Stereoselective Direct Copper-Catalyzed Alkenylation of Oxazoles with Bromoalkenes

Direct Heterocycle C-H Alkenylation via Dual Catalysis Using a Palladacycle Precatalyst: Multifactor Optimization and Scope Exploration Enabled by High-Throughput Experimentation

One of the major challenges in developing catalytic methods for C-C bond formation is the identification of generally applicable reaction conditions, particularly if multiple substrate structural classes are involved. Pd-catalyzed direct arylation reactions are powerful transformations that enable direct functionalization of C-H bonds; however, the corresponding direct alkenylation reactions, using vinyl (pseudo) halide electrophiles, are less well developed. Inspired by process development efforts toward GSK3368715, an investigational active pharmaceutical ingredient, we report that a Pd(II) palladacycle derived from tri-tert-butylphosphine and Pd(OAc)2 is an effective single-component precatalyst for a variety of direct alkenylation reactions. High-throughput experimentation identified optimal solvent/base combinations for a variety of HetAr-H substrate classes undergoing C-H activation without the need for cocatalysts or stoichiometric silver bases (e.g., Ag2CO3). We propose this reaction proceeds via a dual cooperative catalytic mechanism, where in situ-generated Pd(0) supports a canonical Pd(0)/(II) cross-coupling cycle and the palladacycle effects C-H activation via CMD in a redox-neutral cycle. In all, 192 substrate combinations were tested with a hit rate of approximately 40% and 24 isolated examples. Importantly, this method was applied to prepare a key intermediate in the synthesis of GSK3368715 on multigram scale.

Sustainable Production of Bioactive Molecules from C-Lignin-Derived Propenylcatechol

Catechyl lignin (C-lignin) is a naturally occurring linear homogeneous biopolymer composed solely of caffeyl alcohol subunits with cleavable benzodioxane linkages. The inherent structural features of propenylcatechol, a direct depolymerized product of castor seed coats C-lignin, render it a sustainable and promising platform for the synthesis of bioactive molecules. Herein, diversified transformations of propenylcatechol, including C=C bond difunctionalization, β-modification, β,γ-rearrangement, and γ-methyl derivatization, were reported based on known or developed methods. A series of functional molecular skeletons involved in the current synthetic routes for the preparation of pharmaceuticals and bioactive molecules were obtained. Starting from castor seed coats, annuloline (natural product) and CC-5079 (antitumor) were synthesized using facile and inexpensive reagents in only four- and five-sequence reactions, respectively, thereby demonstrating a superior step-efficiency to that of reported synthetic routes. Almost all atoms in the C-lignin biopolymer were incorporated into the final products owing to the intrinsic structures of naturally occurring C-lignin. Bioactive molecules produced from C-lignin integrate a low-carbon footprint with high-quality and economical manufacture of pharmaceuticals.

Palladium-Catalyzed Dehydrogenative C-2 Alkenylation of 5-Arylimidazoles and Related Azoles with Styrenes

The construction of carbon–carbon bonds by direct involvement of two unactivated carbon–hydrogen bonds, without any directing group, ensures a high atom economy of the entire process. Here, we describe a simple protocol for the Pd(II)/Cu(II)-promoted intermolecular cross-dehydrogenative coupling (CDC) of 5-arylimidazoles, benzimidazoles, benzoxazole and 4,5-diphenylimidazole at their C-2 position with functionalized styrenes. This specific CDC, known as the Fujiwara–Moritani reaction or oxidative Heck coupling, also allowed the C-4 alkenylation of the imidazole nucleus when both 2 and 5 positions were occupied.

Cu-Catalyzed C-H Alkenylation of Benzoic Acid and Acrylic Acid Derivatives with Vinyl Boronates

An efficient Cu-catalyzed C-H alkenylation with acyclic and cyclic vinyl boronates was realized for the first time under mild conditions. The scope of the vinyl borons and the compatibility with functional groups including heterocycles are superior than Pd-catalyzed C-H coupling with vinyl borons, providing a reliable access to multisubstituted alkenes and dienes. Subsequent hydrogenation of the product from the internal vinyl borons will lead to installation of secondary alkyls.

Copper(i)-catalyzed benzylation of triazolopyridine through direct C-H functionalization

A general and efficient copper-catalyzed benzylation reaction of triazolopyridine with N-tosylhydrazones was developed. This reaction forms a C(sp²)-C(sp³) bond through cross-coupling, and represents an exceedingly practical method to afford 3-benzylated triazolopyridines in moderate to good yields. A proposed mechanistic pathway underlying this reaction was outlined. This catalytic transformation should enable broad synthetic applications in functionalization chemistry, allowing the synthesis of new pharmaceutically relevant triazolopyridine derivatives.

Copper-catalyzed synthesis of pyrazolo[5,1-a]isoquinoline derivatives from 2-gem-dipyrazolylvinylbromobenzenes

An efficient copper-catalyzed synthesis of pyrazolo[5,1-a]isoquinolines from 2-gem-dipyrazolylvinylbromobenzenes under mild conditions has been developed.

Palladium-catalyzed trisallylation of benzoxazoles and 2-aryl-1,3,4-oxadiazoles with alkyne

In this report, a palladium catalyzed direct trisallylation of azoles with alkyne via a novel subsequent mono C(sp²)–H allylation/olefin isomerization/y-selective double C(sp³)–H allylation process was developed. A wide range of substituted trisallylated azoles bearing all quaternary carbon centers has been synthesized efficiently and with high atom economy.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

A stereoselective synthesis of (E)- or (Z)-β-arylvinyl halides via a borylative coupling/halodeborylation protocol

A new highly selective method for the synthesis of (E)-arylvinyl iodides and (E)- or (Z)-β-arylvinyl bromides from easily accessible styrenes and vinyl boronates, on the basis of a one-pot procedure via borylative coupling/halodeborylation is reported.

Cu-catalyzed controllable C–H mono-/di-/triarylations of imidazolium salts for ionic functional materials

The first Cu-catalyzed direct C–H mono-, di- and triarylations of imidazolium salts with aryl iodides/bromides are reported.

CuCl-Catalyzed direct C–H alkenylation of benzoxazoles with allyl halides

An efficient and concise CuCl-catalyzed C2-alkenylation reaction of benzoxazoles with allyl halides has been established.

Regioselective copper-diamine-catalyzed C–H arylation of 1,2,4-triazole ring with aryl bromides

A bench-top protocol using simple Cu-diamine catalysis for the regioselective C5–H arylation of 1,2,4-triazole ring with (hetero)aryl bromides.

Copper-catalyzed direct C–H fluoroalkenylation of heteroarenes

Efficient copper-catalyzed direct C–H fluoroalkenylation of heterocycles using various gem-bromofluoroalkenes as electrophiles is reported for the first time.

Ligand-free Pd-catalyzed and copper-assisted C-H arylation of quinazolin-4-ones with aryl iodides under microwave heating

A microwave-assisted method for the palladium-catalyzed direct arylation of quinazolin-4-one has been developed under copper-assistance. This method is applicable to a wide range of aryl iodides and substituted (2H)-quinazolin-4-ones. This protocol provides a simple and efficient way to synthesize biologically relevant 2-arylquinazolin-4-one backbones.

Base-mediated direct fluoroalkenylation of 2-phenyl-1,3,4-oxadiazole, benzothiazole and benzoxazole with gem-difluoroalkenes

Direct α-fluorovinylation of 2-phenyl-1,3,4-oxadiazole, benzothiazole and benzoxazole with gem-difluoroalkenes under the assistance of KHMDS or NaH at room temperature was developed.

Trichloroisocyanuric Acid/Triphenylphosphine-Mediated Synthesis of Benzimidazoles, Benzoxazoles, and Benzothiazoles

A new and efficient method for preparation of benzimidazoles, benzoxazoles, and benzothiazoles from reactions of different carboxylic acids with o-phenylenediamine, o-aminophenol, and o-aminothiophenol in the presence of triphenylphosphine/trichloroisocyanuric acid system is presented. The desired products have been characterised on the basis of spectral (infrared, NMR, mass spectrometry) data, and the mechanism of their formation is proposed. The remarkable advantages are the inexpensive and readily available reagent, simple procedure, mild conditions, and good-to-excellent yields.

Ligand promoted Pd-catalyzed dehydrogenative alkenylation of hetereoarenes

An efficient Pd-catalyzed cross-dehydrogenative coupling of heteroarenes with styrenes and other olefinic substrates has been developed. This alkenylation paradigm encompasses a wide range of substrates and provides a straightforward approach toward C2-E-alkenylated azole motifs.

A copper-mediated cross-coupling approach for the synthesis of 3-heteroaryl quinolone and related analogues

An efficient and practical method for the direct cross-coupling between quinolones and a range of azoles was developed via copper-mediated C–H functionalization.

C-H alkenylation of azoles with enols and esters by nickel catalysis

Rather u(Ni)que: Two new C-H alkenylation reactions, that is C-H/C-O alkenylation and decarbonylative C-H alkenylation, of azoles are uniquely catalyzed by Ni/dcype. These azole alkenylation reactions are successfully applied to the convergent formal synthesis of siphonazole B.

Direct functionalization processes: a journey from palladium to copper to iron to nickel to metal-free coupling reactions

The possibility of finding novel disconnections for the efficient synthesis of organic molecules has driven the interest in developing technologies to directly functionalize C-H bonds. The ubiquity of these bonds makes such transformations attractive, while also posing several challenges. The first, and perhaps most important, is the selective functionalization of one C-H bond over another. Another key problem is inducing reactivity at sites that have been historically unreactive and difficult to access without prior inefficient prefunctionalization. Although remarkable advances have been made over the past decade toward solving these and other problems, several difficult tasks remain as researchers attempt to bring C-H functionalization reactions into common use. The functionalization of sp(3) centers continues to be challenging relative to their sp and sp(2) counterparts. Directing groups are often needed to increase the effective concentration of the catalyst at the targeted reaction site, forming thermodynamically stable coordination complexes. As such, the development of removable or convertible directing groups is desirable. Finally, the replacement of expensive rare earth reagents with less expensive and more sustainable catalysts or abandoning the use of catalysts entirely is essential for future practicality. This Account describes our efforts toward solving some of these quandaries. We began our work in this area with the direct arylation of N-iminopyridinium ylides as a universal means to derivatize the germane six-membered heterocycle. We found that the Lewis basic benzoyl group of the pyridinium ylide could direct a palladium catalyst toward insertion at the 2-position of the pyridinium ring, forming a thermodynamically stable six-membered metallocycle. Subsequently we discovered the arylation of the benzylic site of 2-picolonium ylides. The same N-benzoyl group could direct a number of inexpensive copper salts to the 2-position of the pyridinium ylide, which led to the first description of a direct copper-catalyzed alkenylation onto an electron-deficient arene. This particular directing group offers two advantages: (1) it can be easily appended and removed to reveal the desired pyridine target, and (2) it can be incorporated in a cascade process in the preparation of pharmacologically relevant 2-pyrazolo[1,5-a]pyridines. This work has solved some of the challenges in the direct arylation of nonheterocyclic arenes, including reversing the reactivity often observed with such transformations. Readily convertible directing groups were applied to facilitate the transformation. We also demonstrated that iron can promote intermolecular arylations effectively and that the omission of any metal still permits intramolecular arylation reactions. Lastly, we recently discovered a nickel-catalyzed intramolecular arylation of sp(3) C-H bonds. Our mechanistic investigations of these processes have elucidated radical pathways, opening new avenues in future direct C-H functionalization reactions.

Efficient synthesis of oxazoles by dirhodium(II)-catalyzed reactions of styryl diazoacetate with oximes

An efficient one-step synthesis of multi-functionalized oxazole derivatives is achieved in high yield by dirhodium(II)-catalyzed reactions of styryl diazoacetate with aryl oximes.

Room temperature copper(II)-catalyzed oxidative cyclization of enamides to 2,5-disubstituted oxazoles via vinylic C-H functionalization

A copper(II)-catalyzed oxidative cyclization of enamides to oxazoles via vinylic C-H bond functionalization at room temperature is described. Various 2,5-disubstituted oxazoles bearing aryl, vinyl, alkyl, and heteroaryl substituents could be synthesized in moderate to high yields. This reaction protocol is complementary to our previously reported iodine-mediated cyclization of enamides to afford 2,4,5-trisubstituted oxazoles.