Friedlander synthesis of highly functionalized isoxazolyl quinoline libraries via addition of C(sp3)–H bond to aldehydes

A novel protocol for ionic liquid (IL)-mediated C(sp³)–H bond functionalization of 3,5-dimethyl-4-nitroisoxazoles 4 to substituted o-amino benzaldehydes 5 was developed in excellent yields. Isoxazolyl aryl ethanones 7 have been synthesized from isoxazolyl aryl ethanol synthon 6. Furthermore, utilizing the later as synthons for IL promoted Friedlander synthesis of highly functionalized isoxazole substituted quinoline libraries 9. The merit of this synthesis is easily available and economical starting materials, effective utilization of all the reactants, and simple workup procedure. It is noteworthy that ionic liquid used in C(sp³)–H bond functionalization and Friedlander synthesis reactions can be recycled and reused five times without significant decrease in activity.     

Cationic iridium-catalyzed enantioselective activation of secondary sp3 C–H bond adjacent to nitrogen atom

A cationic Ir(I)–tolBINAP complex catalyzed an enantioselective C–C bond formation, which was initiated by secondary sp³ C–H bond cleavage adjacent to nitrogen atom. A wide variety of 2-(alkylamino)pyridines and alkenes were selectively transformed into the corresponding chiral amines with moderate to almost perfect enantiomeric excesses. Alkynes were also investigated as coupling partners. The effect of alkyl structure in substrates and directing groups were studied. This transformation represents the first example of a highly enantioselective C–H bond activation of a methylene group, not at allylic or benzylic position.     

Challenges in C–C bond formation through direct transformations of sp2 C–H bonds

In the past several decades, organic chemists have made significant contributions to approach direct C–H transformations. However, limited substrate scope and reaction classifications, harsh condition, and the use of noble and toxic late transition-metal catalysts typically with high loadings clogged its applications. This article summarizes our recent efforts to explore new reaction types and develop new catalytic systems in this field, which may open a new channel to consider organic synthesis from easily available chemicals.     

C–H bond addition and copolymerization reactions of N-arylmaleimides: Fundamentals of coagent-assisted polymer cross-linking

Solid-state rheometry and model compound reactions are used to investigate free radical reactions of N-arylmaleimide coagents with saturated and unsaturated polymers. N,N′-m-phenylene dimaleimide (BMI) is shown to provide superior cross-link densities over diacrylate and diallyl coagents for all of the polymers studied, including linear low density polyethylene (LLDPE), poly(ethylene oxide) (PEO), cis-poly(butadiene) (PBD) and cis-poly(isoprene) (PIP). Studies of the N-phenylmaleimide (NPM) + cis-cyclooctane system show that C–H bond addition to yield N-aryl-2-alkylsuccimide grafts is the predominant reaction pathway, as opposed to maleimide homopolymerization. In contrast, peroxide-initiated reactions of cis-cyclooctene with small NPM concentrations generate highly alternating poly(cycloctene-alt-N-phenylmaleimide) in high yield, indicating that unsaturated mers in materials such as PBD engage maleimides in an efficient alternating copolymerization between electron-rich and electron-deficient monomer pairs. Factors that affect the reactivity of different polymers in these C–H bond additions and alternating copolymerizations are discussed.     

Sulfonylation of C(sp3)–H bond for synthesis of 2-sulfolmethyl azaarenes catalyzed by TBAI in water

Research on Chemical Intermediates - A tetrabutylammonium iodide (TBAI)-catalyzed method for synthesis of 2-sulfolmethyl quinolone has been developed. Using water as solvent, a wide range of...     

Rh(I)-catalyzed ring-opening of cyclobutanols via C–C bond activation: Synthesis of cis-olefin with a remote aldehyde

A Rh(I)-catalyzed ring-opening of cyclobutanols has been developed with ring opening products bearing cis-olefin and a remote aldehyde. Various substrates bearing different substituted aryl groups, heterocyclic groups and alkyl groups were compatible with the mild reaction conditions. A β-C elimination pathway was proposed based on the results of preliminary mechanistic studies.     

Sequential Pd2(dba)3 participated C–C bond cleavage of O-bromophenyl cyclobutanones/Michael addition en route to benzospirones

A tandem Pd₂(dba)₃ participated C–C bond cleavage of O-bromophenyl cyclobutanone derivatives/Michael addition reaction sequence was realized. We disclosed the first intramolecular C–Br bond triggered ring opening reaction of arylcyclobutanones, distinct from related reports in which the reactions were initiated by arylboron, silane or unsaturated chemical motifs, among others. The in situ generated palladium species underwent ring expansion process leading to methyleneindanones, which further reacted with dba to provide benzospirones in one step.     

Highly selective insertion of arynes into a C(sp)-O(sp3) σ bond

Arynes react with ethoxyacetylene to afford 2-ethoxyethynylaryl derivatives through a highly chemo- and regioselective formal insertion of the aryne into the C(sp)-O(sp(3)) bond of the alkyne. Computational studies suggest that the reaction does not proceed through a mechanism initiated by the nucleophilic addition of the oxygen atom to the aryne as previously proposed but by the addition of the triple bond of the alkyne to the aryne.     

One-pot synthesis of ortho-acylphenols by palladium-catalyzed phenol C–H addition to nitriles

The regioselective one-pot synthesis of ortho-acylphenols via the palladium-catalyzed addition of phenols to nitriles and subsequent hydrolysis is reported. The acylation reaction proceeded smoothly using the Pd(OAc)₂/DMSO system and TFA as the additive. This method also provides a direct strategy for the synthesis of a salicylketoxime scaffold.     

Oxidative Addition of a Mechanically Entrapped C(sp)–C(sp) Bond to a Rhodium(I) Pincer Complex

By use of a macrocyclic phosphinite pincer ligand and bulky substrate substituents, we demonstrate how the mechanical bond can be leveraged to promote the oxidative addition of an interlocked 1,3-diyne to a rhodium(I) center. The resulting rhodium(III) bis(alkynyl) product can be trapped out by reaction with carbon monoxide or intercepted through irreversible reaction with dihydrogen, resulting in selective hydrogenolysis of the C−C σ-bond.     

Iron-catalyzed alkenylation of cyclic ethers via decarboxylative sp(C)–sp(C) coupling

An efficient Fe(acac)₃-catalyzed decarboxylative C(sp²)–C(sp³) coupling reaction via oxidation of C–H bond adjacent to an oxygen atom has been developed successfully, in which cyclic ethers are selectively transformed into the corresponding alkenylation products with good chemical yields and excellent stereoselectivities. The mechanism was studied and the reaction was supposed to proceed through a radical oxidative coupling process.     

Revealing Silylation of C(sp2)/C(sp3)–H Bonds in Arylphosphines by Ruthenium Catalysis

The first aromatic C−H silylation between arylphosphines and hydrosilanes enabled by a ruthenium complex has been developed. The excellent ortho-selectivity results from a four-membered metallacyclic intermediate involving phosphorus chelation. The developed system can be extended to the benzylic C−H silylation of arylphosphines. Diverse silylated arylphosphines are produced, exhibiting broad functional group compatibility. Further functionalization of the products under mild conditions renders the formed compounds useful building blocks.     

Copper(I) promoted CC bond forming reactions: direct activation of allyl alcohols

Allylic alcohols, acetates, carbonates and chlorides can be activated by copper(I) salts towards nucleophilic substitution by carbon nucleophiles under relatively mild conditions.     

Solvent-free and direct C(sp)–H amination of adamantanes by grinding

A facile, direct and environmentally benign conversion of C(sp³)–H bonds to C(sp³)–N bonds using substoichiometric amount of aprotic superelectrophiles polyhalomethane–AlX₃ has been achieved by grinding under solvent-free conditions at room temperature in air. It is a general and simple method for the direct amination of adamantanes, and a series of aminoadamantanes of azoles, arylamines or heteroarylamines were obtained in good to excellent yields. The advantages of this amination are atom economy, solvent-free, chemoselectivity, short reaction time and high yields.     

Indium trichloride catalyzed sp C–H bond functionalization of 2-alkyl azaarenes under microwave irradiation

Microwave promoted indium trichloride (10 mol %) catalyzed sp³ C–H bond functionalization of 2-alkyl azaarenes 1 or 4 has been observed to construct C–C bond either with but-2-ene-1,4-diones 2 or (E)-3-(2-oxo-2-phenylethylidene)indolin-2-one (6) giving access to 2-((quinolin-2-yl)methyl)butane-1,4-diones 3, 2-((pyridin-2-yl)methyl)butane-1,4-diones 5, or 3-(quinolin-2-yl)propan-2-yl)indolin-2-ones 7 in good yields using 1,4-dioxane as solvent.     

Practical synthesis of C1–8 fragment of autolytimycin via a chelation-controlled diastereoselective addition of diisopropenylzinc to α-methoxy aldehyde

The C1–8 fragment of autolytimycin was synthesized via a reliable 10-step route capable of delivering 41% overall yield at multi-gram scale. As a key step, a chelation-controlled isopropenylation of α-oxygenated aldehydes was established with a reagent combination of diisopropenylzinc, magnesium halide, and a dichloromethane/toluene mixed solvent. Cram-chelate isopropenylation products dominated for aldehydes with a small α-substituents, such as –OMe and –OBn groups, while the Felkin product could be obtained with a bulky –OTBS group.     

Ortho-substituent effect promoted rapid cleavage of amide C–N bond under mild conditions

This article reports the ortho-substituent effect on the cleavage of the amide C–N bond. In the structure of N ^α -Phth-N-pyridinyl-amide, when the hydrogen atom in the 3-position of pyridine ring was replaced by alkoxy group, the amide C–N cleavage can take place in the 2-position of the pyridine ring. This transformation can proceed rapidly in methylamine ethanol solution under mild conditions to afford 2-amino-3-alkoxy pyridines.     

Palladium-catalyzed ortho-selective C–H bond chlorination of aromatic ketones

A palladium-catalyzed ortho-selective C–H bond chlorination reaction for the preparation of 2-chloro aromatic ketones was described. Both electron-withdrawing and electron-donating groups on the aromatic rings are well tolerated under the optimized conditions. The 2-chloro aromatic ketones obtained by our method could be applied to synthesize the derivatives of 1H-indazole or benzo[d]isoxazole.     

Carbon–hydrogen (C–H) bond activation at PdIV: a Frontier in C–H functionalization catalysis

The direct functionalization of carbon–hydrogen (C–H) bonds has emerged as a versatile strategy for the synthesis and derivatization of organic molecules. Among the methods for C–H bond activation, catalytic processes that utilize a Pd^II/Pd^IV redox cycle are increasingly common. The C–H activation step in most of these catalytic cycles is thought to occur at a Pd^II centre. However, a number of recent reports have suggested the feasibility of C–H cleavage occurring at Pd^IV complexes. Importantly, these latter processes often result in complementary reactivity and selectivity relative to analogous transformations at Pd^II. This mini review highlights proposed examples of C–H activation at Pd^IV centres. Applications of this transformation in catalysis as well as mechanistic details obtained from stoichiometric model studies are discussed. Furthermore, challenges and future perspectives for the field are reviewed.     

Pd-catalyzed ligand-free C(5)–H arylation of 1,4-disubstituted 1,2,3-triazoles promoted by microwaves

Pd(OAc)₂-catalyzed regioselective C(5)–H arylation of 1,4-disubstituted 1,2,3-triazoles was achieved without ligand under microwave conditions in 1?h, generating 1,4,5-trisubstituted 1,2,3-triazoles with good to excellent yields. The obtained molecules can be easily converted into 4,5-disubstituted 1,2,3-triazoles through the debenzylation process with one-pot manipulation.