anhydride

The reaction of acetic [¹³C]formic anhydride with sodiumpentacarbonylmanganate proceeds rapidly at 0°C to give ¹³CO substituted pentacarbonylmanganese hydride as the predominant product. The results are consistent with the formation of a short-lived neutral formyl complex, (CO)₅Mn¹³CHO.     

Reversible α‐Hydrogen and α‐Alkyl Elimination in PC(sp3)P Pincer Complexes of Iridium

Despite significant progress in recent years, the cleavage of unstrained C(sp³)? C(sp³) bonds remains challenging. A C? C coupling and cleavage reaction in a PC(sp³)P iridium pincer complex is mechanistically studied; the reaction proceeds via the formation of a carbene intermediate and can be described as a competition between α‐hydrogen and α‐alkyl elimination; the latter process was observed experimentally and is an unusual way of C(sp³)? C(sp³) bond scission, which has previously not been studied in detail. Mechanistic details that are based upon kinetic studies, activation parameters, and DFT calculations are also discussed. A full characterization of a C? C agostic intermediate is presented.     

Palladium‐Catalyzed γ‐C(sp3)−H Arylation of Thiols by a Detachable Protecting/Directing Group

Reported herein is a palladium‐catalyzed, directed γ‐C(sp³)?H arylation of protected thiols. The key is to utilize Michael acceptors as a dual reagent to install a protecting/directing group on thiols by a thiol‐Michael click reaction, and remove it later under basic conditions. The C?H arylation proceeds with high functional‐group tolerance and the deprotected thiols can be further transformed into other sulfur‐containing compounds. This unique mode of activation could open the door for site‐selective functionalization of thiols or other sulfur‐containing compounds at unactivated positions.     

Preparation of 4,4a,9,9a-tetrahydrocarbazoles and 1,3a,4,8b-tetra-hydrocyclopenta[b]indoles

Halocyclization of mesylates or tosylates of 2-(cycloalk-2-en-1-yl)anilines gives N-methanesulfonyl-or N-toluenesulfonyl-1-halo-1,2,3,4,4a,9a-hexahydrocarbazoles, heating of which in DMF at 160°C or in piperidine at 110°C leads to 4,4a,9,9a-tetrahydro-3H-carbazoles. Heating N-methanesulfonyl-1-iodo-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole in DMF at 180—200°C gives 1,3a,4,9b-tetrahydrocyclopenta[b]indole, while in the presence of an ortho-methyl substituent the dehydroiodination reaction proceeds in piperidine at 110°C in high yield. The effect of the nature of the ortho substituent of N-methyl-1-iodo-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole on the conformational equilibrium of the cyclopentane ring has been established by ¹H NMR spectroscopy. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1184–1190, August, 2006.     

The loss of a methyl radical and the retro diels—Alder reaction in the electron impact mass spectrometry of 2-cyclohexen-1-ol and related compounds

The electron impact mass spectra of 2-cyclohexen-1-ol and of several of its ²H and ¹³C labelled analogues show that the molecular ions lose a methyl radical by a completely different means from the mechanism described previously. Moreover, the retro Diels–Alder reaction also proceeds in a non-classical way; in addition to the elimination of an olefinic molecule from unrearranged molecular ions, a second more important route implies a formal 1,3 allylic rearrangement prior to the retro Diels–Alder reaction. The mass spectra of a series of alkyl substituted homologues show that the competition between the two processes is closely related to the size of the olefinic moiety that is expelled.     

Stereochemical peculiarities of the catalytic reduction of sym-octahydrothioxanthylium salts

The three-dimensional structures of the products of catalytic hydrogenation of sym-octahydrothioxanthylium salts in the presence of palladium on carbon were studied by ¹³C NMR spectroscopy. It was shown that the reaction proceeds stereoselectively with the formation of cis-syn-cis isomers. The stereochemistry of the reduction products is in agreement with the previously proposed reaction mechanism.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1187–1190, September, 1987.     

Cobalt(II)-Catalyzed C(sp3)−C(sp3) Coupling for the Direct Stereoselective Synthesis of 2-Deoxy-C-Glycosides from Glycals

We report a ligand-controlled Co^II-catalyzed C(sp³)−C(sp³) coupling hydroalkylation for direct and β-selective synthesis of 2-deoxy-C-glycosides from glycals. This reaction proceeds by a radical pathway for alkyl halide activation and is β-selective through ligand control. This approach may inspire the development of further stereoselective coupling reactions with potential application in the field of carbohydrates.     

Construction of Hexahydrophenanthrenes By Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Substituted Internal Alkynes through C−H Activation

The treatment of benzylallene‐substituted internal alkynes with [RhCl(CO)₂]₂ effects a novel cycloisomerization by C(sp²)?H bond activation to produce hexahydrophenanthrene derivatives. The reaction likely proceeds through consecutive formation of a rhodabicyclo[4.3.0] intermediate, σ‐bond metathesis between the C(sp²)?H bond on the benzene ring and the C(sp²)?Rh^III bond, and isomerization between three σ‐, π‐, and σ‐allylrhodium(III) species, which was proposed based on experiments with deuterated substrates.     

Synthesis of Substituted 2‐Amino‐1,3‐oxazoles via Copper‐Catalyzed Oxidative Cyclization of Enamines and N,N‐Dialkyl Formamides

A facile synthesis of 2‐amino‐1,3‐oxazoles via Cu^I‐catalyzed oxidative cyclization of enamines and N ,N ‐dialkyl formamides has been developed. The reaction proceeds through an oxidative C−N bond formation, followed by an intramolecular C(sp²)−H bond functionalization/C−O cyclization in one pot. This protocol provides direct access to useful 2‐amino‐1,3‐oxazoles and features protecting‐group‐free nitrogen sources, readily available starting materials, a broad substrate scope and mild reaction conditions.     

Rhodium(II)-Catalyzed Selective C(sp3)−H Amination of Alkanes

The development of an efficient catalytic system enabling the conversion of alkanes to valuable nitrogen-containing building blocks is reported. Light alkanes can be selectively functionalized by an intermolecular C(sp³)−H amination reaction that proceeds at room temperature in the presence of only 1 mol % of a dirhodium(II) complex. Selective amination of tertiary C(sp³)−H within acyclic or cyclic alkanes used as limiting components leads to the corresponding amides isolated with yields in the 51–96 % range. The reaction, that can be performed on a gram-scale, applies with equal levels of efficiency and selectivity to more complex hydrocarbons.