Pd-Catalyzed Amination of Base-Sensitive Five-Membered Heteroaryl Halides with Aliphatic Amines

Rapid Aminations of Functionalized Aryl Fluorosulfates in Water

Aryl fluorosulfates of varying complexities have been used in amination reactions in water using a new Pd oxidative addition complex (OAC-1) developed specifically to match the needs of the fine chemicals industry, not only in terms of functional group tolerance, but also reflecting time considerations associated with these important C-N couplings. Also especially noteworthy is that they replace both PFAS-related triflates and nonaflates, which are today out of favor due to recent government regulations. The new complex based on the BippyPhos ligand is used at low loadings and under aqueous micellar conditions. Moreover, it is easily prepared and stable to long term storage. DFT calculations on the OAC precatalyst compare well with the X-ray structure of the crystals with π-complexation to the aromatic system of the ligand and also confirm the NMR data showing a mixture of conformers in solution that differ from the X-ray structure in rotation of the phenyl and t-butyl ligand substituents. An extensive variety of coupling partners, including pharmaceutically relevant APIs, readily participate under mild and environmentally responsible reaction conditions.

Development of a Deactivation-Resistant Dialkylbiarylphosphine Ligand for Pd-Catalyzed Arylation of Secondary Amines

Despite the prevalence of N-heteroarenes in small-molecule pharmaceuticals, Pd-catalyzed C-N cross-coupling reactions of aryl halides and amines containing these rings remain challenging due to their ability to displace the supporting ligand via coordination to the metal center. To address this limitation, we report the development of a highly robust Pd catalyst supported by a new dialkylbiarylphosphine ligand, FPhos. The FPhos-supported catalyst effectively resists N-heteroarene-mediated catalyst deactivation to readily promote C-N coupling between a wide variety of Lewis-basic aryl halides and secondary amines, including densely functionalized pharmaceuticals. Mechanistic and structural investigations, as well as principal component analysis and density functional theory, elucidated two key design features that enable FPhos to overcome the limitations of previous ligands. First, the ligated Pd complex is stabilized through its conformational preference for the O-bound isomer, which likely resists coordination by N-heteroarenes. Second, 3',5'-disubstitution on the non-phosphorus-containing ring of FPhos creates the ideal steric environment around the Pd center, which facilitates binding by larger secondary amines while mitigating the formation of off-cycle palladacycle species.

Adaptive Photochemical Amination via Co(II) Catalysis

Transition-metal-catalyzed amination of aryl halides is one of the most employed methods for constructing N-arylation adducts. However, the broad success of these reactions largely relies on the screening of precatalysts, elaborated ligands, and case-by-case optimization of reaction conditions (solvent, base, additive, temperature, etc.) for electronically or structurally diverse nucleophiles. Herein, we report an adaptive photochemical C-N coupling of aryl halides with various nitrogen nucleophiles (aliphatic and aromatic amines, amides, sulfonamides, pyrazoles, and ammonium salts) by Co(II) catalysis under the same reaction conditions (same precatalyst, same ligand, same base, same solvent, same temperature) without the addition of any exogenous photocatalyst. This photochemical amination features a wide substrate scope (>130 examples, up to 95% yield) with excellent functional group tolerance. Mechanistic studies indicate that these C-N coupling reactions may proceed via a Co(I)/Co(III) catalytic cycle.

Copper-Catalyzed Amination of Aryl Chlorides under Mild Reaction Conditions

We report a mild method for the copper-catalyzed amination of aryl chlorides. Key to the success of the method was the use of highly sterically encumbered N1,N2-diaryl diamine ligands which resist catalyst deactivation, allowing reactions to proceed at significantly lower temperatures and with a broader scope than current protocols. A sequence of highly chemoselective C-N and C-O cross-coupling reactions were demonstrated, and mechanistic studies indicate that oxidative addition of the Cu catalyst to the aryl chlorides is rate-limiting. We anticipate that the design principles disclosed herein will help motivate further advances in Cu-catalyzed transformations of aryl chlorides.

Synthesis and Reactivity of Dipalladated Derivatives of Terephthalaldehyde

The polynuclear complex [{μ-C1,C4,N,N″-C6H2{C(H)=N(nBu)}2-2,5}{Pd(μ-OAc)}]2 (I) reacts with tbbpy (4,4'-di-tert-butyl-2,2'-bipyridine) and TlOTf to form the dinuclear complex [{μ-C1,C4,N,N″-C6H2{C(H)=N(nBu)}2-2,5}{Pd(tbbpy)}2] (1). The hydrolysis of I with acetic acid in a 5:1 acetone/water mixture, in the presence of two equivalents of tbbpy and excess NaX (X = Br, I), yields the dipalladated terephthalaldehyde complexes [C6H2{PdX(tbbpy)}2-1,4-(CHO)2-2,5] [X = Br (2a), X = I (2b)], which are the first fully characterized complexes of this type. The reaction of 2a,b with CO results in the insertion of CO into both aryl-Pd bonds, forming [C6H2{C(O){PdX(tbbpy)}}2-1,4-(CHO)2-2,5] [X = Br (3a), X = I (3b)], which are the first examples of complexes with CO inserted into two separate aryl-metal bonds involving the same ligand. The bromo complex 2a reacts with excess XylNC in acetone, causing the precipitation of the dinuclear complex 2,3,6,7-tetrahydrobenzo[1,2-c:4,5-c']dipyrrole-1,5-dione-2,6-dixylyl-3,7-bis{=C(NHXyl)-C(=NXyl)-[PdBr(CNXyl)2]} (4), which is the result of the insertion of three molecules of the isocyanide into each aryl-Pd bond and the nucleophilic attack of one of them at each formyl group. When complex 4 reacts with TlOTf and residual water in 1,2-dichloroethane at 70 °C, depalladation occurs, and the organic compound 2,3,6,7-tetrahydrobenzo[1,2-c:4,5-c']dipyrrole-1,5-dione-2,6-dixylyl-3,7-bis{=C(NHXyl)-C(O)NHXyl} (5) can be isolated. The crystal structures of 1·4CHCl3, 4·2CH2Cl2·3hexane, and 5·2CDCl3 have been determined by X-ray crystallography.

Cu-Catalyzed Amination of Base-Sensitive Aryl Bromides and the Chemoselective N- and O-Arylation of Amino Alcohols

We report a general and functional-group-tolerant method for the Cu-catalyzed amination of base-sensitive aryl bromides including substrates possessing acidic functional groups and small five-membered heteroarenes. The results presented herein substantially expand the scope of Cu-catalyzed C-N coupling reactions. The combination of L8, an anionic N1,N2-diarylbenzene-1,2-diamine ligand, along with the mild base NaOTMS leads to the formation of a stable yet reactive catalyst that resists deactivation from coordination to heterocycles or charged intermediates. This system enables the use of low catalyst and ligand loadings. Exploiting the differences in nucleophile deprotonation in C-O and C-N coupling reactions catalyzed by Cu·L8 we developed a method to chemoselectively N- and O-arylate a variety of amino alcohol substrates. Employing NaOt-Bu as the base resulted exclusively in C-O coupling when the amino alcohols featured primary alcohols and more hindered amines or aniline groups. Utilizing NaOTMS enabled the ability to override the steric-based selectivity of these reactions completely and exclusively promoted C-N coupling regardless of the structure of the amino alcohol. The ability to invert the observed chemoselectivity is distinct from previously described methods that require protecting group manipulations or rely entirely on steric effects to control reactivity. These results substantially improve the scope of Cu-catalyzed C-N coupling reactions using N1,N2-diarylbenzene-1,2-diamine ligands and introduce a new chemoselective method to arylate amino alcohols.

Reductive Transamination of Pyridinium Salts to N-Aryl Piperidines

Saturated N-heterocycles are found in numerous bioactive natural products and are prevalent in pharmaceuticals and agrochemicals. While there are many methods for their synthesis, each has its limitations, such as scope and functional group tolerance. Herein, we describe a rhodium-catalyzed transfer hydrogenation of pyridinium salts to access N-(hetero)aryl piperidines. The reaction proceeds via a reductive transamination process, involving the initial formation of a dihydropyridine intermediate via reduction of the pyridinium ion with HCOOH, which is intercepted by water and then hydrolyzed. Subsequent reductive amination with an exogenous (hetero)aryl amine affords an N-(hetero)aryl piperidine. This reductive transamination method thus allows for access of N-(hetero)aryl piperidines from readily available pyridine derivatives, expanding the toolbox of dearomatization and skeletal editing.

A strategy for the controllable generation of organic superbases from benchtop-stable salts

Organic superbases are a distinct class of strong base that enable numerous modern reaction applications. Despite their great synthetic potential, widespread use and study of superbases are limited by their air sensitivity and difficult preparation. To address this, we report air-stable carboxylate salts of BTPP and P2-t-Bu phosphazene superbases that, when added to solution with an epoxide, spontaneously generate freebase. These systems function as effective precatalysts and stoichiometric prereagents for superbase-promoted addition, substitution and polymerization reactions. In addition to improving the synthesis, shelf stability, handling and recycling of phosphazenes, this approach enables precise regulation of the rate of base generation in situ. The activation strategy effectively mimics manual slow addition techniques, allowing for control over a reaction's rate or induction period and improvement of reactions that require strong base but are also sensitive to its presence, such as Pd-catalyzed coupling reactions.

Synthesis of alkenylphosphine oxides via Tf2O promoted addition-elimination of ketones and secondary phosphine oxides

Herein, we describe an efficient method for the synthesis of alkenylphosphine oxides via a Tf2O promoted addition-elimination process. Various diarylphosphine oxides and alkylarylphosphine oxides react with ketones smoothly and produce alkenylphosphine oxides in moderate to excellent yields with abundant functional group compatibility. In addition, several transformations and applications of the product also demonstrate the potential value of the methodology.

Anhydrous and Stereoretentive Fluoride-Enhanced Suzuki-Miyaura Coupling of Immunomodulatory Imide Drug Derivatives

Immunomodulatory imide drugs form the core of many pharmaceutically relevant structures, but Csp2-Csp2 bond formation via metal-catalyzed cross coupling is difficult due to the sensitivity of the glutarimide ring ubiquitous in these structures. We report that replacement of the traditional alkali base with a fluoride source enhances a previously challenging Suzuki-Miyaura coupling on glutarimide-containing compounds with trifluoroborates. These enabling conditions are reactive enough to generate these derivatives in high yields but mild enough to preserve both the glutarimide and its sensitive stereocenter. Experimental and computational data suggest a mechanistically distinct process of π-coordination of the trifluoroborate enabled by these conditions.

Aminative Suzuki-Miyaura coupling

The Suzuki-Miyaura and Buchwald-Hartwig coupling reactions are widely used to form carbon-carbon (C-C) and carbon-nitrogen (C-N) bonds, respectively. We report the incorporation of a formal nitrene insertion process into the Suzuki-Miyaura reaction, altering the products from C-C-linked biaryls to C-N-C-linked diaryl amines and thereby joining the Suzuki-Miyaura and Buchwald-Hartwig coupling pathways to the same starting-material classes. A combination of a bulky ancillary phosphine ligand on palladium and a commercially available amination reagent enables efficient reactivity across aryl halides and pseudohalides, boronic acids and esters, and many functional groups and heterocycles. Mechanistic insights reveal flexibility on the order of bond-forming events, suggesting potential for expansion of the aminative cross-coupling concept to encompass diverse nucleophiles and electrophiles as well as four-component variants.

Ligated Pd-Catalyzed Aminations of Aryl/Heteroaryl Halides with Aliphatic Amines under Sustainable Aqueous Micellar Conditions

Sustainable technology for constructing Pd-catalyzed C-N bonds involving aliphatic amines is reported. A catalytic system that relies on low levels of recyclable precious metal, a known and commercially available ligand, and a recyclable aqueous medium are combined, leading to a newly developed procedure. This new technology can be used in ocean water with equal effectiveness. Applications involving highly challenging reaction partners constituting late-stage functionalization are documented, as is a short but efficient synthesis of the drug naftopidil. Comparisons with existing aminations highlight the many advances being offered.

Nanoparticles as Heterogeneous Catalysts for ppm Pd-Catalyzed Aminations in Water

A general protocol employing heterogeneous catalysis has been developed that enables ppm of Pd-catalyzed C-N cross-coupling reactions under aqueous micellar catalysis. A new nanoparticle catalyst containing specifically ligated Pd, in combination with nanoreactors composed of the designer surfactant Savie, a biodegradable amphiphile, catalyzes C-N bond formations in recyclable water. A variety of coupling partners, ranging from highly functionalized pharmaceutically relevant APIs to educts from the Merck Informer Library, readily participate under these environmentally responsible, sustainable reaction conditions. Other key features associated with this report include the low levels of residual Pd found in the products, the recyclability of the aqueous reaction medium, the use of ocean water as an alternative source of reaction medium, options for the use of pseudohalides as alternative reaction partners, and associated low E factors. In addition, an unprecedented 5-step, one-pot sequence is presented, featuring several of the most widely used transformations in the pharmaceutical industry, suggesting potential industrial applications.

First atroposelective Chan-Lam coupling for the synthesis of C-N linked biaryls

The first atroposelective Chan-Lam coupling for the synthesis of C-N axial enantiomers is reported with good yields and ee. MnO2 additive is crucial for the success of the coupling. The longstanding problem of the lack of enantioselective synthesis to make chiral C-N linked atropisomers is solved.

A General Strategy for N-(Hetero)arylpiperidine Synthesis Using Zincke Imine Intermediates

Methods to synthesize diverse collections of substituted piperidines are valuable due to the prevalence of this heterocycle in pharmaceutical compounds. Here, we present a general strategy to access N-(hetero)arylpiperidines using a pyridine ring-opening and ring-closing approach via Zincke imine intermediates. This process generates pyridinium salts from a wide variety of substituted pyridines and (heteroaryl)anilines; hydrogenation reactions and nucleophilic additions then access the N-(hetero)arylpiperidine derivatives. We successfully applied high-throughput experimentation (HTE) using pharmaceutically relevant pyridines and (heteroaryl)anilines as inputs and developed a one-pot process using anilines as nucleophiles in the pyridinium salt-forming processes. This strategy is viable for generating piperidine libraries and applications such as the convergent coupling of complex fragments.

Solvent-Free Buchwald-Hartwig Amination of Heteroaryl Chlorides by N-Heterocyclic Carbene-Palladium Complex (SIPr)Ph2Pd(cin)Cl at Room Temperature

Using the robust N-heterocyclic carbene-palladium complex (SIPr)Ph2Pd(cin)Cl, a highly efficient and easy-to-operate method has been developed at room temperature for the solvent-free Buchwald-Hartwig amination of heteroaryl chlorides with various amines. The amount of catalyst can be as low as 0.05 wt %. The system was demonstrated on 47 substrates and successfully applied to the synthesis of commercial pharmaceuticals and candidate drugs with high yields. Furthermore, the protocol can be used to prepare aniline derivatives on a multigram scale without yield loss.