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.

Chiral, air stable, and reliable Pd(0) precatalysts applicable to asymmetric allylic alkylation chemistry

Stereoselective carbon-carbon bond formation via palladium-catalyzed asymmetric allylic alkylation is a crucial strategy to access chiral natural products and active pharmaceutical ingredients. However, catalysts based on the privileged Trost and Pfaltz-Helmchen-Williams PHOX ligands often require high loadings, specific preactivation protocols, and excess chiral ligand. This makes these reactions uneconomical, often unreproducible, and thus unsustainable. Here we report several chiral single-component Pd(0) precatalysts that are active and practically-applicable in a variety of asymmetric allylic alkylation reactions. Despite the decades-long history and widespread use of Trost-type ligands, the precatalysts in this work are the only reported examples of stable, isolable Pd(0) complexes with these ligands. Evaluating these precatalysts across nine asymmetric allylic alkylation reactions reveals high reactivity and selectivity at low Pd loading. Importantly, we also report an unprecedented Pd-catalyzed enantioselective allylation of a hydantoin, achieved on gram scale in high yield and enantioselectivity with only 0.2 mol% catalyst.

Enolate addition to bicyclobutanes enables expedient access to 2-oxo-bicyclohexane scaffolds

We report the synthesis of 2-oxo-bicyclo[2.1.1]hexanes (2-oxo-BCHs) from bicyclobutanes (BCBs) and readily available enolate precursors. Glycine-derived enolates directly give protected 2-oxo-3-amino-BCH derivatives that can be further functionalized. Arylacetate derivatives are also suitable enolate precursors, giving 2-oxo-3-aryl-BCH scaffolds from readily available starting materials.

A broadly applicable quantitative relative reactivity model for nucleophilic aromatic substitution (SNAr) using simple descriptors

We report a multivariate linear regression model able to make accurate predictions for the relative rate and regioselectivity of nucleophilic aromatic substitution (S_NAr) reactions based on the electrophile structure. This model uses a diverse training/test set from experimentally-determined relative S_NAr rates between benzyl alcohol and 74 unique electrophiles, including heterocycles with multiple substitution patterns. There is a robust linear relationship between the experimental S_NAr free energies of activation and three molecular descriptors that can be obtained computationally: the electron affinity (EA) of the electrophile; the average molecular electrostatic potential (ESP) at the carbon undergoing substitution; and the sum of average ESP values for the ortho and para atoms relative to the reactive center. Despite using only simple descriptors calculated from ground state wavefunctions, this model demonstrates excellent correlation with previously measured S_NAr reaction rates, and is able to accurately predict site selectivity for multihalogenated substrates: 91% prediction accuracy across 82 individual examples. The excellent agreement between predicted and experimental outcomes makes this easy-to-implement reactivity model a potentially powerful tool for synthetic planning.

Beyond Bioisosteres: Divergent Synthesis of Azabicyclohexanes and Cyclobutenyl Amines from Bicyclobutanes

The development of two divergent and complementary Lewis acid catalyzed additions of bicyclobutanes to imines is described. Microscale high-throughput experimentation was integral to the discovery and optimization of both reactions. N-arylimines undergo formal (3+2) cycloaddition with bicyclobutanes to yield azabicyclo[2.1.1]hexanes in a single step; in contrast, N-alkylimines undergo an addition/elimination sequence to generate cyclobutenyl methanamine products with high diastereoselectivity. These new products contain a variety of synthetic handles for further elaboration, including many functional groups relevant to pharmaceutical synthesis. The divergent reactivity observed is attributed to differences in basicity and nucleophilicity of the nitrogen atom in a common carbocation intermediate, leading to either nucleophilic attack (N-aryl) or E1 elimination (N-alkyl).

A reactivity model for oxidative addition to palladium enables quantitative predictions for catalytic cross-coupling reactions

Making accurate, quantitative predictions of chemical reactivity based on molecular structure is an unsolved problem in chemical synthesis, particularly for complex molecules. We report an approach to reactivity prediction for...

An evaluation of palladium-based catalysts for the base-free borylation of alkenyl carboxylates

Palladium catalysis can achieve the base-free borylation of alkenyl carboxylates, enabling direct access to functionalized enones and heterocycles.

The influence of additives on orthogonal reaction pathways in the Mizoroki–Heck arylation of vinyl ethers

Translating microscale high-throughput screening hits into preparative scale chemistry often requires an understanding of scale-dependent factors, such as the effect of additives on catalyst activation pathways.

High length-to-width aspect ratio lead bromide microwires via perovskite-induced local concentration gradient for X-ray detection

Well-oriented PbBr₂ microwires with a length-to-width ratio of up to 5000 were grown using a concentration gradient in co-crystallization with perovskite. Planar-integrated microwires showed a response to X-ray photons.

C–O Bond Activation as a Strategy in Palladium-Catalyzed Cross-Coupling

The activation of strong C–O bonds in cross-coupling catalysis can open up new oxygenate-based feedstocks and building blocks for complex-molecule synthesis. Although Ni catalysis has been the major focus for cross-coupling of carboxylate-based electrophiles, we recently demonstrated that palladium catalyzes not only difficult C–O oxidative additions but also Suzuki-type cross-couplings of alkenyl carboxylates under mild conditions. We propose that, depending on the reaction conditions, either a typical Pd(0)/(II) mechanism or a redox-neutral Pd(II)-only mechanism can operate. In the latter pathway, C–C bond formation occurs through carbopalladation of the alkene, and C–O cleavage by β-carboxyl elimination.

1 Introduction

2 A Mechanistic Challenge: Activating Strong C–O Bonds

3 Exploiting Vinylogy for C–Cl and C–O Oxidative Additions

4 An Alternative Mechanism for Efficient Cross-Coupling Catalysis

5 Conclusions and Outlook

Oxidative addition of activated aryl-carboxylates to Pd(0): divergent reactivity dependant on temperature and structure

With the exception of activated sulfonate esters, oxidative addition of Ar-O bonds to Pd(0) complexes is extremely rare. This has led to a general perception that Pd-catalyzed cross-coupling is not feasible with O-based electrophiles such as aryl esters. We report that pyrone and coumarin esters do undergo oxidative addition to Pd(PCy3)2, with Pd insertion into either the acyl-O or Ar-O bond. Addition of the acyl-O bond to Pd(0) is kinetically favoured and reversible, while addition of the Ar-O bond is thermodynamically favoured. Using a larger and more electron-rich pivalate derivative disfavours acyl-O cleavage, enabling selective oxidative addition of the Ar-O bond and corresponding catalytic cross-coupling.

High-Throughput Discovery and Evaluation of a General Catalytic Method for N-Arylation of Weakly Nucleophilic Sulfonamides

Through targeted high-throughput experimentation (HTE), we have identified the Pd/AdBippyPhos catalyst system as an effective and general method to construct densely functionalized N,N-diaryl sulfonamide motifs relevant to medicinal chemistry. AdBippyPhos is particularly effective for the installation of heteroaromatic groups. Computational steric parametrization of the investigated ligands reveals the potential importance of remote steric demand, where a large cone angle combined with an accessible Pd center is correlated to successful catalysts for C-N coupling reactions.

Convergent Synthesis of the NS5B Inhibitor GSK8175 Enabled by Transition Metal Catalysis

A convergent eight-stage synthesis of the boron-containing NS5B inhibitor GSK8175 is described. The previous route involves 13 steps in a completely linear sequence, with an overall 10% yield. Key issues include a multiday S_NAr arylation of a secondary sulfonamide using HMPA as solvent, multiple functional group interconversions after all of the carbon atoms are installed (including a Sandmeyer halogenation), use of carcinogenic chloromethyl methyl ether to install a protecting group late in the synthesis, and an unreliable Pd-catalyzed Miyaura borylation as the penultimate step. We have devised an orthogonal approach using a Chan-Lam coupling between a halogenated aryl pinacol boronate ester and an aryl methanesulfonamide. This reaction is performed using a cationic Cu(I) precatalyst, which can be easily generated in situ using KPF₆ as a halide abstractor. High-throughput screening revealed a new Pd catalyst system to effect the penultimate borylation chemistry using simple monodentate phosphine ligands, with PCyPh₂ identified as optimal. Reaction progress analysis of this borylation indicated likely mass-transfer rate limitations under standard conditions using KOAc as the base. We have devised a K₂CO₃/pivalic acid system as an alternative, which dramatically outperforms the standard conditions. This new synthesis proceeds in eight stages with a 20% overall yield.

Titanium amidate complexes as active catalysts for the synthesis of high molecular weight polyethylene

A series of titanium and zirconium bis(amidate) complexes of the type L2MX2, where L is an amidate ligand, and X is either –NMe2 or –Cl, were prepared in 60%–83% yield and fully characterized. Multiple binding motifs are observed as the amidate ligand can bind in κ1- and κ2-modes. These complexes were then subjected to screening the catalytic polymerization of ethylene. All catalysts, after reaction with suitable co-catalyst, were functional for ethylene polymerization, though not for the copolymerization of ethylene and longer linear 1-alkenes. Polyethylene was formed in the range of 1000–4000 kDa, and with PDI values as low as 1.3. These long polymeric chains are considered as ultra-high molecular weight polyethylene (UHMWPE).

A palladium-catalysed multicomponent coupling approach to conjugated poly(1,3-dipoles) and polyheterocycles

Conjugated polymers have emerged over the past several decades as key components for a range of applications, including semiconductors, molecular wires, sensors, light switchable transistors and OLEDs. Nevertheless, the construction of many such polymers, especially highly substituted variants, typically involves a multistep synthesis. This can limit the ability to both access and tune polymer structures for desired properties. Here we show an alternative approach to synthesize conjugated materials: a metal-catalysed multicomponent polymerization. This reaction assembles multiple monomer units into a new polymer containing reactive 1,3-dipoles, which can be modified using cycloaddition reactions. In addition to the synthetic ease of this approach, its modularity allows easy adaptation to incorporate a range of desired substituents, all via one-pot reactions.

The requirement for multistep synthesis can render the fabrication of highly substituted polymers particularly troublesome. Here, the authors take advantage of metal-catalysed multicomponent polymerization to synthesize a large family of such materials with ease from single-pot reactions.

Upgrading light hydrocarbons via tandem catalysis: a dual homogeneous Ta/Ir system for alkane/alkene coupling

Light alkanes and alkenes are abundant but are underutilized as energy carriers because of their high volatility and low energy density. A tandem catalytic approach for the coupling of alkanes and alkenes has been developed in order to upgrade these light hydrocarbons into heavier fuel molecules. This process involves alkane dehydrogenation by a pincer-ligated iridium complex and alkene dimerization by a Cp*TaCl2(alkene) catalyst. These two homogeneous catalysts operate with up to 60/30 cooperative turnovers (Ir/Ta) in the dimerization of 1-hexene/n-heptane, giving C13/C14 products in 40% yield. This dual system can also effect the catalytic dimerization of n-heptane (neohexene as the H2 acceptor) with cooperative turnover numbers of 22/3 (Ir/Ta).

Asymmetric hydroamination catalyzed by in situ generated chiral amidate and ureate complexes of zirconium — Probing the role of the tether in ligand design

Simple chiral proligands have been synthesized from inexpensive chiral starting materials. These amidate and ureate ligands support zirconium complexes that successfully catalyze intramolecular hydroamination with up to 26% ee. Several elements necessary for successful ligand design are highlighted and discussed. In particular, the strict control of metal geometry through multidentate tethered ligands is determined to be an essential aspect of future ligand development.

An Easy-To-Use, Regioselective, and Robust Bis(amidate) Titanium Hydroamination Precatalyst: Mechanistic and Synthetic Investigations toward the Preparation of Tetrahydroisoquinolines and Benzoquinolizine Alkaloids

Amidate-supported titanium amido complexes are efficient and regioselective precatalysts for intermolecular hydroamination of terminal alkynes with primary amines. The synthesis and characterization of the first bis(amidate)-supported titanium-imido complex is reported. Its role as the active catalytic species is suggested in the course of product distribution studies using deuterated substrates. The bis(amidate)-supported precatalysts exhibit good functional-group tolerance, even performing hydroaminations in the presence of ester and amide groups. This functional-group tolerance was exploited in the synthesis of a variety of 1-substituted tetrahydroisoquinoline alkaloids and a one-pot hydroaminative procedure for the high yielding preparation of the benzo[a]quinolizine skeleton.

Intramolecular hydroamination of unactived olefins with Ti(NMe2)4 as a precatalyst

Commercially available Ti(NMe(2))(4) has been used effectively as a precatalyst in a facile protocol for the intramolecular hydroamination of aminoalkenes to yield pyrrolidine and piperidine heterocyclic products with isolated yields up to 92%. Geminally substituted substrates display the highest reactivity. This precatalyst is also effective for the hydroamination of activated internal alkenes, providing access to more complex heterocyclic target molecules.