A series of geometry‐constrained iminopyridyl‐palladium chlorides were synthesized and characterized. These phosphine‐free palladium complexes were explored for their catalytic activities in both Suzuki and Heck cross‐coupling reactions, achieving turnover numbers as high as 106 towards various aryl bromides, even those containing various functionalities. In addition, the influence of substituents with steric and electronic factors was reflected by the differences observed in their activities.
Cobalt‐catalyzed C−H amination of arenes with alkylamines by the assistance of 8‐aminoquinoline as auxiliary through sp2 C−H bond functionalization has been achieved. Attractive features of this protocol include the low loading of the cobalt catalyst and the readily available reagents.
A palladium‐catalyzed domino strategy has been developed for the synthesis of 2,3‐disubstituted benzofuran derivatives. This cascade reaction sequence involves intermolecular carbopalladation and C(sp3)−H functionalization followed by isomerization.
The importance of phosphines in synthetic chemistry cannot be underestimated. Catalytic hydrophosphination offers an ideal method to prepare P−C bonds without the need for harsh reaction conditions or stoichiometric amounts of waste by‐product. We herein report our studies into two biocompatible iron(III) complexes in hydrophosphination chemistry using diphenylphosphine under mild and benign reaction conditions (room temperature, solvent‐free) and our extended exploration of hydrophosphination with phenylphosphine, which can be tuned to operate in the absence of catalyst under thermal conditions for single hydrophosphination or solvent‐free with an iron(III) precatalyst to generate the products of double hydrophosphination.
A series of well‐defined iron(II) complexes of the types [Fe(PNP)Br2] and [Fe(PNP)(CO)Br2] with PNP pincer ligands based on triazine and pyridine backbones were prepared and fully characterized. These complexes were tested as catalysts for the alkylation of amines by alcohols. The high‐spin complexes [Fe(PNP)Br2] are catalytically inactive. The low‐spin complexes [Fe(PNP)(CO)Br2] bearing a carbonyl co‐ligand efficiently and selectively convert primary alcohols and aromatic and benzylic amines selectively into mono‐N‐alkylated amines in good to excellent isolated yields. A mechanistic proposal is given.
The improved synthesis of γ‐, δ‐ and ϵ‐lactones using a dinuclear N‐heterocyclic carbene (NHC)‐gold(I) catalyst is reported. This solvent‐free process provides access to γ‐ and δ‐lactones in high regio‐ and stereoselectivity. Reactions were performed at low catalyst loadings and without the need for any additives. The use of a digold pre‐catalyst provides a new synthetic route to functionalised ϵ‐lactones, poorly accessible using previous methodologies.
An inexpensive ruthenium‐catalyzed direct C–H olefination of aromatic carboxamides with allyl acetate was developed. This external oxidant‐free protocol provided an efficient route for the synthesis of useful trans‐styrene derivatives in high to excellent yields. The olefination reaction exhibited excellent regioselectivities and allowed the presence of a wide range of functional groups, such as OMe, F, Cl, Br, I, CF3, NMe2, as well as NO2.
A mild, metal‐free synthesis of polyfunctionalized N‐acyl‐N,O‐hemiacetals was developed via the nucleophilic addition of unactivated amides to ketones. The protocol demonstrated a wide substrate scope, with good isolated yields. Additionally, their O‐acetylated products serve as a precursor of α,α‐difunctionalized N‐acylimines. An addition reaction of broad scope of nucleophiles to generate N‐acylimines is also reported.
An iron(III)‐catalyzed C‐3 functionalization of flavones has been achieved using tert‐butyl peroxybenzoate (TBPB)/potassium persulphate (K2S2O8) oxidant combinations with a suitable solvent. In the presence of iron(III)/tert‐butyl peroxybenzoate/potassium persulphate, the reaction of flavones in cycloalkanes afforded exclusive C‐3 cycloalkylation via C –C coupling, whereas the solvent N,N‐dialkylformamide provided C‐3 amidation via C –C coupling. Under identical reaction conditions just by switching the solvent to chlorobenzene, C‐3 methylated flavones were obtained where tert‐butyl peroxybenzoate (TBPB) served as the source of the methyl group.
NHC‐rhodium complexes (NHC=N‐heterocyclic carbenes) have been widely used as efficient catalysts for hydrosilylation reactions. However, the substrates were mostly limited to reactive carbonyl compounds (aldehydes and ketones) or carbon‐carbon multiple bonds. Here, we describe the application of newly‐developed chelating bis(tzNHC)‐rhodium complexes (tz=1,2,3‐triazol‐5‐ylidene) for several reductive transformations. With these catalysts, the formal reductive methylation of amines using carbon dioxide, the hydrosilylation of amides and carboxylic acids, and the reductive alkylation of amines using carboxylic acids have been achieved under mild reaction conditions.
A metal‐free synthesis of biologically important benzazepines is achieved through a single synthetic operation involving an oxidative C–H bond functionalization and ring expansion with diazomethanes as key reagent. This represents a new, strong methodology for the straightforward construction of the seven‐ring N‐heterocyclic structures under mild conditions using a 2,2,6,6‐tetramethylpiperidine 1‐oxyl (TEMPO) oxoammonium salt as oxidant. Moderate to good yields are achieved from simple, readily available tetrahydroisoquinolines, and this methodology has been further successfully applied for the synthesis of the 3‐benzazepine drug Lorcaserin. A possible mechanistic pathway for the ring expansion step, comprising the extrusion of nitrogen in a concerted asynchronic process, is proposed based on both mechanistic proof and density function theory (DFT) calculations.
An efficient rhodium(III)‐catalyzed vinylic sp2 C–H bond activation of 7‐azaindoles with alkynes is reported, leading to the controlled synthesis of stereodefined diene and fused polycyclic heteroarene products. The novel strategy for the synthesis of diverse coupling products was controlled by the reaction conditions and the substitutions on the vinyl group.
4H‐Pyran units are frequently present in molecules with significant biological and pharmaceutical activities. Herein, we present the first enantioselective formal [3+3] cycloaddition between 2‐(1‐alkynyl)‐2‐alken‐1‐ones and β‐keto esters catalyzed by a cyclohexyldiamine‐based thiourea‐tertiary amine bifunctional catalyst. Under the mild and eco‐friendly conditions, a wide range of polysubstituted 4H‐pyrans were obtained in moderate yields with good enantioselectivities.
A palladium‐catalyzed cross‐coupling of activated amides with arylboronic acids has been developed via C–N bond cleavage. This approach demonstrated high tolerance to a variety of alkyl, aryl, heterocyclic and vinyl substituents. Unsymmetrical ketones could be achieved in excellent yield under mild conditions with 1% catalyst loadings.
A tunable and practical synthesis of electrophilic sulfenylating reagents, thiosulfonates and disulfides, from inexpensive and easily available sulfonyl chlorides, has been developed. By appropriate choice of solvents, the reaction of sulfonyl chlorides and tetrabutylammonium iodide gave the target products in good to excellent yields, respectively. These transformations probably proceed through a reducing–coupling pathway.
An efficient, metal‐free, silicon–hydrogen bond functionalization based on the microwave‐assisted reaction of readily available enynones and silanes is reported. This process seemingly proceeds through a 2‐furyl carbene species, a particularly elusive intermediate. Preliminary studies on the metal‐free oxygen–hydrogen and nitrogen–hydrogen bond functionalization of representative alcohols, azoles and sulfonamides are also provided.
A pair of epimeric l ‐proline derived chiral auxiliaries have been developed for the diastereoselective lithiation–substitution of η6‐arene chromium tricarbonyl complexes. Lithiation of the anti epimer affords planar chiral imidazolones in >95:5 dr upon electrophile quench. Application of the same reaction sequence to the syn epimer provides products with opposite planar chirality in equally high levels of diastereomeric purity. The ortho‐substituted epimeric imidazolones give rise to planar chiral enantiomers upon acid‐induced elimination, thus precluding the need to prepare a d ‐proline‐derived starting material.
The acyloxy group of 2‐acyloxy‐2H‐azirines has been displaced by carboxylic acids to generate the corresponding 2‐acyloxy‐2H‐azirines by using potassium iodide as a promotor. This metal‐free method can be performed with a wide scope of substrates readily generating products in moderate to high yields. It also represents an example of nucleophilic substitution between esters and carboxylic acids under metal‐free reaction conditions for the first time.
